5 year old YT video that still rocks and provided the information I was looking for. I picked this same unit up today off eBay for my starter electronics bench. Thanks.
I just got one very similar from sphere.bc and was wondering why I saw voltage to ground on the output with my Fluke but not with my cheap Walmart meter. Ghost voltages, evidently the fluke input impedance is high enough to not pull the voltage induced between ground and the hot wire to zero. The Walmart meter does though. I see I did good by not buying a dual ended C13-C14 cable and went with a 5-15 (regular outlet) cable. Now I just need to remove ground from the output. A recommendation for secondary probes, remove the ground lead so that risk is removed, you'll get all the ground reference to the scope that you need from the first probe ground clip.
I also just bought a similar one (1000 VA Powertronix). It also said OUTPUT FLOATING - NEUTRAL CONDUCTOR NOT GROUNDED and has 2 standard 15A receptacles. When I removed the chassis ground connection to each of the receptacles, I found out that the receptacle grounds were still grounded to the chassis ground via the receptacle mounting screw. I replaced the receptacles with orange “isolated ground”receptacles and now the receptacle grounds are not grounded to chassis ground.
I know this is an old video, but I have to thank you for it. I've watched so many videos and watched so many forum posts trying to figure out exactly what I can and can't do and why, but I always walked away even more confused. Thanks for making it simple and explaining specifically what to watch out for.
Doug. You are truly an honest national treasure. Thank God for you and that are with us. Overwhelmed with gratitude. You have given me a new career. You have made it all so clear. Thank you.
This video covers one of the most important topics we ampaholics need to understand! Uncle Doug, your explanations are clear, thorough and very, very informative. This is one video that deserves multiple viewings! Thank you so much for your wonderful videos and kudos on your teaching style. Your former students were very lucky to have you for a teacher!
I have watched this video a few times as well as several others to gain a firm understanding of the topic. It has been years since I made my isolation transformer from a "medical" grade power conditioner. I opened it up, removed everything except the transformer, outlets, switch and fuse. I then made sure there was NO connection from the secondary to ANY other line or the chassis. I then wired the outlets to the 2 wires from the secondary side of the transformer. It was an easy conversion. To triple check everything, I used the ohmeter and checked between all 3 prongs of the primary plug to all 3 connections of the outlet plugs on the secondary side. THEN I clearly labeled the power conditioner that it was ISOLATION ONLY, NO GROUND CONNECTED just in case someone gets ahold of it and I am no longer around to explain the unit. This is SUCH an important safety device on the test bench!!!!
This information is exceptional and necessary for anyone attempting this type of testing. In this world where no one wants to take responsibility for their own actions, this is critical to deflect any liability, but also to insure proper knowledge. Make yourself aware of all the information available and proceed with all due care. But I have found that there are always new ways to get in trouble. My favorite saying as I have traveled through this life has become "If you're going to be Dumb, you had better be Tough !" Something I unfortunately witness more often than I'd like. Once again Uncle Doug, a public service message and useful knowledge. Thanks jj
Only just found this video although it's 4 years old. Very good advice here. I have always said NEVER ground the secondary side of the isolation transformer, it will make it worse than useless. I say worse than useless because any accidents happening on the secondary side will not trip the RCD (GFCI) on the primary side because no imbalance will happen.
Thanks Uncle Doug, I bought a Toroid Medical Grade Isolation Transformer ISB-100W 1000VA last week and I was thinking about the ground connection topology for about a week and I just saw this video , It's like you where reading my mind or something ha ha. Thank you again sir, Keep up all the good work you do.
I got several Powertronix toroid transformers. One is a beefy 1500VA with 20A receptacles with green dot indicators. The neutral is floating, but the grounds are tied to the mains ground at the chassis... AFAIK, the reason the isolated receptacle grounds are still tied to mains ground is that hospital ground wires are insulated and tie to a mains bus bar separately from the rest of the building ground wires (such as metal wall boxes) before all grounds tie back to a common Earth. Perhaps some medical transformers are meant for use in hospital wiring environments, while others were meant for in-home use where hospital-grade wiring is not present.
At 29:50 I would say that since the first ground is already connected you don’t need to even connect the second one since it is already done. Great video thank you! Everyone mentions the oscilloscope ground problem but almost everyone fails to clearly explain it as you did, thanks Uncle Doug and keep up the great videos!
Excellent video. I have an isolation transformer with dual toroidal transformers that I converted to a tech service unit. I disconnected the ground terminal connections to the output receptacles and tested the receptacle ground to make sure it was disconnected. The receptacle still had a connection to the chassis ground through the receptacle mounting ears. I solved this by installing isolated ground receptacles which have no redundant ground connection to the mounting ears. Thank you for all your videos.
Thank you for this PRACTICAL *demonstration* of potentially dangerous grounding situations. I've read about the theory, watched the videos that talk about it but I never fully understood until now. As a visual learner demonstrations like this are invaluable.
This is a great video! Everything explained in detail, I now have a clear picture of isolation transformers and how earth grounding and chassis grounding work. Thanks a million Doug!!!
This is the best video on iso transformers that I've ever seen. I didn't fully understand how this all works until I watched this video a few times. Thank you Uncle Doug!
Another military precision video, excellently planned, executed and payload delivered. At least we can rely on your vids to cut through the bs. Thanks for your time producing these vids.
Another good'n. I agree as to the mis-leading multiple opinions floating around the internet. A quick thought exercise one afternoon for me concluded the same as you've shown (grounding the DUT chassis essentially defeats the IT). By providing the detailed empirical evidence through your experiment you have removed any doubts and provided fact over opinion. Thank you. As we say in engineering, "Without data, you're just another person with an opinion." Or... "I'm from Missouri on this one, Show Me !"
Thanks so much, CA, especially for recognizing the benefits of empirical evidence over mere supposition. The more I read on the Internet, the more confused I became.....thus the need to actually discover the truth through experimentation.
ADDITIONAL "RULES": 1.) NEVER operate an ungrounded oscilloscope. Depending on where you attach the ground clips, the chassis and external metal parts may acquire a voltage potential that is lethal......to you. 2.) Although we must be careful with oscilloscope ground clips, the probes are extremely high impedance (1Meg to 10Meg) and thus tolerant of relatively high voltages; HOWEVER, each probe has a maximum rated voltage limit, beyond which you risk breaching the insulation between signal and shield......with disastrous results. 3.) It has come to my attention that simply disconnecting the (green) receptacle-grounding wires from the chassis of the I.T. may not completely eliminate the earth-grounding capability of the receptacles, since the receptacles may also have an internal connection to the chassis. It is safer to use a 3 to 2 adapter to avoid this problem. 4.) For those who use a Frequency Generator connected to the DUT, be aware that in most cases, the shield of the FG cable is earth-grounded and will give ground reference to the chassis of the DUT. 5.) Although the Medical I.T. used in the video costs about $650 new, they are commonly available on Ebay for around $65 (what I paid).
Hi Uncle Doug--I take your point, however, as a working EE, I can say that using a scope with a "cheater plug" that eliminates the scope's connection to the safety ground is standard operating practice. The key to doing this safely is understanding the device you are testing. For example, if you are probing equipment that has a 3-wire cord, and you know it is connected correctly, then connecting the scope's ground clip to the chassis of the device under test grounds the scope and all is well--even when using a cheater plug with the scope. The most common reason to disconnect the scope ground is to avoid a ground loop between the scope and a grounded piece of equipment. In general, the best situation is to have one connection to ground. Otherwise the ground loop created by grounding both the scope and the device under test can cause false noise to appear on the scope trace. This false signal can drive one crazy and cause all kinds of wasted time chasing a problem that isn't there. My usual rule is simple--if the device under test is not grounded, then leave the scope grounded. If the device under test is grounded, then unground the scope if the observed noise floor is oddly high. BTW, one good test is to connect the scope's ground clip to the device under test, and then connect the scope's probe to the exact same spot. Turn the sensitivity of the scope channel all the way up and observe the scope trace. If you see noise there, leave the scope probe connected to its own ground, but disconnect from the device under test. If the trace shows less noise, then you know you have a ground loop.
Excellent information, as always, Roiderien. Since most of the viewers are novices with oscilloscope use, I felt it best to keep the instructions as clear, basic, and concise as possible. The moment that alternatives are presented, confusion arises. Thanks again for your counsel.
That's why my entire bench is ground-lifted. It is not uncommon for tube amps to draw 40 to 50 watts idling. These 50 watts transformers were already too close to their limit to provide adequate voltage. The idea is good, though.
Dear Uncle Doug, Thank you for your video on Isolation Transformers (IT). Your experiments with an IT greatly helped me to observe its value in Test and Measurement. Your story about horses and barns within that video ended up to be the most important lesson for me. Struggles to Understand two issues Your story’s value became apparent as I struggled to understand the impact on my safety of an oscilloscope’s connection to Earth Ground when a power isolation transformer was used in a manner that did not expose the test circuit to a ground. That struggle also revealed that I did not understand how multiple grounds (at least scope ground and power ground) might impact not only my safety but also the safety of test and measurement gear and the circuit being tested. Understanding Issues with your horse story In that struggle to create a story that enables me to assess a strange lab situation so that I make decisions which protect my personal safety and gear safety, I kept replaying your story of the electron horses leaving their stable (Secondary Winding of the isolation transformer) and returning to the stable independent of any Earth Ground attached to the circuit. I heard that the horses are never interested in deviating from their route (stable to mtns to stable) but I could not think with that information. My story - shift in emphasis The story that I finally created that helped me understand the impact of Earth Ground on my safety in test and measurement shifted emphasis from “Earth Ground” to “Short Circuit”. It took me awhile to realize that the reason the horses did not try the Earth Ground route had less to do with "Earth Ground" but more to do with the fact that the Earth Ground path never led back to the stable. So to shift emphasis in my story, I replaced "Earth Ground" explaining Safety Issues (Using a Scope) with the concept of a Short Circuit. I start with a “wire” that is connected to the test circuit but hanging out in space touching nothing but air. We are NOT going to entice the horses to go down our wire hanging out in space until we attach the wire's other end to a place in the circuit that short circuits at least one of the circuit elements. Then the horses will readily gallop down the wire because it is so much easier for them than going through a short circuit than a circuit element on their way back to the stable. The horses galloping down our wire is fundamentally due to a short circuit, not “Earth Ground” in my story. The essence of the above story, hence, becomes “Discovering unintended short circuits” when we engage in Test and Measurement. Any time we can offer the horses an easier path home, they will take it. If our body can become an unintended path back to the stable, we should understand how that could occur. So the story then focuses on “what are the various ways a short circuit can occur”. Less abstractly, what kinds of wire can we put across any circuit element(s) creating a short circuit. Aside from a wire made out of metal we readily have two other virtual wires. (1) One virtual wire is above the circuit board: Air. Imagine a wire composed of air bypassing any circuit element just like a piece of metal wire. Air that is humid acts somewhat like a metal wire if the horses are moving fast enough. You can imagine the air has the shape of a wire as the horses are seeking to go as directly as possible from one of the wire ends to the other. (2) the other virtual wire is below the circuit board. Imagine a wire composed of dirt or “ground” by-passing any circuit element just like a piece of metallic wire. Ground that is humid enough acts just like a virtual wire "Oh, but wait", you say. "The Ground virtual wire is not touching the circuit board". So lets help the Ground virtual wire. Let’s connect a conductive wire to one end of a Ground virtual wire and another conductive wire to the other end of the Ground virtual wire. We can now connect those conductive wires across at least one or more circuit elements. The horses will absolutely run down one conductive wire, then through the Ground virtual wire, and then back out the other conductive wire to the stable. The Ground virtual wire acts as a "wire" as the horses are seeking to go as directly as possible from one of the connected conductive wires to the other connected conductive wire. Many of us fail to recognize that those two conductive wires connected to the Ground virtual wire actually exist. One conductive wire is easy: the human body. It can touch the circuit in a location and allow the horses to gallop through the human body to a body part that is touching the Ground virtual wire. The other conductive wire coming out of the Ground virtual wire is not as obvious. There are actually two conductive wires available. The Neutral wire in a power line is connected to the Ground virtual wire and the "power ground" wire in a power line is connected to the Ground virtual wire. If we use an isolation transformer, we eliminate the Neutral wire as a connection to the Ground virtual wire. But the Power ground wire can still there in a sneaky way. The scope probe always looks at the voltage ACROSS circuit elements. The probe hook connects to one side of circuit elements and the probe alligator clip is connected to the other side of the circuit elements. The probe alligator clip is connected to the Ground virtual wire through the three prong power cord of the scope To summarize this last point: we can short out any circuit element by touching one side of a circuit element with our body which allows the horses to run down or on our body to the Ground virtual wire....where they run directly to the power ground wire of the scope....then through the scope and out the probe cable to the probe alligator clip where they connect to bypassed circuit element(s) and race back to their stable. To summarize the big picture, when engaged with test and measurement, I now look for all ways that I might inadvertently short out a portion of the circuit being tested. One way that I can short out a circuit Is through my body into the Ground virtual wire which is connected to the scope power ground which is connected to the scope alligator wire connected to the circuit being tested. Multiple Grounds Multiple grounds ( ie: a ground pin of an isolation transformer outlet connected to power ground as well as to the chassis of a circuit being tested) is another wire through which the horses can run. In terms of human safety, a short circuit can take place through a human body and the power ground as well as through the scope ground. There is also an equipment safety issue. A short can take place with the short circuit being the Virtual Wire Ground and at least two grounds one on each end of the Virtual Wire Ground. Considerable current could run through the scope alligator ground and through the scope chassis. In addition to the above current due to by-passing a circuit element, the two grounds, even if just inches apart, are not likely to be at the same potential thus inducing current flow through the alligator clip.
Brilliant idea! I've never considered using an isolation transformer to protect my oscilloscope, or me. Excellent advice, humorous, and exceptionally grammatically correct! You sir, are the Jack Benny of amp repair videos.😁
Absolutely invaluable information here. I learned the basics on tube amps from my tech-then-mentor in the late-90s. I learned on his lovely Tek 2465B. As a musician I learned just enough to do my own basic repair and maintenance and was able to help out friends with low-voltage repairs on pedals and things. I always wanted to get an oscilloscope but I was just never able to make it a priority. Fairly recently, I sat down and passed my Tech and General amateur radio exams and it got me wanting to get serious about electronics again. I now have two scopes, a 2465B and a DS1054Z, and as I mentioned, this series has been invaluable to me getting back into it. Many thanks for this very clear and concise content!
Great info Doug. I'm an old school electronics tech who received a ASEET degree. The double "E" says I was trained in power systems, motors, etc. As well as "electronics". Your video clears up what "ground" means in different scenarios. There are many oscilloscopes out there that thank you. :) Thanks and keep up the great training you do. I'm a patron and will always be.
I certainly hope you and your channel are active on youtube. Our knowledge bases cross in part, but the important point is you remind me of my mentor. His name was Roger Tokarz and he had the same gift of explaining relatively complex concepts with easy to understand basic analogies. You Sir, have the same gift. I am listening to your multiple videos (with the same gusto I had for Roger’s explanations on how resonant circuits worked in telephony). You have made my day, please, if you are able acknowledge your channel as active and is it possible to contact you privately (if not, I totally understand). Thank you from the bottom of my heart as you even sound like Roger, with the same command of the English langue. It has brought tears to my eyes as I lost my mentor a few years ago. Certain people touch your life with extraordinary results, Roger did that for me. Thank you so much for the effort and time required to make these videos. You have a new FAN! Jeff in Michigan
Thanks for the very nice comments, Jeff. We generally post around one lengthy video per month. Hopefully, our latest offering will be available in about two weeks.
Uncle Doug, this video inspired me to do the same. I purchased a medical isolation transformer and disconnect the ground wires on the outlets. Thanks for the very helpful videos you produce.
Excellent. A few weeks ago I had just studied the confusing subject of grounding and the different ways isolation transformers can be wired. You video explains everything. The best out there about the subject.
Thanks for another excellent video! You do such an amazing job explaining and making what is difficult for my tiny brain to get wrapped around so much less confusing. Thanks for addressing and making the IT mysteries much less mysterious! Probably saved someone from wrecking equipment and possibly another from injury. You're a good man, Uncle Doug! Thanks!
Thank You So Very Much For Your Time & Effort! I just bought a N.O.S. BK 2120 scope and powered it up on a variac, it works like a champ. Now seeing your video I "Will" be safe using my scope.
Thank you for your video, I am picking the same IT you have in the video. Found one in used one on the Facebook marketplace. Thank you for keeping us safe.
greetings uncle Doug, you are one awesome teacher on every one of your videos...you inspired me to buy my first oscilloscope after being intimidated in the past about owning one because so many buttons on them and not knowing what they do but thanks to your oscilloscope videos i now have one the reason you got higher dc voltages in the plate of the tube you tested in this video though, is because the isolation XFMR is for 120v input, but now the voltages coming from the wall are not longer 120v. today voltages from the wall are more than 120v possible 123v. or 120v. the more the input, the higher the output. thanks to your isolation XFMR videos i now have an ACONE transformer that was stored inside of an old desk at my job. for many years i knew it was there but i never paid any interest in it because in only had one receptacle outlet and the XFMR was only labeled as "power supply''. so no big deal! but after watching these video series i went and took a look at it and found out it had a 2.4 amp in the secondary side of the XFMR with a receptacle that had the neutral terminals connected to ground via the mounting plate on such receptacle. i replaced the receptacle with an isolated ground receptacle and now i have a real tech grade XFMR that is big enough for what i will use it. i am also working on a 4.2 amp on the secondary side that i found on e bay for 35 dls. plus 25 dls. shipping. thanks so much for posting your very instructive videos.
Hi, Another good empirical presentation. Effectively what you are creating is a bathroom shaver socket arrangement. These are always 2 pin to prevent ground reference and you wouldn't open up your shaver and ground it in the name of safety ;-)
Thank you very much for making this video. It cleared up a lot about this subject that i wasnt sure about. Very valuable information. All the best to you, sir!
Just had to comment on your feral cat and his love of catnip. I have a male cat that I reward with catnip that I grow in my yard. He loves it and does the same as Olie by rolling in it except my cat, Pinto, eats it also. My female cat doesn't seem to even notice it. From what I have noticed over the years, it seems only male cats love the catnip. Not sure if that is really true but it is what I have seen. So good to see you posting again.
Thanks so much, David. It's always good to hear from you. We grow catnip too and have found, like you, that different cats respond differently to it. I did some research and found that "catnip appreciation" is a genetic trait. Jack strictly eats the catnip, Ollie rolls in it, and (no doubt) some cats ignore it. I'm not sure if it's a sex-linked trait, but will observe to see if that's the case.
i have a female cat that eats it (literally chews up the dried vegetative matter - strange). She doesn't really exhibit any behavioral changes, though. Certainly doesn't do the rolling around business.
I was going to say another premium video, but ALL of your videos are always premium. I always endorse all of them, especially with new kids on the block. My workbench has, and always will have an iso tranny. I tend to favor the old RCA ISO-Tap which was designed for techs. The receptacles are 2 prong and are fully isolated. Talking about creating a deadly ground reference with a scope probe brings me back to a serious mistake I once made but was extremely lucky in the end. (Yes, even us veterans made them). I remember working on an AA5 AC/DC radio with a hot chassis. I connected my scope to check to see if the local oscillator was functioning. I made the mistake of not connecting the DUT to the iso tranny. When I touched the chassis with the scope's ground lead, Bang!! The ground lead created a fireworks display and blew off the probe. Luckily, the ground lead acted as a fuse, and I got lucky that no damage was done to my Textronix 2465B scope. I do however, recommend investing in a Differential Probe for your scope. Granted, they are quite expensive, but the probes are isolated from any ground potential if used properly. Keep the great videos coming, your viewership is intense...many of your viewers are also professional working technicians such as myself and many others. It's good for the new kids, and us veterans...(refresher) :) Has Jack finished his electronics training yet? He will need the theory under his hat to be the official shop cat :)
Greetings and thanks, Frank. I'm glad to hear that you enjoy the videos and, despite all your years of experience, still appreciate a little refresher once in a while. Yes, Jack has been studying hard and soon will be serving his apprenticeship in videography.
I was given a medical isolation transformer and the neutral was bonded to the case which was grounded. I removed the bonded neutral and all is well. The isolated outlets still have the ground pin grounded, I only plug in 2 prong line connected radios in them anyway so there should be no problem. As for using a scope I never ever connect the scope ground clip to anything but the chassis, I would never have a reason to connect a scope ground clip to anything but the chassis. BTW, I once had a PHILLIPS oscilloscope that had a 2 wire cord and plug, it came that way.
I agree, OG, that the ground clip is generally attached to the amp chassis. I guess at one time, all electronic gear came with two-wire power cords......but not any more.
I have Philips PM3540 oscilloscope (with rudimentary logic analyzer included) that is made mainly for TV repairs and works this way. It specifically says with big scary letters on first page of the manual that it's wired with floating ground, they consider it a feature. By the way Doug, if you don't want to wait for the seller and want something a bit less how you doin' for connecting stuff to this isolation transformer, ask/search for "UPS cords" or power strips (it's the typical application, as most UPS are equipped with these IEC female sockets instead of normal ones - it saves space if you actually connect computers via female-female IEC cable and it's harder to connect a toaster "accidentally") :)
Thank you so much Uncle Doug!! I found the identical ISB-060W I.T. on e-bay for $77.00 shipped to my door. I dremeled down the prongs on the 3/2 prong adapter, so it will plug into the funny shaped receptacle, works pretty good.
Hi Doug ,I remember as an apprentice in a TV repair shop 1970's the whole workshop was floating, the floor had a thick rubber mat which covered the whole floor., even so at my home work space is also fully floating.
Great information! One note: the reason the chassis is usually grounded for a device. The mains negative is generally referenced to ground. If something happens inside your device and the chassis becomes energized, because mains is referenced to ground, you would have a return current through your body to ground (could be lethal). If the chassis is grounded, then if it ever gets energized the current flows directly to ground (in modern homes this would trip a ground breaker linked to mains as well, indicating there's a short or defect in some equipment).
Now if the Device Under Test is using a floating supply, like you've showed, of course this isn't an issue! Because its power supply negative isn't linked to ground, in the case there is a defect and the chassis is energized *with the isolated supply* there would be usually no return path from your body through ground to the isolated negative/return, so you would be safe. If your device chassis is tied to negative as well, then a short would occur (which is also dangerous), i.e. lots of current, but usually it would trigger a mains breaker or hopefully a fuse in your equipment (to prevent fire). To all newbies out there: don't mess with mains voltage unless you have a good theory understanding!
I believe the reason your homemade isolation transformer didn’t work is because your transformers were rated for 4 amps on the secondary (12 volt) windings. This, of course, would be only 0.4 amps on the primary (120 volt) windings. You would need 40 amp transformers to make it work the way you intended. Those would be some giant mothers! Great work as always, Doug!
You're exactly right, Gary, and this is why I took the precaution of buying the big I.T. I thought that inclusion of this experiment would be a good lesson for viewers in the realization that not all seemingly logical experiments work out......and I was curious just how close it would come to actually working.
@@UncleDoug could you simply plug the occiliscope into the isolation transformer with a 2 prong cord and plug the amp in a common 3 prong recepticle ? would this be a way to protect the occiliscope without removing ground from the chassis ?
For the DIYers, a good source for a isolation transformer is from an old 150 watt high pressure sodium light transformer. its a one to one winding. Just eliminate the exciter, center tap and capacitor on the output. Found mine at a recycle center as most are being replaced by leds. most have multiple mains taps.
Great demonstration. When I was going to buy my isolation transformer I looked all over for a good explanation of what to buy and could not find one as encompassing as this. One thing I see in the comment section is a bit of confusion over the term hospital grade. You can buy the same power rated isolation transformer, from the same company, either hospital grade or not. Hospital grade will cost considerably more for the same amp capability. The main difference to make either a tech bench isolation transformer, is in the secondary or output plugs. With hospital grade, they have plugs with a green upside down triangle or marked as yours were. Non hospital grade, you will have to replace the outlets with ones with the green triangle or always try to use an 3 to 2 adapter. That is because normal outlets are internally strapped from neutral to the safety ground so when you disconnect the safety ground internally it is still connected. As for a green dot on the primary plug, it just means the safety ground has been verified active.
Seeing is believing ! Outstanding demonstration ! Previously, I couldn’t get my head around the two types of ground or neutral wires in a three prong plug. The demonstrations in this video helped me understand how neutral and earth/ground are different. Thank you, Uncle Doug ! BTW, I couldn’t help but notice that king crab legs were $19.99 a pound. That’s a pretty good price ! :)
Interesting and salient advice. My isolation transformer / variac combo came with earth ground on the input, case and output socket earth. As we require an earth pin to be present on all UK plugs because they operate the socket shutters on the live and neutral connectors, I added a sufficiently rated switch to enable me to disconnect the earth if I require it for testing but easily reconnect it if needed. I used a switch with a safety cover that connects ground earth when in the closed position as this is the safest position, although not necessarily best for a connected scope.
If you buy a tech grade isolation transformer you still need to open it up and check it wiring. I bought an goldtool grip-1200 and when I opened it up all of the grounds were connected. It is set up so that it is super easy to set it up for electrical work, but if I had not checked it would have been bad news.
One individual tried to implement an electric heater on the floor of his appartment. He embedded the heater wire into the concrete of the floor. This was done without isolating from ground reference. The heater wire touched the reebar inside the floor and thus it led flowing big current through the part of heater wire and circuit interrupt(wire has broken).
Good stuff Doug. Some years back I got a hold of what I thought was an isolation transformer for the work bench. Found out it was a commercial unit for noise reduction in computers and such. So I had to go in and make sure the earth ground was truly disconnected from the outputs and now I have a proper and safe transformer. I think there are a lot of people out there that may believe they have a isolation unit and in fact do not. Let us hope they watch this video. Thanks buddy.
You're welcome. Was your neutral return grounded to the chassis, Carl ? It usually is in Medical I.T.'s. I sincerely hope people will determine if their I.T.'s are legit.
I made sure that the secondary transformer is floating with no connection to ground where it goes to outputs,,, and mine was not a toroid. Right off hand I can't recall all the steps I took to make it right, but it's as safe as an isolation transformer can be now.
Just found this - a really useful and clearly presented feature on isolation transformers. Thank you. Regarding the initial DIY attempt - I note that the Jameco transformers used are only rated at 50VA (12.6 4A). I guess the amplifier would have been taking maybe 30VA, thus losing efficiency in the cores and the windings? It would be interesting to repeat this but with larger transformers at, say, 100VA or even 250VA ratings to assess the voltage drop across the combination under load. Indeed, I shall try this since I have the parts and also a regular 500VA high noise isolation transformer from Topaz.
Thanks so much, GG. Agreed, my experiment was rather ill-conceived, but it did demonstrate an important consideration when dealing with transformers. Good luck with yours.
Another option at 23:36 is to measure voltage relative to ground using a DMM that has a LoZ setting. (The Mastech you're using doesn't appear to have that option). That would have a low impedance mode (about 2K instead of the typical 10 M ohm ) and would drain any ghost voltages to near zero.
I have a couple of very nice old 5 watt amps with two prong cords . A "White" made by Fender for Forest White and an old Harmony 304A . What I do is put some water on the concrete floor , then plug the amp in each way , with the two prong plug so hot and neutral are reversed and see which way delivers a shock when lightly touching the chassis with my toe in the water!!!. I pinpoint which polarity does not shock and mark the plug with a plus on the side I will always from then on make sure goes into the small slot of the AC outlet ! Primitive but it keeps the amp quiet and safer though I should probably just add a three prong with the switch on the hot side !
That sounds like a better method than a friend of mine (now deceased) used. He would dangle his nads in a pail of water and touch the chassis with his tongue :)
Same Gretch 6150 amp I used to have, totally original ...death cap and all. Unfortunately I had to pawn it a few months back... I have a medical isolation transformer similar to the one you show, I've been using it as a safety device for all my series filament garage sale amps - assuming isolation meant isolation. Now I'm going to have to double check .... Thanks!
I'm a retired electrician and even people in the trade don't fully understand grounding for one the green or bare wire tied to a metal chassis on even a hot water heater is called the equipment ground. The white and sometimes yellow is the neutral and is considered a current carrying conductor. Gfci outlets watch both hot and neutral for fast interrupt. If you were in say a shop that has a sub panel from say your house, if you look inside the neutral is floating on a insulator and the ground is separate tied to the panel and the two cannot be interconnected as in your house service panel where the neutral and equiptment ground are by code tied together. I really like the way you teach getting to the meat of the issue see you on patreon
Uncle Doug, with respect to your advice about not using two ground clips from 2 different probes and attaching the ground clips to different points in the amp, in an ungrounded chassis;. My question to you is, if your using multiple probes, then why would you bother hooking up two ground clips ? Since the chasis on the scope and all BNC connector ground are all booked together, once you hook one ground clip onto the DUT, then you shouldn't need to connect another ground clip if your poking around the amp with a second probe, since the ground clip from the first probe will be referencing the ground for the second probe, so no need to hook it up at all right ? Tell me if my thinking is flawed ?
Yes, since both ground clips are connected to the scope chassis, they are equivalent and it is only necessary to connect one of them to the device being tested......however, beginners and "forgetful people" do tend to connect both. The warning is intended for them, to be sure they are aware of the hazard from connecting the ground clips to areas containing different potentials.
@@UncleDoug okay Uncle Doug, that is so much for your reply ! I have spent my entire day watching your videos and they are superb, thanks for taking the time to do this ;)
It depends what you are working on. In general, if the circuit is high current, or high frequency, you want the ground for the probe to be connected as close to the measurement point as possible. High current can cause voltage drop on ground lines that can cause a phantom signal to show up on the scope. See "ground loop". For high frequencies, inductance raises the impedance of ground lines, also causing phantom signals to show up. This was often a trap for young players when troubleshooting CRT TVs, where you have a combination of high voltage/high frequency, and high current sections of the circuit. Sometimes you end up chasing a phantom signal that doesn't exist, because you used a long ground wire and didn't move it when moving your scope probe.
Hi Uncle Doug. Thank you for inspiring me and helping me recall some long forgotten information. I am slowly building my bench tools and equipment with the intent of playing with tube amps. I purchased a used medical grade iso trans. As you described, and as expected, my medical isolation transformer is not a tech grade.. no problem! I removed the ground connection to the output AND removed the Neutral buss connection to the chassis. Now the output of this overly massive transformer (7.5” diameter @¿@ ) is wired direct to the neutral and hot of the IEC C13 outputs. .. those funny looking power inputs and outputs are called IEC C13 (the six outputs) and IEC C14 (the input from the wall). Knowing that terminology you can now search the web for all types of versions of these. I found the C14 part as a clamp on/cable mount style, that allows making your own adapter /extension cord with a C14 end. (The C14 part will plug into one of your six C13 outputs) - thanks again!
I bought myself a lab tech isolation transformer from Tuvasson, it's 6A on 240VAC, which feed my Variac and a dimbulb tester. And different test equipment like current and volt meters. This transformer has a mid-shield for clean power. It wasn't cheap, but working with 2 toroidal transformers from 230VAC - 12VAC and 12VAC to 220VAC wasn't that convenient. And they went very hot as well, when used.
I used those exact two 149771 transformers and only lost about 4 volts total. The reason is that the transformers have center taps on both sides. According to the datasheet, the primary side is fed by 115v on the center tap (black and red). My voltage on the secondary side and NOT using the center tap gives me around 13.7 VAC (120v in my case). Reversing this gives me around 117 to 118VAC on the other. I added a switch to the ground so can decouple it fully. Once I get everything fully assembled, I'll test it to see what kind of current I can draw without exceeding the 4 AMP rating on the secondary side.
Thanks for the video. Excellent follow up to your previous video. Your explanation of the grounding certainly cleared up some doubts I had from other information.
You're welcome, Bryan. "Seeing is believing", and, like you, I had grown weary and confused by all the conflicting claims I read and saw on the Internet.
Love the humour in your videos! Lots of things you can do with power transformers and re-purposing. Lots of voltages and current outputs can be achieved from stacking identical transformers in different configurations. For anyone who may be looking in their spare parts bins or scrap, the magic specification to pay attention to in transformers is their power rating (VA or KVA in larger transformers), and understanding a few simple principles. Ebay is a great place. Industrial control transformers make for the cheapest 500w+ to re-purpose into isolation... given you are willing to run 240V to your test bench if you do not live in a country that uses 240V. Or you can buy two, and piggy back them to get around the 240V requirement. (These transformers usually have 120x240 on one side, and 240x480 on the other, with both sides configured to use 240V, hence the 240V requirement). You could run it at 120V... but your power would be 1/4 of the VA rating. This is because the VA is rated at the stated voltage of the coil... which gives you the current rating of the wire used in the coil. This is primarily what limits things severely when you drop input voltage from what it is intended. Some voltage converters make cheap isolation transformer sources too, but the biggest thing people need to watch out for, is staying away from units that employ auto-transformer method. Generally... the size and cost of transformers are proportional to their power capabilities. You can expect to pay about $60+USD for a 500VA isolation transformer, no matter how you approach achieving it. Buying a dedicated isolation transformer probably is the easiest, and isnt too much more costly when you find the right time and place. Like right now, for 115V people, there is a Hammond Manufacturing 169G on ebay for $76 cost to your door. 500VA, It has output taps for anywhere from 90V to 130V, which is a bonus if your wall powerpoint is too "hot". ------------------------------------------------- Warning - boring transformer maths ahead! ------------------------------------------------- Voltage and current are inversely proportional. Based on turns ratio. If you step up output voltage (more turns in secondary then primary), your output current decreases proportionally. Likewise, if you step down output voltage (less turns in secondary vs primary), you increase output current***. The 149771 transformers have a rating of 50.4VA VA is (V)olt(A)mp. It basically is your power rating in watts. On a 50.4VA transformer with a primary of 120 volts, your input current would be 50.4 / 120, or 0.42A Likewise, on the output you have 12.6VCT, so output current is 50.4 / 12.6, or 4 amps. IF that transformer had two independent 6.3V output coils instead of centre tapped, you would be able to wire the secondaries in parallel, and have 8 amps (50.4 / 6.3). The important key here, is the wattage is the same regardless of the voltage or coil configuration. That is the ideal transformer. In the real world, there are always losses, but that is the general gist of the maths. ***There are of course some deeper technicalities, such as core saturation, core material, losses, cross sectional area, etc... But for general stuff, VA rating is most you will need.
@@UncleDoug Your welcome and thank you for all your videos and contributions! I still have a lot to watch. You have earned my respect, esp the few times I saw you own up when you are wrong and you share the learning experiences. I wish more people your age would embrace YT as you do. At least, I'd be tickled pink if I had an uncle like you, who would actually understand the stuff I talk about instead of me feeling like I'm speaking Greek or something LOL. This is the biggest reason I love YT and forums... Interacting with people who actually are on the same level :) Thank you again, and hope to see the videos keep coming! I know they take time (which is why I have not had any uploaded in a long time!)
@@UncleDoug well looks can be deceiving and i will leave high voltage to specialist anyway XD. Not going to mess with things im not feeling proficient at
Just what I was looking for Doug! I have not seen your channel before, but just purchase a medical grade issolation transformer (also from Toroid) and am super excited about it. I wanted to see if anyone else had one opened up and was happy to see such an informative video. This has definitely given me a few things to think more about.. I tell you what though, the build quality on that transformer looks top notch! That's why I chose to get it over the new import ones. I am a bit lucky in the sense that mine will have normal Nema 5-15p sockets.. But It is going to be nice to have something to mount under my bench out of the way.
Doug, congrats on crossing the 50K+ subscribers! Hey y'all, throw some dough and kibble his way, every little bit pulls Doug and Jack more towards electronics than gasoline. Thanks for the video, stay safe.
You're welcome, Craig. Thanks for the promo......things are going rather well with both Patreon and PayPal, and we've regained our enthusiasm for 'ampliology'. We're currently upgrading to broadband, high speed Internet to facilitate video uploads.
I have a Tektronix 224 and it operates from a wall transformer or lead acid battery. The battery went to battery heaven many years ago. Why I love this little scope is because its isolated from the mains by the transformer, and the scope has 2 channels that are even isolated from each other and have a 6-kV Peak Surge Rating. Floatable to ±850 V/channel
Very good video Audiophiles forget that earth ground is a second return path if there is a catastrophic failure of the neutral conductor not a sink for noise they are trying to get rid off
Good point, Doc, but since 60Hz hum is AC in nature, providing it a short, alternative path back to its original source can reduce or eliminate ground loops and other sources of hum often by effectively shielding signal wires, reverb tanks, etc.
@@UncleDoug hi yer Uncle Doug that was the house wiring electrician in me not an electronics engineer very good video and fully copper shielded cable helps as well just one question fully copper shield then insulation layer then Aluminium layer then insulation then final stainless steel braid on top 3 layer shielding should stop any noise ??
@@UncleDoug through my investigations and reading about EMC shielding materials each metal layer brings different benefits I.E outgoing and incoming a cable that lives in a null feild is that not the ideal cable. I have one question for you why does the US use 2 voltages ( 110/220 volts ) why dont you just use 220 its safer lower ampage just curious Iam in the UK so we are 240 volt and EU is 220 thanks for your time Doug
@@drmindbender8616 That's a good question, Doc. I think we are so involved with 110VAC that it would be impossible to change. The vast majority of existing electrical devices would be obsolete.
Great video. If I'm understanding this correctly, the issue with grounding the chassis is that a lot of equipment ties the 0v rail of the internal DC PSU to chassis ground. The amp in your example must do the same if you're measuring full plate voltage at the plate connection?
The chassis is the HV DC return route to the center-tap of the HV winding......but isolation is used only for the AC primary circuit, to prevent a grounded tech from becoming part of the return circuit.
Great video as always, Uncle Doug ! One advantage with making a 'high-low-high' isolation transformer is that the current is multiplied by the turn-down ratio of the lower voltage. So it is easier to detect current change. I have a beautiful old 5 Amp. AC G.E. analog meter that would barely twitch when used with120V, since most of what I work on is
Hi there Doug. Watched your videos for a good number of years and find them very informative but have just caught this one. I would like to pitch an observation if I may? I've just gotten hold of an IT off GreedBay which is exactly the same as the one in your video (but it hasn't been shipped to me yet so I haven't gotten my hands on it to play around with test meters). But; if you leave the isolation transformer unplugged from the wall socket, plug your amp into the isolation transformer (nothing is live but the ground pin will have continuity throughout the circuit as it is not switched): Bearing in mind the neutral/return on the IT outlet is floating in this transformer, if you do a continuity test between the amp chassis and the earth ground pin on the IT power socket plug (with the amp plugged in to the IT obviously) if you get continuity between the amp chassis and the earth/ground pin on the IT plug that goes into the wall socket then surely your amp chassis has a ground earth reference as the earth ground socket on the IT is not actually isolated inside the IT so you have earth/ground continuity between the amp chassis and the wall outlet, so you should actually isolate the ground/earth receivers in the IT. Where you do your lightbulb test at 19mins into the video and touch the yellow alligator clip to the return there is no circuit BUT if you touched the yellow clip to the green/yellow ground/earth coming out of the IT then you would close the circuit via ground/earth which is undesirable???? Surely the object of the exercise is to allow the amp chassis to get its ground/earth reference through the scope ground clip only?
What an elaborate inquiry, David. I agree that isolation of the circuit being tested (CBT) from earth ground is the goal of using an IT. For this reason, the chassis of the IT is generally grounded via a 3-wire power cord, but the CBT chassis is not connected to the chassis of the IT, thus avoiding an earth ground reference. Any continuity between the CBT chassis and earth ground would therefore not be desirable and would negate the value of the IT.
@@UncleDoug So the best option for safety for the scope and more importantly the Tech when testing a ground referenced CUT that has a three pin plug to ground reference the chassis is to disconnect the ground pin in the IT socket that is supplying your CUT (same as using a two-prong plug to the CUT so the chassis is completely floating from ground reference). What I shall do when I get my IT is to disconnect the ground reference pins on right-hand bank of outlets to emulate a two-prong CUT and leave the left-hand bank as is and annotate the IT case accordingly so I have the option. Thanks Doug.
@@UncleDoug Hi there Doug. I got my I.T. which is same as yours and put my meters on it (I use a Fluke 87V and a Simpson 260-6P). I live in the UK so my I.T. is set for 240v on the primary winding and 240v on the secondary (it actually gives out 250v when tested so I'll turn it down with my Variac when using it on tube amps). I did a slightly different test to yours in the video. I left bank A (left output bank) wired as existing but disconnected the green earth strap from the chassis binding post for bank B (right output bank) and insulated it. These were my metered results: on bank A between hot and ground there was a potential difference of 182V and between neutral/return and ground there was a PD of 83V (I assume that this is the case as whilst the neutral/return and ground reference on this transformer are not connected within its case, if we trace the wall socket supply back to the Electric Companies power switch and fuse board [we call it Consumer Unit in the UK] the neutral/return and ground are bonded). On bank B (totally floating) there was a PD of 22V between hot and ground and 8V between neutral/return and ground. I am going to investigate further as to why there should be a PD at all on the totally floating bank B as I was expecting no PD at all noted between the ground pin and hot, and the ground pin and neutral/return.
Doug - at 20:30 in the video I think you might have inverted the Line and Neutral wires of your plug. Since the US uses White for Neutral and Black for Line then the black wire should be on the left of the kettle plug and white on the right. It doesn't make a difference to your experiments tho.
I have a question. If you already have one oscilloscope lead grounded, why you need to have the second one too? Doesn’t the oscilloscope leads share the same earth ground?
In Australia, safety requirement always dictates that the power plug to have 3 pin. So with this Medical grade isolation transformer, it's basically still earth reference, not floating. You still have to remove the green wire to the chassis of the transformer to make sure it's isolated.
Thanks for the Reply I understand about the Earth when testing the amp with a Oscilloscope My concern was that when using the amp there will be no earth connection and because the transformer isolates the input power therefore if there is a fault in the amp , circuit breakers would not work as the amp is floating. Thinking that there needs to be an earth neutral link on output of step down transformers. Regards Geoff
I am not an expert in step-down transformers and have no idea how yours is wired, Geoff. The best you can do is to use a multi-meter to assess the situation.
Great tutorial Doug! I have a couple of (I believe) the same toroidal transformers (Hammond 182L60) so I should put an isolation together in case my variac is not. I’ll likely put in a ground on/off switch. Thank you!
I'm trying to get a good understanding of these - please correct me with any mistakes. The main point of removing ground in an isolation transformer is to remove the connection to the physical ground. In your home or somewhere near your home is a rod physically planted deep into the ground. That means that depending on the physics, your feet might complete a circuit to your hands touching something energized on your bench. When you remove the ground, you and nothing around you will complete a circuit that will electrocute you through your chest. That does not mean that high voltage danger is eliminated, but it is definitely safer. The reason why the light bulb would light up when connected to the ac receptacle's earth pin is that in your breaker box, by law/code, the neutral wire is connected there to a reference ground aka the metal pole in the physical ground. However, for some reason I'll need to research more (edit: conclusion below), the circuit operates entirely between the hot and neutral wires. Even though the neutral wire is connected to ground, they are not the same. Something about impedance and the design where the ground reference is used to quickly trip the breaker to prevent electrocution / fire is much more effective than old fashioned hot/neutral with fuses. An isolation transformer is also extremely effective at removing noise from tube amplifiers.
Paragraph 1 sounds good. Paragraph 2: I understand your confusion between neutral and ground.....if they are jumpered at the fuse box then why aren't they equivalent, and thus redundant? To my understanding, the neutral wire provides an operational path for current to flow back to the AC transformer (on the pole behind your house). The ground connection is only activated when the hot wire becomes shorted to the chassis, providing a pathway for the current, preventing you from providing that pathway and most likely blowing a fuse. If no ground was provided, the chassis would be hot, dangerous, and no fuse would blow.
@@UncleDoug Thanks Doug. Your explanation helped me get to the crux of the explanation. After a lot of searching, the answer lies in Ohm's law. As current flows through the neutral wire, it develops a voltage drop along its length due to the wire’s resistance. The neutral wire carries return current from the load back to the transformer on your street, completing the circuit. In the event of a fault (e.g., if the hot wire contacts a metal chassis), the ground wire actually creates a new path back to the breaker box, where it is bonded to the neutral wire. This pathway allows a large enough current to flow rapidly to trip the breaker or blow the fuse, preventing the chassis from becoming energized and dangerous. This is key for fire prevention and of course electrocution on the bench. Neutral as the Primary Path: Electrical current always takes the path of least resistance. In a properly functioning electrical system, the neutral wire provides a direct, lower-resistance path back to the transformer than the ground pin. There are actually two earth grounds: Yep you read that right, the center tap of the transformer outside your home is tied to earth just as Doug showed in the first video. Your main breaker also has a separate ground connection. The transformer’s ground (at the utility pole) stabilizes the entire power grid. The home’s ground (at your breaker box) protects your home and ensures safety. They are tied together through the neutral wire, so they operate as a single, unified system. Reference voltage is critical to the operation of your system. And don't forget that 120 volts is RMS, your home AC voltage is realistically more in the 300 volts region - thanks Doug for showing us that on the oscilloscope. Ground as a Safety Mechanism: In the event of a fault, the ground wire provides a low-resistance path back to the breaker box because it is typically tied to the chassis, quickly passing current and allowing the breaker to trip. The earth connection itself (via a grounding rod) has much higher resistance and serves only as a secondary safety mechanism in extreme cases, such as a disconnected ground wire. Voltage on Neutral: Even though neutral and ground are bonded at the breaker box, neutral can have a slight voltage above ground at other points in the system. This voltage difference occurs because current flows through the neutral wire, and its resistance creates a small voltage drop. In contrast, the ground wire remains at 0 volts because it is not part of the circuit and is connected to the earth as a reference point. Now while you might think, ok then the current should go to the ground, the neutral still has a lower resistance. Why Ground is at 0 Volts: The earth serves as a large "neutral" reservoir and is used as a consistent reference potential. By connecting the ground wire to a grounding rod driven into the earth, the system ensures that the ground wire remains at 0 volts. This connection stabilizes the electrical system and provides a safety mechanism to dissipate fault currents when necessary. Why Current Flows to Ground in a Short: The Fault Creates a New Path: When a short circuit occurs (e.g., the hot wire touches a metal chassis), the ground wire becomes directly connected to the energized hot wire via the fault. This creates a direct path for current to flow back to the breaker box, where the ground and neutral are bonded. In this scenario, the neutral wire is bypassed because the fault path through the ground wire is the most direct route for the current.
Thanks Uncle Doug. You have a gift for teaching. Your gift is similar to Mr. Pete who teaches on UA-cam how to use a lathe. He was a shop teacher. Your gift must just be God given. I have watched your lesson on Isolation transformers several times, but can’t get past my confusion of the difference it makes connecting earth ground from the isolation transformer or from the scope. It seems to me that the chassis is earth grounded in either case so why is there a problem for potentially blowing up the scope? What am I missing?
Yes, thanks for your quick reply. I understand; the IT has its ground disconnected, but don’t you earth reference ground as soon as you connect the scope ground to the chassis anyway? What difference does it make if the same ground reference comes from the scope, or from the IT(assuming you left it connected). I do understand your excellent presentation, but I’m missing something very simple I guess. You say we don’t want the earth ground from the IT, but we supply it from the scope anyway. So the chassis gets earth reference either way. Earth ground is earth ground, no? Or is it like having a potential difference with the two probes having to be at the identical location for test. Sorry, for being so dense, but I have watch all the UA-cam videos on the subject and although yours is absolutely the best, I’m missing some little detail that I can’t get over. Thanks so much for you WONDERFUL (really wonderful) videos. You do such beautiful presentations.
Great video, good Sir, especially the solution with this Medical Grade iso-transformer. If you fancy meaty toroidals, I recommend searching for old, burnt out PA-amps (Crown, QSC ...) You get those broken ones for cheap, they have massive transformers, and due to the amp-stage-designs their secondaries are usually ~ 2x60V, meaning: no rewinding-work on the secondaries needed for your folks from 110V-land. Other than that: Ollie's catnip-debauchery and Peeping Jack cracked me up :) Very good show!
Great video, U. Doug. I ended up getting up getting that oscilloscope for $125 and he threw in a Hammond 1:1 isolation transformer, a Wavetek LCR55 ohm/Henry/capacitance meter and a fused 10 outlet power strip. The probes are a bit work, but I figure I can get replacements online. That Hammond is heavy!
Wow, what a major score, B55 !!! There are wonderful bargains to be had in used electronic gear. I keep an eye on Craigslist and snap up every bargain I can.
Uncle Doug The man was 76 and had finally decided to sell his business of making electronic equipment for the government. It was a cleared facility, so I had to be escorted, but that was no problem. He had a ton of stuff that would have been great, but not necessary. Spectrum analyzers, almost a dozen scopes to choose from. A ton of metal brakes for building chassis's of all sorts of sizes. I couldn't afford those. I forgot, but he threw in a big spool of 44 gauge Formvar, in case I wanted to try winding some pickups (they would be hot, I suppose, or small). Funny, he asked me what I was going to use the stuff for and when I told him, he said that he did his thesis in 1966 on tube amplification. Small world. Nice guy. I'd love to pick his brain for no doubt a ton of knowledge.
@@UncleDoug Understood. All I'm saying is that a DMM in continuity mode with one test lead in the neutral and one in the ground would have visually (and audibly) confirmed lack of continuity without the need to hook up a bulb.
Noise can also come from the ground line to the sensitive medical machine, thus it makes sense not to connect it or let the groung wire bypass the filtering.
I am confused by this statement, Janne, but agree that if your purpose for the medical grade I.T. is to eliminate electrical noise, then you should leave the connection between the neutral and ground intact.
But you expect that EMR noise can not come from ground or neutral. It can. EMR spikes and noise can affect all leads. If you have an car ignition coil making sparks and 3 wire cord nearby, the noise will spread to whole length of wire, reflect from the underground end and taken to ground from the grounded end. It works like an long wire radio transmitter antenna. At RF frequencies the noise is a spreading waveform in an conductor.
NB! Simply removing the IT internal green/ground wire from the IT receptacle ground lug may not be good enough. The receptacle mounting yoke is probably directly connected to the IT receptacle ground lug internally thereby connecting it to the chassis via the holding screws UNLESS the receptacle has the orange triangle emblem and/or the round hospital grade emblem on it's face showing it is a truly isolated ground receptacle. The isolated ground receptacle can be purchased and installed if you don't want to use the 3 to 2 prong adapter. Thanks for the lesson video! Forgive me if I am repeating previous information as I haven't read all the comments. My comments come from making continuity checks last night on my so-called isolation transformer. It most definitely is not an isolation transformer. I am using the 3 to 2 prong adapter until I change the receptacles to the true isolation ground models. The isolation ground receptacle is either the color ivory/white or red/orange and can be more expensive than the standard grounded receptacle.
@Uncle Doug - I don't know if you will ever see this but I had to try! On another Group on FB I have been asking questions about Isolation Transformers. Many people say... use one on your "device under test" to isolate it's ground from the test equipment on the bench. Others say...."what is the sense in doing that? As soon as you hook up a signal generator to the input.....you ARE ground referenced!!!! So... don't worry about an isolation transformer"! I am thinking that if you DO hook up a signal generator..... THAT establishes your grounding point for THAT PART of the troubleshooting effort. Can you offer any thoughts or comments on this?
The primary purpose of an IT is to protect the technician from AC shock while testing and repairing a circuit. Generally, this is done using a DMM, and the IT does its job......but when you begin to use AC powered equipment, problems can arise. You can "float" a signal generator (run it ungrounded) but oscilloscopes present another challenge. Please see my oscilloscope video series for a detailed discussion of shock hazards and how to prevent them.
@@UncleDoug I really think that I am getting a "grip" on what would be a safe arrangement FOR ME!!!! The way some of these guys are telling me I don't need to worry about isolation.... I can only guess that when they change oil on their cars/trucks they use 4x4s on end instead of jack stands!!!! Anyway, on another channel a guy brought up building a little "audio isolation" box using BNCs and a little 1:1 audio transformer (10K:10K) to isolate the generator from the radio. I might play with that idea! Thank you sir for responding, I hope you are well!!!!!
In the Uk we have 230 vac our mains have live neutral and earth. We have large house size tranformers not on posts like the usa. Nuetral is earthed at the sub station tranformer, earth is a local earth. In uk electronic work shops we have rubber mats, the bench suppy is via a 1kw 230vac to 230vac isolation tranformer, there is no earth on bench sockets and if there is arial sockets they must be isolated with capacitors and no earth is alowed for 1 meter around the bench, we also have rubber on our benches
Also, we don't use 230vac - Europe insisted we switch from 240vac but the cost of replacing all those substation Txs was billions so, as the mains used to be about +/- 15v anyway, they just relabelled everything to 230vac. This is worth knowing as American gear has two 115v primaries wired in series for 230v use here but the 240v means their HT voltage is way higher than it should be. I typed this out for those who don't know - sounds like you do! :o)
5 year old YT video that still rocks and provided the information I was looking for. I picked this same unit up today off eBay for my starter electronics bench. Thanks.
You're welcome, Mr. D.
I just got one very similar from sphere.bc and was wondering why I saw voltage to ground on the output with my Fluke but not with my cheap Walmart meter. Ghost voltages, evidently the fluke input impedance is high enough to not pull the voltage induced between ground and the hot wire to zero. The Walmart meter does though. I see I did good by not buying a dual ended C13-C14 cable and went with a 5-15 (regular outlet) cable. Now I just need to remove ground from the output. A recommendation for secondary probes, remove the ground lead so that risk is removed, you'll get all the ground reference to the scope that you need from the first probe ground clip.
I also just bought a similar one (1000 VA Powertronix). It also said OUTPUT FLOATING - NEUTRAL CONDUCTOR NOT GROUNDED and has 2 standard 15A receptacles. When I removed the chassis ground connection to each of the receptacles, I found out that the receptacle grounds were still grounded to the chassis ground via the receptacle mounting screw. I replaced the receptacles with orange “isolated ground”receptacles and now the receptacle grounds are not grounded to chassis ground.
@@UncleDoug Excellent just a quick one is this 12V ac output ?
@@juangizirian9810 Isolation transformers output the same AC voltage they are plugged into, Juan. So, no, 120VAC in and out.
I know this is an old video, but I have to thank you for it. I've watched so many videos and watched so many forum posts trying to figure out exactly what I can and can't do and why, but I always walked away even more confused. Thanks for making it simple and explaining specifically what to watch out for.
You're quite welcome, Nigel :)
I feel the same way. It's why I haven't hooked up either of my isolation transformers.
Man same here. I just got an old Oscilloscope from an auction and was confused as hell….just been playing with low voltage battery powered stuff
Doug. You are truly an honest national treasure. Thank God for you and that are with us. Overwhelmed with gratitude. You have given me a new career. You have made it all so clear. Thank you.
Wow....that's great to hear, Pete. Thanks so much.
Uncle Doug's soothing voice and sage advice are like catnip for my soul . . .
You're too kind, Aero.....Thanks !!
This video covers one of the most important topics we ampaholics need to understand! Uncle Doug, your explanations are clear, thorough and very, very informative.
This is one video that deserves multiple viewings!
Thank you so much for your wonderful videos and kudos on your teaching style. Your former students were very lucky to have you for a teacher!
You're welcome. Thanks so much for all your very nice comments, Steven :)
I have watched this video a few times as well as several others to gain a firm understanding of the topic. It has been years since I made my isolation transformer from a "medical" grade power conditioner. I opened it up, removed everything except the transformer, outlets, switch and fuse. I then made sure there was NO connection from the secondary to ANY other line or the chassis. I then wired the outlets to the 2 wires from the secondary side of the transformer. It was an easy conversion. To triple check everything, I used the ohmeter and checked between all 3 prongs of the primary plug to all 3 connections of the outlet plugs on the secondary side. THEN I clearly labeled the power conditioner that it was ISOLATION ONLY, NO GROUND CONNECTED just in case someone gets ahold of it and I am no longer around to explain the unit. This is SUCH an important safety device on the test bench!!!!
I like your approach, Wayne, systematic and thorough. Thanks for sharing this with us.
This information is exceptional and necessary for anyone attempting this type of testing. In this world where no one wants to take responsibility for their own actions, this is critical to deflect any liability, but also to insure proper knowledge. Make yourself aware of all the information available and proceed with all due care. But I have found that there are always new ways to get in trouble. My favorite saying as I have traveled through this life has become "If you're going to be Dumb, you had better be Tough !" Something I unfortunately witness more often than I'd like. Once again Uncle Doug, a public service message and useful knowledge. Thanks jj
You're welcome, Robert. Thanks for your input.
Only just found this video although it's 4 years old. Very good advice here. I have always said NEVER ground the secondary side of the isolation transformer, it will make it worse than useless. I say worse than useless because any accidents happening on the secondary side will not trip the RCD (GFCI) on the primary side because no imbalance will happen.
Thanks for your helpful input, Chris.
A great no-nonsense video! But I clenched every time your fingers approached the temporary line connections!!
Just a little added suspense, Paul. Thanks :)
Thanks Uncle Doug, I bought a Toroid Medical Grade Isolation Transformer ISB-100W 1000VA last week and I was thinking about the ground connection topology for about a week and I just saw this video , It's like you where reading my mind or something ha ha. Thank you again sir, Keep up all the good work you do.
You're welcome, Vijay. I'm glad the video was helpful and timely :)
I got several Powertronix toroid transformers. One is a beefy 1500VA with 20A receptacles with green dot indicators. The neutral is floating, but the grounds are tied to the mains ground at the chassis... AFAIK, the reason the isolated receptacle grounds are still tied to mains ground is that hospital ground wires are insulated and tie to a mains bus bar separately from the rest of the building ground wires (such as metal wall boxes) before all grounds tie back to a common Earth. Perhaps some medical transformers are meant for use in hospital wiring environments, while others were meant for in-home use where hospital-grade wiring is not present.
At 29:50 I would say that since the first ground is already connected you don’t need to even connect the second one since it is already done. Great video thank you! Everyone mentions the oscilloscope ground problem but almost everyone fails to clearly explain it as you did, thanks Uncle Doug and keep up the great videos!
Correct :)
Thanks for making this video, it is a great service to us newbies at working on amps for safety. Excellent demo's !
You're welcome, GF.
Excellent video. I have an isolation transformer with dual toroidal transformers that I converted to a tech service unit. I disconnected the ground terminal connections to the output receptacles and tested the receptacle ground to make sure it was disconnected. The receptacle still had a connection to the chassis ground through the receptacle mounting ears. I solved this by installing isolated ground receptacles which have no redundant ground connection to the mounting ears. Thank you for all your videos.
Thanks, Jimmy. Excellent information, thanks for sharing it. This seems like a good reason to simply use a 3 to 2 adapter.
Thank you for this PRACTICAL *demonstration* of potentially dangerous grounding situations. I've read about the theory, watched the videos that talk about it but I never fully understood until now. As a visual learner demonstrations like this are invaluable.
You're welcome, FP. I'm glad you found the video to be informative.
This is a great video! Everything explained in detail, I now have a clear picture of isolation transformers and how earth grounding and chassis grounding work. Thanks a million Doug!!!
That's great news, Deo. I'm glad it was helpful.
This is the best video on iso transformers that I've ever seen. I didn't fully understand how this all works until I watched this video a few times. Thank you Uncle Doug!
You're welcome, Matt. Thanks !!!
The best bulletproof guide for isolation transformer ever.
Thanks, Robert :)
Another military precision video, excellently planned, executed and payload delivered. At least we can rely on your vids to cut through the bs. Thanks for your time producing these vids.
You're welcome, ISO. Thanks for your very kind assessment :)
Another great tutorial on isolation transformers Uncle Doug. I do appreciate te time you have taken to provide this information.
You're welcome, RD.....glad you liked it :)
It was really great explanation. BTW, Got my tektronix 465 yesterday. Uploaded part one of the video this morning. Fun scopes to work with.
Another good'n. I agree as to the mis-leading multiple opinions floating around the internet. A quick thought exercise one afternoon for me concluded the same as you've shown (grounding the DUT chassis essentially defeats the IT). By providing the detailed empirical evidence through your experiment you have removed any doubts and provided fact over opinion. Thank you.
As we say in engineering, "Without data, you're just another person with an opinion."
Or... "I'm from Missouri on this one, Show Me !"
Thanks so much, CA, especially for recognizing the benefits of empirical evidence over mere supposition. The more I read on the Internet, the more confused I became.....thus the need to actually discover the truth through experimentation.
Hands down The best iso-trans/safety Video . Thank you
Thanks, E :)
ADDITIONAL "RULES": 1.) NEVER operate an ungrounded oscilloscope. Depending on where you attach the ground clips, the chassis and external metal parts may acquire a voltage potential that is lethal......to you. 2.) Although we must be careful with oscilloscope ground clips, the probes are extremely high impedance (1Meg to 10Meg) and thus tolerant of relatively high voltages; HOWEVER, each probe has a maximum rated voltage limit, beyond which you risk breaching the insulation between signal and shield......with disastrous results. 3.) It has come to my attention that simply disconnecting the (green) receptacle-grounding wires from the chassis of the I.T. may not completely eliminate the earth-grounding capability of the receptacles, since the receptacles may also have an internal connection to the chassis. It is safer to use a 3 to 2 adapter to avoid this problem. 4.) For those who use a Frequency Generator connected to the DUT, be aware that in most cases, the shield of the FG cable is earth-grounded and will give ground reference to the chassis of the DUT. 5.) Although the Medical I.T. used in the video costs about $650 new, they are commonly available on Ebay for around $65 (what I paid).
Hi Uncle Doug--I take your point, however, as a working EE, I can say that using a scope with a "cheater plug" that eliminates the scope's connection to the safety ground is standard operating practice. The key to doing this safely is understanding the device you are testing. For example, if you are probing equipment that has a 3-wire cord, and you know it is connected correctly, then connecting the scope's ground clip to the chassis of the device under test grounds the scope and all is well--even when using a cheater plug with the scope. The most common reason to disconnect the scope ground is to avoid a ground loop between the scope and a grounded piece of equipment. In general, the best situation is to have one connection to ground. Otherwise the ground loop created by grounding both the scope and the device under test can cause false noise to appear on the scope trace. This false signal can drive one crazy and cause all kinds of wasted time chasing a problem that isn't there. My usual rule is simple--if the device under test is not grounded, then leave the scope grounded. If the device under test is grounded, then unground the scope if the observed noise floor is oddly high. BTW, one good test is to connect the scope's ground clip to the device under test, and then connect the scope's probe to the exact same spot. Turn the sensitivity of the scope channel all the way up and observe the scope trace. If you see noise there, leave the scope probe connected to its own ground, but disconnect from the device under test. If the trace shows less noise, then you know you have a ground loop.
Excellent information, as always, Roiderien. Since most of the viewers are novices with oscilloscope use, I felt it best to keep the instructions as clear, basic, and concise as possible. The moment that alternatives are presented, confusion arises. Thanks again for your counsel.
That's why my entire bench is ground-lifted. It is not uncommon for tube amps to draw 40 to 50 watts idling. These 50 watts transformers were already too close to their limit to provide adequate voltage. The idea is good, though.
Thanks, TB. You're right.
one RULE I learned a looong time ago . . always keep one hand in your back pocket! m_m
Dear Uncle Doug,
Thank you for your video on Isolation Transformers (IT). Your experiments with an IT greatly helped me to observe its value in Test and Measurement. Your story about horses and barns within that video ended up to be the most important lesson for me.
Struggles to Understand two issues
Your story’s value became apparent as I struggled to understand the impact on my safety of an oscilloscope’s connection to Earth Ground when a power isolation transformer was used in a manner that did not expose the test circuit to a ground.
That struggle also revealed that I did not understand how multiple grounds (at least scope ground and power ground) might impact not only my safety but also the safety of test and measurement gear and the circuit being tested.
Understanding Issues with your horse story
In that struggle to create a story that enables me to assess a strange lab situation so that I make decisions which protect my personal safety and gear safety, I kept replaying your story of the electron horses leaving their stable (Secondary Winding of the isolation transformer) and returning to the stable independent of any Earth Ground attached to the circuit. I heard that the horses are never interested in deviating from their route (stable to mtns to stable) but I could not think with that information.
My story - shift in emphasis
The story that I finally created that helped me understand the impact of Earth Ground on my safety in test and measurement shifted emphasis from “Earth Ground” to “Short Circuit”. It took me awhile to realize that the reason the horses did not try the Earth Ground route had less to do with "Earth Ground" but more to do with the fact that the Earth Ground path never led back to the stable.
So to shift emphasis in my story, I replaced "Earth Ground" explaining Safety Issues (Using a Scope) with the concept of a Short Circuit.
I start with a “wire” that is connected to the test circuit but hanging out in space touching nothing but air.
We are NOT going to entice the horses to go down our wire hanging out in space until we attach the wire's other end to a place in the circuit that short circuits at least one of the circuit elements. Then the horses will readily gallop down the wire because it is so much easier for them than going through a short circuit than a circuit element on their way back to the stable. The horses galloping down our wire is fundamentally due to a short circuit, not “Earth Ground” in my story.
The essence of the above story, hence, becomes “Discovering unintended short circuits” when we engage in Test and Measurement. Any time we can offer the horses an easier path home, they will take it. If our body can become an unintended path back to the stable, we should understand how that could occur.
So the story then focuses on “what are the various ways a short circuit can occur”. Less abstractly, what kinds of wire can we put across any circuit element(s) creating a short circuit.
Aside from a wire made out of metal we readily have two other virtual wires.
(1) One virtual wire is above the circuit board: Air. Imagine a wire composed of air bypassing any circuit element just like a piece of metal wire. Air that is humid acts somewhat like a metal wire if the horses are moving fast enough. You can imagine the air has the shape of a wire as the horses are seeking to go as directly as possible from one of the wire ends to the other.
(2) the other virtual wire is below the circuit board. Imagine a wire composed of dirt or “ground” by-passing any circuit element just like a piece of metallic wire. Ground that is humid enough acts just like a virtual wire
"Oh, but wait", you say. "The Ground virtual wire is not touching the circuit board".
So lets help the Ground virtual wire. Let’s connect a conductive wire to one end of a Ground virtual wire and another conductive wire to the other end of the Ground virtual wire. We can now connect those conductive wires across at least one or more circuit elements. The horses will absolutely run down one conductive wire, then through the Ground virtual wire, and then back out the other conductive wire to the stable. The Ground virtual wire acts as a "wire" as the horses are seeking to go as directly as possible from one of the connected conductive wires to the other connected conductive wire.
Many of us fail to recognize that those two conductive wires connected to the Ground virtual wire actually exist.
One conductive wire is easy: the human body. It can touch the circuit in a location and allow the horses to gallop through the human body to a body part that is touching the Ground virtual wire.
The other conductive wire coming out of the Ground virtual wire is not as obvious. There are actually two conductive wires available. The Neutral wire in a power line is connected to the Ground virtual wire and the "power ground" wire in a power line is connected to the Ground virtual wire. If we use an isolation transformer, we eliminate the Neutral wire as a connection to the Ground virtual wire. But the Power ground wire can still there in a sneaky way.
The scope probe always looks at the voltage ACROSS circuit elements. The probe hook connects to one side of circuit elements and the probe alligator clip is connected to the other side of the circuit elements. The probe alligator clip is connected to the Ground virtual wire through the three prong power cord of the scope
To summarize this last point: we can short out any circuit element by touching one side of a circuit element with our body which allows the horses to run down or on our body to the Ground virtual wire....where they run directly to the power ground wire of the scope....then through the scope and out the probe cable to the probe alligator clip where they connect to bypassed circuit element(s) and race back to their stable.
To summarize the big picture, when engaged with test and measurement, I now look for all ways that I might inadvertently short out a portion of the circuit being tested. One way that I can short out a circuit Is through my body into the Ground virtual wire which is connected to the scope power ground which is connected to the scope alligator wire connected to the circuit being tested.
Multiple Grounds
Multiple grounds ( ie: a ground pin of an isolation transformer outlet connected to power ground as well as to the chassis of a circuit being tested) is another wire through which the horses can run.
In terms of human safety, a short circuit can take place through a human body and the power ground as well as through the scope ground.
There is also an equipment safety issue. A short can take place with the short circuit being the Virtual Wire Ground and at least two grounds one on each end of the Virtual Wire Ground. Considerable current could run through the scope alligator ground and through the scope chassis.
In addition to the above current due to by-passing a circuit element, the two grounds, even if just inches apart, are not likely to be at the same potential thus inducing current flow through the alligator clip.
Thanks for sharing your very detailed thought processes with us, William.
This is an outstanding video explaining all hazards of not using an isolation transformer and correcting the misinformation that existed! Lou
Thanks, Louis. Glad you enjoyed it.
Brilliant idea! I've never considered using an isolation transformer to protect my oscilloscope, or me. Excellent advice, humorous, and exceptionally grammatically correct! You sir, are the Jack Benny of amp repair videos.😁
Thanks, Remley :)
Absolutely invaluable information here. I learned the basics on tube amps from my tech-then-mentor in the late-90s. I learned on his lovely Tek 2465B. As a musician I learned just enough to do my own basic repair and maintenance and was able to help out friends with low-voltage repairs on pedals and things. I always wanted to get an oscilloscope but I was just never able to make it a priority. Fairly recently, I sat down and passed my Tech and General amateur radio exams and it got me wanting to get serious about electronics again. I now have two scopes, a 2465B and a DS1054Z, and as I mentioned, this series has been invaluable to me getting back into it. Many thanks for this very clear and concise content!
Thanks for your kind comments, Brad. We're glad the video was helpful and wish you the best with your electronic pursuits.
Thanks for this excellent tutorial. In my opinion this is __by far__ the most practical and understandable material regarding isolation transformers.
Wow....thanks so much, Georg.
Great info Doug. I'm an old school electronics tech who received a ASEET degree. The double "E" says I was trained in power systems, motors, etc. As well as "electronics". Your video clears up what "ground" means in different scenarios. There are many oscilloscopes out there that thank you. :) Thanks and keep up the great training you do. I'm a patron and will always be.
Thanks so much for your nice comments and patronage, CA :)
I certainly hope you and your channel are active on youtube. Our knowledge bases cross in part, but the important point is you remind me of my mentor. His name was Roger Tokarz and he had the same gift of explaining relatively complex concepts with easy to understand basic analogies. You Sir, have the same gift.
I am listening to your multiple videos (with the same gusto I had for Roger’s explanations on how resonant circuits worked in telephony). You have made my day, please, if you are able acknowledge your channel as active and is it possible to contact you privately (if not, I totally understand).
Thank you from the bottom of my heart as you even sound like Roger, with the same command of the English langue. It has brought tears to my eyes as I lost my mentor a few years ago. Certain people touch your life with extraordinary results, Roger did that for me.
Thank you so much for the effort and time required to make these videos. You have a new FAN!
Jeff in Michigan
Thanks for the very nice comments, Jeff. We generally post around one lengthy video per month. Hopefully, our latest offering will be available in about two weeks.
Uncle Doug, this video inspired me to do the same. I purchased a medical isolation transformer and disconnect the ground wires on the outlets. Thanks for the very helpful videos you produce.
You're welcome, GS. I'm glad the videos are helpful :)
Excellent. A few weeks ago I had just studied the confusing subject of grounding and the different ways isolation transformers can be wired. You video explains everything. The best out there about the subject.
Thanks so much, Hermann :)
Thanks for another excellent video! You do such an amazing job explaining and making what is difficult for my tiny brain to get wrapped around so much less confusing. Thanks for addressing and making the IT mysteries much less mysterious! Probably saved someone from wrecking equipment and possibly another from injury. You're a good man, Uncle Doug! Thanks!
You're welcome, Riloh. Thanks for your nice comments.
Thank You So Very Much For Your Time & Effort! I just bought a N.O.S. BK 2120 scope and powered it up on a variac, it works like a champ. Now seeing your video I "Will" be safe using my scope.
You're quite welcome, TT. That is our goal.
Thank you for your video, I am picking the same IT you have in the video. Found one in used one on the Facebook marketplace. Thank you for keeping us safe.
You're welcome, Ray. Good luck.
greetings uncle Doug, you are one awesome teacher on every one of your videos...you inspired me to buy my first oscilloscope after being intimidated in the past about owning one because so many buttons on them and not knowing what they do but thanks to your oscilloscope videos i now have one
the reason you got higher dc voltages in the plate of the tube you tested in this video though, is because the isolation XFMR is for 120v input, but now the voltages coming from the wall are not longer 120v. today voltages from the wall are more than 120v possible 123v. or 120v.
the more the input, the higher the output.
thanks to your isolation XFMR videos i now have an ACONE transformer that was stored inside of an old desk at my job. for many years i knew it was there but i never paid any interest in it because in only had one receptacle outlet and the XFMR was only labeled as "power supply''. so no big deal!
but after watching these video series i went and took a look at it and found out it had a 2.4 amp in the secondary side of the XFMR with a receptacle that had the neutral terminals connected to ground via the mounting plate on such receptacle.
i replaced the receptacle with an isolated ground receptacle and now i have a real tech grade XFMR that is big enough for what i will use it. i am also working on a 4.2 amp on the secondary side that i found on e bay for 35 dls. plus 25 dls. shipping. thanks so much for posting your very instructive videos.
You're welcome, 4H.
Hi,
Another good empirical presentation. Effectively what you are creating is a bathroom shaver socket arrangement. These are always 2 pin to prevent ground reference and you wouldn't open up your shaver and ground it in the name of safety ;-)
Thanks, OB. Good comparison :)
Thank you very much for making this video. It cleared up a lot about this subject that i wasnt sure about. Very valuable information. All the best to you, sir!
You're welcome, S3. Glad it was helpful :)
Just had to comment on your feral cat and his love of catnip. I have a male cat that I reward with catnip that I grow in my yard. He loves it and does the same as Olie by rolling in it except my cat, Pinto, eats it also. My female cat doesn't seem to even notice it. From what I have noticed over the years, it seems only male cats love the catnip. Not sure if that is really true but it is what I have seen. So good to see you posting again.
Thanks so much, David. It's always good to hear from you. We grow catnip too and have found, like you, that different cats respond differently to it. I did some research and found that "catnip appreciation" is a genetic trait. Jack strictly eats the catnip, Ollie rolls in it, and (no doubt) some cats ignore it. I'm not sure if it's a sex-linked trait, but will observe to see if that's the case.
I had never seen a cat do that. I don't know much about cats.
I thought catnip was somethings cat ate! Not role around in it!
They seem to react differently, PK.....some eat it and some roll in it and some do both :)
i have a female cat that eats it (literally chews up the dried vegetative matter - strange). She doesn't really exhibit any behavioral changes, though. Certainly doesn't do the rolling around business.
I was going to say another premium video, but ALL of your videos are always premium. I always endorse all of them, especially with new kids on the block. My workbench has, and always will have an iso tranny. I tend to favor the old RCA ISO-Tap which was designed for techs. The receptacles are 2 prong and are fully isolated. Talking about creating a deadly ground reference with a scope probe brings me back to a serious mistake I once made but was extremely lucky in the end. (Yes, even us veterans made them). I remember working on an AA5 AC/DC radio with a hot chassis. I connected my scope to check to see if the local oscillator was functioning. I made the mistake of not connecting the DUT to the iso tranny. When I touched the chassis with the scope's ground lead, Bang!! The ground lead created a fireworks display and blew off the probe. Luckily, the ground lead acted as a fuse, and I got lucky that no damage was done to my Textronix 2465B scope. I do however, recommend investing in a Differential Probe for your scope. Granted, they are quite expensive, but the probes are isolated from any ground potential if used properly. Keep the great videos coming, your viewership is intense...many of your viewers are also professional working technicians such as myself and many others. It's good for the new kids, and us veterans...(refresher) :) Has Jack finished his electronics training yet? He will need the theory under his hat to be the official shop cat :)
Greetings and thanks, Frank. I'm glad to hear that you enjoy the videos and, despite all your years of experience, still appreciate a little refresher once in a while. Yes, Jack has been studying hard and soon will be serving his apprenticeship in videography.
I was given a medical isolation transformer and the neutral was bonded to the case which was grounded.
I removed the bonded neutral and all is well.
The isolated outlets still have the ground pin grounded, I only plug in 2 prong line connected radios in them anyway so there should be no problem.
As for using a scope I never ever connect the scope ground clip to anything but the chassis, I would never have a reason to connect a scope ground clip to anything but the chassis.
BTW, I once had a PHILLIPS oscilloscope that had a 2 wire cord and plug, it came that way.
I agree, OG, that the ground clip is generally attached to the amp chassis. I guess at one time, all electronic gear came with two-wire power cords......but not any more.
I have Philips PM3540 oscilloscope (with rudimentary logic analyzer included) that is made mainly for TV repairs and works this way. It specifically says with big scary letters on first page of the manual that it's wired with floating ground, they consider it a feature. By the way Doug, if you don't want to wait for the seller and want something a bit less how you doin' for connecting stuff to this isolation transformer, ask/search for "UPS cords" or power strips (it's the typical application, as most UPS are equipped with these IEC female sockets instead of normal ones - it saves space if you actually connect computers via female-female IEC cable and it's harder to connect a toaster "accidentally") :)
Thanks for the suggestion, Karol. Fortunately, the adapter cables have already arrived and the I.T. is ready to go.
Doug this is excellent ❗ you have demystify this subject possibly saving people from damaging their equipment or much much worse ☆☆☆☆☆
Thanks, Bret. I hope so :)
I work with mains wires all over my test bench and im still alive! Don't remember how may times ive electrocuted myself
@@TheSlimShady87 well let's hope you're not standing in a bucket of salt water running a shipyard welding machine 😉
Thank you so much Uncle Doug!! I found the identical ISB-060W I.T. on e-bay for $77.00 shipped to my door. I dremeled down the prongs on the 3/2 prong adapter, so it will plug into the funny shaped receptacle, works pretty good.
That sounds like a good buy to me, Mr. T. They cost about $650 new :O Your solution to the weird receptacle problem sounds like a good one to me.
Hi Doug ,I remember as an apprentice in a TV repair shop 1970's the whole workshop was floating, the floor had a thick rubber mat which covered the whole floor., even so at my home work space is also fully floating.
That sounds like a safe working environment, Paul.
@@UncleDoug At 66 I'm told old for shocks anymore Doug :)
Great information! One note: the reason the chassis is usually grounded for a device. The mains negative is generally referenced to ground. If something happens inside your device and the chassis becomes energized, because mains is referenced to ground, you would have a return current through your body to ground (could be lethal). If the chassis is grounded, then if it ever gets energized the current flows directly to ground (in modern homes this would trip a ground breaker linked to mains as well, indicating there's a short or defect in some equipment).
Now if the Device Under Test is using a floating supply, like you've showed, of course this isn't an issue!
Because its power supply negative isn't linked to ground, in the case there is a defect and the chassis is energized *with the isolated supply* there would be usually no return path from your body through ground to the isolated negative/return, so you would be safe. If your device chassis is tied to negative as well, then a short would occur (which is also dangerous), i.e. lots of current, but usually it would trigger a mains breaker or hopefully a fuse in your equipment (to prevent fire).
To all newbies out there: don't mess with mains voltage unless you have a good theory understanding!
Thanks for your input, Gustavo.
I believe the reason your homemade isolation transformer didn’t work is because your transformers were rated for 4 amps on the secondary (12 volt) windings. This, of course, would be only 0.4 amps on the primary (120 volt) windings. You would need 40 amp transformers to make it work the way you intended. Those would be some giant mothers! Great work as always, Doug!
You're exactly right, Gary, and this is why I took the precaution of buying the big I.T. I thought that inclusion of this experiment would be a good lesson for viewers in the realization that not all seemingly logical experiments work out......and I was curious just how close it would come to actually working.
@@UncleDoug could you simply plug the occiliscope into the isolation transformer with a 2 prong cord and plug the amp in a common 3 prong recepticle ? would this be a way to protect the occiliscope without removing ground from the chassis ?
@@AndyX No. As described in the video, oscilloscope grounded, amp chassis isolated.
For the DIYers, a good source for a isolation transformer is from an old 150 watt high pressure sodium light transformer. its a one to one winding. Just eliminate the exciter, center tap and capacitor on the output. Found mine at a recycle center as most are being replaced by leds. most have multiple mains taps.
Thanks for the tip, Joe.
Great demonstration. When I was going to buy my isolation transformer I looked all over for a good explanation of what to buy and could not find one as encompassing as this. One thing I see in the comment section is a bit of confusion over the term hospital grade. You can buy the same power rated isolation transformer, from the same company, either hospital grade or not. Hospital grade will cost considerably more for the same amp capability. The main difference to make either a tech bench isolation transformer, is in the secondary or output plugs. With hospital grade, they have plugs with a green upside down triangle or marked as yours were. Non hospital grade, you will have to replace the outlets with ones with the green triangle or always try to use an 3 to 2 adapter. That is because normal outlets are internally strapped from neutral to the safety ground so when you disconnect the safety ground internally it is still connected. As for a green dot on the primary plug, it just means the safety ground has been verified active.
Thanks, Kevin. We also appreciate your helpful input on the topic of I.T.'s.
Seeing is believing ! Outstanding demonstration ! Previously, I couldn’t get my head around the two types of ground or neutral wires in a three prong plug. The demonstrations in this video helped me understand how neutral and earth/ground are different. Thank you, Uncle Doug ! BTW, I couldn’t help but notice that king crab legs were $19.99 a pound. That’s a pretty good price ! :)
Thanks for your kind appraisal, JJ. Next time you're in town, you must join Jack and me for some crab legs ;)
Man, you're the best, Doug. I've been resurrecting my electronics interest and leveling up with your videos!
Thanks, Greg. Continued good luck with your re-education.
Interesting and salient advice.
My isolation transformer / variac combo came with earth ground on the input, case and output socket earth. As we require an earth pin to be present on all UK plugs because they operate the socket shutters on the live and neutral connectors, I added a sufficiently rated switch to enable me to disconnect the earth if I require it for testing but easily reconnect it if needed. I used a switch with a safety cover that connects ground earth when in the closed position as this is the safest position, although not necessarily best for a connected scope.
Thanks for the interesting information, Pete. Glad you liked the video :)
Thanks for explaining the “medical grade” death trap!! Saved a life :) thank you.
You're welcome, Nef. Glad to hear that the video was helpful.
If you buy a tech grade isolation transformer you still need to open it up and check it wiring. I bought an goldtool grip-1200 and when I opened it up all of the grounds were connected. It is set up so that it is super easy to set it up for electrical work, but if I had not checked it would have been bad news.
One individual tried to implement an electric heater on the floor of his appartment. He embedded the heater wire into the concrete of the floor. This was done without isolating from ground reference. The heater wire touched the reebar inside the floor and thus it led flowing big current through the part of heater wire and circuit interrupt(wire has broken).
There is no cure for ignorance, I guess. Thanks for sharing, Nik.
Thank you for all your videos. They are really valuable. Greetings from Croatia. All best 👍
That's great to hear, T1. Likewise to you from West Texas, USA :)
Good stuff Doug. Some years back I got a hold of what I thought was an isolation transformer for the work bench. Found out it was a commercial unit for noise reduction in computers and such. So I had to go in and make sure the earth ground was truly disconnected from the outputs and now I have a proper and safe transformer. I think there are a lot of people out there that may believe they have a isolation unit and in fact do not. Let us hope they watch this video. Thanks buddy.
You're welcome. Was your neutral return grounded to the chassis, Carl ? It usually is in Medical I.T.'s. I sincerely hope people will determine if their I.T.'s are legit.
Yes, it sure was Doug.
And you removed it from its chassis (ground) connection ?
I made sure that the secondary transformer is floating with no connection to ground where it goes to outputs,,, and mine was not a toroid. Right off hand I can't recall all the steps I took to make it right, but it's as safe as an isolation transformer can be now.
Well explained. It's a confusing topic, and this made things a lot clearer.
Thanks, MW. I agree. It is a complex topic that is tough to explain and understand. I'm glad the video was helpful.
Just found this - a really useful and clearly presented feature on isolation transformers. Thank you.
Regarding the initial DIY attempt - I note that the Jameco transformers used are only rated at 50VA (12.6 4A). I guess the amplifier would have been taking maybe 30VA, thus losing efficiency in the cores and the windings? It would be interesting to repeat this but with larger transformers at, say, 100VA or even 250VA ratings to assess the voltage drop across the combination under load. Indeed, I shall try this since I have the parts and also a regular 500VA high noise isolation transformer from Topaz.
Thanks so much, GG. Agreed, my experiment was rather ill-conceived, but it did demonstrate an important consideration when dealing with transformers. Good luck with yours.
Another option at 23:36 is to measure voltage relative to ground using a DMM that has a LoZ setting. (The Mastech you're using doesn't appear to have that option). That would have a low impedance mode (about 2K instead of the typical 10 M ohm ) and would drain any ghost voltages to near zero.
Thanks for your helpful input, BB.
Your Show is the best , I learn so much and watch episodes over & over. I also miss Rusty.
Thanks, James. I'm glad you enjoy our videos. We miss him too :(
I have a couple of very nice old 5 watt amps with two prong cords . A "White" made by Fender for Forest White and an old Harmony 304A . What I do is put some water on the concrete floor , then plug the amp in each way , with the two prong plug so hot and neutral are reversed and see which way delivers a shock when lightly touching the chassis with
my toe in the water!!!. I pinpoint which polarity does not shock and mark the plug with a plus on the side I will always from then on make sure goes into the small slot of the AC outlet ! Primitive but it keeps the amp quiet and safer though I should probably just add a three prong with the switch on the hot side !
That sounds like a better method than a friend of mine (now deceased) used. He would dangle his nads in a pail of water and touch the chassis with his tongue :)
Same Gretch 6150 amp I used to have, totally original ...death cap and all.
Unfortunately I had to pawn it a few months back...
I have a medical isolation transformer similar to the one you show, I've been using it as a safety device for all my series filament garage sale amps - assuming isolation meant isolation.
Now I'm going to have to double check .... Thanks!
You're welcome, TG.
I'm a retired electrician and even people in the trade don't fully understand grounding for one the green or bare wire tied to a metal chassis on even a hot water heater is called the equipment ground. The white and sometimes yellow is the neutral and is considered a current carrying conductor. Gfci outlets watch both hot and neutral for fast interrupt. If you were in say a shop that has a sub panel from say your house, if you look inside the neutral is floating on a insulator and the ground is separate tied to the panel and the two cannot be interconnected as in your house service panel where the neutral and equiptment ground are by code tied together. I really like the way you teach getting to the meat of the issue see you on patreon
Thanks so much for your helpful and knowledgeable input, Robert :)
Uncle Doug, with respect to your advice about not using two ground clips from 2 different probes and attaching the ground clips to different points in the amp, in an ungrounded chassis;. My question to you is, if your using multiple probes, then why would you bother hooking up two ground clips ? Since the chasis on the scope and all BNC connector ground are all booked together, once you hook one ground clip onto the DUT, then you shouldn't need to connect another ground clip if your poking around the amp with a second probe, since the ground clip from the first probe will be referencing the ground for the second probe, so no need to hook it up at all right ? Tell me if my thinking is flawed ?
Yes, since both ground clips are connected to the scope chassis, they are equivalent and it is only necessary to connect one of them to the device being tested......however, beginners and "forgetful people" do tend to connect both. The warning is intended for them, to be sure they are aware of the hazard from connecting the ground clips to areas containing different potentials.
@@UncleDoug okay Uncle Doug, that is so much for your reply ! I have spent my entire day watching your videos and they are superb, thanks for taking the time to do this ;)
You're welcome, Scotty.
It depends what you are working on. In general, if the circuit is high current, or high frequency, you want the ground for the probe to be connected as close to the measurement point as possible.
High current can cause voltage drop on ground lines that can cause a phantom signal to show up on the scope. See "ground loop".
For high frequencies, inductance raises the impedance of ground lines, also causing phantom signals to show up.
This was often a trap for young players when troubleshooting CRT TVs, where you have a combination of high voltage/high frequency, and high current sections of the circuit. Sometimes you end up chasing a phantom signal that doesn't exist, because you used a long ground wire and didn't move it when moving your scope probe.
Hi Uncle Doug. Thank you for inspiring me and helping me recall some long forgotten information. I am slowly building my bench tools and equipment with the intent of playing with tube amps. I purchased a used medical grade iso trans. As you described, and as expected, my medical isolation transformer is not a tech grade.. no problem! I removed the ground connection to the output AND removed the Neutral buss connection to the chassis. Now the output of this overly massive transformer (7.5” diameter @¿@ ) is wired direct to the neutral and hot of the IEC C13 outputs. .. those funny looking power inputs and outputs are called IEC C13 (the six outputs) and IEC C14 (the input from the wall). Knowing that terminology you can now search the web for all types of versions of these. I found the C14 part as a clamp on/cable mount style, that allows making your own adapter /extension cord with a C14 end. (The C14 part will plug into one of your six C13 outputs) - thanks again!
You're welcome, Racer.
I bought myself a lab tech isolation transformer from Tuvasson, it's 6A on 240VAC, which feed my Variac and a dimbulb tester. And different test equipment like current and volt meters. This transformer has a mid-shield for clean power. It wasn't cheap, but working with 2 toroidal transformers from 230VAC - 12VAC and 12VAC to 220VAC wasn't that convenient. And they went very hot as well, when used.
Sounds like a nice acquisition, Gunther.
Very informative! Thank you for finally clearing up the ground/don't ground question with facts.
You're welcome, Richard. Thanks !!
I used those exact two 149771 transformers and only lost about 4 volts total. The reason is that the transformers have center taps on both sides. According to the datasheet, the primary side is fed by 115v on the center tap (black and red). My voltage on the secondary side and NOT using the center tap gives me around 13.7 VAC (120v in my case). Reversing this gives me around 117 to 118VAC on the other. I added a switch to the ground so can decouple it fully.
Once I get everything fully assembled, I'll test it to see what kind of current I can draw without exceeding the 4 AMP rating on the secondary side.
Thanks for the helpful input, Greg.
Thanks for the video. Excellent follow up to your previous video. Your explanation of the grounding certainly cleared up some doubts I had from other information.
You're welcome, Bryan. "Seeing is believing", and, like you, I had grown weary and confused by all the conflicting claims I read and saw on the Internet.
Love the humour in your videos!
Lots of things you can do with power transformers and re-purposing. Lots of voltages and current outputs can be achieved from stacking identical transformers in different configurations. For anyone who may be looking in their spare parts bins or scrap, the magic specification to pay attention to in transformers is their power rating (VA or KVA in larger transformers), and understanding a few simple principles.
Ebay is a great place. Industrial control transformers make for the cheapest 500w+ to re-purpose into isolation... given you are willing to run 240V to your test bench if you do not live in a country that uses 240V. Or you can buy two, and piggy back them to get around the 240V requirement. (These transformers usually have 120x240 on one side, and 240x480 on the other, with both sides configured to use 240V, hence the 240V requirement). You could run it at 120V... but your power would be 1/4 of the VA rating. This is because the VA is rated at the stated voltage of the coil... which gives you the current rating of the wire used in the coil. This is primarily what limits things severely when you drop input voltage from what it is intended.
Some voltage converters make cheap isolation transformer sources too, but the biggest thing people need to watch out for, is staying away from units that employ auto-transformer method.
Generally... the size and cost of transformers are proportional to their power capabilities. You can expect to pay about $60+USD for a 500VA isolation transformer, no matter how you approach achieving it.
Buying a dedicated isolation transformer probably is the easiest, and isnt too much more costly when you find the right time and place. Like right now, for 115V people, there is a Hammond Manufacturing 169G on ebay for $76 cost to your door. 500VA, It has output taps for anywhere from 90V to 130V, which is a bonus if your wall powerpoint is too "hot".
-------------------------------------------------
Warning - boring transformer maths ahead!
-------------------------------------------------
Voltage and current are inversely proportional. Based on turns ratio. If you step up output voltage (more turns in secondary then primary), your output current decreases proportionally. Likewise, if you step down output voltage (less turns in secondary vs primary), you increase output current***.
The 149771 transformers have a rating of 50.4VA
VA is (V)olt(A)mp. It basically is your power rating in watts. On a 50.4VA transformer with a primary of 120 volts, your input current would be 50.4 / 120, or 0.42A
Likewise, on the output you have 12.6VCT, so output current is 50.4 / 12.6, or 4 amps.
IF that transformer had two independent 6.3V output coils instead of centre tapped, you would be able to wire the secondaries in parallel, and have 8 amps (50.4 / 6.3).
The important key here, is the wattage is the same regardless of the voltage or coil configuration. That is the ideal transformer. In the real world, there are always losses, but that is the general gist of the maths.
***There are of course some deeper technicalities, such as core saturation, core material, losses, cross sectional area, etc... But for general stuff, VA rating is most you will need.
Wow, thanks for all the detailed and helpful info, Salcon. We appreciate it.
@@UncleDoug Your welcome and thank you for all your videos and contributions! I still have a lot to watch. You have earned my respect, esp the few times I saw you own up when you are wrong and you share the learning experiences. I wish more people your age would embrace YT as you do. At least, I'd be tickled pink if I had an uncle like you, who would actually understand the stuff I talk about instead of me feeling like I'm speaking Greek or something LOL. This is the biggest reason I love YT and forums... Interacting with people who actually are on the same level :)
Thank you again, and hope to see the videos keep coming! I know they take time (which is why I have not had any uploaded in a long time!)
Thanks very much for the video. I am just getting started with bench electronics and so far still breathing. :-)
I hope you remain that way, Boyd. You're welcome :)
I have never had so many heart attack moments watching any other video compared to this one xd. The way a guy just do everything bare-handed is crazy
Not really, Neuro. The camera angle makes the distances seem much closer than they really are.
@@UncleDoug yeah but even then it looks terrifying. in my mind it is touching live wires like this or something
@@Neuroszima Start relying on your actual eyes, rather than your imagination, N ;)
@@UncleDoug well looks can be deceiving and i will leave high voltage to specialist anyway XD. Not going to mess with things im not feeling proficient at
@@Neuroszima A wise decision, N.
Just what I was looking for Doug! I have not seen your channel before, but just purchase a medical grade issolation transformer (also from Toroid) and am super excited about it. I wanted to see if anyone else had one opened up and was happy to see such an informative video. This has definitely given me a few things to think more about.. I tell you what though, the build quality on that transformer looks top notch! That's why I chose to get it over the new import ones. I am a bit lucky in the sense that mine will have normal Nema 5-15p sockets.. But It is going to be nice to have something to mount under my bench out of the way.
Yes, indeed, HI. I too was quite pleased with the build quality and price. I hope your new IT serves you well.
Doug, congrats on crossing the 50K+ subscribers! Hey y'all, throw some dough and kibble his way, every little bit pulls Doug and Jack more towards electronics than gasoline. Thanks for the video, stay safe.
You're welcome, Craig. Thanks for the promo......things are going rather well with both Patreon and PayPal, and we've regained our enthusiasm for 'ampliology'. We're currently upgrading to broadband, high speed Internet to facilitate video uploads.
Glad to hear. Don't get me wrong, we love the cars as well. I doubt anyone stops watching when you go outside.
Thanks, Craig. It's been 105+ degrees here lately, so I don't tend to go outside very often.
Thanks for your advice and help Doug
You're welcome, Michael.
I have a Tektronix 224 and it operates from a wall transformer or lead acid battery. The battery went to battery heaven many years ago.
Why I love this little scope is because its isolated from the mains by the transformer, and the scope has 2 channels that are even isolated from each other and have a 6-kV Peak Surge Rating.
Floatable to ±850 V/channel
Wow.....quite impressive, PK.
What does "PK" stand for? I see you use it often in your other responses.
Googled it, and it came back as "Player Killer"!
It's your initials, Pirate Kitty.......PK. I respond to virtually all comments by the initials of the viewer.
Ah. Did not know that.
Very good video Audiophiles forget that earth ground is a second return path if there is a catastrophic failure of the neutral conductor not a sink for noise they are trying to get rid off
Good point, Doc, but since 60Hz hum is AC in nature, providing it a short, alternative path back to its original source can reduce or eliminate ground loops and other sources of hum often by effectively shielding signal wires, reverb tanks, etc.
@@UncleDoug hi yer Uncle Doug that was the house wiring electrician in me not an electronics engineer very good video and fully copper shielded cable helps as well just one question fully copper shield then insulation layer then Aluminium layer then insulation then final stainless steel braid on top 3 layer shielding should stop any noise ??
@@drmindbender8616 Three layers sounds like overkill to me, Doc, but there is something to be said for wretched excess.
@@UncleDoug through my investigations and reading about EMC shielding materials each metal layer brings different benefits I.E outgoing and incoming a cable that lives in a null feild is that not the ideal cable. I have one question for you why does the US use 2 voltages ( 110/220 volts ) why dont you just use 220 its safer lower ampage just curious Iam in the UK so we are 240 volt and EU is 220 thanks for your time Doug
@@drmindbender8616 That's a good question, Doc. I think we are so involved with 110VAC that it would be impossible to change. The vast majority of existing electrical devices would be obsolete.
Thanks for that great vid Doug ! It seems simple but it's not that simple... but simpler with you !
You're welcome, PA :)
Great video. If I'm understanding this correctly, the issue with grounding the chassis is that a lot of equipment ties the 0v rail of the internal DC PSU to chassis ground. The amp in your example must do the same if you're measuring full plate voltage at the plate connection?
The chassis is the HV DC return route to the center-tap of the HV winding......but isolation is used only for the AC primary circuit, to prevent a grounded tech from becoming part of the return circuit.
Great video, very informative. Those funny output connectors are IEC-C13 Female connectors, a very standard connector in the UK.
Thanks for the info, RPS :)
@@UncleDoug And lots of oscilloscopes have them.
Great video as always, Uncle Doug ! One advantage with making a 'high-low-high' isolation transformer is that the current is multiplied by the turn-down ratio of the lower voltage. So it is easier to detect current change. I have a beautiful old 5 Amp. AC G.E. analog meter that would barely twitch when used with120V, since most of what I work on is
You do indeed. Thanks, Christopher :)
Hi there Doug. Watched your videos for a good number of years and find them very informative but have just caught this one. I would like to pitch an observation if I may? I've just gotten hold of an IT off GreedBay which is exactly the same as the one in your video (but it hasn't been shipped to me yet so I haven't gotten my hands on it to play around with test meters). But; if you leave the isolation transformer unplugged from the wall socket, plug your amp into the isolation transformer (nothing is live but the ground pin will have continuity throughout the circuit as it is not switched): Bearing in mind the neutral/return on the IT outlet is floating in this transformer, if you do a continuity test between the amp chassis and the earth ground pin on the IT power socket plug (with the amp plugged in to the IT obviously) if you get continuity between the amp chassis and the earth/ground pin on the IT plug that goes into the wall socket then surely your amp chassis has a ground earth reference as the earth ground socket on the IT is not actually isolated inside the IT so you have earth/ground continuity between the amp chassis and the wall outlet, so you should actually isolate the ground/earth receivers in the IT. Where you do your lightbulb test at 19mins into the video and touch the yellow alligator clip to the return there is no circuit BUT if you touched the yellow clip to the green/yellow ground/earth coming out of the IT then you would close the circuit via ground/earth which is undesirable???? Surely the object of the exercise is to allow the amp chassis to get its ground/earth reference through the scope ground clip only?
What an elaborate inquiry, David. I agree that isolation of the circuit being tested (CBT) from earth ground is the goal of using an IT. For this reason, the chassis of the IT is generally grounded via a 3-wire power cord, but the CBT chassis is not connected to the chassis of the IT, thus avoiding an earth ground reference. Any continuity between the CBT chassis and earth ground would therefore not be desirable and would negate the value of the IT.
@@UncleDoug So the best option for safety for the scope and more importantly the Tech when testing a ground referenced CUT that has a three pin plug to ground reference the chassis is to disconnect the ground pin in the IT socket that is supplying your CUT (same as using a two-prong plug to the CUT so the chassis is completely floating from ground reference). What I shall do when I get my IT is to disconnect the ground reference pins on right-hand bank of outlets to emulate a two-prong CUT and leave the left-hand bank as is and annotate the IT case accordingly so I have the option. Thanks Doug.
@@UncleDoug Hi there Doug. I got my I.T. which is same as yours and put my meters on it (I use a Fluke 87V and a Simpson 260-6P). I live in the UK so my I.T. is set for 240v on the primary winding and 240v on the secondary (it actually gives out 250v when tested so I'll turn it down with my Variac when using it on tube amps). I did a slightly different test to yours in the video. I left bank A (left output bank) wired as existing but disconnected the green earth strap from the chassis binding post for bank B (right output bank) and insulated it. These were my metered results: on bank A between hot and ground there was a potential difference of 182V and between neutral/return and ground there was a PD of 83V (I assume that this is the case as whilst the neutral/return and ground reference on this transformer are not connected within its case, if we trace the wall socket supply back to the Electric Companies power switch and fuse board [we call it Consumer Unit in the UK] the neutral/return and ground are bonded). On bank B (totally floating) there was a PD of 22V between hot and ground and 8V between neutral/return and ground. I am going to investigate further as to why there should be a PD at all on the totally floating bank B as I was expecting no PD at all noted between the ground pin and hot, and the ground pin and neutral/return.
Doug - at 20:30 in the video I think you might have inverted the Line and Neutral wires of your plug. Since the US uses White for Neutral and Black for Line then the black wire should be on the left of the kettle plug and white on the right. It doesn't make a difference to your experiments tho.
Thanks for your very helpful input, Ed.
Just subscribed to learn about working on tube amplifiers, what a wealth of simply Great Information!
Thanks so much, John, for subscribing and for your nice comments.
@@UncleDoug Your Welcome!
I have a question. If you already have one oscilloscope lead grounded, why you need to have the second one too? Doesn’t the oscilloscope leads share the same earth ground?
It's better to have it connected to a ground than flopping around, possibly shorting something in the circuit.
In Australia, safety requirement always dictates that the power plug to have 3 pin. So with this Medical grade isolation transformer, it's basically still earth reference, not floating. You still have to remove the green wire to the chassis of the transformer to make sure it's isolated.
It's easier to do as I recommend in the video and use a 2 prong to 3 prong adapter.
Thanks for the Reply
I understand about the Earth when testing the amp with a Oscilloscope
My concern was that when using the amp there will be no earth connection and because the transformer isolates the input power therefore if there is a fault in the amp , circuit breakers would not work as the amp is floating.
Thinking that there needs to be an earth neutral link on output of step down transformers.
Regards Geoff
I am not an expert in step-down transformers and have no idea how yours is wired, Geoff. The best you can do is to use a multi-meter to assess the situation.
Great tutorial Doug! I have a couple of (I believe) the same toroidal transformers (Hammond 182L60) so I should put an isolation together in case my variac is not. I’ll likely put in a ground on/off switch.
Thank you!
That sounds like an interesting project, Byron. Best of luck with it.
Fantastic video. Potential life saver. Well done, Uncle. You’re amazing and I have learned a ton.
Thanks, BEW :)
I'm trying to get a good understanding of these - please correct me with any mistakes.
The main point of removing ground in an isolation transformer is to remove the connection to the physical ground. In your home or somewhere near your home is a rod physically planted deep into the ground. That means that depending on the physics, your feet might complete a circuit to your hands touching something energized on your bench. When you remove the ground, you and nothing around you will complete a circuit that will electrocute you through your chest. That does not mean that high voltage danger is eliminated, but it is definitely safer.
The reason why the light bulb would light up when connected to the ac receptacle's earth pin is that in your breaker box, by law/code, the neutral wire is connected there to a reference ground aka the metal pole in the physical ground. However, for some reason I'll need to research more (edit: conclusion below), the circuit operates entirely between the hot and neutral wires. Even though the neutral wire is connected to ground, they are not the same. Something about impedance and the design where the ground reference is used to quickly trip the breaker to prevent electrocution / fire is much more effective than old fashioned hot/neutral with fuses.
An isolation transformer is also extremely effective at removing noise from tube amplifiers.
Paragraph 1 sounds good. Paragraph 2: I understand your confusion between neutral and ground.....if they are jumpered at the fuse box then why aren't they equivalent, and thus redundant? To my understanding, the neutral wire provides an operational path for current to flow back to the AC transformer (on the pole behind your house). The ground connection is only activated when the hot wire becomes shorted to the chassis, providing a pathway for the current, preventing you from providing that pathway and most likely blowing a fuse. If no ground was provided, the chassis would be hot, dangerous, and no fuse would blow.
@@UncleDoug
Thanks Doug. Your explanation helped me get to the crux of the explanation. After a lot of searching, the answer lies in Ohm's law.
As current flows through the neutral wire, it develops a voltage drop along its length due to the wire’s resistance. The neutral wire carries return current from the load back to the transformer on your street, completing the circuit. In the event of a fault (e.g., if the hot wire contacts a metal chassis), the ground wire actually creates a new path back to the breaker box, where it is bonded to the neutral wire. This pathway allows a large enough current to flow rapidly to trip the breaker or blow the fuse, preventing the chassis from becoming energized and dangerous. This is key for fire prevention and of course electrocution on the bench.
Neutral as the Primary Path:
Electrical current always takes the path of least resistance. In a properly functioning electrical system, the neutral wire provides a direct, lower-resistance path back to the transformer than the ground pin.
There are actually two earth grounds:
Yep you read that right, the center tap of the transformer outside your home is tied to earth just as Doug showed in the first video. Your main breaker also has a separate ground connection.
The transformer’s ground (at the utility pole) stabilizes the entire power grid. The home’s ground (at your breaker box) protects your home and ensures safety. They are tied together through the neutral wire, so they operate as a single, unified system. Reference voltage is critical to the operation of your system. And don't forget that 120 volts is RMS, your home AC voltage is realistically more in the 300 volts region - thanks Doug for showing us that on the oscilloscope.
Ground as a Safety Mechanism:
In the event of a fault, the ground wire provides a low-resistance path back to the breaker box because it is typically tied to the chassis, quickly passing current and allowing the breaker to trip. The earth connection itself (via a grounding rod) has much higher resistance and serves only as a secondary safety mechanism in extreme cases, such as a disconnected ground wire.
Voltage on Neutral:
Even though neutral and ground are bonded at the breaker box, neutral can have a slight voltage above ground at other points in the system. This voltage difference occurs because current flows through the neutral wire, and its resistance creates a small voltage drop. In contrast, the ground wire remains at 0 volts because it is not part of the circuit and is connected to the earth as a reference point. Now while you might think, ok then the current should go to the ground, the neutral still has a lower resistance.
Why Ground is at 0 Volts:
The earth serves as a large "neutral" reservoir and is used as a consistent reference potential. By connecting the ground wire to a grounding rod driven into the earth, the system ensures that the ground wire remains at 0 volts. This connection stabilizes the electrical system and provides a safety mechanism to dissipate fault currents when necessary.
Why Current Flows to Ground in a Short:
The Fault Creates a New Path:
When a short circuit occurs (e.g., the hot wire touches a metal chassis), the ground wire becomes directly connected to the energized hot wire via the fault. This creates a direct path for current to flow back to the breaker box, where the ground and neutral are bonded.
In this scenario, the neutral wire is bypassed because the fault path through the ground wire is the most direct route for the current.
Wow....Thanks for all the info, Jules.@@julesl6910
Thanks Uncle Doug. You have a gift for teaching. Your gift is similar to Mr. Pete who teaches on UA-cam how to use a lathe. He was a shop teacher. Your gift must just be God given.
I have watched your lesson on Isolation transformers several times, but can’t get past my confusion of the difference it makes connecting earth ground from the isolation transformer or from the scope. It seems to me that the chassis is earth grounded in either case so why is there a problem for potentially blowing up the scope? What am I missing?
Thanks so much, Larry. There is no earth ground from the IT. It isolates the circuit being tested from earth ground, as well as the chassis.
Yes, thanks for your quick reply. I understand; the IT has its ground disconnected, but don’t you earth reference ground as soon as you connect the scope ground to the chassis anyway? What difference does it make if the same ground reference comes from the scope, or from the IT(assuming you left it connected). I do understand your excellent presentation, but I’m missing something very simple I guess. You say we don’t want the earth ground from the IT, but we supply it from the scope anyway. So the chassis gets earth reference either way. Earth ground is earth ground, no? Or is it like having a potential difference with the two probes having to be at the identical location for test. Sorry, for being so dense, but I have watch all the UA-cam videos on the subject and although yours is absolutely the best, I’m missing some little detail that I can’t get over. Thanks so much for you WONDERFUL (really wonderful) videos. You do such beautiful presentations.
Great video, good Sir, especially the solution with this Medical Grade iso-transformer. If you fancy meaty toroidals, I recommend searching for old, burnt out PA-amps (Crown, QSC ...) You get those broken ones for cheap, they have massive transformers, and due to the amp-stage-designs their secondaries are usually ~ 2x60V, meaning: no rewinding-work on the secondaries needed for your folks from 110V-land. Other than that: Ollie's catnip-debauchery and Peeping Jack cracked me up :) Very good show!
Thanks, KS.....glad you enjoyed the video :) Thanks for the transformer tip.
Great video, U. Doug. I ended up getting up getting that oscilloscope for $125 and he threw in a Hammond 1:1 isolation transformer, a Wavetek LCR55 ohm/Henry/capacitance meter and a fused 10 outlet power strip. The probes are a bit work, but I figure I can get replacements online. That Hammond is heavy!
Wow, what a major score, B55 !!! There are wonderful bargains to be had in used electronic gear. I keep an eye on Craigslist and snap up every bargain I can.
Uncle Doug The man was 76 and had finally decided to sell his business of making electronic equipment for the government. It was a cleared facility, so I had to be escorted, but that was no problem. He had a ton of stuff that would have been great, but not necessary. Spectrum analyzers, almost a dozen scopes to choose from. A ton of metal brakes for building chassis's of all sorts of sizes. I couldn't afford those. I forgot, but he threw in a big spool of 44 gauge Formvar, in case I wanted to try winding some pickups (they would be hot, I suppose, or small). Funny, he asked me what I was going to use the stuff for and when I told him, he said that he did his thesis in 1966 on tube amplification. Small world. Nice guy. I'd love to pick his brain for no doubt a ton of knowledge.
You are in good hands with all state
Better than with solid state :)
Don't understand the bulb testing at 18:30. Why not just check for continuity between the outlet neutral and ground?
It's a video, BB. Visualization is both possible and preferable.
@@UncleDoug Understood. All I'm saying is that a DMM in continuity mode with one test lead in the neutral and one in the ground would have visually (and audibly) confirmed lack of continuity without the need to hook up a bulb.
@@Jnglfvr Thanks for your input. BB.
Hay thanks for the good information. As soon as my new scope shows up I'm going to give it another try.
You're welcome, TT. Best of luck with the new scope.
Noise can also come from the ground line to the sensitive medical machine, thus it makes sense not to connect it or let the groung wire bypass the filtering.
I am confused by this statement, Janne, but agree that if your purpose for the medical grade I.T. is to eliminate electrical noise, then you should leave the connection between the neutral and ground intact.
But you expect that EMR noise can not come from ground or neutral. It can. EMR spikes and noise can affect all leads. If you have an car ignition coil making sparks and 3 wire cord nearby, the noise will spread to whole length of wire, reflect from the underground end and taken to ground from the grounded end. It works like an long wire radio transmitter antenna. At RF frequencies the noise is a spreading waveform in an conductor.
Wow, your points are very clear and well explained.
That's great to hear, Andy. Thanks :)
NB! Simply removing the IT internal green/ground wire from the IT receptacle ground lug may not be good enough. The receptacle mounting yoke is probably directly connected to the IT receptacle ground lug internally thereby connecting it to the chassis via the holding screws UNLESS the receptacle has the orange triangle emblem and/or the round hospital grade emblem on it's face showing it is a truly isolated ground receptacle. The isolated ground receptacle can be purchased and installed if you don't want to use the 3 to 2 prong adapter. Thanks for the lesson video! Forgive me if I am repeating previous information as I haven't read all the comments. My comments come from making continuity checks last night on my so-called isolation transformer. It most definitely is not an isolation transformer. I am using the 3 to 2 prong adapter until I change the receptacles to the true isolation ground models. The isolation ground receptacle is either the color ivory/white or red/orange and can be more expensive than the standard grounded receptacle.
Thanks for the detailed input, Ted.
@Uncle Doug - I don't know if you will ever see this but I had to try! On another Group on FB I have been asking questions about Isolation Transformers. Many people say... use one on your "device under test" to isolate it's ground from the test equipment on the bench. Others say...."what is the sense in doing that? As soon as you hook up a signal generator to the input.....you ARE ground referenced!!!! So... don't worry about an isolation transformer"!
I am thinking that if you DO hook up a signal generator..... THAT establishes your grounding point for THAT PART of the troubleshooting effort.
Can you offer any thoughts or comments on this?
The primary purpose of an IT is to protect the technician from AC shock while testing and repairing a circuit. Generally, this is done using a DMM, and the IT does its job......but when you begin to use AC powered equipment, problems can arise. You can "float" a signal generator (run it ungrounded) but oscilloscopes present another challenge. Please see my oscilloscope video series for a detailed discussion of shock hazards and how to prevent them.
@@UncleDoug I really think that I am getting a "grip" on what would be a safe arrangement FOR ME!!!!
The way some of these guys are telling me I don't need to worry about isolation.... I can only guess that when they change oil on their cars/trucks they use 4x4s on end instead of jack stands!!!!
Anyway, on another channel a guy brought up building a little "audio isolation" box using BNCs and a little 1:1 audio transformer (10K:10K) to isolate the generator from the radio. I might play with that idea!
Thank you sir for responding, I hope you are well!!!!!
In the Uk we have 230 vac our mains have live neutral and earth. We have large house size tranformers not on posts like the usa. Nuetral is earthed at the sub station tranformer, earth is a local earth. In uk electronic work shops we have rubber mats, the bench suppy is via a 1kw 230vac to 230vac isolation tranformer, there is no earth on bench sockets and if there is arial sockets they must be isolated with capacitors and no earth is alowed for 1 meter around the bench, we also have rubber on our benches
Wow......very complicated. Thanks for the info.
Also, we don't use 230vac - Europe insisted we switch from 240vac but the cost of replacing all those substation Txs was billions so, as the mains used to be about +/- 15v anyway, they just relabelled everything to 230vac. This is worth knowing as American gear has two 115v primaries wired in series for 230v use here but the 240v means their HT voltage is way higher than it should be.
I typed this out for those who don't know - sounds like you do! :o)