PC power supply

Discussion in 'General Electronics Chat' started by mrosh, Nov 25, 2009.

  1. mrosh

    Thread Starter New Member

    Nov 25, 2009
    3
    0
    Hi. I am a newbie in electronic. I have a Compaq Evo PC, which has that long power supply unit with the standard 20 pins connector to the motherboard and the other 4 pins connector.
    Recently, it developed an electronic fault whereby it powers on in an instant then it goes off.
    I took it out and opened it, tried to discharge the main electrolytic capacitor first and it didnt indicate any high voltage, so i assumed it could be the capacitors which are faulty, changed them some new ones and still the problem was the same. it powers on in an instant, then switches off, any assistance please
     
  2. bill l

    Member

    Oct 11, 2009
    30
    0
    are you sure your motherboard is working properly?

    if you used another power supply to verify your mother board is working properly, then you can deduce that the fault lies with the supply obviously.

    now if the evo supplies are like any other supply, the +5 volt circuitry in the supply will either turn the supply off if there is not a load on it, or keep it running if there is the proper load on it.

    your caps may have been discharged due to the fact that the clock driving the supply's transformers is not being turned on and producing the dc on them, therefor, turning your supply off.

    computer supplies are somewhat worthless to repair if they are under 300 watts now days.

    your best bet, replace it.
     
    Last edited: Nov 25, 2009
  3. mrosh

    Thread Starter New Member

    Nov 25, 2009
    3
    0
    The motherboard is okay, because i tested the power supply with another load other than the motherboard and powered it on, still it behave the same. The problem with replacing it is that it is hard to find the exact power supply around here
     
  4. bill l

    Member

    Oct 11, 2009
    30
    0
    i'm not familiar with the evos. i tend to stick with standard atx form stuff.

    good luck to you on this as schematics for computer supplies are not easily found and if you aren't familiar with switching supplies, that makes it all the more difficult to work on.
     
  5. mrosh

    Thread Starter New Member

    Nov 25, 2009
    3
    0
    Thanks 4 trying 2 help
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Testing the power supply on a different load does not mean it will work correctly with the motherboard because different loads behave differently.

    Have you tested all the outputs of the power supply?

    If you don't know how to repair it, don't play with it because then you will need a new motherboard too. ;) Just buy a new one.
     
  7. bill l

    Member

    Oct 11, 2009
    30
    0
    well, that must be a new one on me because every computer power supply i have ever messed with would not work properly if there was an issue with any of the supply voltages to the motherboard.
     
    Last edited: Nov 26, 2009
  8. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    I didn't say that if the power supply works fine with a static load (load current is constant) it will not work with a dynamic load (load current varies). It may work but it is not the same thing to test power supplies with static loads and then expect it to work perfectly for a dynamic load. When the current changes rapidly in a dynamic load then the power supply must adapt its internal state as to supply that current and keep the output voltage constant the same time. This has to do with its transient response which is very important for rapid current changing loads. If the transient response is poor (slow) then the output voltage will change during a rapid current change and the power supply may detect an over/undervoltage and shut down. Another issue with dynamic loads is the stability of the output voltage. If the power supply is not properly designed then it may become unstable with its output oscillating during a rapid load current change.
     
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