How to Check Your PC Power Supply

When diagnosing computer issues, the power supply is often overlooked. However, checking the power supply first can save you a lot of time when troubleshooting. If your computer shows a Blue Screen of Death (BSOD), has a faulty hard drive, or simply fails to boot, you might be dealing with a power supply problem. Follow the quick steps below before spending money on expensive new hardware.

Turn off the computer. Once the computer is powered down (or unresponsive), flip the switch on the back of the power supply and unplug the power cord.

Open the case. Disconnect the power cables from the internal components. Trace each cable running from the power supply to the other parts to ensure everything is properly disconnected.

• Note where each cable is plugged in so you can reconnect it when assembling the case later.

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

• The paperclip will act as the pins in the power supply, sending the signal to turn on.

Locate the 20/24-pin connector that typically plugs into the motherboard. This is usually the largest connector on the power supply.

Find the green and black pins (pins 15 & 16). Insert one end of the paperclip into the green pin and the other end into the black pin. Before proceeding, ensure the power supply switch is turned off and that it is fully disconnected from power and all other components in the computer.

• The green pin is typically in position 15 on the pinout diagram.

Insert the paperclip. Once the paperclip is inserted into pins 15 & 16, set it aside. Plug in the power supply and turn on the switch at the back.

Check the fan. Once the power supply is powered on, you should hear or see the fan spinning. This indicates that the power supply is at least functioning. If the fan doesn't turn on, double-check the connections (after unplugging) and try again. If it still doesn't work, the power supply is likely faulty.

• This method only verifies that the power supply is running, but it doesn't confirm normal operation. To determine if the power supply provides stable output, proceed to the next method.

Check the output using software. If the computer is operating normally and the operating system has booted up, you can check the power supply output using software. SpeedFan is a free program that can read diagnostics and report the temperature and voltage of your computer. Check the information to make sure everything is within acceptable ranges.

• If the computer isn't working, move on to the next step.

Turn off the computer. Unplug the power cord and switch off the power supply at the back. Open the case and disconnect the power supply from all internal components. Trace each power supply cable to ensure everything is properly disconnected.

Test the power supply with a specialized tool. You can purchase a power supply tester at computer stores or online for a reasonable price. First, locate the 20/24-pin connector on the power supply, which is usually the largest 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 tester's light will illuminate.
  • Some testers need to be powered on via a switch or button, while others turn on automatically.
• Check the voltage. The 20/24-pin connector has multiple outputs, but the following are the four key voltages to monitor:
  • +3.3 VDC
  • +5 VDC
  • +12 VDC
  • -12 VDC
• Verify the voltage readings are within acceptable tolerance. +3.3, +5, +12 VDC should have a tolerance of +/- 5%. The -12 VDC should fluctuate within +/- 10%. If any reading falls outside this range, the power supply is unstable and needs replacement.
• Check other connectors. After verifying the main power connector is stable, proceed to test each additional connector. Be sure to unplug the power supply and turn it off between each test.

Test the power supply with a multimeter. Straighten a paperclip into a 'U' shape. Find the green pin on the 20/24-pin connector. Insert one end of the paperclip into the green pin (pin 15) and the other end into the black pin next to it. This will simulate a connection to the motherboard.

• Plug in the power supply and turn on the switch.
• Refer to the pinout diagram to identify which pins provide voltage.
• Set your multimeter to VDC. If the multimeter doesn’t automatically adjust the range, 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 you want to check. Be sure to note the voltage reading.
• Check that the voltage on each pin is within acceptable tolerance. If any voltage is out of range, the power supply is faulty.
• Repeat this process for the other power supply connectors. Be sure to check the specific pinout diagram for each connector to know where to test.

Reassemble the computer. After checking all the power connectors, you can reassemble the computer. Refer to your notes to ensure all devices are properly connected and that all motherboard connectors are in place. Once reassembled, power on the system.

• If the issue persists or the computer doesn’t start, move on to further troubleshooting steps. The first component to check at this point is the motherboard.