If you are experiencing problems with your computer system and would like to find out if the problem may be caused by a power supply with voltages that are out of specification, it is recommended to use a digital multi-meter (DMM).
First, make sure your DMM is capable of reading DC voltage and that it can read voltages to the 100th place (aka: a "resolution" of .01V.) Find an unused power connector and insert the probes into the appropriate pins. Apply the DMM's black probe (-) to a ground wire. Black is always ground, and it doesn't matter which ground you use as all grounds in the power supply terminate to the same location. Apply the DMM's red probe (+) to a colored wire that corresponds with a voltage you want to test. Red is +5V, yellow is +12V and orange is +3.3V.
Above, the probe is on a red wire and a black wire to test +5V.
Above, the probe is on a yellow wire and a black wire to test +12V.
Voltages are within specification if they are within 5% of the labeled voltage. For example: Tolerance for +12V would be 11.4V to 12.6V, +5V would be 4.75V to 5.25V, etc.
Range of acceptable DC voltages for computer power supply.
Another way to probe for voltages is to take the 20+4-pin power connector from the motherboard, remove the +4 and use the exposed pins in the motherboard connector for your DMM's probe. The exposed pin directly below the red wire on the main power connector is another +5V. Below that is a common ground. On the next column of pins, the pin below the last yellow wire is another +12V. The pin below this is +3.3V.
Above, the probes are on a ground and a +3.3V pin.
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