varying voltage from PC power supply

Thread Starter


Joined 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.)


Joined 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.
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Joined 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.

Thread Starter


Joined 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?
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Joined 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?