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I was recently using an old 375 watt PSU as a benchtop power supply and managed to break it trying to disable the short circuit/overvoltage/undervoltage protection. Meh. Anyway, naturally I took it apart and started desoldering the components. I pulled a transformer out that was mounted to the board with screws with 3 wires coming out and being soldered to the board elsewhere. The transformer had a sticker on it that reads on 2 lines "37-C6371104 ROHS HP OK/0706 SC REV.B" I did a quick google for the hyphenated number but didn't come up with anything. It was taped up with yellow tape and under the first layer was some thin copper foil wrapped around in a single layer, under that was the coils and iron core. There are 3 wires, blue, black, and white. It looks like the blue and white wire goes to one of the coils and the black wire goes to the other, which confuses me. I'm trying to identify the rating of it and I'm nervous to just hook it up to 120VAC and use a multimeter. If anyone could help me figure it out, that'd be awesome. I can take a picture of it if that would help.

Hopefully someone can shed some light and thanks in advance!

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    \$\begingroup\$ Unless "an old PSU" means from the seventies, that transformer is meant for switch mode conversion and is custom-wound for that specific PSU, designed to run at around 100 kHz. There's little you can do. \$\endgroup\$
    – pipe
    Aug 2, 2016 at 13:12
  • \$\begingroup\$ I thought there was something weird about it. Mainly that the two windings seem to be exactly the same size and same gauge wire. I guess I'll need to continue my search for a sufficient transformer for another project. Thanks :) \$\endgroup\$
    – HaLo2FrEeEk
    Aug 2, 2016 at 13:14

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Based on your description, chances are that it is a filter inductor to keep the high frequency switching noise from backing up into the power mains. Of course each winding has at least TWO ends, so there is a fourth "wire" somewhere. Perhaps one of the pins that was soldered to the PC board?

Like virtually ALL switch-mode power-supplies (SMPS) the transformers and inductors are FULL CUSTOM parts designed and manufactured specifically for THAT model of power supply. You will not find any information on the inductors because they were never sold on the general market. They were OEM custom parts.

ABSOLUTELY NOT RECOMMENDED to connect this to the power mains until you understand A LOT MORE about what it is and how it works.

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I know, i'm late, but I have wrecked open another powersupply and it has a "transoformer", too. Similar in P/N: 37-C5251601

I think it was mounted to the top of the PSU. Thats because it is not a Transformer :D

It is a "choke", mine even only had two wires. It is used to block high frequency ac and only let trough DC and low freq AC.
It is mounted to the top, so the magnetic field doesn't interfere with the rest of the circuit.
Just to help future lookers!

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Better late than never...

It is actually a choke designed to help improve the power factor of the power supply. This is known as "passive" power factor correction.

More modern computer power supplies almost always use "active" power factor correction and would not contain such a large choke. Active PFC is more effective and less costly to implement than passive PFC.

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