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Computer switch mode power supply suddenly fails

Discussion in 'Troubleshooting and Repair' started by Proschuno, Oct 1, 2012.

  1. Proschuno

    Proschuno

    87
    1
    Aug 1, 2011
    So suddenly my power supply ( switch mode) stops working. Now I notice whenever I would try to sink to much current from/they it, it shuts off as a safety feature. Knowing this, and my insane mind lol, I've triggered this to many times to count. But now it won't even turn on, and I'm not in the mood to spend $$$ on a supply. I know this is quite vague, but where do I even start diagnosing?
     
  2. grant1842

    grant1842

    11
    0
    Oct 1, 2012
    I have worked on computer SMPS.(ATX, etc)

    Most often i find bad caps, and all so the Bridge rectifier.

    Start by taking it apart. Check the fuse.
    If the fuse is blown then be sure to check the Bridge Rectifier for short.

    If the fuse is NOT blown then most likely CAPS.
    Look at the caps I bet you see some swollen CAPS (secondary side) .

    I have seen the switching transistors all so short to ground and blow fuses .
    Some have MosFets for switching. so check them as well.

    If you need help on how to test the single package bridge rectifier let me know I will show you.

    P.S If you have not worked on SMPS before then you need to learn how to bleed the Primary filter CAPS(THe Big Ones).
    Safety first.
     
    Last edited: Oct 1, 2012
  3. Proschuno

    Proschuno

    87
    1
    Aug 1, 2011
    Sorry I also forgot to say that it's from a Macintosh g5 if that rings a bell for anyone. I'm not finding anything that would seem to be a rectifier..... All I'm finding near the power input is capacitors and a toroidal transformer. Also no bad fuses or caps. Would it be whatever prevents excessive current flow, like a thermistor or something?
     
  4. grant1842

    grant1842

    11
    0
    Oct 1, 2012
    Can you post a picture of it . and model ?
     
  5. Proschuno

    Proschuno

    87
    1
    Aug 1, 2011
    Model is PCF451601A... I'm using an iPad so I can't post pictures.
     
  6. grant1842

    grant1842

    11
    0
    Oct 1, 2012
    Well usually dead with a good fuse you need to test , Bridge Rectifier, Power Fets, secondary Diodes, etc.

    If after checking these items and still no luck its time for some live voltage testing.
    If you do not understand how a SMPS works it is dangerous to do voltage testing.

    There is a Hot side and a cold side . Probing the wrong ground with the wrong side will get a big blue flame and ruin your DVOM :).

    I hate to guide you and you get hurt.

    Safety first . If you have not done live voltage testing on a SMPS before then do not try this.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    Or give your family the opportunity to attend your funeral.
     
  8. Proschuno

    Proschuno

    87
    1
    Aug 1, 2011
    Lol

    I also forgot to note that as it failed the fans would try to turn over then shut off.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    Look for a shorted rectifier diode, or possibly dead filter capacitors.
     
  10. djradu

    djradu

    19
    0
    Oct 5, 2012
    Every electronic gear has its limits,pushing something over does not bring any good,as in this case "playng" with the source "killed" it,the fact that has a over current protection does not make sence to blow it.Have you checked the FUSE,it has to have one,the protection circuit may be damaged and needs replacing,the crowbar circuit ( even if its for overvoltage needs to be tested,
    http://en.wikipedia.org/wiki/Crowbar_(circuit)
    http://power-topics.blogspot.ro/2008/09/over-current-protection-in-power.html

    Check the 2 links
     
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