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atx power supply

Discussion in 'Electronic Repair' started by cm, Oct 20, 2006.

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  1. cm

    cm Guest

    I would appreciate it if someone would tell me how to test an atx power
    supply with digital volt meter. (how to rig the atx plug to turn on the
    supply so I can check it)
    Thanks a bunch,
  2. I guess it depends how rigorous a test you want to do.

    To do it right requires a bunch of high-power adjustable resistors.

    To do it at all usually requires at least one substantial load
    resistor, as most switching power supplies don't run at all with no

    And you do have to load down all the outputs, as a supply may be able
    to put out 12 volts with no load, but not with any realistic load.

    For very rough starters, get a 1 ohm 50 watt resistor and place it
    across the 5 volt output, that will draw 25 watts. Or a half-ohm
    resistor, that's 50 watts. Or more of these in parallel to draw
    whatever wattage you want to test at.

    Same for the 3.3 volt output, only there a one ohm resistor draws about
    10 watts.

    For the +12 output, a two ohm resistor draws about 70 watts, a one
    ohmer 144 watts.

    For the low-current outputs like -5 and -12, a 10 ohm resistor draws
    2,.5 watts and 14.4 watts respectively.

    Oh, there's also the always-on outputs, be sure to load them too.

    the formula is watts = voltage squared over ohms.

    or watts = current squared times ohms.

    Go ahead and have fun, just don't burn yourself on the hot resistors!

    pins., then ground the remote startup pin for a second, then go ahead
    and measure the voltages.
  3. cm

    cm Guest

    Thank you, you've helped a bunch. I had forgotten about the remote
    startup... thanks again
  4. Heavy G

    Heavy G Guest

    On the atx connector theres a green wire. take a paperclip or small piece of
    wire, push it into the hole of the green wire, and then to any of the black
    wires. the unit will turn on. you can check the voltages, remember, yellow
    wires should be +12v red +5v and I believe orange is -5v. but. to properly
    test a ATX supply, you should get a ATX tester. it can test it for you. and
    without any load on the ps, it will be hard to diagnose any voltage
    problems. by itself the voltages might look fine but when actually under
    load, they can be way off. when ti is under load it should still be within
    the tolerance for + and - if it isnt then its time to replace it. if the
    voltages are under while running the hardware normally, you could do damage
    to it. same if they are over beyond tolerance.

    ive lost a hard drive due to a faulty PS over voltage damaging the drive. so
    better to be safe than sorry. maybe someone else here has alot more
    experience with testing them to be able to offer other suggestions. :)
  5. Jim Land

    Jim Land Guest

    So you load down the outputs and turn on the ps and measure the
    voltages. If one of the voltages is off, you'll know the ps is bad.
    But what if all the voltages are within spec? Can you conclude that the
    ps is OK? No, you can't! Because all the voltages may be OK for hours,
    then one of them has a glitch. Your PC may not run well under those
    conditions. And you may never see it, even if you stare at your
    voltmeter for hours. Conclusion: substitute a known-good ps. (Power
    supplies are cheap.) If your PC problems go away, be happy.
  6. w9gb

    w9gb Guest

    You did not happen to read thsi group's FAQ?

    Sam Goldwasser a web page -- Troubleshooting and Repair of Small Switchmode
    Power Supplies.
    A little education is always useful.
    Also remember that switch mode power supplies -- have high voltage ( > 300
    volts) inside - that is part of the design!

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