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After ordering this 200VA, 32V transformer, I realized that it doesn't have a center tap on each secondary, which I need to have. (duh on my part) So I want to use 110V across the 220V primary leads to give me half the voltage at each secondary, resulting in a single secondary of 32V and a center tap.

I know that this works because I have done it and observed that the output is 32V RMS with a center tap, but I want to make sure I'm not throwing away half the VA rating by doing so. I read TONS about transformers trying to get a definitive answer to this, and I think that the core saturation and the secondary wire size and associated voltage drop and heat are the main concerns, along with the fact that the ratio of number of turns is proportional to the ratio of the voltages and inversely proportional to the ratio of the currents in the primary and secondary. So on those points, I think I'm not reducing the usable 200VA, with the possible exception of secondary wire heating.

To illustrate what I mean using the image below, I would wire-nut the middle black and red primary wires together and connect 110V across the top red and bottom black wires. And on the secondary, I would connect the middle blue and green wires and use that connection as my center tap. I would use the top green and the bottom blue wires as my supply rails and there would be 32V RMS across these rails. If that's confusing for me to say rails, I'll add that I'm using a bridge rectifier and 2 shunt capacitors to make them into +/- 45-ish volt rails for an amplifier.

enter image description here

Yes, I know that this is NOT standard, and this is not for a commercial product, so please don't reply simply that this is not standard or that it will void my warranty.

There is a similar question here with 2 answers to my question, but unfortunately those two answers contradict each other exactly. Also, here is a question with a very promising title, but the answers don't answer my question. The other similar questions don't have answers which address the current-rating-doubling part.

Please weigh in if you know whether I can safely get the full 200VA in this way.

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You will halve the rated VA of the transformer if you run it at half the rated input voltage.

The rated maximum VA is the product of maximum allowable Volts and maximum allowable Amps. The Amps will be unchanged, that's limited by wire heating. You've halved the Volts, so you've halved the VA, to first order at least. Running the core at half the field reduces the core losses, so the thermal load on the transformer is not quite so high. This means you can increase the thermal loading on the copper a little, at the expense of voltage drop aka voltage regulation. Maybe then the rated VA has only fallen to 55% and not to 50% of what it was.

If you have a centre-tapped 32 V, ie 16-0-16, you can bridge rectify that to about +/- 22 V. If you want +/- 45 V (that is, a total of 90 V from positive to negative, with ground in the middle), then the simplest way to get there is with a centre-tapped 64 V secondary (two 32 V secondaries in series), leaving your transformer configured and supplied as it sounds like it was meant to be, and like my audio amp transformer is.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Yes, so I can use the transformer as it was designed, with 110V across each primary in parallel. I was taking for granted that since 32V RMS gives me 45V peak (90V p-p), then two 32V secondaries in series would give me 90V peak (180V p-p), but that is not correct and I had to draw it out to prove it to myself. Instead it means that the capacitors only have to hold a charge for half of each cycle instead of the full cycle. At least I was thinking correctly when I chose and bought the transformers. (one for each channel with plenty of headroom) Thanks for making me think outside of my wrong box. \$\endgroup\$ – Kent Weigel Dec 30 '20 at 4:51
  • \$\begingroup\$ Suggested edit: "The VA rating is the product of the maximum allowable Volts and the maximum allowable Amps". It makes it a bit more clear that it isn't just the product of whatever happens to be on the transformer at any given moment. \$\endgroup\$ – TimWescott Dec 30 '20 at 6:54
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Transformer manufacturers generally don't leave much margin between what their device is capable of, and how they advertise it. If the transformer is advertised to handle 200VA, with two 32V secondaries, then it's nominal operating condition would be 3.125A in each secondary or a total of 6.25A. (6.25*32=200VA). Can you run 6.25A through those wires wired in series? Well, I would say no. You might be lucky, but there is a good chance there will soon be smoke coming out of your transformer.

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