I have this big transformer taken from an old UPS. I wanted to use it to build a linear high current 12V power supply, so I was testing if it was suitable for the task. Unfortunately it has no ratings, but I assume it would be capable of delivering at least 10A. Here are a couple of pictures of the transformer: https://i.sstatic.net/92mCX.jpg

These are the resistance of the windings:

  • grey-violet 1.61 ohm
  • brown-blue 5.22 ohm
  • black-red(right) 0.15 ohm
  • white-red(left) 0.15 ohm

I connected brown-blue to the mains (230V) using a regular PC cable (rated 2.5A 250V) and took some measurements. In this configuration, both black and white with the respective reds were 13,8VAC, but grey-violet was out of range with my multimeter (UT61E).

Then I tried to connect grey-violet to the mains, and this is where the PC cable immediately burned.

My question is, why there was this big current absorption if there was no load connected (just my multimeter)? And why only when I connected grey and violet? Is this transformer still safe to use, provided that the resistance measurements remained the same and its cable show no signs of burning (maybe the 2.5A cable acted as a fuse)?

  • 1
    \$\begingroup\$ If you burnt the cable, odds are you damaged transformer windings. \$\endgroup\$ Commented May 24, 2021 at 7:45
  • \$\begingroup\$ At 1/3 of the impedance you could reasonably expect 1/sqrt(3) the voltage or in the range of 130VAC. What range was your DMM on? And what possessed you to try connecting it to mains? The transformer is probably now unsafe to use. \$\endgroup\$
    – user16324
    Commented May 24, 2021 at 12:06
  • \$\begingroup\$ the DMM has autorange, it should be capable of measuring 130V without problems, so I think the voltage was much higher. I just don't understand why, since this was just a UPS transformer. \$\endgroup\$
    – Marc
    Commented May 24, 2021 at 14:21

1 Answer 1


My question is, why there was this big current absorption if there was no load connected

Because you connected a low voltage winding to 230V. It's not the resistance of a winding on a core that stops excessive current but its inductance. Overvoltage saturates the core before a half-period is over and before the core gets demagnetized again by the negative voltage half-period. So the inductance for all the additional voltage-time in an AC half-period drops to nearly zero and the only thing that limits current is the winding resistance.

  • \$\begingroup\$ So how do I understand what is the purpose of that winding? I thought it was for converting the 12V from the lead batteries of the UPS to 110V, while the other two were for 230V. Also, do you know why I got out of range when measuring grey-violet while brown-blue was connected to 230V? \$\endgroup\$
    – Marc
    Commented May 24, 2021 at 7:51
  • \$\begingroup\$ Do grey–violet and brown–blue have the same cross section? If yes, brown-blue is a 230V winding and grey-violet is a 80V winding. Computer racks in the US often have three-phase 480V mains = 277V live to neutral voltage. They need both windings connected in series then. Leave open for 230V. \$\endgroup\$
    – Janka
    Commented May 24, 2021 at 8:41
  • \$\begingroup\$ Thank you a lot for your help. Is there a way to know if they have the same cross-section without disassemblying? All I see is that they come from the same "side", but everything is immersed in that yellow-ish glue(?). The UPS was sold in Italy (we have 230V here), but I don't have any other information, only that it was connected to a regular pentium3 PC. \$\endgroup\$
    – Marc
    Commented May 24, 2021 at 9:01

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