A digital multimeter has many different settings to help diagnose electrical problems.
The power supply on a computer transforms the alternating current from the wall outlet into a direct current flow that the computer's components can use. Testing the DC voltage output of the power supply using a digital multimeter can help you isolate any potential issues and determine if you have a bad power supply. Always use the probes to touch wires and terminals -- never use your fingers. If you can't hold the wires or terminals with the probes, use a stand or binder clip to secure them so you don't have to hold them with your hand.
1.
Test the outlet and surge protector into which the power supply has been plugged. Power off the computer, unplug it from the surge protector, and then unplug the surge protector from the wall outlet.
2.
Set your digital multimeter to measure AC voltage. The AC symbol is a "V" with a "~" near it. Set it to the range of 200 V. Insert one probe into the left side of the outlet and the other into the right. Since the current is alternating, the probe color doesn't matter. You should see a reading of between 110 and 120 volts.
3.
Plug the surge protector into the outlet, and then test the surge protector. The reading should still be between 110 and 120 volts.
4.
Set your multimeter to measure resistance; the symbol is "Ω." If there is a symbol on your dial that looks like an arrow pointing to sound waves, use that one. The multimeter makes a sound if there is continuity along a wire. Remove the cord connected to the power supply on the back of the computer. Touch one of the plug terminals with one of the DMM probes. Put the other probe into the female end of the cord. If none of the holes produce a tone on your multimeter, then the cord is bad and needs to be replaced. If you have continuity for each of the three wires in the power supply cable, then there is an issue with the power supply.
5.
Remove your computer's case, and then locate the place where the power supply attaches to the motherboard. The ATX style power supply won't operate unless it is connected to the motherboard, so you must insert the probes into the back side of the connector to make contact with the wires.
6.
Note the orientation of the clip that secures the connector to the motherboard; the clip is between pins 15 and 16. The pins on that side are 11 through 20, and the pins on the opposite side are 1 through 10. Power is carried through pins 1, 2, 4, 6, 9, 10, 11, 19 and 20. Use a pin diagram to identify the different testing locations (link in Resources).
7.
Set the multimeter to read DC voltage. The symbol is "V" and a straight line over a broken line. Set the range to 20 volts.
8.
Insert the black probe into the back of the connector with any black wire. Insert the red probe into the back of pin 10. Power on the computer. The multimeter should read between 11 and 12 volts. If it's too high or too low, the power supply is failing and needs to be replaced. Test pin 12 as well.
9.
Test pins 4, 8, 9, 19 and 20 for 5 volts. If the voltage is too high or low, the power supply is failing.
10.
Test pins 1, 2 and 11 for 3.3 volts.
Warning
Don't attempt to salvage or repair a failing power supply, as doing so may damage your computer.
The power supply on a computer transforms the alternating current from the wall outlet into a direct current flow that the computer's components can use. Testing the DC voltage output of the power supply using a digital multimeter can help you isolate any potential issues and determine if you have a bad power supply. Always use the probes to touch wires and terminals -- never use your fingers. If you can't hold the wires or terminals with the probes, use a stand or binder clip to secure them so you don't have to hold them with your hand.
1.
Test the outlet and surge protector into which the power supply has been plugged. Power off the computer, unplug it from the surge protector, and then unplug the surge protector from the wall outlet.
2.
Set your digital multimeter to measure AC voltage. The AC symbol is a "V" with a "~" near it. Set it to the range of 200 V. Insert one probe into the left side of the outlet and the other into the right. Since the current is alternating, the probe color doesn't matter. You should see a reading of between 110 and 120 volts.
3.
Plug the surge protector into the outlet, and then test the surge protector. The reading should still be between 110 and 120 volts.
4.
Set your multimeter to measure resistance; the symbol is "Ω." If there is a symbol on your dial that looks like an arrow pointing to sound waves, use that one. The multimeter makes a sound if there is continuity along a wire. Remove the cord connected to the power supply on the back of the computer. Touch one of the plug terminals with one of the DMM probes. Put the other probe into the female end of the cord. If none of the holes produce a tone on your multimeter, then the cord is bad and needs to be replaced. If you have continuity for each of the three wires in the power supply cable, then there is an issue with the power supply.
5.
Remove your computer's case, and then locate the place where the power supply attaches to the motherboard. The ATX style power supply won't operate unless it is connected to the motherboard, so you must insert the probes into the back side of the connector to make contact with the wires.
6.
Note the orientation of the clip that secures the connector to the motherboard; the clip is between pins 15 and 16. The pins on that side are 11 through 20, and the pins on the opposite side are 1 through 10. Power is carried through pins 1, 2, 4, 6, 9, 10, 11, 19 and 20. Use a pin diagram to identify the different testing locations (link in Resources).
7.
Set the multimeter to read DC voltage. The symbol is "V" and a straight line over a broken line. Set the range to 20 volts.
8.
Insert the black probe into the back of the connector with any black wire. Insert the red probe into the back of pin 10. Power on the computer. The multimeter should read between 11 and 12 volts. If it's too high or too low, the power supply is failing and needs to be replaced. Test pin 12 as well.
9.
Test pins 4, 8, 9, 19 and 20 for 5 volts. If the voltage is too high or low, the power supply is failing.
10.
Test pins 1, 2 and 11 for 3.3 volts.
Warning
Don't attempt to salvage or repair a failing power supply, as doing so may damage your computer.