Viewer Questions: Back to Back Transformers

A viewer asked about connecting transformers back to back for applications such as isolation. I demonstrate that and show how the magnetizing current can reduce the available current or power from the second transformer. I also show how a capacitor can be used to reduce the magnetizing current, something that is done in real power transmission applications.
For a more insight into cascaded transformers, look at this video about an EM Chain: • Let's Experiment with ...
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[@kenny-tu2bu]

I've done this for a local sign company. The issue was voltage drop. Used a 120/240-480 volt on a 500 ft cable run, then another 480-120/240 on the load end. Works as it should.

[@ElectromagneticVideos]

Your own high voltage transmission line - how cool! Great example. Assuming you were t doing this to make use of an existing cable (which would be a great application for this), I wonder wonder what the break even length is between increasing cable cost and the cost of the transformers.

[@robertedwards3147]

That was a good example of power factor correction

[@ElectromagneticVideos]

Exactly!

[@Timothycan]

This one was interesting for me, as many years ago now I inherited an old 'Farm Radio' (that is, a battery powered tube radio). It has been in my family since it was new, in 1946. Designed to run on a battery that provided 1.5V for the filaments and 90v for the B+ (or the HT, as we say here in the UK). I decided to cobble together a circuit to provide those voltages, from our 240V AC mains. I did this with bits I happened to have lying around. So I used a 240v:6.3V transformer, and then a 240v:16V transformer used 'backwards'. Thus I could rectify the 6.3V and add dropping resistors and smoothing caps and a voltage regulator to get the 1.5V DC, and then also step the 6.3V up again to get the HT, which (luckily) after rectifying and smoothing gave me almost exactly 90v. OK, there is a bit of 50Hz hum, but it works, and has done for many years! The transformers I used are fairly meaty, so they seem to handle it OK.

[@ElectromagneticVideos]

That must have been a wonderfully satisfying project! And what a great example of what I showed here. Sometimes using what you have on hand makes way more sense than buying a perfect part. The hum - I trust you tried increasing the size of the filter capacitors? The other thing that can cause it is the stray field from the transformers in interacting with the electron flow in the tubes/valves. So if you can, try moving the transformers away from the tubes. Or put a piece of steel sheet/plate between the transformers and the radio to provide magnetic shielding. There is something really cool about old tube radios. I have a few myself - used to be great fun listening to short wave from all over the world. Sadly thats mostly gone thee days.

[@Timothycan]

@@ElectromagneticVideos Many thanks. I found it very sensitive to any variation in filament voltage. More difficult to get that one right than the high voltage. I may try some of your ideas there, too.

[@ElectromagneticVideos]

@@Timothycan Oh - I never thought of the filament - which I guessing is the directly heated cathode and at 1.5V probably uses a lot of current. Maybe a 1F (not uF!) supercap to smooth the filament voltage?

[@Timothycan]

@@ElectromagneticVideos Yes that could be worth a try, thanks.

[@jerril42]

I hope you have a speedy recovery from the strep. The EM chain video is one of the first videos I watched of yours.; it is fantastic. This is a perfect follow up on those concepts, demonstrating a real world application. I'll be re-watching the EM Chain video again soon. Thanks Dr. Jones. Take care.

[@ElectromagneticVideos]

Thanks - I;m doing much better now! So glad you liked the EM chain. It really is one of my favourites. It was actually inspired by a much smaller EM chain in the Deutsches Museum in Munich that I saw a kid. If you ever get a chance to visit that museum, its well worth it. Someone else reported an EM chain in a museum in London, so it not quite unique....

[@MrWaalkman]

I made this setup to give me 110-ish volts of isolated power for use in identifying which wire I was trying to locate out of hundreds that we had to choose from. We were re-purposing a 53' semi trailer to be used as a control shack for a converted Gencor asphalt burner to dirt-burner. We used it to clean tainted soil that had been contaminated by fuel spills at the site of the spill. By setting up the dirt-burner at the place of the spill rather than hauling thousands of tons of soil to our plant in Mecca, CA we saved tons of money for our customers, and we didn't have the dirt to deal with once it had been cleaned. In any case, we left the "neutral" disconnected on the secondary and used a non-contact tester to get an idea which wire it was that we were looking for. The wires were all labeled, this just got us (safely) into the ballpark of where our wire was. We could handle the wires safely without fear of a shock hazard. Worked okay.

[@ElectromagneticVideos]

I'm always fascinated at the things people like yourself have done. Great way to rig up a safe way for identifying wires. That must have been a fascinating project. I actually could have used you services 15 years ago when a friend and I did a small housing development on some land that had fuel spill contamination. The (government approved) remediation was to remove a dozen truckloads of contaminated soil and replace with clean soil. Your device would have been perfect, although maybe our job would have been too small for you equipment.

[@MrWaalkman]

@@ElectromagneticVideos Thanks! That's what it was intended for, but it might take up a good sized chunk of your yard. :) Our first project for it was at Naval Base Point Loma, CA. IIRC, it could "burn" 20 tons of dirt an hour. But don't quote me on that. It's been awhile (2010). The customer had to provide natural gas and electric hookups, and bring in/haul out their dirt. This unit took seven truckloads to transport and could be setup anywhere in the States in a week. The last that I had heard, it was burning dirt in North Dakota.

[@ElectromagneticVideos]

@@MrWaalkman It really says something that is still in operation. I'm surprised there isn't a company manufacturing your machines - there is so much contaminated soil that at times needs to be remediated. Here (Ontario, Canada) there is apparently a plume of contamination around every (no longer used) small town garbage dump, gas station and some dry cleaners (who would apparently dump the carbon tetrachloride out the back door). Im so the cleanup companies are making $$$$$ these days.

[@MrWaalkman]

@@ElectromagneticVideos Oh, there is. We used an old semi-mobile Gencor asphalt mixer as the basis of the machine. Gencors, with the exception of their control shack, are mobile. So you can easily move around 80% of your system. Unfortunately, the last 20% is nailed down and hard-wired. I think that the (sneaky) intent was that you bought a system that could be moved around as long as you paid Gencor to take it apart and re-commission it when it got to its new home. Ours had 11 (IIRC) Meltrics plugs that our guys would unplug and plug back in once it got to its new home. Easy-peasy. Keep in mind that given the age of the machine, mobility might not have been a major selling point back then. I noticed that the "G" series Gencor machines look to actually be mobile. The control shack on a Gencor is a room built upon stilts and is not mobile at all. I guess those would be disassembled and shipped. We scrapped ours. The entire controls system had to be re-designed from scratch, and all new wiring. Total cost was somewhere north of $100k IIRC. Which I am sure was easily made up on the first job. The old controls were 1970's era, so a bit finicky to deal with. Mostly legacy transistor amps and such. The prints (all 10 or so pages of them) were the old school sepia prints. Not as old as an actual blueprint, but mercifully no longer an active media for print outs. For what I am doing these days, I'm helping with a retro-fit of the RAID Fluid Recovery System (FRS) to make it easier to store the electronics during the Antarctic winter. You can't leave them behind and expect them to work the next season. So we re-packaged the controls into something that is quite a bit smaller and can be handled by one person. BTW, the RAID guys are looking for a PhD to head up the project, and Duluth is not that far north of Toronto. :) BTW, I worked at the Bramelea Chrysler plant back in the 90's on a new body shop. Loved Canada!

[@ElectromagneticVideos]

@@MrWaalkman Makes sense from so many standpoints including generating future revenues when the machine gets moved. You certainly are involved in a lot of fascinating stuff! Are you getting a visit to the Antarctic? That would be an amazing business trip! So you worked in Canada for a while - wonderful! That must have been when Lee Iacocca had recued the compnay and it was doing OK. Sad how its a shell of its former self today. I'm on the outskirts of Ottawa so a bit North East of where you were.

[@GemunuJayasinghe]

Clear explanations. Very useful video . Sir , thank you so much for sharing your knowledge . I am from Sri Lanka .

[@ElectromagneticVideos]

Thanks you so much! Great to hear from Sri Lanka! Greetings from Canada on almost the other side of the world!

[@Scott-s9u3n]

At my last job, the hospital beds had isolation transformers...some were adjustable, with a switch that allowed you to use 108-130 volts in, with a steady 120 out... In the '70s, I saw a back to back transformer setup used to power a neon relaxation oscillator as part of a model train setup. If you had a 12 volt center tap transformer, I guess you could drive two six volt transformers. Your setup would make a good educational display for a high school physics or shop class.

[@ElectromagneticVideos]

So are you saying the transformers had manually (and easily accessible) taps to adjust the input voltage? When I was a kid an we lived in Germany, we had one transformer with taps like that on the output to select 110, 114, 117,120, 125 or some such series of voltages. Your hospital transformers wouldn't have been constant voltage transformers by any chance? I had one of those in the early days of home computers - ran very hot - an a selectable input voltage would have been a good way to keep it from overheating. Interesting about the model train setup. Probably a nice safe way of generating isolated 120V or so using transformers they probably had crates of. We sure weren't soiled back then - a blinking light - wow! You certainty could could drive two 6v transformers that way. As far as educational uses - I do hope my videos have been inspiring a good number of teachers/instructors/professors to create similar demos experiments for their classes. I have had emails from a few who wanted more details to to do some of the experiments in high schools, universities and even a museum.

[@Scott-s9u3n]

@ElectromagneticVideos These transformers had a rotary switch you needed a screwdriver to turn - you were adjusting for input voltage-..it was recessed, so it couldn't be done accidentally. I honestly don't know if they ran hot..they really weren't continuously used anywhere near capacity...it had a bed motor, a nightlight, and a small amplifier. The nightlight and amplifier might use...10 watts? The motor used a lot more, but was just intermittent. Hospital beds have an impressive amount of safety features...

[@ElectromagneticVideos]

@@Scott-s9u3n So maybe just an isolation transformer. I'm glad to hear they have a lot of safety features and isolation transformers. Once when I was grad student a (really good) biomedical group was doing some sort of experiments where they had electrodes attached to volunteers to measure some sort of nerve or muscle activity. A few of the volunteers got a good jolt of electricity from some stray leakage currents. Someone suggested they ask me for help and ended up getting them setup with medical grade isolation transformers. So I am somewhat aware how dangerous things can be when you are wired up to who knows how many pieces of equipment, each possibly adding some leakage current. That was my one and only experience with that sort of thing but it left a lasting impression on me.

[@Scott-s9u3n]

@@ElectromagneticVideos Part of tests we had to do at the job was measuring leakage current..I forgot the exact limit, but it was micro amperes to ground. The IV pumps were all battery powered ( at that time, 12' volt sealed gel cell lead acid batteries) to prevent leakage current through the patient..an IV in your arm is a conductive path to your heart. The IV tubing itself is part of the pump ( imagine a series of padded " fingers" gently squeezing the tubing) , and that isolates you from the machine. From a fire and electrical standpoint, hospitals are very safe places.

[@ElectromagneticVideos]

@@Scott-s9u3n Very interesting - I guess you cant do better than batteries in terms of reducing leakage. I never thought about it but given that most IVs are salty, the fluid itself would be a good conductor at least from a mA or microA perspective. Here is a question - I have heard that in some/all(?) hospitals in Canada and the US, the ground prong of AC receptacles is oriented up to somewhat block things falling on the live prong. Any truth to that?

[@ThriftyToolShed]

Excellent video with great explanations! Hope you feel much better soon!

[@ElectromagneticVideos]

Thanks - hope your doinge OK and the hurricane cleanup is almost over!

[@ThriftyToolShed]

@ElectromagneticVideos It's going well. I have a lot of work replacing fence, but mainly back to normal. It will be months still before roads side are ckean. They are working 7 days a week hauling. I think they are even grinding/chipping at night. Crazy amount of trees and debris. Thanks for asking!

[@ElectromagneticVideos]

@@ThriftyToolShed Hard to imagine - and shocking to think how there still is to clean up.Glad things are going OK!

[@davida1hiwaaynet]

Glad to see you back online! Hopefully the vacation was enjoyable, in spite of the strep which happened after you came home. This is interesting, and I have had to do similar before to "make something work." The piece of equipment I was trying to use had 240V motors and 120V controls. They originally used line to neutral for control power; but the supply to this machine only had L1, L2, and ground; but no neutral. I used a 240V to 24V transformer on the input side and a 24V to 120V transformer on the output side, to take the place of a 240:120V transformer I didn't have. It was, of course a get-by but it got the 120V control circuit working without a neutral available; using what I had on hand. When you are poor you have to use what's in the junk box LOL! I still remember your electromagnetic chain! That is very neat but for some time it is also quite humorous!

[@ElectromagneticVideos]

You know, a few other people have also mentioned using back to back transformers to make do with what they had on hand. I was not expecting that. It sure show how much ingenuity is out there! Cool that you managed to get things going that way. Hope your doing OK - you haven't posted in a while either? Were you in the path of the hurricane a while back? @ThriftyToolShed is apparently still cleaning up from it.

[@bretfuzz925]

I have seen this method done in the past to generate a plate power supply for a tube. It also provides mains isolation.

[@ElectromagneticVideos]

Its funny - I always learn so much from the comments. Quite a few comments like yours using this to generated isolated HV - I had no idea so many people had used something like this for real applications. It would even be useful today now that I think about - two $2 transformers like these would make a much cheaper HV source than most isolation transformers.

[@_a.z]

I moved from UK to the US for a while and took a 240 to 120v 1kW transformer with me. I was surprised how warm it got when using it reversed, to give 240 out!

[@ElectromagneticVideos]

Its interesting that it got warmer when used in reverse. I wonder if it was designed for a slightly lower voltage - say 230:115 since a lot of Europe seems to be more around 230V these days. Add to that AC in the US and Canada is often 120 to 125V, the transformer may be operating somewhat more towards magnetic saturation and drawing more magnetization current that it did in the UK. Interesting observation!

[@StanleyRestall]

@@ElectromagneticVideos Wouldn't the change in frequency from 50Hz to 60Hz make a difference too. With the frequency being higher would it cause an decrease in the magnetising current ? If it was unloaded and working as an inductor, the reactance would be the same but a higher frequency in would tend to diminish the current due to XL = (2 * pi * F * L). I'm probably barking up the wrong tree and there's more than likely some large number of research papers telling me I am 🙂

[@ElectromagneticVideos]

@@StanleyRestall Your absolutely right! A change in frequency would make a difference and as you pointed out the magnetizing current would be less for the higher frequency. So generally things like transformers designed for 50Hz tend to draw less magnetizing current at 60Hz. 60Hz transformers draw more if taken to places where 60s Hz is used. And the capacitor used to provide the magnetizing current provides more current at 60Hz than 50Hz. So it would have to be tweaked if the frequency was changed (really tuning a resonant circuit).

[@HazeAnderson]

One of my favorite circuits inserts a diode ring in between two (audio) transformers ... 😏welcome back and I hope you are 100% healthy again! 👍

[@ElectromagneticVideos]

I getting close to 100% thanks :) You certainly got my curiosity with that statement. I have never heard of that - closest I can think of is an RF AM modulator done like that. Is that to somehow use one audio signal to modulate another? Do you have a video on that? Just subscribed to you channel - looks like you have all sorts of interesting stuff linking music and electronics!

[@mickeythompson9537]

@@ElectromagneticVideos It's a ring modulator - so your're correct.

[@ElectromagneticVideos]

@@mickeythompson9537 Neat - that must create quite the interesting sound!

[@HazeAnderson]

@@ElectromagneticVideos KZbin ate my reply :/

[@ElectromagneticVideos]

@@HazeAnderson I hate it when that happens. It sometimes seems to pick on certain people for no reason. I checked for held comments and nothing there.

[@Apismeliffera]

I was once tasked to build a three phase AC to DC power supply intended to charge a large capacitor bank. The cap bank was only one part of a test setup. The electronic parts department didn't have three phase rectifiers. My supervisor told me to use three bridge rectifiers. Wired as phase A to B for the first rectifier. B to C for the second and C to A for the third. Then connect the three (-) terminals together and the three + terminals together. I told him the rectifiers will blow up. He didn't believe me. So I built it and let him turn it on. He did check my wiring first. All three bridge rectifiers blew up as I expected they would. So I added a pair of small 120 to 16 volt transformers back to back to independently isolate each bridge rectifier from each phase. That worked as it eliminated the DC path caused by directly connecting the bridge rectifiers to the line. As a bonus I added a small 7&1/2 watt 120 volt lamp in series with each transformer pair's primary. The lamps worked as inrush current limiters. The circuit had 18 capacitors in parallel with each one being 2,400Uf @ 450 volts. The power supply worked perfectly. Once it was turned on, it did take a couple minutes for the voltage to come up and stabilize. And yes it did have a permanent load resistor on it to discharge it when not in use.

[@ElectromagneticVideos]

That must have been "exciting"! It sure demonstrated who understood things and who didn't :) Incandescent lamps as current limiters - they work so well for that and are sadly overlooked these days. And as bonus provide a great visual indication of a short or other issue.

[@myself248]

@@ElectromagneticVideos To this day, I have a rubber-base Edison socket (a Leviton 55) with alligator clips on the leads. Stole the idea from Western Electric who issued these to telephone switch service techs for use as capacitor bank prechargers/dischargers. When you're working on a filter cap bank, you take the fuse out and connect the bulb across the bank to discharge it, then leave it there. If some goober reinstalls the fuse while you're working, the light warns you!

[@ElectromagneticVideos]

@@myself248 I should make myself one of those! Could be really handy in many ways. Its those simple things that work so well.

[@d614gakadoug9]

A few people in the comments have mentioned re-purposing transformers salvaged from microwave ovens. While this is certainly feasible, precautions are necessary. These transformers are far from "conservative designs." The expectation is that they will have forced air cooling. That allows using "less copper" than you would in a typical small power transformer for general purpose use. In typical general purpose transformers you can get a reasonable idea of how much power one will handle just from its weight. Catalogs from manufactures such as Hammond can help you get an idea of this. A transformer from a (say) 700 watt oven might need to be derated to four to five hundred watts for general purpose use. (It is quite acceptable to fairly significantly "overload" a transformer if the duty cycle is low and the ON time is short; allowable maximum temperature in the hottest part of the winding is the limiting factor. Transformer turns ratios are often not exactly as expected from the ratio of winding voltages. Typically the turns ratio is adjusted so that the secondary delivers the nominal voltage at the full rated current. This means that the secondary voltage may be quite a lot higher than nominal with light or no load. Probably covered in other videos in the series: Inductance is proportional to the square of the number of turns. If you have a transformer with an _actual_ turns ratio of 10:1, for example, the primary inductance will be 100 times the secondary inductance.

[@ElectromagneticVideos]

Great summary of all the transformer factors to consider. I'll add one more thing about microwave transformers: to anyone using them, be aware how deadly lethal the HV secondary is. The 1000V output is more than enough voltage to punch though some cracked insulation and dry skin, and the 1A current more than enough to kill or at a minimum, result in sever burns. There have been people who have been killed or maimed when re-purposing these transformers. Be careful - and remove the secondary before using them!

[@lmwlmw4468]

Great video.

[@ElectromagneticVideos]

Thanks!!!!!!!!!!!

[@RoboticsDIY]

Wow, I'm really happy I found your channel. My understanding of transformers, instructor and capacitors is high school level. U1/U2=Number of turns1/Number of turns2. In real life its more complicated than with ideal components. Plus some bit of extra from playing with Arduino's, making simple circuits bust and buck converters on prototyping board. But the more I learn the more I understand how much I don't know. I have been thinking when can I put two secondary wingdings of the same transformer or multiple transformers in series and paralel. Either to get the voltage i need from the stuff i have or to get a higher voltage. Lets say 500V with 50V or so increments for testing tubes. I get that you can add multiple wingdings in series, but I'm not sure about saefty, what do i need to take in account. In this video I was wondering how the output voltage change of the first 15V winding and whats the actual voltage at the second transformers 12V terminals. I been thinking about making some experiments and include in video about building an LM7812 and LM7805 power supply, but Im hesitant because I don't wan't to give bad advice. If you can, make some videos explaining relation between Watts and VA's. How to determine what is the maximum load in W can apply to a transformer with given output Voltage and VA rating. How to know what maximum current and power can I draw if I use a single diode rectifier, a capacitor bank, and resistive load. And how it changes with full bridge rectifier. I been wondering if its half fith a single diode, not to exceed the maximum current in wire, or its more about heat disipiation, and it averages out. I imagine there is more current in one half cycle, and zero in other after the rectifier diode. About power factor, perhaps some simple circuits for correcting it. I used to wok in place making vacuum tube amplifiers and when testing the most powerful ones, you could hear hum in other devce headphones. Old engineer working there told me this is aniche item, generaly in modern devices such simple power supplies (transformer + diode + capacitor) are not allowed, as they create electromagnetic noise due current spikes when charging the capacitor. One simple solution I have found is putting an inductor in serries, and it suposedly doesnt matter if it is before or after the diode. But it has to be before the capacitor. Its not a perfect power factor solution, but it makes the current spike more gentle. One friend who works with CNC told me not to bother, its only important for industrial equipment. I tend to disaree, its worth atleast in something as big as computer power supplies, right? About series incuctors to filter noise in circuits - weither ferite beads or inductors, do they have a downside of electromagnetic radiattion? If so, any advice to deal with it? I could probably add more questions abut transformers, but this comment is getting long as is. If you're reading this, i hope you will be interested to make more educational videos about those topics. Perhaps something about controllable rectifiers in the future? Field effect transistors, thirystors... One can find circuits online, but i havent seen a good material how ith goes together with transformers, the current and power draw. What are the "rules of thumb" how much % of transformer VA should I leave as a reserve. In short it would be interesiting to see your take on a power supply series, something like "Building a Linear Power Supply, Part 1 The Transformer" by @Vocademy-Electronics-Tech kzbin.info/www/bejne/j5qukJl6lLyanac Not sure if it is your thing, but also about designing chasis. For example can you share a load of amplifier or other device on two transforemers of the same type and put them in oposing directions within the chasis, to reduce the radiated noise? Grounding is another rabbit hole. Im not interested that much in audiophile stuff, but i often stumble upon their forums, and its not easy to distinguish what is true engineering know how and what is just odd supertitions. How to connect grounds within a device, cant find it now, but there was a video recomending cutting the power cord and connecting the ground wire outside of metal chasis of whatever you are building. I wanted to ask someone knowledgable but not audiophile about validity of that.

[@ElectromagneticVideos]

Hi Edgars, Well I'm thrilled you found my channel! I have a large number of European viewers, but not sure how many from Latvia! I have a list of video topics and viewer questions and I just copied your post to it. You bring up a lot of potential topics. Watts and VA's - that will be the next video! And after that, I want to try and explain how to use Ohms law type circuit calculations for AC circuits with indicators and capacitors. A few quick answers: using transformer winding in series or parallel: yes - its done all the time. For parallel output they better be identical windings with the primary fed with identical voltages. If not, you end up with what amounts to a short. For series connection to increase voltage - you have to be careful how much you do it. The insulation of the windings has a limited max voltage. Stack ten transformers with 240V out to get 2400V and the insulation will probably fail. Rule of thumb - usually doubling V is OK but much more than that without knowing what the insulation rating is and you are asking for trouble. Diode rectifier currents - hats a hard one for the reasons you describe - there is a spike at the peak of the AC cycle. A bridge rectifier is better - with that a smaller spike or positive and negative AC peaks. Rule of thumb - use a transformer with 2 or 3 times the AC current rating of the DC current you need. Use a diode of 10 times the current rating (diodes are cheap). And yes - and inductor will help a lot, but you need a big one - using the secondary of a second transformer (primary unattached) identical to your power transformer is good start. LM7812 and LM7805 - great things to start with! Look up things like "LM7805 Application Note" and you will get plenty of suggestions from manufacturers like TI as to how to make a power supply with their ICa. And try it and make a video and ask for suggestions how to improve it. I learn so much from peoples posts! Hope thats a good start! Best wishes from Canada!

[@carlubambi5541]

Thank you

[@ElectromagneticVideos]

You welcome!

[@jamest.5001]

I think I'd just get a large iron toroid or microwave transformer core, wind two 120v coils on it and use that for isolation. You can disassemble two microwave cores, remove the high voltage coils and put the 120v coils on a single core that should get you about 1kw to near 1800 max watts. Depending on specifics.

[@ElectromagneticVideos]

That would sure be a great way to repurposed two microwave transformers if you has some identical ones. Or use them as a 120:240 or vice versa autotransformer.

[@schwinn434]

Was very interesting!

[@ElectromagneticVideos]

Thanks!

[@jeffschroeder4805]

Did I miss you verifying that the output of the 2nd transformer successfully restored the voltage back to 120V. I was very curious whether adding or removing the capacitor would affect the final output voltage. I am guessing that it would mostly affect the overall efficiency of the circuit.

[@ElectromagneticVideos]

Your right - I didn't show that - like so much stuff in every video I do, it got removed in editing. The output voltage was 115V without the capacitor and no load, so 5V was lost. Not a huge surprise given how cheaply made these transformers were. I didn't measure the output voltage with the capacitor attached. My guess is there would be a slight increase due to the the capacitor lowering the current coming from the secondary of the first transformer and therefor reducing the restive voltage drop along the secondary which would raise the low voltage (15V) provided to the second transformer slightly.

[@paradiselost9946]

considering the current trend for DIY generators and the like, should do a video on "jacobi's law" or maximum power transfer theorem... seems that most people forget about the "other half" of a circuit. another subject is "reflected impedances", and why an MOT is deadly because of it...

[@ElectromagneticVideos]

You know, that might be a very interesting topic for a video! Something like how power transfer is not max efficiency and why we go for one or the other at different times. And the whole bit of reflected impedance- and "impedance" of things like audio transformers. I'm thinking of an impedance video - so might be a great followup. Microwave oven transformers - why they are deadly might be a good video just by itself. There have apparently been deaths and serious injury from people mimicking youtube videos use them. I actually have a bunch of microwaves sitting in my workshop for a faraday cage video - that might be a good use the the transformers... Thanks for the ideas!

[@garbo8962]

Its probably been 40 years but thought that I read that most transformers under 1 KVA are not compensated so they are only designed as a step down transformer. You could install a GFCI device on the 120 VAC on output transformer. You do not need a ground for GFCI''S to work. They have a torrid coil that measure current unbalance between the two circuit conductors.

[@ElectromagneticVideos]

I hadn't heard that - and there are plenty of sub-1kva step up transformers. And there really isn't anything inherently different between stepping up or stepping down. Your right about being able to use a GFCI on the output and not needing a ground. But it wouldn't help in this situation unless there was a short or insulation failure in the transformer coil since there is no back path outside of the torroid to cause an imbalance. If one of the two legs of the isolated legs of the transformer were grounded (or it was a autotransformer) it certainly could be lifesaving!

[@BjornV78]

Hi, big thanks for demonstrating this question. A few questions about the use of the capacitor. 1) Is there a limitation on the maximum value that you can use ? and 2) when you added this capacitor, the current on the primary side of the first transformer, and the current between both secundary sides dropped almost with 50%, but does this mean that the output voltage on the second transformer also is dropped from in theory 120V with no capacitor, to arround 60V with capacitor ? Grtz

[@ElectromagneticVideos]

Your welcome! 1) You have to consider that the can be thought of as consisting of an in phase "real" part (like the current though a resistor) and a 90 degree out of phase "imaginary" part from the lagging current due to inductance. "real" and "imaginary" refer to real and imaginary numbers for those that are mathematically inclined. The capacitor adds a leading 90 degree out of phase current. The total current is the (vector sum) of the real an imaginary parts. The total can the thought of the length of the hypotenuse of a right triangle with the real in phase part on one of the sides and imaginary 90 degree out of phase par on the remaining side. To minimize current, we need to have a the capacitor current equal to the inductor current since it has opposite (180 degree phase). This should cancel the out of phase part of the current leaving only the real "power" current. If the capacitor is too big, the sum will have leading out of phase current making things worse. So how to determine the capacitor? I figure out the real current from the power - the power can be read from a power meter. Use that power to calculate the real component of the current. Measure the actual current in with an ammeter to get the (vector) sum of the real an imaginary currents = hypotenuse of a triangle. With the real current as one of the sides, calculate the imaginary (our of phase) current. You will need a capacitor drawing that amount of out of phase current. The reactance of the capacitor is obtained from ohms law R = V/(imaginary current). Use any of the online capacitance-reactance calculators to find the capacitor size needed for that reactance at 50Hz. Or - get a bunch of say 1uF capacitors and add them to the circuit till the current stops dropping and you will have the value. Note that capacitor of this size are often quite crude in terms of how close they are to the stated capacitance. 2) No change in voltage! The capacitor supplies the magnetizing current - the core still draws the same magnetizing current - so the voltage remains the same. 3) It doesn't matter which side of the transformer has the capacitor. You could use a 120V rated capacitor of suitable scaled down capacitance on the 120V side of the second transformer to achieve the same thing. Just remember that if disconnected the capacitor could retain a 120V charge! Hope that helps!

[@miltonthecat2240]

Does the capacitor value used "resonate" with the primary inductance at 60Hz? An ideal parallel L-C tank circuit draws zero current (i.e., has infinite input impedance) at the resonant frequency, if I remember correctly.

[@ElectromagneticVideos]

@@miltonthecat2240 Thats it exactly! The combination of L and C when perfectly matched draw zero current so appear to have an infinite impedance, while a large current flows back and forth between them at the resonant frequency! So then the only current left going into the transformer is the current supplying the power loss - which in the end looks like a resistor across the tank circuit.

[@jackhreha4907]

How about a buck boost with that set up. It is a cheap way to make diffrent tube voltages. like how you use test equipment that a poor guy can cobble up. Good show Thanks.

[@ElectromagneticVideos]

You mean like adding a few transformers on the right side of the diagram with the 12V windings in parallel and 120V windings in series to create a few hundred volts for tube power supplies. That would actually be a great way to replace the now harder and harder to get tube transformers. The only issue might be the breakdown voltage of the insulation on 120V side that probably works find to maybe get 240V, but when raised to something like 480V (4 in series) might not do too well. One way to get the higher voltages might be to use a 12:240V transformer and put the two of them in series. The 240V winding insulation should handle 480V and since 240V power is more common worldwide than 120V, cheap transformers of this kind should be easily available. In case anyone's interested, I got these transformers from $2 Newark on clearance, so they were really cheap.

[@button-puncher]

Thanks for another excellent video! I saw Mr Carlson build an isolation transformer and he added a cap to reduce the loses but he never explained why. Is there a relatively simple formula to calculate the capacitance needed? Also, could you add a GFCI to the isolated side?

[@ElectromagneticVideos]

There isn't an easy way because few transformer secs have the reactive current specified. So 1) You could measure the total current and the real power using a power meter, calculate the real in phase current from the real current and total current where the total current is the hypotenuse of a right triangle and the real and imaginary currents are the sides. Use Pythagorus rule. 2) Instead of the above, measure the current when the transformer is unloaded and guess that about half is reactive. So now we know the reactive current we need to compensate for. 3) Use ohms law Zc = V/I where V is voltage into the transformer and I is the reactive current. Zc is is the reactance we need. 4)Use Zc = 1/(2πfC) to get C. 5) So start with C close to the value from above, and try a bit more and a bit less till you find the minimum current draw and you have it. Note you need to use continuous operation AC capacitors, and be sure to discharge them after removal from the circuit do you dont get zapped. In most situations with small transformers its rarely worth the effort unless you have a good reason to reduce the current a bit. GFCI - generally not much help at the end of an isolation transformer since in theory the transformer should prevent any stray current. It could be of use if a short developed in a transformer - and one can debate if its better to have the GFCI on the in or out side of the transformer. In one video I mentioned my isolation transformer was plugged into a GFCI - did I ever get criticized for that. No sure why - more protection is better. By the way - a GFCI st time is better than an isolation transformer since it shuts down the circuit. If you touch both terminals of the isolation transformer output, it wont stop and can be lethal. So each use case should be carefully considered as to which (or both) is safer.

[@button-puncher]

​@@ElectromagneticVideos PERFECT! THANK YOU!!! I'm saving your reply for future reference. I stumble onto a clearance deal a long time ago on two 24V 40A landscape lighting transformers. Connecting them back to back worked but the because of the idle power draw, I haven't used them for that purpose since. It'll be nice to dust them off and put them to good use.

[@ElectromagneticVideos]

@@button-puncher That sounds like the perfect transformers for that sort of thing. So when you do it, measure the current in the 24V connection between the two while adding capacitors. You can use smaller capacitance higher voltage capacitors on the isolated 120V winding output or large capacitance lower V on the 24V middle, whichever is easier to get. Most likely the 120V ones will be easier to get - motor run capacitors would be a common component that would be easy to get.

[@button-puncher]

@@ElectromagneticVideos I knew that you can make bi-polar capacitors using two 'lytics back-to-back (for speaker crossovers) so I was thinking of going that way, but thanks for the tip on using a cap on the 2nd transformers output. I have a bunch of furnace fan run capacitors that I can experiment with.

[@ElectromagneticVideos]

@@button-puncher The furnace run ones would be perfect. If you do go with the back to back route, also a good idea to put back to back diodes in parallel with the capacitors since you cant trust the capacitors to be identical in terms of leakage currents. Also, for this sort of use you have to be wary of heat generation in the capacitor due to current flow - so not every electrolytic is suitable for long term use like this.

[@InssiAjaton]

If I had done this same presentation, I would have added just one more measurement - the resulting final output voltage, no load. Maybe a few words about the normal practice of winding all transformers with a little higher than the nominal (loaded) secondary voltage, like in this case 15 V rather than 12 V. In the industry, the catalog listings talk about “% Regulation”.

[@ElectromagneticVideos]

Yes - in hindsight, I should have shown the voltage at the end. I always struggle with how much to cut - sadly KZbin audiences have a very short attention span and and I have to ruthless as to how much I cut from each video. I did feel that 15V for a 12V winding was a bit high, but this is a cheapo low efficiency transformer so I guess they were a bit aggressive in that respect.

[@hadibq]

I love these videos with clear explainations and comfy human proper english 😄

[@ElectromagneticVideos]

Thanks - I really appreciate that - its exactly what I am trying to do!

[@miltonthecat2240]

Strep throat! That sounds serious. Hope you are well on your way to recovery. It is my understanding that most all of the transformers of the type you are using in this demo are designed to operate somewhat into the saturation region of the magnetic core. It would be interesting to measure the waveform of the magnetizing current, to see how much it is distorted from the effects of saturation. It would be even more interesting to see how the waveform distortion of the magnetizing current changes with input voltage, say when the input is powered from a Variac. I imagine that many hobbyists have done what I did when first playing around with step-down transformers, which was to reverse the primary and secondary in an attempt to step up the voltage. Which fails, probably dramatically so. Although I later discovered that I could get away with doing this if I had a source of AC power at a frequency higher than 50/60Hz, such as 500Hz.

[@steam7087]

Objawem pracy w pobliżu nasycenia, jest przede wszystkim nagrzewanie rdzenia w stanie biegu jałowego. Mam starsze transformatory projektowane z założeniem mniejszej indukcji magnetycznej w rdzeniu, i te bez obciążenia pozostają chłodne. Szczególnym przypadkiem są transformatory do zasilania żarówek w semaforach kolejowych. W nich prąd magnesowania niebciążonego teansformatora nie przekracza 80 mA przy 110V. Jednak taki transformator o mocy kilkudziesięciu watów ma rdzeń o wymiarach charakterystycznych dla transformatorów o mocy dziesięciokrotnie większej.

[@ElectromagneticVideos]

I am doing much better a now thanks. The real issue was the initial symptoms weren't that bad so had it for over a week before getting a strep throat rapid test. I think your right about everything! 1.4W is a lot of ideal power loss for a 7W transformer (I thought it was 15V but the spec sheet says 7!) . The 15V out on the "12V" winding also shows they skimped on wire size expecting a significant voltage drop on full load. I should look look at the magnetizing current- I'll bet you will be proven correct! A while ago I did look at transformer magnetization currents when I demod magnetic amps and magnetic memory cells. They do get pretty spiky ass one goes into saturation. Higher frequencies - funny you should mention that. I have have an up to 2kHz variable frequency drive that a got to do exactly that with the 120 to 10kV ignition transformers I have to hopefully get around 20kv (or 40kV between two outputs of two oppositely phased transformers). The long term issue may be eddy current heating because the lamination are designed for 60Hz. (and for my HV transformers, insulation breakdown).

[@ElectromagneticVideos]

@@steam7087 They sure dont build them as well as they sued to! Its all a bout saving money these days. Interesting to hear about the semaphore bulb transformers - I guess the compromise there is more money spent on the transformer reduces long terms power costs. One thing we could do over here in 120V counters is use 240V transformers to keep the core well out of saturation and reduce long term power consumption of the transformer is idle a lot. Just as you described - ending up with a way larger than normal transformer for a given power.

[@miltonthecat2240]

@@ElectromagneticVideos "Eddy current heating" jogged my memory. Yes, the step-down transformer I used in reverse to step-up, using 500Hz input power (the upper limit of the variable frequency drive I had available at the time), was a spendy toroidal transformer with a powdered-iron core that had low eddy current losses. It was a fairly hefty transformer, and the output it produced truly scared me. It probably took me ten times as long to build a relatively safe enclosure for the thing, with proper interlocks and fusing.

[@ElectromagneticVideos]

@@miltonthecat2240 That must have been really cool! Did you create a jacobs ladder or an arc with it? My motivation for doing something similar is to show how a high frequency arc - or DC arc - is harder to extinguish in breakers and switches than a 50 or 60Hz arc because there is no time between cycles for the air/plasma to cool down. If you did an arc, was it noticeably more stable?

[@kylewellman402]

If they want an isolated 120VAC, they make 1:1 transformers. No need to double the inductive current needed.

[@ElectromagneticVideos]

I think many were using this sort thing because the had the stuff on hand, or already had low voltage AC and really only needed a tiny amout of higher voltage AC.

[@kylewellman402]

@ElectromagneticVideos that's a very good point. I suppose if I already had 2 older style power bricks that used transformers I would do the same rather than buying a 1:1 transformer 😅 although it might be a fun challenge using those 2 transformers and pulling the secondary coils out and making a singular transformer out of the 2 primary coils. On the subject of transformers, I am currently working on an experiment to remove eddy currents entirely, rather than making them very small by using the stacks of thin steel sheets as the core.

[@ElectromagneticVideos]

@@kylewellman402 I was actually amazed at the of people who had done the back to back transformer thing for one reason or another. My initial thought was like yours. Someone mentioned taking two identical microwave transformers, removing the HV windings and slipping the primary from one core onto the other to make an isolation transfomer. Since many have lousy cores with all the lamination Es on one side and Is on the other, held together by a shallow, should be easy to do. Removing eddy currents - if you can do that it would be a very significant invention. I wont ask how becuase if you really have something you should patent it. You can stake your claim up to a year before you do a formal patent application by filing a provisional patent in case you are not aware of that.

[@kylewellman402]

@ElectromagneticVideos thank you for that information. I was not aware of that about the provisional patent deal. I'm almost ready for physical testing. Trying to think of the best and most thorough way to test though. I know i can apply power to primary and measure across open secondary, but that includes hysteresis losses. Of course, if I remove the eddy currents there should still be a noticeable difference between the readings that way, but surely there has got to be a more thorough and precise way to measure the eddy current losses themselves. I thought about seeing if I can take a current reading across the core material itself, but don't think that will really work. I would love to hear your ideas if you have any and wouldnt mind sharing?

[@ElectromagneticVideos]

@@kylewellman402 Its been a while since I did provisional patent so what I did say might be out of day, so look into it a bit. Measuring across the secondary wont give you the core loss which includes eddy currents. All you need is the primary - leave the secondary unconnected and measure the power in the primary which will include all core losses. Compare that to a "normal core" Your core should consume less power - in the end it doesn't matter how - all that matters is the total power savings. Now you have calculate the cost of your solution vs $ saved in power all related to the cost of the transformer. I dont know what % of core loss is eddy currents these days, but it does have to make significant financial sense for people to pay to license your patent and also costs of changing manufacturing etc. Bottom line: it you may have a great idea, but be really careful about figuring out if there actually is money to be made before you invest $$$ into it - or spend $20L or more to have a patent filed.

[@MyProjectBoxChannel]

I recently got my hands on quite a large variac for just 10 bucks. I was thinking to connect it to a DIY isolation transformer, to make a variable voltage, safety isolated supply. Im planning to connect two microwave oven Transformers, back to back. Connecting the two high voltage sides together. So step up to step-down again. I olso curious if I could use them back to back but connected in reverse, feeding the high-voltage side first. So step down then back up again?

[@ElectromagneticVideos]

$10 for a variac - wow- what a steal of a deal! You could certainly do that with microwave transformers, but if you do, be really really really careful with the HV. With the current they can supply and the ability of 1kV or so to jump a mm or 2 in air and puncture the insulating layer of our skin, even an unnoticed crack in the insulation can be lethal. One other commenter suggested taking two identical microwave transformers, separating the windings (they often are on separate plastic forms), discarding the HV windings, and putting the two LV (120V or 240V) windings on the same core to make KW size isolation transformer. Much safer in the end! If you do want see two HV transformers back to back I did it here: kzbin.info/www/bejne/pqO0pKZ_gZWJo6M And if you are doing anything with lethal voltages, consider using/making a dead mans switch like this one (it has saved me a few times!) kzbin.info/www/bejne/kJmmp4d5mqypg8U . As a side note, I find the dead mans switch even more important when making vidoes - between running the camera, doing the narration, and doing the experiment, its so easy to get distracted and forget something is energized.

[@MyProjectBoxChannel]

@ElectromagneticVideos I have always been absolutely terrified of the HV side microwave oven transformers. In the past I've always removed/knocked out, the HV winding, in favor of high-current/Low voltage winding, experiments with microwave oven transformers. Making spot wellers and other high current devices to melt metal, is a lot of fun, And relatively safe. Unfortunately my two transformers are not identical. But I can remove the high voltage windings, and link the two transformers with a single fat winding/. Mabe a copper bar, pipe or some really thick wire. It used to be very common to find discarded microwave ovens. So maybe I just need to be patient and find two identical transformers, and put the two primarys side by side. You're absolutely correct the "dead man switch" is a must.

[@ElectromagneticVideos]

@@MyProjectBoxChannel I am so pleased to hear your "absolutely terrified of the HV side microwave oven transformers". I am too! So many people dont understand the danger. I try to gently educate people about things like that but there is a fine line between making suggestions and being too forceful and have having people ignore the advice. Some thick wire should do it. As a general rule, just fill up the area that the HV had with as many turns as possible of thick wire and it will end up close to optimum. Of course if the transformers are different enough you may have to tweak the number of turns around one core or the other to get a 120V in 120V out situation. If you use really thick wire, keep it less than a cm (1/2inch) diameter so you dont have skin effect coming into play. I think whats happening with microwave ovens is the metal recycles really look out for them because of the copper in the transformer. Interesting a while back they started using copper coated aluminum for the windings to reduce costs messing up things for the recyclers. And of course now they have moved to switching power supplies ("inverter") to reduce the transformer size even more and making the transformers useless for other applications.

[@MyProjectBoxChannel]

@@ElectromagneticVideos I don't think matching the in to output voltage of my DIY isolation transformer is super important, because I will have the veriac to adjust the voltage. Maybe a slightly higher "step up" voltage could be a advantage 🤔. Copper coated aluminium is definitely everywhere nowadays😩😔. It's nasty stuff, but I guess if sufficiently derated and used with spring-loaded connectors, to deal with creep effect. It becomes a viable economic option? Still copper is best 👌 and aluminium is too much of a compromise for me.

[@ElectromagneticVideos]

@@MyProjectBoxChannel Of course - I forgot about the variac. Where I find copper coated aluminum seems to really have made inroads is Ethernet cable which is a bit crazy since the cable is so small and the the Al is more likely to break. And for short cables the cost difference is negligible, and long lung the installation cost of laying the cable so outweighs the cost of the cable itself. Plain Al cable is still widley used for thicker power handling cables such as for bringing power into the home for the pole. I just had a 80A electric boiler installed installed for central heat and Al was used for that - Cu would have cost $1K more. And is seems to be OK for that sort of use. (I will be doing a video in the pros/cos of electric heat).

[@emilalmberg1096]

You have made a nice little intro video, well done. But I get so worried when you put your fingers on the transformers like that, can't you use a stick or something?

[@ElectromagneticVideos]

Thanks and I appreciate you concern. The 120V power is though a GFCI and where I was attaching things in the middle was on the isolated 15V windings. The biggest area of concern is the 120V out to the second transformer on the right which is isolated from the GFCI. I should have covered its 120V terms to be safe!

[@SteveM45]

4:10 electroboom style

[@ElectromagneticVideos]

But when he does it, isn't it supposed to explode in brilliant display of sparks? Unfortunately my videos are rarely that interesting :)

[@Iowa599]

You should go the other way. 120v -> 1800v -> 120v. Reduce current losses...

[@ElectromagneticVideos]

Assuming the windings are suitably sized, it really should make a difference. I certainly wouldnt do that for a demo like this - the sort of voltage is really lethal at the currents that would be present even for a 15W transformer.

[@Iowa599]

@ElectromagneticVideos isn't that what demos are for? ElecroBOOM shows what happens when doing things wrong...

[@ericlondon5731]

I kept cringing every time you grabbed the transformer while it was energized just to help express which one you are talking about.

[@ElectromagneticVideos]

Well I appreciate the concern! The transformer ion the left is powered though a GFCI so if I did touch the 120V terminal it would (hopefully) trip. The 2nd transformer's output is the more dangerous one in terms of being isolated from the GFCI - so it would not trip the GFCI Since I was only holding it with one hand worst case would a shock from one finger to another finger. Nasty but a well contained current path. And since the the coil is isolated, touching one one of the terminals would have no effect.

[@aleksandartomic5515]

There is reason why he did not show output voltage.

[@mygreatbigfoot1679]

GFCI?

[@ElectromagneticVideos]

Ground fault circuit interrupter. Also called RCD in most of the world outside of North America.