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  1. #1
    Senior Moment Tony's Avatar
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    Default Power supply question

    If I have a naked PSU on the bench top and connect it to power, when I turn it on, should I see the fan move?
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  2. #2
    VoidMaster
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    Default Re: Power supply question

    You need to have a 12volt car bulb as load on the 12volt line between red and black negative wires and activate the power on pin 14, by putting the green wire to black negative wires.

    If the PSU is good you'll see the fan spinning.

    Do not be tempted to open the cage unless you know what you are doing, power capacitors will store the charge for ages if their discharge resistors are open-circuit, and they often are, offering the possibility of a nasty shock and dropping heavy things on feet etc.
    Last edited by zqwerty; 21-06-2021 at 03:30 PM.
    It's not the least charm of a theory that it is refutable. The hundred-times-refuted theory of "free will" owes its persistence to this charm alone; some one is always appearing who feels himself strong enough to refute it - Friedrich Nietzsche

  3. #3
    Senior Moment Tony's Avatar
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    Default Re: Power supply question

    Quote Originally Posted by zqwerty View Post
    You need to have a 12volt car bulb as load on the 12volt line between red and black negative wires and activate the power on pin 14, by putting the green wire to black negative wires.

    If the PSU is good you'll see the fan spinning.

    Do not be tempted to open the cage unless you know what you are doing, power capacitors will store the charge for ages if their discharge resistors are open-circuit, and they often are, offering the possibility of a nasty shock and dropping heavy things on feet etc.
    Thanks for that. I've just about reached the limit of my electrical knowledge here, so this follow-up question may be silly:
    If I put that PSU into a PC case and connect the 4-pin ATX 12V plug and the multi-pin EATXPWR plugs on the mobo, and power on, what should I see then?

    Context: I blew up a PC the other day. See this thread's latest posts 2980-2982. I'm trying to establish what is actually defunct. I have a spare power supply that I believe is good, but when I connect it and try to power on I get the green LED on the mobo, but nothing else happens.
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  4. #4
    Enterprise IT Consultant chiefnz's Avatar
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    Default Re: Power supply question

    Check that the motherboard is not making contact with any bare metal inside the case... there could be a short somewhere. Also double check all your PSU connections...start with just the motherboard/CPU and a GPU if the CPU doesn't have integrated graphics.

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  5. #5
    Senior Moment Tony's Avatar
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    Default Re: Power supply question

    Thanks for the feedback everyone. I guess my primary question is: how can I verify that my spare PSU is actually working? (I'm assuming the one with the melted wires is definitely munted )
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  6. #6
    VoidMaster
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    Default Re: Power supply question

    With wires burnt and insulation melted like that the associated PSU has probably given its last into a solid short circuit somewhere in the circuits it was supplying, so be careful. as soon as you connect the short circuited board/psu/dvd player/motherboard the problem will occur again until you locate the faulty whatever using an ohm-meter.

    Theoretically the PSU should be shot-circuit protected and may still be ok, can't say for sure but you'd have to replace the melted wires etc to get it going again, probably not worth it and potentially dangerous if you do not have experience repairing mains powered printed circuit boards.

    I told you what to do in my post above to verify the spare PSU. Not much use though, finding that it is good, only to put it in place of the original one with the now burnt out wires and do it again without finding the cause of the excessive current flow.
    It's not the least charm of a theory that it is refutable. The hundred-times-refuted theory of "free will" owes its persistence to this charm alone; some one is always appearing who feels himself strong enough to refute it - Friedrich Nietzsche

  7. #7
    amateur expert dugimodo's Avatar
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    Default Re: Power supply question

    There's not really a simple way to do much more than verify a PSU will turn on and output power without some test instruments most people don't have and zqwerty already described how to do that. Otherwise you need to connect it to a working pc to see what happens which could be risky.
    Shorting the power on pin to ground as zqwerty described should power on the fan but it is best to have a load as he mentioned with the bulb idea. You could also put a multimeter across the various power supply voltages to verify them.

    This might explain better about how to start the PSU https://www.overclockersclub.com/gui...x_psu_startup/
    And this https://www.lifewire.com/atx-24-pin-...pinout-2624578
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  8. #8
    Senior Moment Tony's Avatar
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    Default Re: Power supply question

    This is all definitely getting above my pay grade. I'm going to make an executive decision to not worry about it. The munted PC was on the list to be replaced/scrapped so it has just shot to the top. When/if I replace it I'll worry about everything else then.

    Thanks again for the advice.
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  9. #9
    Senior Member 1101's Avatar
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    Default Re: Power supply question

    All you can do is verify it turns on

    Faulty power supplies can still turn on & spin up the fans. Can have intermittent faults that cause the PC to shutdown/restart every other day (or week)

    They need to be scoped under load. A meter wont show if the output is clean or dirty (spiky)
    If in doubt , its best to just replace it .

  10. #10
    Senior Member piroska's Avatar
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    Default Re: Power supply question

    Quote Originally Posted by Tony View Post
    : how can I verify that my spare PSU is actually working? (I'm assuming the one with the melted wires is definitely munted )
    No reason it shouldn't be....
    Plug it in, if you killed the motherboard then not much will happen in the way of booting or bios info, the PSU will go....

    Or buy one of those little gadgets
    https://hungrypc.co.nz/product/atx-power-supply-tester/
    Ex-pctek

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