How to Test a PC Power Supply

In the process of diagnosing problems on a computer, the power supply is often forgotten. However, checking the power supply first can help you shorten a lot of problems in troubleshooting. If your computer displays a blue screen of death (BSOD: Blue Screen of Death), has a faulty hard drive, or simply won't boot, you likely have a power supply problem. Perform these quick checks below before purchasing any expensive new hardware.

Check if the power is on or not

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Turn off the computer. After the computer turns off (or cannot turn on), slide the switch behind the power supply and unplug the computer.

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Open the case. Disconnect the power cable from the components inside the case. Trace each cable from the power supply to the other components to ensure that all are properly disconnected.

Note which cable goes where so you can connect it when you reinstall the case.

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Test with a paperclip. Use a paperclip to 'fool' the power supply into thinking a connection has been established. Before proceeding, bend the paperclip into a 'U' shape.

The paper clip will act as pins to the power supply and provide an on signal.

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Look for the 20/24 pin connector that typically plugs into your computer's motherboard. This is usually the largest connector on the power supply.

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Find the green and black pins (pin numbers 15 & 16). Plug one end of the paperclip into the green pin, and the other end into the black pin. Before proceeding, you need to check whether the power supply switch is turned off and whether the power supply is completely disconnected from the power source as well as other parts in the computer.

The green pin is usually located at position 15 in the pin diagram.

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Insert the paperclip. After inserting the paper clip into pins 15 & 16, set this cable aside. Plug in the power supply and turn on the switch on the back.

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Check the fan. Once the power supply is energized, you will hear or see the blower rotate. This shows at least that the power supply is still working. If the power supply does not turn on, check the pins again (after unplugging) and try again. If nothing happens, the power supply is most likely broken.

This method cannot test the normal operation of the power supply, but only shows that it is functional. To know if the power supply has stable output or not, you will need to proceed with the next method.

Check the output

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Check output through software. If the computer operates normally and the operating system boots, try checking the power supply output using software. SpeedFan is a free program that can read diagnostics and report your computer's temperature and voltage. Check the information to make sure everything is within acceptable levels.

If the computer doesn't work, proceed to the next step.

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Shutdown. Unplug the power and turn off the switch located behind the power supply. Then, open the case and disconnect the power supply from all internal components. Trace each power supply cable to make sure everything is properly disconnected.

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Check the power supply with specialized equipment. You can buy power supply testing equipment at computer stores or online at a not too expensive price. First, find the 20/24-pin connector on the power supply. This is usually the largest sized cable.

Connect the power supply tester to the 20/24 pin connector.

Plug in the power supply and turn on the switch. The power supply will automatically turn on and the light on the tester will light up.

Some power supply testers need to be turned on via a switch or button. The remaining types will turn on automatically.

Check voltage. 20/24-pin connectors have many outputs, but here are four important metrics to watch:

+3.3 VDC

+5 VDC

+12 VDC

-12 VDC

Check that the voltage readings are within acceptable tolerances. +3.3, +5, +12 have a tolerance of +/- 5%. The -12 index can fluctuate within +/- 10%. If any index is outside this range, it means the power supply is unstable and needs to be replaced.

Check other connectors. After determining that the main connector is providing stable power, check each remaining connector. Remember to unplug and turn off the power supply between each test.

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Check the power supply with a multimeter (electrical measuring instrument). Bend the paperclip straight into a 'U' shape. Find the green pin on the 20/24-pin connector. Insert one end of the paper clip into the green pin (number 15), the other end into the black pin next to it. Thus, the connector will receive a false signal that the cable is connected to the motherboard.

Plug in the power supply and turn on the switch.

Find the power supply's pin diagram to determine which pins provide the voltage.

Set the multimeter to VBDC mode. If the multimeter does not automatically adjust the range, you need to set it to 10V.

Connect the multimeter's negative probe to the ground (black) pin on the connector.

Connect the positive probe to the first pin to be tested. Remember to take note of the displayed voltage.

Check that the voltage of the pins is within tolerance. If any indicator is outside the tolerance range, the power supply is defective.

Repeat this process with the other power supply connectors. You should refer to the specific pin diagram of each plug to know the pin location to check.

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Reassemble the computer. After checking all the power connectors, you can reassemble the computer. Look in the notes to make sure the devices are connected properly and that all motherboard connectors are in the correct positions. After completing the assembly, turn on the device.

If the error continues or the computer won't start, move on to the troubleshooting steps. At this point, the first part you need to check is the motherboard.