I have a power transformer that I pulled out of a UPS a couple of years ago and I'd like to re use it in a linear power supply.

It has 2 wires on one side and 3 on the other. Currently, I don't know which side is primary and which one is secondary but I suspect, maybe wrongly, that the side with 2 wires is primary. The side with the two wires has screw terminals (a ring to put a screw through) and the other side has plug terminals that if I remember well used to plug to the board.

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Now, since I'm not sure about a lot of things about this transformer, I don't want to just wire it up and measure it while connected in the mains, especially since I'm not very experienced with mains voltages. (I do work servicing 3-phase dimmers but it's different when you have the safety measures we have at work).

My thought was to connect it to a signal generator and scope (I have a USB combo) and see if the other side is of higher or lower voltage in order to identify the primary and secondary sides. Since this was part of the power supply of a UPS, in theory the primary should take in 230V and the secondary should give out a voltage that when rectified would be sufficient to charge 12V batteries + maybe a second secondary for +5V logic(?). So I estimate that I expect a 20something:1 drop in voltage on one secondary and a 50ish:1 on the other.

To formalize this question a bit:

How do I safely measure a power transformer using a signal generator and scope without damaging my equipment?

Is a 1V (peak to peak), 50Hz sine wave signal sufficient/suitable for the measurement?

I suspect I need to measure the secondaries across a load. Will a resistor around 1MΩ (1/4W) be sufficient (or necessary)?

Finally, please let me know if I'm moving in totally the wrong direction.

  • \$\begingroup\$ I measure 0 Ohms on the side with two wires, and on the other side (white, yellow, brown), I have 1.8Ohms between white and yellow, 2.1Ohms between white and brown and 0.3Ohms between yellow and brown. \$\endgroup\$ Commented Dec 16, 2017 at 9:19
  • 1
    \$\begingroup\$ Most likely designed to support both 115V and 230V operation then, a common arrangement. \$\endgroup\$
    – user133493
    Commented Dec 16, 2017 at 9:26
  • \$\begingroup\$ Ah, that would make sense \$\endgroup\$ Commented Dec 16, 2017 at 9:27
  • \$\begingroup\$ Size and weight of the transformer? Maybe a picture? It is possible that it's a lightweight transformer that will only work at high frequencies in a switch-mode power supply. In which case using it at 50 or 60Hz would be disastrous. \$\endgroup\$
    – user16324
    Commented Dec 16, 2017 at 10:14
  • \$\begingroup\$ @BrianDrummond - Picture added. The weight is pretty large. I'd say maybe 5kg, maybe more. \$\endgroup\$ Commented Dec 16, 2017 at 11:15

2 Answers 2


Use a multimeter instead. Identify the primary with higher resistance winding and secondary, thicker wires, lower resistance. Connect the primary on 220VAC and then measure the output at secondary. That's all, you don't need hi-tech instruments.

Weight the transformer and you will find approximately what is the nominal power of it.

  • \$\begingroup\$ Hmm. If I get what you're saying, it seems I have 2 primaries. \$\endgroup\$ Commented Dec 16, 2017 at 9:26

My advice is to use incandescent lamp in series of the winding, when you are connecting it to mains. If everything is ok the lamp will not light up and it's non-linear resistance will be quite low to neglect voltage across it. In case of any error you'll see just normal light of the lamp, neither smoke nor other bad things.


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