I just had a project that required an Iso Trans and I did not have one. Your project inspired and instructed me on how to get it done. Thank you! A friend gave me two model SUA1500s. I already have them stripped down and I am configuring the rebuild. I thought a single 1500 might have two 750s in it, meaning two transformers. No, they have what I think is the exact same transformer as yours (best I can tell by the writing on them,) even though I am on USA 120VAC. I do have more room than you in my case, I think. My rear bank of power sockets has the common wall-type, 3 pin configuration. Meaning, they are very usable and I do not need to build in a different type of socket. To facilitate their use, I am changing the "workflow" of the device to be back to front. The sockets and fuse are now on what will be the front of the unit. All that was needed to accomplish this was to route the main power cable through the front and to discard the front cover. And, to remove the common ground bus bar from the sockets.
Thank you for explaining the entire procedure Robert. Every process & every decision you executed, was thought out and explained well. I was able to follow you through to the very end. Thank you Sir!👍 New subscriber here! The newer UPS's are made cheap. Most do not have the big Power Transformers anymore.
A second option what can be used for creating a isolationtransformer, is if you have a old ATX computer powersupply , and a 12V to 230V inverter (car, camping, etc....) lie arround, is to combine these. The 12V output of the ATX powersupply can easy supply 5A or more, to feed the 12V to 230V inverter. The power output will not be to great, but enough to power small devices.
That's an fantastic idea! The efficiency is probably significantly higher than my two iron core transformer solution. However, a true/real sine output 12V to 230V inverter will set you back at least 100 bucks. And I don't care if the Chinese sell "real sine wave" inverters for 30 bucks or less - I don't trust those.
This video was extremely helpful to me - I really appreciate the clear explanations, diagrams and practical examples. I wrongly assumed today that the fat wires were the primary side of my transformer, and tripped the breaker in my house haha but I don't seem to have damaged the transformer at least. Your video has helped me to understand a lot better, thank you. I have three old UPSs which I bought for 5e, I think it's probably just dead batteries as everything looks very clean inside. I want to use two of the transformers in series to get a 24v AC supply for a [what I now realise is not very well regarded] DIY 30v lab power supply kit. If I can manage it, it should make a nice project as I can use the case of the UPS, the heatsinks, and the original power switch. I still have a huge amount to learn though....I have subscribed to your channel. thanks again.
You're welcome! Regarding your little mishap: That secondary winding should be able to take short current peaks of up to 90A, so your breaker really never stood a chance :-) Anyway, good luck with your project! A 40A or so lab power supply no less!
A while ago i did some tests with a used 500VA transformer that i purchased for a isolation transformer project. This transformer has multiple taps from 0 to 500V, it can convert 230V mains to 110V, 230V, 400V and 500V, during some tests, i discovered the phenomenon called "transformer polarity", which comes to play when using 2 transformers back to back. When changing the "polarity" , this can give a influence on the output voltage. Also to mention, these transformers in UPS work reversed. The 230V side is in fact the secundairy, and the low voltage side is the primairy. The primary side gets feeded by a chopped AC sine wave created by a SMPS power supply + (chopped) AC sine wave combo circuit. The more expensive UPS can create a pure sine wave on the output. A second part of the SMPS power supply is for charging the UPS battery. And last, the newer UPS don't have these big transformers anymore and use small transformers with high frequency , and in the last stage the signal is converted to a 50/60Hz AC wave to feed the 230V power outlet socket(s). If the transformer has a -5a10% / +5a10% tap on the 230V side, these extra taps between can deliver a small voltage what can be used for powering LED(s) or in combination with a buck converter to a 5V USB outlet.
Thanks a lot for all the info! The APC Smart UPS units are technology wise stuck in the past I guess (see my teardown of one for more information: ua-cam.com/video/TT9je5yo7yM/v-deo.html ). The high voltage side of the smart UPS has several taps and is used as autotransformer (switched by relays) to raise/lower incoming AC mains in case of undervoltage/overvoltage (I guess there is also some "black magic" switched energy input on the low voltage side in the undervoltage case). When everything is well the low voltage side is used to charge the battery. In case of a total power loss switched energy is fed from the battery into the low voltage side keep the output AC up (I assume you'll get a "modified sine wave" for the money). So, a single cheap transformer does it all! Though efficiency is probably near that of a internal combustion engine :-)
Several microfarads 60v+ capacitance (not electrolytic) across the low voltage primary of the second transformer should improve the power factor losses caused by running a reactive load on the first transformer. You will need to change the values until the current in the low voltage side is at a minimum when off load and at maximum load. Then either use a mean value across the range or a value for minimum current at your most commonly expected load current or switch values in and out ( complicating a simple project).
Don't worry about complicating a simple project. I thank you for this great input. I'll have to see if I can get my hands on some Microfarad range 60V capacitors. Preferably Y or X types so they won't short out the high current / low voltage side when they fail. But then, the primary transformer has probably enough steam to vaporize any capacitor that shorts out.
@@robertssmorgasbord Hi, I can't take all the credit for this suggestion. But its good to share! Like yourself I am building my own bench supply rig with an isolation transformer, variac and Dim Bulb Tester. Although I am an experienced electronics tech (now retired some years), I like to see how others tackle their similar projects and it can also refresh technical detail long forgotten or ignored. One guy I subscribe to is Mr Carlson his videos are very clear and informative. In his build of my current project he has an issue with matching his variac and iso trans. See the following link and start at about 12 mins in. ua-cam.com/video/51mjt9nFoeA/v-deo.html. He matches his using about 12 microfarads of 200v non pol caps. I will have the same issue betwwen my iso trans and variac but I will need 600v caps as the voltage here in the UK is 240V. Your intermediate voltage between the back to back transformers is less than 20v so a bunch of 2 and 4 mfd 40 -60V non pol caps should suffice to reduce that 2.7A no load standing current, reduce that hum and improve output reg. Bof (Dave)
Hello Sir. Thanks for such a wonderful project with proper explanation. I have constructed the same with two different brand UPS and it is working like charm. Once again thanks for this simple concept which most of us can have it. The one which I created does not produce any humming noise at all sir.
You're welcome! And sorry for the late reply. Anyway, maybe the UPSs you've used have better transformers or enclosures that are less prone to vibration. What types/brands of UPSs did you use?
@@robertssmorgasbord Hello Sir. I have used two different brands one from APC and other from ACM. Both have a bulky transformers. Probably noise could be from transformer plates if they are loose. Thanks once again.
I am considering how much air flow I will need in the case. How many fans/what size/etc. did you end up using and how well are they working? I have a 120mm/120vac unit and plenty of case space. That would give me a lot of airflow and eliminate the need for a dc power supply. I do have the two 93mm(?)/24vdc fans, from the original cases. What do you suggest?
I certainly would reuse at least one of the fans from the original cases! These transformers are build to a price, not for maximum efficiency. And you're putting two of them into a case (though the thermal load from the electronics is gone).
@@robertssmorgasbord I decided to use the single, 120vac/120mm fan. I will mount it on the rear wall and pull air through the side vents and out the back.
At 6:50... I understand how you are measuring resistances on the primary legs... But, what was the determining factor to let you know that White is Neutral? Is it that the White wire has the greatest resistances between the remaining three wires?
You already got it right. One wire (the white/neutral one) has a (relatively large) resistance to the other three wires (live). The resistances between the other three (live) wires should be very small compared to that. See also the diagram at 7:18 - the lengths of the windings are proportional to their resistance.
I think that I saw that you have grounded your case to earth. If so, doesn't this flow through to grounding the transformer bodies? If so, that would seem counter-intuitive, when the point is to isolate the DUT. What did you do and why. Please and thank you.
Yes I did. But no current will flow through the transformer bodies. Both the primary and the secondary winding are isolated from a transformer's body. The earth connection is however essential: In case there is an isolation failure in the mains AC side transformer's primary winding, any failure current is going to mains earth and not you ;-) And if the failure current is big enough the fuse will trigger. Side note: The changing magnetic field from the (primary) winding will induce some (very low) voltage into the transformer body, which in this kind of transformer acts kinda 1-turn winding. But that voltage is floating, respectively, not referenced to anything, and so no current will flow through mains earth.
I'm trying to make one out of a salvaged microwave Transformer. I very carefully drilled out the secondary coils. Im trying to figure out just how much wire I'll need for the second coil. I made 5 complete rotations & when I operate the Transformer at over 100 volts the amperage draw goes High! At 100V. I have 1.93 amp At 124.5V it jumps to 9.0 amp. Could this possibly be because I do not have enough copper or Eddy currents in my secondary coil yet? I had only 5 complete loops for testing the voltage on the secondary side. I haven't fully wrapped it up yet.
The transformer, regardless of the number of windings on the secondary coil, shouldn't behave like that. Though drawing 1.1kW (124.5V x 9A) is not unusual for a microwave transformer, it shouldn't do that with no load on the secondary winding (I'm assuming you're just measuring the voltage across the secondary winding and not shorting it out). My best guess is that the core or the primary winding unfortunately got damaged. The sharp jump of amperage between 100V and 124.5V points to some kind of isolation failure in the primary winding. Sorry I don't have better news for you.
Microwave transformers are a poor starting point. Apparently, they don't have enough iron in the core to operate unloaded. They depend on the fact that, in the microwave oven, they always have the full magnetron load on the secondary, which drops the primary voltage due to resistive losses. Also, they are always fan cooled by a high-airflow fan, so the transformer doesn't have to be very efficient. To make the transformer behave well, you need to add some extra primary turns to keep the core out of saturation. Other problems: you don't want the magnetic shunts across the core, so they should be removed. Your 5-turn secondary is suitable if you're making a welder, when you want a secondary winding output of perhaps 5-10 V at 100 A, but not for half of an isolation transformer. What makes the most sense is to wind a new secondary with the same number of turns as the primary, so you only need 1 transformer.
@@dmmartindale Thanks for the info! So the cores of microwave transformers go into saturation without load - turning the primary windings basically into air coils (slight exaggeration here :-) )! And they need active cooling (I always thought the fan in a microwave is for the magnetron)! I didn't know either, but it makes total sense. These microwaves are sold by the millions, so of course they try to shave off every cent they can on manufacturing cost. Regarding removing the magnet shunt: Of course that improves the efficiency of the transformer, but doesn't that also make the transformer less resilient to shorting out the secondary winding?
@@robertssmorgasbord The shunt acts as a current limiter. That's probably a good thing to have if you're building a DIY welder, but normally you don't want a power transformer to drop its output voltage any more than necessary under load. Instead, you protect it with a fuse or thermal switch.
@@dmmartindale Yup, that's what I meant (shunt limiting the current). And yes, you're right. In this application you might wanna get rid of that shunt. Adding a primary side fuse is of course a must!
If you would like progress updates/pictures/etc. of my project, feel free to send me a private message with your email address, through my UA-cam channel/About. I have made good progress with cutting the metal work. Drilling screw holes and assembly are to follow.
I tested my two transformers, in the method that you instructed. Both sets of Primaries measured... Black = 0.52 Ohms, Yellow = 0.57 Ohms and Blue = 0.62 Ohms. This is a tenth of your readings, so I checked with another meter and got the same results. You used the Diode setting, but I used the Ohms setting? I am not an AC, or Transformer, guy, but whole Ohm resistances in a transformer seems high to me. The information painted on the top of my transformers reads the same as yours for lines two and three. My first line is different than yours and mine are different from each other. They are both LEI-4 E154515. One is 430-1211B and the other is 430-1211A R.4. I just read that transformers used in Switch-Mode applications (ours were) are different than regular transformers. As that is an internet comment, who knows if it can be trusted, or if it matters for our use. An important point that you did not cover is that the (signal) phase of the incoming mains power supply must be maintained, throughout the circuit. If it gets inverted, strange performance will occur. For my transformers... For each transformer... The Primary White Wire is in phase to the Secondary White Wire. And, by attaching the White Secondary Wires to each other (and Black to Black, of course,) the proper phase was/is maintained.
Measuring resistances within the single Ohms range is a hit and miss with the multimeter I used ;-) So I wouldn't be concerned if your measurements are an order of magnitude off (plus, you're using 120V transformers and I was using 230V transformers). The important thing here is to get reliable relative (!) measurements between the wires, so you can determine which is which. And depending on the transformer one Ohm of resistance is quite OK ;-) That primary winding consists of a very long, relatively thin copper wire. In fact in your transformer - 120V vs 230V primary - should have about half the windings with copper wire twice as thick. I've long since given up to decode the labels on the APC transformer - they are probably custom made for APC or APC makes them in-house. Anyway, your transformer should definitely have a somewhat different label (120V AC vs 230V AC). That switch-mode stuff is certainly true, because switch-mode transformers are operated with frequencies between 10th of kHz and MHz. In this application we run the transformers at 60Hz (well, mine runs at 50Hz). These APC transformers are very interesting because the combine different principles: The primary side in itself is a (50Hz/60Hz) mains AC autotransformer. The secondary side is used as (A) a conventional secondary side when charging the battery and (B) as primary side of a kinda "switch mode" transformer when the power fails. Regarding maintaining the phase: In theory that shouldn't matter - and I not only didn't mentioned it, I didn't thought about it - my bad! In practice there will be some magnetic coupling between these two transformers, and messing up the phase will indeed having a negative impact on the performance! Thanks for bringing that point up!
I just found your channel today, and you have some interesting video's, but i noticed that the audio of all the video's after ""Simple" Constant Current Load (7)" have only audio on 1 side (mono), while the previous video's have sound on both sides. Besides that, keep up the good work and i will be following for more video's.
Uh oh, thanks for the hint! Guess something changed when I did some software updates without me noticing it. I'll look into it and you should be able to enjoy stereo sound again starting with the 1st of March upload. Thanks again!
@@BjornV78 :-) Found the problem: Since the last iOS/iPadOS update i did, videos were recorded in mono. Fortunately, that's quite easy to fix in the software I use (daVinci Resolve). Currently I'm re-rendering the video scheduled for the 23rd of February. So you'll be able to enjoy my German accented stuttering on both your ears again a weak earlier than I anticipated ;-) Thanks so much again for bringing that problem to my attention!
@@robertssmorgasbord nice to hear that you found the problem. At first, i though that my right speaker was blown, then i switched back to an older video and my right speaker worked again :-)
You're welcome! I can't be certain, but the transformers in a Smart UPS 1000 should be of similar construction, just bigger ;-) If you're qualified to work with AC mains (had to write that), you should characterize the transformers like I did in the video, just to be sure. The transformers in the Smart UPS 1000 are specified for 1.000VA/670W. Derate that at least 20% (800VA/536W) to account for the additional losses (two transformers coupled). Anyway, you'll see what voltage you get out at what load and if something gets hot ;-)
At 15:15... Your Secondary test to Ground was with your bare hands. I am quiet confident that you know the dangers of a bare-hand test, but, if you are going to do one, do not use two-hands... Using two hands makes the current path go across your heart. For all AC tests, I suggest the one-hand-behind-your-back method.
Yeah, I shouldn't have shown that on video - it's a really bad practice - my bad! To my defense, I did that only after I tested the isolation of the transformer ;-) But you're absolutely right about the (left hand) behind your back method (right hand if your hard is on the right side - about 0.007% of the populace).
I have completed my unit, to the point of being able to do the first power-on test. Meaning, I have not closed up the unit, pending charging my camera battery and making photos. And, prettying things up... Simple First Test Setup Mains/Wall AC Supply = 123.1VAC. Isolated Secondary @ No Load = 121.9VAC; a loss of 1.2VAC across the transformers. DUT = 60W Incandescent Light Bulb. The light operated at full brightness. The isolated secondary dropped to 120.9VAC. The Standard allowable drop for USA mains AC is 5%. For 120VAC mains, this equals 6 VAC. Therefore, 114VAC is the minimum acceptable voltage. The unit dropped one volt, for 60 watts. This would indicate that the unit is capable of supplying (120 -114) x 60 = 360 watts. 360 watts/120VAC = 3 Amps. This is higher than my original guess-imate and I am pleased, well enough. Of course, this needs to be proven out with stress testing. I think my model 1500 case is bigger than what you used. I had plenty of room for the two transformers and a good bit of room left over, at the front and back, to add the 120V fan and the switch, fuse, sockets, wiring, etc... I was also considering a "Dim Bulb" circuit, to limit the AC current. However, I saw a YT video that suggested just using additional transformers, with the Isolation Transformer, to drop the voltage and limit the current. This means that the DUT would be operated at something less than 120VAC, i.e. whatever voltage and amperage the transformer would supply. But given the ease of construction and the smaller size of such a unit, it is well worth considering. We are talking about just sticking an additional transformer of acceptable voltage and wattage in line. All that would be needed is a selection of transformer, to keep ready at-hand. That sounds so much more appealing than a big ol' clunky, fragile Dim Bulb Tester. Pictures pending.
Nice to hear your project is going that well! I think your decision not to use a "Dim Bulb" circuit is a good one. Light bulbs are PTCs, and so they are not very good at limiting switch-on current spikes and such. I'm not sure about the usage of another in-line transformer. You'll have to experiment with that. Keep in mind that you can fine tune the output voltage by switching between the three primary windings of the transformers too. Looking forward to see some pictures ;-)
I just had a project that required an Iso Trans and I did not have one. Your project inspired and instructed me on how to get it done. Thank you!
A friend gave me two model SUA1500s. I already have them stripped down and I am configuring the rebuild.
I thought a single 1500 might have two 750s in it, meaning two transformers. No, they have what I think is the exact same transformer as yours (best I can tell by the writing on them,) even though I am on USA 120VAC. I do have more room than you in my case, I think.
My rear bank of power sockets has the common wall-type, 3 pin configuration. Meaning, they are very usable and I do not need to build in a different type of socket. To facilitate their use, I am changing the "workflow" of the device to be back to front. The sockets and fuse are now on what will be the front of the unit. All that was needed to accomplish this was to route the main power cable through the front and to discard the front cover. And, to remove the common ground bus bar from the sockets.
You're welcome! And it sounds like you have a nice little project ahead of you :-) Anyway, I'll answer your comment in chronological order ...
Thank you for explaining the entire procedure Robert.
Every process & every decision you executed, was thought out and explained well. I was able to follow you through to the very end. Thank you Sir!👍
New subscriber here!
The newer UPS's are made cheap. Most do not have the big Power Transformers anymore.
You welcome and thank you for the praise! And it's not only newer UPSs. Newer equipment in general contains less and less parts that you can reuse :-(
A second option what can be used for creating a isolationtransformer, is if you have a old ATX computer powersupply ,
and a 12V to 230V inverter (car, camping, etc....) lie arround, is to combine these.
The 12V output of the ATX powersupply can easy supply 5A or more, to feed the 12V to 230V inverter.
The power output will not be to great, but enough to power small devices.
That's an fantastic idea! The efficiency is probably significantly higher than my two iron core transformer solution. However, a true/real sine output 12V to 230V inverter will set you back at least 100 bucks. And I don't care if the Chinese sell "real sine wave" inverters for 30 bucks or less - I don't trust those.
This video was extremely helpful to me - I really appreciate the clear explanations, diagrams and practical examples. I wrongly assumed today that the fat wires were the primary side of my transformer, and tripped the breaker in my house haha but I don't seem to have damaged the transformer at least. Your video has helped me to understand a lot better, thank you.
I have three old UPSs which I bought for 5e, I think it's probably just dead batteries as everything looks very clean inside. I want to use two of the transformers in series to get a 24v AC supply for a [what I now realise is not very well regarded] DIY 30v lab power supply kit. If I can manage it, it should make a nice project as I can use the case of the UPS, the heatsinks, and the original power switch. I still have a huge amount to learn though....I have subscribed to your channel. thanks again.
You're welcome! Regarding your little mishap: That secondary winding should be able to take short current peaks of up to 90A, so your breaker really never stood a chance :-) Anyway, good luck with your project! A 40A or so lab power supply no less!
A while ago i did some tests with a used 500VA transformer that i purchased for a isolation transformer project.
This transformer has multiple taps from 0 to 500V, it can convert 230V mains to 110V, 230V, 400V and 500V, during some tests,
i discovered the phenomenon called "transformer polarity", which comes to play when using 2 transformers back to back.
When changing the "polarity" , this can give a influence on the output voltage.
Also to mention, these transformers in UPS work reversed. The 230V side is in fact the secundairy, and the low voltage side is the primairy.
The primary side gets feeded by a chopped AC sine wave created by a SMPS power supply + (chopped) AC sine wave combo circuit. The more expensive UPS can create a pure sine wave on the output.
A second part of the SMPS power supply is for charging the UPS battery.
And last, the newer UPS don't have these big transformers anymore and use small transformers with high frequency , and in the last stage the signal is converted to a 50/60Hz AC wave to feed the 230V power outlet socket(s).
If the transformer has a -5a10% / +5a10% tap on the 230V side, these extra taps between can deliver a small voltage what can be used for powering LED(s) or in combination with a buck converter to a 5V USB outlet.
Thanks a lot for all the info! The APC Smart UPS units are technology wise stuck in the past I guess (see my teardown of one for more information: ua-cam.com/video/TT9je5yo7yM/v-deo.html ). The high voltage side of the smart UPS has several taps and is used as autotransformer (switched by relays) to raise/lower incoming AC mains in case of undervoltage/overvoltage (I guess there is also some "black magic" switched energy input on the low voltage side in the undervoltage case). When everything is well the low voltage side is used to charge the battery. In case of a total power loss switched energy is fed from the battery into the low voltage side keep the output AC up (I assume you'll get a "modified sine wave" for the money). So, a single cheap transformer does it all! Though efficiency is probably near that of a internal combustion engine :-)
Several microfarads 60v+ capacitance (not electrolytic) across the low voltage primary of the second transformer should improve the power factor losses caused by running a reactive load on the first transformer. You will need to change the values until the current in the low voltage side is at a minimum when off load and at maximum load. Then either use a mean value across the range or a value for minimum current at your most commonly expected load current or switch values in and out ( complicating a simple project).
Don't worry about complicating a simple project. I thank you for this great input. I'll have to see if I can get my hands on some Microfarad range 60V capacitors. Preferably Y or X types so they won't short out the high current / low voltage side when they fail. But then, the primary transformer has probably enough steam to vaporize any capacitor that shorts out.
@@robertssmorgasbord Hi, I can't take all the credit for this suggestion. But its good to share! Like yourself I am building my own bench supply rig with an isolation transformer, variac and Dim Bulb Tester. Although I am an experienced electronics tech (now retired some years), I like to see how others tackle their similar projects and it can also refresh technical detail long forgotten or ignored. One guy I subscribe to is Mr Carlson his videos are very clear and informative. In his build of my current project he has an issue with matching his variac and iso trans. See the following link and start at about 12 mins in. ua-cam.com/video/51mjt9nFoeA/v-deo.html. He matches his using about 12 microfarads of 200v non pol caps. I will have the same issue betwwen my iso trans and variac but I will need 600v caps as the voltage here in the UK is 240V. Your intermediate voltage between the back to back transformers is less than 20v so a bunch of 2 and 4 mfd 40 -60V non pol caps should suffice to reduce that 2.7A no load standing current, reduce that hum and improve output reg. Bof (Dave)
Hello Sir. Thanks for such a wonderful project with proper explanation. I have constructed the same with two different brand UPS and it is working like charm. Once again thanks for this simple concept which most of us can have it. The one which I created does not produce any humming noise at all sir.
You're welcome! And sorry for the late reply. Anyway, maybe the UPSs you've used have better transformers or enclosures that are less prone to vibration.
What types/brands of UPSs did you use?
@@robertssmorgasbord Hello Sir. I have used two different brands one from APC and other from ACM. Both have a bulky transformers. Probably noise could be from transformer plates if they are loose. Thanks once again.
I am considering how much air flow I will need in the case. How many fans/what size/etc. did you end up using and how well are they working? I have a 120mm/120vac unit and plenty of case space. That would give me a lot of airflow and eliminate the need for a dc power supply. I do have the two 93mm(?)/24vdc fans, from the original cases. What do you suggest?
I certainly would reuse at least one of the fans from the original cases! These transformers are build to a price, not for maximum efficiency. And you're putting two of them into a case (though the thermal load from the electronics is gone).
@@robertssmorgasbord I decided to use the single, 120vac/120mm fan. I will mount it on the rear wall and pull air through the side vents and out the back.
At 6:50... I understand how you are measuring resistances on the primary legs... But, what was the determining factor to let you know that White is Neutral? Is it that the White wire has the greatest resistances between the remaining three wires?
You already got it right. One wire (the white/neutral one) has a (relatively large) resistance to the other three wires (live). The resistances between the other three (live) wires should be very small compared to that. See also the diagram at 7:18 - the lengths of the windings are proportional to their resistance.
I think that I saw that you have grounded your case to earth. If so, doesn't this flow through to grounding the transformer bodies? If so, that would seem counter-intuitive, when the point is to isolate the DUT. What did you do and why. Please and thank you.
Yes I did. But no current will flow through the transformer bodies. Both the primary and the secondary winding are isolated from a transformer's body. The earth connection is however essential: In case there is an isolation failure in the mains AC side transformer's primary winding, any failure current is going to mains earth and not you ;-) And if the failure current is big enough the fuse will trigger.
Side note: The changing magnetic field from the (primary) winding will induce some (very low) voltage into the transformer body, which in this kind of transformer acts kinda 1-turn winding. But that voltage is floating, respectively, not referenced to anything, and so no current will flow through mains earth.
I'm trying to make one out of a salvaged microwave Transformer.
I very carefully drilled out the secondary coils.
Im trying to figure out just how much wire I'll need for the second coil. I made 5 complete rotations & when I operate the Transformer at over 100 volts the amperage draw goes High!
At 100V. I have 1.93 amp
At 124.5V it jumps to 9.0 amp.
Could this possibly be because I do not have enough copper or Eddy currents in my secondary coil yet? I had only 5 complete loops for testing the voltage on the secondary side.
I haven't fully wrapped it up yet.
The transformer, regardless of the number of windings on the secondary coil, shouldn't behave like that. Though drawing 1.1kW (124.5V x 9A) is not unusual for a microwave transformer, it shouldn't do that with no load on the secondary winding (I'm assuming you're just measuring the voltage across the secondary winding and not shorting it out). My best guess is that the core or the primary winding unfortunately got damaged. The sharp jump of amperage between 100V and 124.5V points to some kind of isolation failure in the primary winding. Sorry I don't have better news for you.
Microwave transformers are a poor starting point. Apparently, they don't have enough iron in the core to operate unloaded. They depend on the fact that, in the microwave oven, they always have the full magnetron load on the secondary, which drops the primary voltage due to resistive losses. Also, they are always fan cooled by a high-airflow fan, so the transformer doesn't have to be very efficient. To make the transformer behave well, you need to add some extra primary turns to keep the core out of saturation.
Other problems: you don't want the magnetic shunts across the core, so they should be removed.
Your 5-turn secondary is suitable if you're making a welder, when you want a secondary winding output of perhaps 5-10 V at 100 A, but not for half of an isolation transformer. What makes the most sense is to wind a new secondary with the same number of turns as the primary, so you only need 1 transformer.
@@dmmartindale Thanks for the info! So the cores of microwave transformers go into saturation without load - turning the primary windings basically into air coils (slight exaggeration here :-) )! And they need active cooling (I always thought the fan in a microwave is for the magnetron)! I didn't know either, but it makes total sense. These microwaves are sold by the millions, so of course they try to shave off every cent they can on manufacturing cost.
Regarding removing the magnet shunt: Of course that improves the efficiency of the transformer, but doesn't that also make the transformer less resilient to shorting out the secondary winding?
@@robertssmorgasbord The shunt acts as a current limiter. That's probably a good thing to have if you're building a DIY welder, but normally you don't want a power transformer to drop its output voltage any more than necessary under load. Instead, you protect it with a fuse or thermal switch.
@@dmmartindale Yup, that's what I meant (shunt limiting the current). And yes, you're right. In this application you might wanna get rid of that shunt. Adding a primary side fuse is of course a must!
If you would like progress updates/pictures/etc. of my project, feel free to send me a private message with your email address, through my UA-cam channel/About. I have made good progress with cutting the metal work. Drilling screw holes and assembly are to follow.
I tested my two transformers, in the method that you instructed. Both sets of Primaries measured... Black = 0.52 Ohms, Yellow = 0.57 Ohms and Blue = 0.62 Ohms. This is a tenth of your readings, so I checked with another meter and got the same results. You used the Diode setting, but I used the Ohms setting? I am not an AC, or Transformer, guy, but whole Ohm resistances in a transformer seems high to me.
The information painted on the top of my transformers reads the same as yours for lines two and three. My first line is different than yours and mine are different from each other. They are both LEI-4 E154515. One is 430-1211B and the other is 430-1211A R.4.
I just read that transformers used in Switch-Mode applications (ours were) are different than regular transformers. As that is an internet comment, who knows if it can be trusted, or if it matters for our use.
An important point that you did not cover is that the (signal) phase of the incoming mains power supply must be maintained, throughout the circuit. If it gets inverted, strange performance will occur. For my transformers... For each transformer... The Primary White Wire is in phase to the Secondary White Wire. And, by attaching the White Secondary Wires to each other (and Black to Black, of course,) the proper phase was/is maintained.
Measuring resistances within the single Ohms range is a hit and miss with the multimeter I used ;-) So I wouldn't be concerned if your measurements are an order of magnitude off (plus, you're using 120V transformers and I was using 230V transformers). The important thing here is to get reliable relative (!) measurements between the wires, so you can determine which is which. And depending on the transformer one Ohm of resistance is quite OK ;-) That primary winding consists of a very long, relatively thin copper wire. In fact in your transformer - 120V vs 230V primary - should have about half the windings with copper wire twice as thick.
I've long since given up to decode the labels on the APC transformer - they are probably custom made for APC or APC makes them in-house. Anyway, your transformer should definitely have a somewhat different label (120V AC vs 230V AC).
That switch-mode stuff is certainly true, because switch-mode transformers are operated with frequencies between 10th of kHz and MHz. In this application we run the transformers at 60Hz (well, mine runs at 50Hz). These APC transformers are very interesting because the combine different principles: The primary side in itself is a (50Hz/60Hz) mains AC autotransformer. The secondary side is used as (A) a conventional secondary side when charging the battery and (B) as primary side of a kinda "switch mode" transformer when the power fails.
Regarding maintaining the phase: In theory that shouldn't matter - and I not only didn't mentioned it, I didn't thought about it - my bad! In practice there will be some magnetic coupling between these two transformers, and messing up the phase will indeed having a negative impact on the performance! Thanks for bringing that point up!
I just found your channel today, and you have some interesting video's, but i noticed that the audio of all the video's after ""Simple" Constant Current Load (7)" have only audio on 1 side (mono), while the previous video's have sound on both sides.
Besides that, keep up the good work and i will be following for more video's.
Uh oh, thanks for the hint! Guess something changed when I did some software updates without me noticing it.
I'll look into it and you should be able to enjoy stereo sound again starting with the 1st of March upload.
Thanks again!
@@robertssmorgasbord thank you. Greetings from Belgium.
@@BjornV78 :-) Found the problem: Since the last iOS/iPadOS update i did, videos were recorded in mono. Fortunately, that's quite easy to fix in the software I use (daVinci Resolve).
Currently I'm re-rendering the video scheduled for the 23rd of February. So you'll be able to enjoy my German accented stuttering on both your ears again a weak earlier than I anticipated ;-)
Thanks so much again for bringing that problem to my attention!
@@robertssmorgasbord nice to hear that you found the problem. At first, i though that my right speaker was blown, then i switched back to an older video and my right speaker worked again :-)
Thank you for your wonderful work . What if i use APC Smart 1000 ?
You're welcome! I can't be certain, but the transformers in a Smart UPS 1000 should be of similar construction, just bigger ;-) If you're qualified to work with AC mains (had to write that), you should characterize the transformers like I did in the video, just to be sure. The transformers in the Smart UPS 1000 are specified for 1.000VA/670W. Derate that at least 20% (800VA/536W) to account for the additional losses (two transformers coupled). Anyway, you'll see what voltage you get out at what load and if something gets hot ;-)
At 15:15... Your Secondary test to Ground was with your bare hands. I am quiet confident that you know the dangers of a bare-hand test, but, if you are going to do one, do not use two-hands... Using two hands makes the current path go across your heart. For all AC tests, I suggest the one-hand-behind-your-back method.
Yeah, I shouldn't have shown that on video - it's a really bad practice - my bad! To my defense, I did that only after I tested the isolation of the transformer ;-) But you're absolutely right about the (left hand) behind your back method (right hand if your hard is on the right side - about 0.007% of the populace).
I have completed my unit, to the point of being able to do the first power-on test. Meaning, I have not closed up the unit, pending charging my camera battery and making photos. And, prettying things up...
Simple First Test Setup
Mains/Wall AC Supply = 123.1VAC.
Isolated Secondary @ No Load = 121.9VAC; a loss of 1.2VAC across the transformers.
DUT = 60W Incandescent Light Bulb. The light operated at full brightness. The isolated secondary dropped to 120.9VAC.
The Standard allowable drop for USA mains AC is 5%. For 120VAC mains, this equals 6 VAC. Therefore, 114VAC is the minimum acceptable voltage.
The unit dropped one volt, for 60 watts. This would indicate that the unit is capable of supplying (120 -114) x 60 = 360 watts. 360 watts/120VAC = 3 Amps. This is higher than my original guess-imate and I am pleased, well enough. Of course, this needs to be proven out with stress testing.
I think my model 1500 case is bigger than what you used. I had plenty of room for the two transformers and a good bit of room left over, at the front and back, to add the 120V fan and the switch, fuse, sockets, wiring, etc...
I was also considering a "Dim Bulb" circuit, to limit the AC current. However, I saw a YT video that suggested just using additional transformers, with the Isolation Transformer, to drop the voltage and limit the current. This means that the DUT would be operated at something less than 120VAC, i.e. whatever voltage and amperage the transformer would supply. But given the ease of construction and the smaller size of such a unit, it is well worth considering.
We are talking about just sticking an additional transformer of acceptable voltage and wattage in line. All that would be needed is a selection of transformer, to keep ready at-hand. That sounds so much more appealing than a big ol' clunky, fragile Dim Bulb Tester.
Pictures pending.
Nice to hear your project is going that well! I think your decision not to use a "Dim Bulb" circuit is a good one. Light bulbs are PTCs, and so they are not very good at limiting switch-on current spikes and such. I'm not sure about the usage of another in-line transformer. You'll have to experiment with that. Keep in mind that you can fine tune the output voltage by switching between the three primary windings of the transformers too. Looking forward to see some pictures ;-)
You can find my pictures here = www.eevblog.com/forum/projects/diy-isolation-transformer-project-back-to-back-method/msg3174352/#msg3174352
Just had I look at your pictures - nice! (did leave a comment on the EEVblog too).