Electrical – Old ATX PSU: Could +5VSB have a minimum amps requirement

atxpower supply

I'm trying to use a small ATX power supply (version 1.0 connector) as a bench power supply. I've found that it only turns on intermittently. I've tried with ~1A load on +5V, and 0.2A on +3.3V .

I accidentally shorted +5VSB to ground, and it turned on. Now I find I can reliably turn the power supply on by momentarily attaching a load to +5VSB, maybe about 0.2A.

Is it possible that this PSU needs a load on +5VSB to start up (but not to keep it running)? An "ATX Design Guide" online lists only 0.0 amps as min current on +5VSB. Is it more likely that the PSU is damaged?

Best Answer

When you just connect your ATX PSU to mains AC, no load, the +5VSB rail should be at +5V, and the PS_ON should be HIGH, either +5V, or +3.3V.

You are saying that +5VSB is normal (+5V and holds some load), but the voltage on PS_ON (green) wire is 0.7V. If the PS_ON is LOW (0.7V as you say) from the very beginning, the PS_ON circuitry is damaged/shorted, possibly by ESD. This pin has a simple pull-up, and must be shorted to ground to turn the PSU on. It is possible that, even when damaged, the remaining PS_ON circuitry needs a power toggling to turn the rest of PSU rails on.

In modern ATX PSUs the PS_ON is managed by a separate specialized IC, and the PS_ON is a simple input, no other active components is there. If the pin is damaged, it can't be repaired, and only IC replacement can fix the problem.

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