DIY Back-To-Back Isolation Transformer - Multiple Secondaries Question Solved - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: July 18, 2023, 03:02:34 pm
content: My BackgroundI took up electronics, when I retired, to keep my brain learning. I am a self-taught EE hobbyist. Meaning, there are large gaps in my knowledge base and understanding (particularly VAC.) Therefore, please explain things in detail and give directions with very specific instructions. I am fortunate to have very nice lab equipment and I can test most things that you need me to test.==================================================================I built an isolation transformer, using the back to back method.You can read about the initial build, here:https://www.eevblog.com/forum/projects/diy-isolation-transformer-project-back-to-back-method/msg3174352/#msg3174352You can read about adding a soft-start circuit to it, here:https://www.eevblog.com/forum/projects/soft-start-circuit-for-isolation-transformer-review-and-design-help-please/msg4957165/#msg4957165The transformers are identical, of course. They both have one primary input and two secondary outputs. When I assembled them, I only used one secondary as the input to one transformer and one secondary as the output from the other.Is it possible to put both secondaries of both transformers in parallel and to use the combined secondaries as their respective inputs and outputs… so long as I am careful to maintain the phases of each and every coil?I do not want, or need, additional current. I am simply wanting to reduce the load on the coils, to ease the heat and wear and tear that they experience. I will keep the device’s power rating as is and limit it with the lower rated fuse that I am already using.Thank you for your help!

author: Ian.M
author_rank: Super Contributor
timestamp: July 18, 2023, 03:45:09 pm
content: It depends on the secondaries (which need to be the same voltage) - if the transformer was originally intended for it to be permissible to directly parallel its secondaries, then yes, but if if not, and there is even a one turn mismatch between them you could get large circulating currents which will increase losses and could result in overheating.If they are the same current rating, a safer option for the special case of two back-to-back transformers of the same type, that doesn't need matched secondaries or even the same voltage ones would be to wire them in series (being careful of phasing so their voltages add, not cancel).

author: bdunham7
author_rank: Super Contributor
timestamp: July 18, 2023, 04:15:49 pm
content: Quote from: t1d on July 18, 2023, 03:02:34 pmAs mentioned, if you have multiple secondaries it makes more practical sense to run them in series. However, I'm confused about your transformers--are these still the recycled APC UPS transformers? Don't those have a primary with multiple taps and then a single large secondary?

author: t1d
author_rank: Super Contributor
timestamp: July 18, 2023, 05:44:34 pm
content: Thank you, Ian.M and bdunham7. You are both correct and you both make good points.Ian's PointThe transformers came out of two of the same model of UPS. While they are likely spec'd to be identical, they are not matched by testing. Meaning, even if it were permissible to parallel them (unknowable,) I would not want to do so, as they are not matched for that purpose and the circulating currents could be a problem.BDunham's PointYes, I totally forgot about the original configuration for the transformers within the UPS. Meaning, I reverted to (my) common thinking of the arrangement of transformers... Single, big wire primary inputs and multiple, smaller wire secondary outputs... and I misspoke. As you point out, if I had changed the wire to the way I was considering, bad things could have happened.I owe both of you, big time, for catching these problem and teaching me about them.And, I am glad that I slowed down enough to ask/double-check my thinking. I made the extra effort, because Mains scare me, which is a healthy habit all Hobbyist should have.I will leave things configured, as they are now... The single inputs/outputs, with the extra set of coils terminated.We will call this one solved.

author: bdunham7
author_rank: Super Contributor
timestamp: July 18, 2023, 05:48:30 pm
content: Quote from: t1d on July 18, 2023, 05:44:34 pmI'm still confused a bit by your picture and explanation. You seem to be implying that there are two separate primary windings and that you have them in parallel. Can you tell us the exact model number of the UPS that the transformers came from?

author: t1d
author_rank: Super Contributor
timestamp: July 18, 2023, 06:11:21 pm
content: Quote from: bdunham7 on July 18, 2023, 05:48:30 pmI apologize for adding more confusion... I have physical challenges and I am in a lot of pain. That makes it hard to think. Maybe I will have to quit for the day. But, I have to quit most days, which does not make for much fun. Sorry to whine. Just trying to explain.The UPSs were both APC 1500. The transformer models are in the picture. I have the unit torn down, presently, to add a soft-start circuit. (I posted about that in a different thread. You helped me, there, as well, IIRC.) So, I tired to label things in red.Please stick with me, until we everything ironed out completely. As I said, Mains scares me. Thank you, so much!!!

author: bdunham7
author_rank: Super Contributor
timestamp: July 18, 2023, 06:35:29 pm
content: Quote from: t1d on July 18, 2023, 06:11:21 pmNo worries, you're not being graded.Here is a picture of a similar APC transformer and a diagram of it. Hopefully this helps you and anyone else reading this that isn't familiar with these transformers. There is one primary winding with 'taps' that allow the UPS to use the transformer as an autotransformer to buck or boost the line output by +/-10% or so without going to backup power. You've selected the lowest tap, using the least amount of the winding, which I'm assuming is what the instructions of whoever came up with this system told you to do. I personally would not have cut those other leads so short just in case you change your mind, but it's probably going to work fine either way. Using the yellow or blue tap on the input would give you less magnetizing current and less idle power loss, but lower output voltage. Using the yellow or blue tap on the output would give you greater voltage.

author: t1d
author_rank: Super Contributor
timestamp: July 18, 2023, 07:43:58 pm
content: Quote from: bdunham7 on July 18, 2023, 06:35:29 pmI know that there is something of jest, in the way you phrased this reply. But, to be honest, it did make me feel a lots better. Thank you.Quote from: bdunham7 on July 18, 2023, 06:35:29 pmYes, very helpful, indeed!Quote from: bdunham7 on July 18, 2023, 06:35:29 pmHonestly, it's been a long time. I will have to see, if I have any notes in that regard.Quote from: bdunham7 on July 18, 2023, 06:35:29 pmOh, no, I did not cut them short. I folded them and covered them with heat shink.Quote from: bdunham7 on July 18, 2023, 06:35:29 pmThank you for the excellent information and your effort to help me. I don't take that light lightly, as you are helping to protect me, in a very real sense.My take-away...1) I have the transformers wired correctly, as to inputs and outputs.2) However, I may want to give consideration to which buck/boost windings that I wish to use. I am intending to have a 1:1 transformer on a 120VAC supply. So, I take it that the yellow and white wires are the way to go.That's all great information. Thank you! Now, I will mark the thread as "Solved."

author: bdunham7
author_rank: Super Contributor
timestamp: July 18, 2023, 07:54:57 pm
content: Quote from: t1d on July 18, 2023, 07:43:58 pm"Solved" really isn't a thing here, and people (including me) will yammer on forever on derivative topics. When it comes to thread drift, we drift more than a Saudi teenager with a Lamborghini.Since I had the transformer dug out and on the bench, I measured some things for you. With 120VAC input (open output) the black tap was 0.30A and 16.3W, the yellow tap was 0.17A and 12.3W and the blue tap was 0.11A and 9.6W. The hum was pretty tame in all cases, so it appears even though they don't want to pay for copper, they at least buy enough iron to make the transformer work. I also measured the inrush current, I'll put that in the other thread. Keep in mind that mine is from an SUA1000 model and presumably rated for 1.0kVA, yours should be a 1.48kVA version. Of course, they could actually be the same, but I doubt it.

author: t1d
author_rank: Super Contributor
timestamp: July 18, 2023, 10:50:45 pm
content: Good information. Thank you!