Isolation Transformers, Variac's and Current Limiters - Which comes first?
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- Опубліковано 1 лип 2024
- When setting up a well equipped electronics lab or test bench, there is a proper sequence in which these items should be connected. They are: Isolation Transformer - Variac - Dim Bulb Current Limiter. This video explores the reasons for this sequence.
Disclaimer: If you are uncomfortable working around high voltages, please seek the help of a professional, licensed electrician or electronics technician. There is nothing to be fearful about working with these devices but a healthy respect for the effects of electrical current on the human body is always advised.
Thanks for sharing a very clear schematic of your setup. This is about the fifth video on this topic I’ve watched today (and the last). But you’ve inspired me to get cracking on building my own; I believe I have couple of old AC voltmeters and ammeters laying around the shop, I didn’t realize I was saving them for this project.
You're very welcome, hope it all works out good for you.
Soothing education. It was like meditation for me.
Thank you sir!
Great video. I also plug that Kill a Watt into the DBT, and the UUT into the Kill a Watt, set to Watts or V.
One of the most thorough explanations I've heard yet, many thanks for taking the time. 👍🏾👍🏾
Thanks for watching Kenny!
Thank you. A good demo video.
Plain language, straight forward and one of the best presentations I have seen! You get my thumbs up. Although the explanation all the way from the utility pole might seem elementary we all need a reminder. 5 stars!
Thanks Odd Job. I appreciate the kind words!
Thanks for the very elegant demonstration of the value of each item, and the sequence to use. I shall incorporate these ideas into my test bench!
You're welcome Dave, thanks for watching!
Thanks for showing the Dim Bulb current limiter. I also learned today that a Variac has only a single winding. A Variac is not constructed the same as a variable transformer.
The variac may also contain several "taps" to control the max. output voltage but it's always (1) continuous winding.
Nice presentation.
Excellent! Thanks for this. Very useful bit of kit for working on older valve (tube) electronics. Now, the pain of all the old bits that were thrown out of the lab years ago suffering from 'I will never need that old thing anymore' syndrome!
Thanks for your reply Anthony. Yes, I suffer from that syndrome too! That's why I keep everything. 😂
Wow so good...! I'm a newbie to all this and have been following a good few amp techies on UA-cam, This has cleared the fog about the current limiter (dim bulb use) Thanks for sharing...Ed..uk..😀
Thanks Edward. There are some really good UA-cam tech's out there worth listening to. Uncle Doug and Mr. Carlson are some of my personal favorites and they really produce some quality, informative and fun videos as well. Probably where I got my inspiration to do these!
Reply to Erik Burman: The human body's resistance varies widely depending on who you are, what your body chemistry is and most importantly, if your skin is moist or sweaty (usually with a lot of salt mixed in). It only takes a few milliamps of current to seize muscles or stop the heart from beating. So yes, there are a lot of variables.
Good video, thanks!
Thank you, I appreciate it!
You're hilarious. And thank you for the well-informed and info
Thank you! I appreciate that. I try not to let my videos get too boring.
I liked the demo towards the end where you show lower voltage on the tested item if the dim bulb has too low of a wattage. I'm hoping with the phase out of incandescents that we won't have shortages of 150 or 300 watt bulbs in the distant future!
I think you'll still be able to find them for quite some time. You could always buy a different type of bulb type like a T3. I just bought (2) 150W 3" long lamps for $2 at Ollie's yesterday. They are used for outdoor security and work lights.
You can still get a 300 watt bulb from Lowes. They only have 1 choice but, they do have it... for now at least (called the G.E. Crystal Clear 300 Watt and 130 Volt). I went ahead and bought a case of 12 just to have so I don't need to worry about it. That should last me the rest of my life lol.
What is important to note is that most variacs have no isolation at all, one side of the output always gonna be live of it and the other just tapped voltage from it pretty much, you want to use your isolation transformer if you measure anything in live voltage to protect your scope. I got my big Variac first actually way back in time but later I got a very big nice isolation transformer that even have virtual ground. I put a switch on the ground circuit to be able to run either isolated live without ground or together with virtual ground. it is a great thing to have if you were to work on a tube radio that works without both ground and transformer, yeah the death radios and I don´t like them. but you never get better than your tools I always say, but your tools also don´t have a brain you you have to use your own. Just because you have fancy isolation and variacs you won´t be completely protected which you always should have in mind. Don´t put too much trust in your extra safety
Thank you for your efforts to teach us! Much appreciated! Suggestions = IMHO, an Emergency All-Stop Switch should be the first device in the chain, from the mains supply wall socket. It should be of the proper industrial design, meaning 1) it is not just a common paddle-type wall switch, but rather 2) an ergonomically designed safety switch, a) meaning it has big button(s) that are easily accessed and brightly colored, as to their function. b) Further, IMHO, the single switch type, having a large mushroom shaped button, is better than the two switch type, because it simply can not be confused, during an emergency and 3) it should be placed in a predominate location, having unimpeded (at all times) access. I am, even presently, designing my AC Supply. I planned to use many 40 watt bulbs, in order to have a great amount of control, when raising the current supply. Accordingly, I purchased Halogen bulbs, because they are considerably smaller in size. After purchasing them, I saw a comment that common incandescent bulbs are better, but there was not an explanation, as to why. Do you know? I would rather adapt for them, now, before finishing the build. Final tip = I suggest that an AC supply system should be designed such that each component is strung together with the next by means of some type of connector. That way, each component can be employed for individual use, in other applications.
Ya know I really don't know if there is much difference between the common incandescent and a halogen. You are of course looking for the inverse resistance properties of the incandescent lamp so, you might do a little experimenting to see which one affords the best resistance properties as they 'incadess' and let us know.
@@DVRproductions52 Thanks. While subscribing, I noticed that your subscriber count is still low. I want to encourage you to keep posting; I like your electronics connect, a lot. You have only been at it for nine months, IIUC. Word of mouth still needs to get around. So, hang in there!
@@t1d100 Yeah I'm pretty new at this YT video stuff although I've been producing broadcast television content for 20 years now. My first love has always been electronics. Just need some more ideas for my channel content.
1:18 also bond vs Jaws on the train, electric bulb electrodes to the dental work, classic ...
Useful video thanks. When do you utilize a gfci or quick blow fuse in your testing bench? The floating vs non floating contradiction in recommendations still baffles me also, especially related to Oscopes. Cheers
Anything AFTER the isolation transformer does not require an GFIC since there is NO return path to the city power grid grounds. That being said, if your test bench is not on the isolation transformer (mine isn't) a GFIC sure wouldn't hurt.
Thank you for the informative video. I am putting together the same device 'isolation transformer -> variac -> dim bulb', and this video helped a great deal. I was thinking about putting the items together in one test box. I noticed your design does not include a fuse or circuit breaker. Would it be beneficial to add one? If so - can you give me your advice on where it should be placed (at the input to the transformer or at the output - or both) and what amp fuse would be best in relation to the transformer you used.
That's a very good question jon. I don't have a fuse in mine because it plugs directly into the wall receptacle but fuses are always a good idea. I would take the maximum rated wattage or amperage of your transformer and calculate the fuse size of around 10-30% over that rating. So, if your transformers max. rating is 5 amps., I'd place a 6 amp. SLOW BLOW fuse between the line cord and the transformer primary. -- If your transformer only shows a wattage rating or KVA rating, divide that by your wall receptacle voltage & use that current value as amperage. Placing the fuse in the primary winding will protect the whole string. Thanks for your question!
Any suggestions for make and model# of an isolation transformer? I've seen lots available but am not completely sure which one I need for working on tube phonographs from the 1950s. I've got some education in college days (so not completely unfamiliar). Just want to start right.
Hi John, The one I used in the video is an old Stancor model P-6415. It's pretty old and I can't remember how I got it but it has (3) taps 105/115/125V all rated at 350VA (Inductive watts) which @ 125V will handle aprrox. 2.8 amps. It's been just right for everything I've ever worked on. Almost any isolation transformer would do well for 50's record player work, they don't draw that much current. There are a bunch of them on eBay at various wattage ratings that would do you just fine.
I have one similar i made. I like the voltage meter! i will incorporate that.I don't have a bypass switch... what i do is have a Two Bulb Y screwed into the bulb socket that way I can add different wattage bulbs to make up different wattage protections, and if i need by pass to full power I just screw in a 15 Amp fuse which will bypass the bulb completely.
Brought up my old signal generator yesterday 10 volts every 15 mins or so, but was wondering what the precise voltage was? I need a AC volt meter in line.jim
Jim, I like the idea of the "Y" that way you can more easily change loads or screw in the bypass etc. The 200 Watt bulb is a good "all around" but there are times when a 75 Watt is more appropriate.
I'm a little confused on this. You said to keep your oscilloscope on the primary side of your transformer. Your oscilloscope is grounded through the ground clip. How does this prevent your oscilloscope from blowing up if you attach your ground clip to something that is live?
Greetings Jason! Since the O-scope is on the primary, your device under test (radio, amplifier etc.) will be isolated from making the 120 VAC circuit through the O-Scope ground to either side of the secondary whether it's the line side or the neutral side. This is the real reason for the isolation transformer, (aside for protecting you from being the ground clip connection LOL). -- I have setup my bench so all the test eq. is on the primary side with all ground pins (if they have them) on the same house circuit. Then everything under test or experimentation is on the secondary of the isolation transformer. -- Don't hesitate to reply if this sounds unclear, sometimes I get a little to in-depth. Haha.
Thanks for the video. If a reason for not putting a variac before an isolation transformer is that the isolation transformer won't develop enough counter emf and could burn out at lower voltage. Then is it also a bad ideal to place a variac before the mains transformer of your favourite Fender Bassman?
It's not so much that you would "burn out" the transformer but I find that you lose the linearity of your Variac as you ramp up the voltage. On mine, as I turn up the variac, the voltage stays low for a while then all of a sudden it jumps up higher as the variac setting increases. Everything seems to work better when the two are chained as I explained them.
Thanks for the reply
Hello, I read that measuring with an oscilloscope, you need an amplifier, you need galvanic separation, I ask from what point of view, when today's amplifiers have only 2 wires. Thank you for any knowledge
Hi Luke, not sure what you are asking, can you elaborate?
I bought from a garage sale what I believed to be an isolation transformer, but when I have the transformer output connected to a working load, if I measure the potential between the output live to earth ground, (to convince myself I am not going to get a shock), I read 129V, and from the output neutral to earth ground I read 43V. However, prior to doing this load test, with every wire disconnected, all Megger tests of the primary and secondary with respect to each other and the earth terminal on the core, show it all as good, i.e. no path of conduction where there shouldn't be. Would your isolation transformer show any potential difference on the output, between itself and earth ground? Can you show that? Is it possible that the primary voltage and secondary voltage levels, just float around with respect to one another, until some load, even infinitely small would couple them, then the PD would become zero? Because when I insert a load where I measure the 129V potential, then measure the potential across that load, it reads zero.
I was gonna suggest you test the leakage with a slight load before I read to the end of your comment. I think your meter may be reading some un-loaded capacitance coupled voltage through the core and windings. Are you using a VTVM to measure it or a 20,000 ohms per volt analog meter? A VTVM or other high input impedance meter can get an unloaded reading by just waving the probe in the air. -- My isolation transformer has NO ground connection whatsoever. I think you'll be ok.
Since the secondary of the variac (and the primary) has no connection to earth ground what is the purpose of the isolation transformer? In its absence you will have the variac connected to the dim bulb and load. If you were to touch line voltage in your load (or attach ground lead of o-scope to it) there is no complete path back to voltage source. Please explain.
Neutral and ground are connected at your panel. The variac doesn’t isolate between the primary and secondary sides of its transformer. So any electricity that comes off the variac is directly tied to your mains electrical wiring. So you have no isolation.
My isolation transformer is only rated 500 va, while my variac is rated 2 kva. Should I still put the isolation transformer first?
Yes, I would. Your 2 KVA Variac won't draw any more current than what you load is. Just keep it under the 500 VA rating of your transformer.
What is the formula to calculate the bulb wattage needed related to the device?
That's always a good question. I tend to use 2 to 3 times the wattage of the device under test. So if you're doing a 50 Watt radio, try a 100 or 150 watt bulb. If the voltage drop is too much and you can't get enough voltage to the radio, (and you know there are no problems with it) try a higher wattage.
I found an old isolation transformer, very heavy 115v .500va. I replaced the incoming cord because the insulation was cracking. The transformer looks unused. I checked for continuity and short to ground on both sides and it was good, it has a single receptacle. After plugging it in to check output voltage at 127v, but there was also 17v on the ground side to chassis ans 96v from hot side to chassis. I am no electrician, is this transformer trash ? Thanks.
It's probably ok. There is always some leakage due to capacitance in the insulation especially if your voltmeter is very sensitive. Unplug the transf. & check with an ohmmeter, that will tell you if there is a short to ground or not.
@@DVRproductions52 I did not detect any short to ground with the meter.
@@forrestgumpv9049 Looks like you have yourself a great isolation transformer! Is it rated at 500VA or point(.)500VA? If it's as heavy as you say, I would think it's a 500VA or in the neighborhood of 500 Watt capacity.
@@DVRproductions52 It is a Triad N-57MG, 500VA , 115V.
I checked it in series to chassis ground one leg at a time with an incandescent light bulb. No lighting, looks like " ghost voltage ".
How can I maintain isolation if my variac has an earth ground plug?
If you are going through an isolation transformer, you ARE isolated from the main service drop even if your ground goes all the way through to the wall outlet. If you want to, you can cut off the ground pin on the variac plug or use an adapter but it's really not an issue as long as you're using an isolation transformer in front of the variac. If you do that, there is NO way that you will have a complete circuit back to the line coming off the power pole.
The ground pin can never be connected to any point of a viriac or transformer. The ground pin only goes to the metal case.
I think I have those exact isolation transformer and variac.
They do work well don't they. I sure wish I could remember exactly when/where I got the iso-transformer. How old would you say it is?
@@DVRproductions52 I found mine at a thrift store, decades ago. Yes, it works wonderfully.
Outlet>Isolation>Variac>Current limiter>What you are working on
Right!
If I could find a large enough transformer, I could put both the test equipment and the device under test on the secondary of the isolation transformer. Is that any "down side" to doing that?
I can't think of any issue other than the voltage may vary under loads more than if you had all your test eq. plugged into the house wiring.
You should watch Uncle Doug and mjlorton videos on this subject (Isolation transformers). While potentially "safe" using your suggested method, there are still ways that you can get shocked or damage your oscilloscope or ground lead on your probe. Most techs (and tech videos on the subject) recommend keeping the oscilloscope grounded (hooked up to a standard outlet with a three prong plug) and only floating the DUT (device under test). Even the way that DVR Productions described in this video does not guarantee user safety or oscilloscope safety, though it can help in most situations, the dim bulb current limiter also adds a level of safety.
Never actually seen a transformer like that anywhere where I live (finland), I only recall having seen a few large stations that took up the space of a house and a yard, wired off with large metal fences and lots of buildings and electrical equipment, whole place makes a hummmmm sound. One station serves many thousands of households.
Most of the ones I've seen here is the U.S. don't have the enclosed "Bell" ends or they are built into an enclosure with various voltage "taps."
$2 for a 350VA isolation transformer? $5 for a metered Variac? Bargains of the century!
Barbecued chips? OR .....secret sauce iso's?
100 or 200 watts is way to high for low current devices like AA5 radios. Just sayen?
Yes, you are right. a 60 watt bulb would be safer and I do change that bulb depending on the device under test. I use a 200 watt as a standard to detect catastrophic short circuits. I still use the 200 watt on larger devices like high powered guitar amps.
...and when incadescent bulbs become banned? In 2022 (even 2021) I would think you would be creating this video with an alternative to a dim bulb limiter...
There will always be some form of incandescent bulb or lamp you could find. i.e. work lights, studio lights, copier fuser lamps etc. I just bought a couple of 100W T3's at Ollie's the other day for 2 bucks.
Hey, just wanted to let you know I scrolled past the stupid key frame of you making an o-face for this video. If there had been a pic of that variac or anything technical, I would have watched.
Are you talking about the Thumbnail image?
If you need a dim bulb then you are the dimmest bulb in the dozen. Learn to use meters with that variac.
The purpose of the bulb is NOT to replace a good ammeter. It's to protect the device under test. For instance, if your radio or guitar amp has a filter cap. that suddenly goes short circuit then the bulb will change its resistance and illuminate, taking up what would have at least blown the bonding wire off the rectifier tube or even worse, wiping out your transformer, ammeter or variac. Another safety it provides would be to take up the massive amount of current in the event you slip with a probe or screwdriver and short the line to neutral. I've had this happen recently and it saved a lot of damage plus the bulb illuminating brightly was an instant warning that I did something stupid! Saved a hard to get vintage power switch.