Hello all, I'm 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:
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:
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 on the +5V line to up the load but the voltage fell to ~3V and the output on the unloaded 12V line went up to +17V! 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.)