Getting A 20 Pin Standard PC Power Supply Working

The switched-mode power supplies used for personal computer applications often need to see a load before you get a measurable output voltage. If you put a meter on the open output pins, you won't read proper values.

I learned that once by trying to test a PS that I thought might have a failed output in a PC I owned, but when I tried to measure it disconnected from the motherboard, none of the outputs worked. It needed the board plugged in to operate.

Do a Google search on "power supply tester" and you will find plenty of options, many of which cost only $10-$15. I have one (not here at hand, so I can't tell you the brand), but it works great - it has LEDs that light for each of the different voltage outputs to verify that they all are at proper voltages.

How To Manually Test a Power Supply With a Multimeter

Don't short anything yet, what are you trying to do?

Are you trying to power a hard drive directly (with or without a PC motherboard connected) instead of using the hard drive power supply plugs provided with the standard PC power supply?

I got it to work. I just shorted the green wire to com and the power supply switched on. It has a sata connection for hard drives and it worked well. I bought a proper power supply as I had heard reviews of the power supplies with the cheap USB to sata kits exploding and destroying the hard drives.

Right, so you were after the wiring connections to simulate the computer start button.

Glad to hear it. That jumper just lets the PS know that it is "properly" installed.

Modern computers with soft start switches no longer turn the power supply on and off as they used to. The power supply is always "on", but is not supplying full power since there is no load.

When the PC is plugged in there is normally a power present indicator of some kind, usually on the moboard.

The soft start or logical power button feature allows the BIOS to take charge of power management (think green!) and reduces the hard power-on/power-off used in older computers. That's why we used to have "reset" buttons on our computers as well...to avoid all of those hard power-ons (required by Windows reboots). Reset buttons are not really needed anymore. The multifunction logical power buttons do both jobs now.

When you connected your meter the load was created...by the meter. That's why you can test/use the power supply without having it actually plugged into the moboard (with the jumper, of course).

Although it seems to be working, it would probably be a good idea to use a voltmeter to check the voltage levels. As another poster mentioned, many of these supplies do need a certain amount of load before they will regulate the output voltage correctly. Power things up, and use your voltmeter to verify the voltages are within tolerance, and not too high.

Tom D.