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Computer power supply capacitors - generic question

Discussion in 'Electronic Repair' started by Sal Holland, Jun 25, 2004.

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  1. Sal Holland

    Sal Holland Guest

    I have a generic power supply that had the two main 470uF/200V caps
    fail about a year ago (physically leaking, one had the top pretty well

    I replaced them with new caps from mouser (computer grade caps and
    105c rated), and all was working well.

    This year, the power supply is again exhibiting problems..upon a cold
    boot, the computer will do about two reboots before it finally powers

    I was wondering if these voltage ratings on the caps, were too
    low..seems like they should have higher rated caps then this I would

    On the -12V line, C1 is rated 16V
    On the +12V line, C2 is rated 16V
    On the +5 or -5V line, C3 is rated 16V
    On the +5 line, C4,5 is rated 10V
    On the +3.3V line, C6, C10 are rated 10V and 50V respectively
    On the +5VSB (+5 Standby I am guessing?), C11,C12, and C13 are all
    rated 16V

    Reading from the BIOS, the computer shows these voltages:

    CPU Core: 1.69v
    +3.3 is reading 2.94V
    +5V reads 4.99V
    +12V reads 11.49V
    -12V reads 11.86
    -5 reads -5.25
    Battery Voltage (should be 3V) reads 2.88V
    Standby Voltage (should be +5) reads 4.80V

    As you can see, +12 Volts is pretty low, as is the +3.3 volt line. I
    suspect these are what is causing the computer to reboot about two

    Are the 10 volt ratings of C4,C5,C6 all too low? Seems to me they
    should have put some capacitors that had a little higher rating in
    order to better handle transient surges, etc?

    Are the 16V caps also too low for the +/- 12 volt lines? Would 25V
    have been better or perhaps 50V caps? Or is 50V too high?
  2. Art

    Art Guest

    Actually the measurements are fairly nominal but as cheap as good power
    supplies are now days I'd just replace it. At least try a new one in your
    system to see if it rectifies the boot up problem.

    Mr TUBEAMPS Guest

    i have fix a few pc supplys,
    try replaceing the caps and resistors
    that connect to the bases of the two
    main switching transistors, you see some
    values like 470k around them as well.
    check all resistors around them transistors,
    you may fine one or two near open or high.
    and check for bad soldering joints.
    these caps and resistors i call starting
    resistors and caps. be cairfull thay are

  4. RubbishRat

    RubbishRat Guest

    You haven't specified the Wattage rating of the PSU or the system you expect
    it to power. If the boot up problems are caused by the supply being
    borderline then no amount of swapping caps is going to fix it. A new higher
    power supply is going to be a lot less trouble and probably the cheapest
    solution in the end.
  5. Don Allen

    Don Allen Guest

    Switching power supplies can be very dangerous, so be careful when you
    are working on the unit. Given the prices of decent supplies today, I
    wouldn't waste your time troubleshooting or replacing caps - just
    replace the supply. Several supply manufacturers, such as Antec, etc.
    offer excellent supplies for a very low cost. Go to, or
    other good online sources, and find you best price on a 300 to 400 watt

  6. I've found that BIOS voltages aren't very trustworthy, and I've
    personally seen one differ as much as 6% from my digital meter's
    reading. Windows software readings are even worse.

    Almost all the capacitor problems with mobos and PSUs were with the
    low voltage capacitors because only they were the low-ESR types
    involved in the counterfeit electrolyte problem that plagued some
    Taiwan capacitor brands a few years ago, including Jackcon, JEE,
    Luxon, and JGE but not Jamicon or Teapo. See and for more details.

    I had a cheap PSU where the 16V capacitors for the +12V rail were
    getting 50V peak spikes, but I saw nothing that high in a better PSU's
  7. RWatson767

    RWatson767 Guest

    had a cheap PSU where the 16V capacitors for the +12V rail were getting 50V
    peak spikes, but I saw nothing that high in a better PSU's +12V.

    From one who has replaced a lot of capacitors.

    DC voltage plus signal voltage/ripple times two. In your case DC plus ripple
    times two. . Use milspec capacitors. I suspect the computer grade thing is more
    marketing hype than anything else.
    Bob AZ

    Mr TUBEAMPS Guest

    if you buy a new supply, dont throw away the old
    one, you can use this supply on other things
    like op amps.
    i built a 16 input mixer useing a pc power supply.
    to run this supply without a computer,
    connect a 5watt 100 ohm resistor across
    the 5 volt out to ground, its a dummy load.
    same for ATX supplys, to fire up an ATX,
    ground the SP-ON wire.

  9. Norm Dresner

    Norm Dresner Guest

    Shouldn't a 1/2 Watt 100 ohm resistor be sufficient? 5V / 100 Ohms = 50
    ma. 50 ma * 5V = 250 mW, right?

  10. TCS

    TCS Guest

    that's like using a used bulldozer for a boxgarden.
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