Which wires do you short to turn on a computer PSU?

[zahc]

I have this old 250W PSU I harvested out of a Dell that was left by the dumpster. For all I know, it was the part that was bad. But if it works, it will be perfect for my HTPC, once I put a quieter fan in. But the thing doesn't have a power switch on the back, only a line voltage selector. Can I short a couple pins to turn the thing on, then test the voltages before I plug it into my motherboard?

I suppose I could just plug it into my motherboard and hit the power switch on the computer, but that seems a bit risky.

[Brad Johnson]

What kind of power connector?

Brad

[Marvin Dao]

I'm assuming it's an ATX supply since AT supplies have separate lines for power on/off.

If 20 pin, short pin 14 to any black wire pin.

If 24 pin, short pin 16 to any black wire pin.

Whatever pin it is, the wire to it should be green or gray. To verify that it's the right pin, plug the supply in and probe that pin. It should read 5VDC referenced to a black wire pin. All other pins should read 0VDC.

Oh yeah, it may be a Dell specific unit and have a Dell specific connector arrangement. The physical connector will still look the same and plug in normally though. You should be able to differentiate it by checking pin 11. On a standard ATX supply, pin 11 will be a yellow +12VDC line. On a Dell specific supply, it's a gray Power On line and will read +5VDC if the supply is plugged in. If that's the case, it's probably better to not use it for this application.

[zahc]

Thanks for the info. I shorted the pin to ground and the psu turned on. All the pins were as shown here

http://www.helpwithpcs.com/courses/power-supply-basics-inc-pinouts.htm

Except there was no PWR_OK wire at all. One of the 12V rails (forgot if it was + or -) was 11.4V though, while the other one was right about 12. Is 11.4 low enough to be a bad power supply?

[Marvin Dao]

Except there was no PWR_OK wire at all. One of the 12V rails (forgot if it was + or -) was 11.4V though, while the other one was right about 12. Is 11.4 low enough to be a bad power supply?

It's right on the edge of the spec (+/- 5%). Questionable performance, but likely still usable unless it sags significantly more when you load it.

[Nick1911]

On a side note, some PSU's don't work right without some loading on the rails.  Plug an old hard drive in or something, then try testing.

[zahc]

I guess I'll try it out then. That's great; I just moved my computer from the living room into the lab, away from the TV. But if this motherboard's onboard graphics turn out to be good enough for 2D it looks like I can build a usable computer with only spare parts. Except a case. I wonder if I could just build one out of foamcore or something.

[Gowen]

Man, I thought I was old for having an old Commodore 64.  It took an hour for the tape drive to load a program.  Then only to have it crash and have to start all over again.  Hey, it was still faster than a 486 at 28.8bps surfing on AOL.