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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?

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  • \$\begingroup\$ I've never seen an ATX power supply with minimum current specified for the +5VSB. You could load up to 1 A or more on the +5VSB to make sure that it's working (a short is a bit severe). I'd suggest you probably have a fault in the turn on circuitry. PS_ON# should measure 5 V before you use it to enable the PS. If you pull PS_ON# low with a multimeter it should draw about 1.6 mA. \$\endgroup\$ – Jack Creasey Feb 10 '17 at 3:36
  • \$\begingroup\$ I had a resistor and LED on +5VSB and it seems to work properly. That's what initially shorted against the case ground. I don't short it any more, usually use 17 ohm resistor. The LED and resistor alone I guess are not enough of a load. I was thinking of measuring a minimum load that will have it turn on. \$\endgroup\$ – user209759 Feb 11 '17 at 3:57
  • \$\begingroup\$ I've measured PS_ON and it is only 0.7 V while the PSU is off. I am getting a negligible mA reading with it on, however that is with a 10A meter input (500mA doesn't appear to be working at all). \$\endgroup\$ – user209759 Feb 11 '17 at 4:01
  • \$\begingroup\$ ... The ATX Design Guide says PS_ON should be 0-0.8V when pulled low. It should be 5V when open circuit. When open circuit, PS_ON seems to work properly (PSU won't turn on, even if +5VSB is shorted), yet it measures like it's pulled low???? So something's wrong with PS_ON? \$\endgroup\$ – user209759 Feb 11 '17 at 4:09
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    \$\begingroup\$ Adding the extra load on +5VSB (to force the power on) drops it from 5.011V to 4.983V. I think +5VSB is fine (Design Guide says overcurrent protection is required on it, so I don't think I damaged it). The PS_ON componentry is the only thing so far measuring out of spec. The PSU turned on properly occasionally before, so I think the .03 V change in +5VSB is enough to overcome the fault. \$\endgroup\$ – user209759 Feb 11 '17 at 7:08
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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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  • \$\begingroup\$ Yes, it seems like the PS_ON circuitry is damaged. I don't think the PSU is very modern. It is a Samsung SFX-108 C Rev B2. I couldn't find a schematic and I would need to remove the PCB from the case to find what PS_ON is connected to. I'm curious to see if I can find damaged parts, but will probably not risk messing with it. For now it works, other than having to "jump start" it. Thanks. \$\endgroup\$ – user209759 Feb 15 '17 at 22:24
  • \$\begingroup\$ @user209759, the same website has older schematics as well. I did check several other schematics, the PS_ON also comes directly into some controller chip. My advice - throw this PSU into recycling, and get a new one. \$\endgroup\$ – Ale..chenski Feb 15 '17 at 23:51
  • \$\begingroup\$ Even in the modern schematic that you linked, PS_ON (labelled "ON/OFF"? in the PS_ON# circuitry in lower middle) is only connected to a resistor, as with other schematics I've glanced at. I figured I might have luck finding a damaged resistor? (or cap, such as C26 in the example) Again, I won't risk trying this without a correct schematic. I plan to keep using the power supply since its voltage is closer to spec than another older AT supply I have, and I am assuming that the damage is limited to the PS_ON circuitry. \$\endgroup\$ – user209759 Feb 17 '17 at 17:18
  • \$\begingroup\$ @user209759, it is very unlikely that any resistor or cap is damaged, although it happens sometimes. The problem is likely in silicon pin, inside the control IC. \$\endgroup\$ – Ale..chenski Feb 17 '17 at 17:35
  • \$\begingroup\$ You're probably right. The same chip is used for OV protection in the example, so maybe it's best to take your advice to recycle it. \$\endgroup\$ – user209759 Feb 17 '17 at 17:45

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