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computer power supplies..

Discussion in 'Electronic Repair' started by larya, Apr 23, 2009.

  1. larya

    larya Guest

    Is there a group for repairing computer power supplies?..
    Question... I have the power supply out of the computer...
    Besides pluging it in... how do you turn it 'on'...
    The front panel on / off switch connects to the mother board..
    How can I turn the power supply 'on' at the power supply?...
  2. If it's ATX, ground the PS_ON pin.

    See Wikipedia for Pinout.

    Michael Karcher
  3. PlainBill

    PlainBill Guest

    Not a group per se, but the BadCaps forum - - has a section on computer
    power supplies. A large part of the discussion is about the
    deficiencies of various vendors. One vendor actually glues iron
    weights inside their power supplies so they weigh more. A heavy power
    supply must be better, right?

    Fry's Electronics sells an inexpensive 'test box' for power supplies,
    but it is very limited.

  4. westom

    westom Guest

    Do you have and use an oscilloscope? Good, now you are ready to
    start learning this stuff. Take lessons on transistors, pulse width
    modulation, diodes, filters, .... You will not believe how many
    functions are required in a power supply if you did not read its full
    page of numeric specs. To fix a power supply means you understand
    what those specs say. Implied - how many self proclaimed computer
    'experts' do not even know what a power supply does. They think it
    only provided voltage. Learn how complex a power supply really is and
    the many functions it performs.

    Of course, learn of the power supply controller – another component
    that many do not even know exists.
  5. Eeyore

    Eeyore Guest

    And be damn careful with the voltages inside.

  6. Eeyore

    Eeyore Guest

    I fixed an ATX supply by replacing 2 very obviously bulged caps on the
    output side with ones from a scrap Mobo !

  7. Eeyore

    Eeyore Guest

    Beware. It may not regulate properly with no load on the 5 or 12 V

  8. john

    john Guest

    just fixed mine yesterday
    replaceing the low imp caps on the
  9. ZACK`

    ZACK` Guest

    just fixed mine yesterday
    replaceing the low imp caps on the
  10. Bob Larter

    Bob Larter Guest

    No, but you're on-topic in here.
    You can find lots of info about ATX power supplies here:
    You need to ground the power-on signal, which is the green wire.
    If you want to use the PSU as a bench supply, you can simply snip the
    green wire from the back of the ATX connector, & solder a switch between
    it & one of the black wires.
  11. Bob Larter

    Bob Larter Guest

    On every PSU I've tried it on, the fan seems to provide enough load for
    the voltages to be okay.
  12. Baron

    Baron Guest

    Bob Larter Inscribed thus:
    A lot of the better ones have suitable loading built in ! More often
    the PSU wont start up if it requires a minimum load, though some will
    but don't regulate the output voltages properly. The regulation is
    done with reference to the 5v rail, so that is the one that should be
    loaded. Its wise to play safe and stick a load on anyway. I use a 10
    ohm 10w resistor wired into a spare connector so that I can add or
    remove it as needed. I also have a LED wired as well so that I
    remember to switch off when working on an open case.
  13. PlainBill

    PlainBill Guest

    That can work. I'm reaching the conclusion that the proce of caps is
    so low (at least here in the USA) that it is better to replace all
    caps with new quality caps. At one time I was using an approach
    similar to yours. After a while I accumulated a number of power
    supplies that APPEARED to work well with a dummy load, but when
    connected to a motherboard the result was an unstable system.

    Bulging caps are a good indication that a cap has failed. Not all
    failed caps bulge.

  14. Eeyore

    Eeyore Guest

    I've heard of some where the feedback is taken as a mix of BOTH the 5V
    and 12V loading. No guarantees though.

    I agree that 'some' load on the 5V is likely best.

  15. Eeyore

    Eeyore Guest

    Not a bad idea.

    Remember the Full Height 5 1/4's ? They took about 30-40 W.

  16. Eeyore

    Eeyore Guest

    The scrap mobo was Dell and used 'name' capacitors that still looked in
    excellent condition ( get an ESR meter if you really need to know ), whereas
    the bulged caps in the PSU were classic 'no name' off- brands.

    The problem I've found is that the caps they tend to use are unusually small
    ( or odd ) form factors. Typically tall and very small diameter. Not many
    stockists have them.

    Absolutely. Guaranteed in fact.

    Indeed. Also look for electrolyte leakage around the base. Usually a white
    deposit. It's corrosive too, so when replacing, clean it away.

  17. Eeyore

    Eeyore Guest

    Very typical.

    Also on a Mobo, check the LV regulator caps around the CPU etc.

  18. Franc Zabkar

    Franc Zabkar Guest

    I haven't repaired many AT/ATX PSUs, but IME they usually (?) regulate
    by sensing a weighted average of the +5V and +12V rails. This means
    that the +5V and +12V rails move in opposite directions.


    - Franc Zabkar
  19. Baron

    Baron Guest

    Hi Franc,
    Yes I've seen that and noticed the effect. But since the 5 volt rail is
    or used to be the most critical to circuit function that is the one
    that is usually most heavily controlled. You can pull the 12 volt rail
    voltage around quite a bit without much effect on the 5 volt one.

    A 5 amp step load on the 12 volt rail (ignoring transients) has almost
    no effect on the 5 volt rail. The reverse is not true though as you

    But its very much academic if the PSU is functioning correctly. Bad
    caps seem to be very much the cause of most PSU ills judging by the
    number that seem to blow up with regular monotony ! Though I don't
    bother repairing them nowadays, it being more cost effective to throw
    them in the scrap metal bin and fit a new one.
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