Ways to combine 2 power transformers for more current capability - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: July 20, 2022, 07:52:29 pm
content: So I'm building a push pull 4 EL84 amp, but the most suitable power transformer I have was only ever used in a radio with 2 EL84's driving 2 speakers in stereo, but I have plenty of those.I am not really comfortable of driving so much current trough that small power transformer. so what ways are there of combining them? of course I know of using resistors to mix both transformers, and that's probably the best solution in my case.but I want to hear if there's any other ways of doing this? like would there be some benefits of mixing the transformer windings instead of the rectifier outputs?

author: bob91343
author_rank: Super Contributor
timestamp: July 20, 2022, 08:20:00 pm
content: Build two separate power supplies and run one pair of tubes from each.

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2022, 09:02:31 pm
content: If you have two identical transformers, couldn't you just wire them in parallel?

author: TimFox
author_rank: Super Contributor
timestamp: July 20, 2022, 09:19:07 pm
content: For power transformers, parallel connection is not wise, but series connection should work.In the latter case, you know that the current through the two series-connected windings is identical, and the voltage will add in the obvious way (assuming you get the phasing correct).For your application, that changes the required secondary voltage, of course.

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2022, 09:21:54 pm
content: Quote from: TimFox on July 20, 2022, 09:19:07 pmWhat would be the issue, and is there a way of mitigating it? For example, by putting separate fuses on each secondary?

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2022, 09:36:07 pm
content: Quote from: ELS122 on July 20, 2022, 09:27:18 pmI'm not sure why that would be the case. TimFox said such a connection was unwise, not that it would fail badly.Ideally, each transformer would provide 50% of the current. If they are slightly unbalanced, then one would supply more than 50% and the other less. It is therefore possible that the one supplying more than 50% could exceed its rated current. However, if the transformers are over specified then even the extra current could be within the rating.There is no reason for one transformer to feed into the other unless, for example, the primary got disconnected on one of them. In that case, the disconnected transformer would present a heavy load on the connected transformer, which might cause overheating and failure. That's where perhaps current fuses or thermal fuses might help.

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2022, 10:08:44 pm
content: Connecting two transformers in parallel is not much different in principle than connecting two batteries in parallel, and people do that all the time with lithium ion cells. Both have a very low output impedance, and both can be slightly unbalanced in voltage and internal resistance.There is really no reason for one transformer to try to feed the other transformer, since both secondary windings are producing more or less the same voltage and these two voltages will cancel each other out. If there is a slight difference in voltage it will cause a slight current to flow from one transformer to the other, but this will be far smaller than the output current feeding the load. The net effect will be that any slight imbalance in voltage will cause a slight imbalance in current sharing.This is one of those things that in theory should work fine, but in practice there may be reasons to avoid it. I am curious about what those practical reasons might be.

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2022, 10:13:42 pm
content: Quote from: ELS122 on July 20, 2022, 10:08:26 pmWhat is the rated output voltage and load current of the transformers we are talking about?

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2022, 10:18:28 pm
content: Quote from: edavid on July 20, 2022, 10:12:49 pmI can do the experiment when I have time and if I can find two identical transformers, but if the transformers are loaded then I predict there won't be any backfeed current. I predict that one transformer may take up a little more of the load current than the other, but if the transformers are the same then the current sharing won't be that much out of balance.

author: Ian.M
author_rank: Super Contributor
timestamp: July 20, 2022, 10:19:34 pm
content: Parallel connection after separate bridge rectifiers should be reasonably safe. Any slight mismatch in voltage will be taken up by the higher voltage one sagging under load till its down to the other one's voltage, and the bridge rectifiers will block circulating AC currents when unloaded.

author: langwadt
author_rank: Super Contributor
timestamp: July 20, 2022, 10:26:30 pm
content: Quote from: ELS122 on July 20, 2022, 08:59:03 pmwhy? if you have two pairs, can't you just power on pair from one supply and another pair from another supply? just as if the supplies were in parallel, except they are not

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 20, 2022, 10:50:59 pm
content: Paralleling identical transformers isn't that much of an issue especially in this case where winding impedances are highish anyway.Another approach could be to have primaries and secondaries in series, you'll have a little less current available on output because of added internal resistance but you'll be putting only 1/2 the power on each transformer.I still think connecting after rectification is the best overall solution. If you feel you must, add series resistance or inductance or a couple of turns to better balance the load...

author: langwadt
author_rank: Super Contributor
timestamp: July 20, 2022, 11:24:11 pm
content: Quote from: ELS122 on July 20, 2022, 11:12:48 pmshow a schematic ...

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 20, 2022, 11:41:07 pm
content: Comments:Deleted, I thought he was trying to parallel the audio Freq Push Pull OUTPUT transformers

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2022, 11:55:09 pm
content: Quote from: ELS122 on July 20, 2022, 11:12:48 pmIf you connect two secondary windings in parallel there can be no voltage difference between them, by definition. Connected points in a circuit always have the same voltage. The combined output voltage will sag under the imposed load and each secondary winding will contribute to the total load current in inverse proportion to their source impedances until the output matches the load. If there were an imbalance of 50 mA this would be 125 mA from one transformer and 175 mA from the other. That would be OK as long as 175 mA is within the current rating of the winding.All through this discussion, there has been the proviso that the transformers should be identical--same manufacturer make and model no. That means same number of turns, same wire diameter, same core dimensions. There's not much reason for any significant imbalance between them.If it were me I would connect the transformers like this and after a few measurements to check for surprises I would have no worries at all about it.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 21, 2022, 01:29:59 am
content: If you have 240V mains, wire the transformers for 120V each and connect primaries in series, secondaries in parallel.

author: Someone
author_rank: Super Contributor
timestamp: July 21, 2022, 03:13:49 am
content: Quote from: mag_therm on July 20, 2022, 11:41:07 pmplease return that post if you can, it was full of gold! (even if not quite on topic)

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 21, 2022, 11:03:05 am
content: Hi ELS122,use two (bridge-) rectifiers and parallel at the dc-side.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 21, 2022, 01:26:35 pm
content: Quote from: Someone on July 21, 2022, 03:13:49 amHi SomeoOne,Maybe better, slightly more on topic for parallel rectifier transformers, to put some photos of my present project which is a 13.8 V UPS with isolated output.The converter transformer is PWM from 32V DC link to 13.8 V 20 Amp output for ham radio .To hopefully reduce RF noise, this transformer will run down at 15 kHz ( for 30 kHz PWM)It has 2 parallel windings on a C-core.Retired, I still like to design transformers occasionally , now I get to build them too!Here is the current and FFT of a smaller 200 Watt DC converter at 70 kHz ( 140 kHz pwm) which I am using to design the present one.https://app.box.com/s/z3g66fl6fdmeai0r66zf5cebwoxb2jv4Here is the connection diagram of the windings and their rectifiers.https://app.box.com/s/28c4dvxd6cv9s5t7quy2i3fls8rlvzd6There will be some imbalance in the core by the half wave rectifiers on each side. I expect that imbalance will be reset over a full cycleThe transformer is electrically two identical windings, cross paralleled, sharing the common c-core.I made the brazed Aluminum core tubes last week.https://app.box.com/s/e8v43d0hb1zs7ucvezxxqsh66rphdgfjThen the windings over last 2 days.https://app.box.com/s/zaleec91vkt3inpvx1xamrxkbgug2w7tNearly ready to do the connections:https://app.box.com/s/wagozvrd6l05u06ty7k4yxym887rp30pI think I will lay the transformer on its side as shown and run short braids from core tubes to chassis.The noise around the transformer circuits has to be contained.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 22, 2022, 01:47:04 pm
content: Quote from: vu2nan on July 22, 2022, 10:25:56 am...exactly. Identical. But the OPs transfos differ.

author: vu2nan
author_rank: Contributor
timestamp: July 23, 2022, 02:17:02 am
content: Hi Roehrenonkel,I'm sorry I overlooked that.Many thanks.Nandu.