# Transformer mains identification

A friend gave me a transformer from an APC UPS that supposedly was capable of $$\1300 VA\$$ (I have my doubts, but it is very large and I am not going to be using nearly so much power, so OK). He doesn't have any other info about it, so I thought I'd experiment. I have a good idea of how these devices work, so it was easy to identify "mains" side vs "secondary" side (of course they would be backwards when using the batteries). The mains side has three wires: white, yellow, and black.

I figured that the this was to accommodate a $$\120/240 V\$$ switch, but on testing them, I get $$\16.4V\$$ from the white/black (expected), but then $$\18.4V\$$ from the white/yellow, which was not expected. If my conjecture had been correct, I would have expected to see half that voltage from the white/yellow mains inputs (I would have been putting $$\120V\$$ into a tap wanting $$\240V\$$ to return $$\16.4V\$$).

So what gives? Does anyone have experience with these transformers that could point me to an explanation?

simulate this circuit – Schematic created using CircuitLab

• The APC UPS I have can select the output voltage depending on how it is programmed. There are 3 settings. UPS output voltage is usually less than mains nominal voltage, to conserve battery. Your values correspond to about 100V and 90V (assuming battery at 14V), which may be fine for many computer systems. – Indraneel Feb 23 at 21:34
• first of all you should measure resistance blk vs wht, blk vs ylw, wht vs ylw. – Marko Buršič Feb 23 at 21:37
• @MarkoBuršič Did that, this is the correct wiring. – TrivialCase Feb 23 at 21:40
• @Indraneel Thanks! So I guess the higher voltage tap gives ~110VAC and the lower ~100VAC or something like that? – TrivialCase Feb 23 at 21:42
• oh sorry, please disregard my comment. I just realized that the input ac may be less than 14V because of the PWM input. So that is an unknown too. – Indraneel Feb 23 at 21:46

I've since found that some of the APC series UPS units have these transformers as part of active circuitry in order to trim/boost output voltage. It all seems to simple now...

simulate this circuit – Schematic created using CircuitLab

"Normal" is the state in the diagram, we see that the AC only goes through XFMR1.

"Trim" is when SW1 is thrown, since then the AC flows backwards through XFMR2, subtracting from the XFMR1 output.

"Boost" is when SW2 is thrown, since then the AC flows forwards through XFMR2, adding to the XFMR1 output.

Mystery solved.

• Yes, they have 3 output voltage settings. Your method might work. – Indraneel Feb 27 at 6:01