Here is a picture of the transformer in question.


I have scrapped this transformer from a 24V UPS for experimentation purposes. All the DIY's available on internet and general transformer guides on the internet talk about transformers having 2 wires on input side, but I see more wires on the input side. What kind of transformer is this?

I want to find out what the purposes are of these wire, and how I should proceed? I want to reverse engineer it without taking it apart. I have a multimeter to probe, but searching on the internet just tells me how to find the primary and secondary which I already know. Is there anything else that can I do to know its functioning and its purpose.

I have a few more question regarding it. Since this was originally built for a 24V supply, can I get 12 volt output from it? I ask this because since the primary has many wires, I may be able to find a positive wire from the set of input which will provide 12 V output.

Will it be safe to do the following things?

  1. Connect the output to voltmeter in AC mode.

  2. Connect input side black wire to neutral terminal of 240VAC

  3. Test the output on the voltmeter by joining the remaining input side one by one to see which input will give me 12V.

  4. I have another doubt, suppose I feed 220V to the black and yellow wire, and I get 9V on output side, after applying a full bridge rectifier on the output, will there be an increase of voltage? If yes how should I calculate the final output voltage that will come after applying rectifier?

  • \$\begingroup\$ Did you measure the inputs / outputs before taking it apart? Was there a circuit diagram inside? \$\endgroup\$ – Solar Mike Jun 4 '18 at 9:12
  • \$\begingroup\$ There were no circuit diagram inside, There was just this transformer with a main board and two 12 V 7.5 AH batteries. The UPS was working properly before I took it apart. I want to see if I can convert it to 12 V inverter by scrapping mosfet and other component. \$\endgroup\$ – Ratna Jun 4 '18 at 9:17
  • 1
    \$\begingroup\$ So, before you took it apart, you did measure the input and output voltages so you know what you have ... \$\endgroup\$ – Solar Mike Jun 4 '18 at 9:20
  • \$\begingroup\$ @Solar Mike, What you say makes sense, but I didn't. Actually I went thru many diys on internet, and all of them had 2 wires for input and 3 wires output i.e 12 0 12, I had expected similar transformer but 24 0 24, never I had expected more than 2 input, I am just stuck in making any sense of the transformer, hence I came here. \$\endgroup\$ – Ratna Jun 4 '18 at 11:00
  • \$\begingroup\$ Try to reverse-engineer the original circuit between the mains input socket and the transformer primary. This part should be quitee simple. It is most likely a matter of 110V/220V configuration, possibly with some jumpers hardwired depending on the destination country, or a selectable switch. \$\endgroup\$ – dim Jun 4 '18 at 12:00

Doing a quick search on the internet gives me this

[EDIT] A quick read through the Wikipedia article, leads me to think that this is a Line-Interactive UPS:

Wikipedia Image


If you have a several-wire primary connection, there are several things it could be.

a) A pair of 120v secondaries, designed to be connected in series for 240v, in parallel for 120v. And maybe a screen/earth wire.

b) A tapped primary, so for instance 0, 220, 230, 240v taps.

c) A combination of the two above.

Your strategy should be

1) Use your meter to identify groups of wires that are connected

2) Use your meter to measure resistance between wires in each group. If you spot two groups that have similar resistances, then you may have identified a pair of 120v primaries. If the group has one pair with high resistance, and one or more pairs with low resistance, then you may have identified a tapped primary.

3) With another low voltage transformer (<40v), energise the highest resistance pair on the transformer you can find. This pair is likely to be the highest voltage, so all other voltages generated should be lower. Measure the voltage on all wires on all groups. If you have two 120v primaries, this is the time to connect them in series to see which polarity is needed for their voltages to add up.

4) Finally, having identified the 240v primary, connect that to the mains in series with an incandescent light bulb for current limiting should you be wrong. Check voltages and current drawn, before replacing the light bulb with a fuse.

A 24v transformer is overwhelmingly unlikely to be able to deliver 12v simply by messing with the primary connections, as that would entail running the core at half the flux. You may be able to find a 12v secondary winding on it as well as 24v.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.