varying voltage from PC power supply

Discussion in 'General Electronics Chat' started by scary, Sep 14, 2011.

  1. scary

    Thread Starter New Member

    Sep 14, 2011
    Hello all, I'm new here & was hoping someone could help me with the DC power supply I'm trying to make. You probably know the sort: a PC power supply with some posts and LEDs stuck on the front\top.

    Well, mine's not an ATX one and I'm having an of issue with variable DV voltages on the +12V line. From the annotation on the circuit board I've worked out the following colour-wire code:

    black: ground
    red: +5V 18A
    yellow: +12V 7A
    blue: -12V 0.3A
    white: -5V 0.3A
    brown: +3.3V 6A & one sense wire
    green: +5VSB 1A
    orange: power good
    purple: DC on/off

    I've connected the green & purple wires to ground and the brown sense wire to another brown wire. When I'm testing it on the bench with 12V 21W automotive bulbs on both +5V and +12V lines and a CPU fan on the +12V too, the voltage on the 12V line varies quite a lot. Like this:

    what's connected 5V line 12V line
    5V line 12V line V A V fan amps bulb amps
    bulb 4.5 0.2
    bulb bulb 5.25 0.2 9 1.43
    bulb bulb & fan 5.25 0.2 9 0.05 1.4
    bulb fan 5.2 0.2 11.5 0.08
    fan 4.7 0.02

    Its been reported that these PSUs need a load on the +5V line to operate and most 'how-to's recommend the use of a 10ohm, 10W power resistor. I'm not sure that the bulb I'm using on the +5V line produces enough load to keep the power supply running. The bulb only has about 1 ohm resistance, measured by DMM, but at 21W I thought it would be adequate. I've tried running three of these in parallel to up the load but the variable voltage issue persists. Can anyone tell me if I've done something wrong that results in these variable voltages or is the PSU duff?

    (I should confess to not knowing if this is entirely normal behaviour for PC PSUs. If so, doh.)
  2. Tealc


    Jun 30, 2011
    I may be way off here but I thought that the 5v load thing was only for much older computer power supplies.

    According to the ATX standard voltages are supposed to be within 5% of nominal, although I'm not sure of what loads this specification comes into effect at. Seeing as yours isn't ATX could mean that it's subject to different tolerances but I would have thought it needs to be closer to nominal than 3v out.

    Sounds like a duff one to me.
    Last edited: Sep 14, 2011
  3. tom66

    Senior Member

    May 9, 2009
    In my experience very few power supplies now have a minimum load on the output; they all have built in load resistors.
  4. scary

    Thread Starter New Member

    Sep 14, 2011
    Thanks for your replies, I have just been replacing four electrolytic capacitors in the PSU and resoldered a large resistor that appeared to be loose. Now when I turn the supply on the +5SB line works okay but when I connect the purple 'power on' to ground the fans twitch or turn for a second and then stop. I'm going to open it up again and check for shorts.

    Another unusual feature of this PSU is the dedicated (-) sense wire that I'm assuming the +3.3V sense wire should connect to? This is confusing me as I've read elsewhere that the (+) sense wire should be connected to another +3.3V line. Should I bundle all three; +3.3V, (+) sense & (-) sense together?
    Last edited: Sep 25, 2011
  5. oldtech33709

    New Member

    Sep 24, 2011
    The (+) sense connects to +3.3V, the (-) sense to the return for the 3.3V.
  6. DkEnrgyFrk

    New Member

    Feb 15, 2010
    The orange wire is 3.3V right... What is powered by that? Heat/fan sensors?
    I read somewhere that you should run prime95 to put the computer at full load to run a more accurate test on the psu. Does that make sense here?