How do I check out power supply? Recently was given an unknown brand of computer, and previous owner mentioned he throught a bad power supply. How can I check out the power supply for a computer without connecting it in? Found that you cannot connect it to wall and expect it to output voltage. There is usually some sort of connection through the motherboard to the switch on front face. I am questioning the functionality of the on/off switch in all this at present. How can I work around this and then measure the output? Wm.

If you short together the green and either of the black connectors which are on the same side as the retaining clip (they are actually pins 14 and 13or15) this will start your ATX PSU. A bent paper clip is the ideal tool for this. But if you're colour blind or not too sure, leave it to the experts!

Above is not always true. Some power supplies will not "start" without a load or other connections hooked to them.

Even that won't work. Pins 15 and 14 will operate the switch. Switch mode power supplies need a load, otherwise they go into runaway mode and burn themselves out, consequently the power supply will cut the outputin the absence of a load. Even if you did apply a load sufficient to get the power supply running, there is no guarantee the voltages would be stable once the rated load was applied. To test the voltages from a power supply you need a low impedance voltmeter and that means a digital voltmeter. An analogue voltmeter will itself present a load to the power supply which will affect the voltage output. Getting a power supply tested properly is not worth the hassle, would cost more than a new one. I would just plug it in try it. If it works fine, if not then buy an new one. They are not expensive. Stuart

Also, any decent techy has got a proper PSU test unit with which to do the test properly.

JOHNR..... There are no green wires coming out of the power supply at all. I have a PC fixum book, but it does not give a diagram of the plugs for the power supply. There are twenty pin holes on the connector. One of which is blank. The label on the power supply also does not have a diagram as to what is relation for each wires coming out of box. How otherwise, do I count for pin numbers? Wm.

Pin 1 is the top right hand pin on the connector when looking at it with the connector key on the bottom. Both pin 1 and pin 2 should be orange. I would be very wary of a power supply that does not colour code its output cables to a recognised specification. Stuart