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I have a 120mm 2-pin fan from an old power supply that I wish to use as a case fan and the closest fan model I can find that matches my fan in appearance is this; however, the color of the wires in the 2-pin fan connector shown are actually swapped in my case.

How can I be sure of this without breaking the fan, PSU, or testing device?

  • I do know where to connect my fan; I have a Molex-to-Fan-Header connector with me that I can connect to my PSU
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  • Use a volt/multimeter to check the voltage (for DC, black is the standardized ground color, whereas for AC it's live: black = death for AC)
    – JW0914
    Commented Apr 1, 2022 at 11:47

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It's extremely unlikely that any fan manufacturer used a red wire for negative and black for positive.

But I have to say this: unless you can find an exact model number match you can't even be sure if the fan takes 12 volts rather than 5. Or something else entirely.

Does the old power supply the fan came from still work? If so, plug in the fan where it was and check the polarity on its power pins with a voltmeter.

Failing that, just connect it in the standard way to a power supply and see what happens. PC power supplies are extremely hard to kill. The fan itself would probably be ok if reverse-powered too. And if it isn't... what have you lost, really? Case fans are cheap.

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Not to mention that most fans are reversible. It is your option to have a negative pressure or positive pressure case. Also a ac fan reverse wired will just run in the opposite direction with no harm to the fan at all.

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  • AC and DC fans are nothing alike - PCs always use DC fans (the only voltage that's AC is the incoming line into the PSU which transforms it to DC for the electronics), whereas AC fans are used in buildings.
    – JW0914
    Commented Apr 1, 2022 at 9:33
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Look for the directional arrows on the edges of the fan housing. The fan will be spinning in the opposite direction if the polarity is incorrect. If the fan is DC brushless though, the fan will not spin in reverse.

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    This is very unlikely to be true. Fans are not simple DC motors -- about 95% of them are brushless with a control IC. Reversing the polarity will typically do nothing (if the fan has reverse polarity protection) or destroy the fan (if it doesn't).
    – David Schwartz
    Commented Sep 12, 2016 at 6:22
  • I'm not sure that reverse polarity would destroy the fan. I'll have to try it on a few of my spares. But aside from that the only quibble I have is that I would change that percentage from 95% to about 99.9%. I really don't think there are ANY small DC fans used in PCs, or in PC PSUs, made with other than DC brushless motors. Which, unlike the little permanent magnet motors that used to be used in hobby work, absolutely will NOT run backwards if reverse polarity is applied.
    – Jamie Hanrahan
    Commented Sep 12, 2016 at 11:30
  • Edited the answer - Tried on a couple of fans, and you are correct - Apologies.
    – JohnnyVegas
    Commented Sep 12, 2016 at 13:36

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