How to Use a Multimeter to Test a Computer Power Supply

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How to Use a Multimeter to Test a Computer Power Supply
Use a multimeter to diagnose computer power-supply issues. (multimeter image by dinostock from Fotolia.com)

A multimeter is a device that measures voltage, and is sometimes the only way to get a truly accurate reading if a computer's power supply is failing. BIOS reports are often faulty, as power-supply issues can cause readings the software doesn't know how to interpret. If your computer begins experiencing sudden shutdowns and restarts, or if there is no power to the CD drive while the hard drive performs normally, these are likely indicators that the power supply is failing. A multimeter will help diagnose the problem beyond any doubt.

Skill level:
Moderate

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Things you need

  • Screwdriver
  • Anti-static wristband
  • Digital multimeter reader

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Instructions

  1. 1

    Open the computer system with a screwdriver to expose the motherboard, while the computer is shut down but still plugged into the wall. Wear an anti-static wristband to avoid damaging internal computer parts. Locate the back of the CD drive. The CD drive is housed in the front of the computer so that discs can be inserted into it, but the back of the CD drive sticks out somewhere close to the motherboard on the inside of the machine. Unplug the connector that has four parallel pins from the CD drive. This is the power cable. Leave the other end of the power cable plugged into the motherboard. Power up the computer.

  2. 2

    Set the multimeter to read 2 to 20 volts in the DC range. There is a dial setting for this adjustment. On the 4-pin connector you have just unplugged, find a black wire and a yellow wire. Insert the multimeter's black probe into the pin opening with the black wire, and insert the red probe into the pin opening with the yellow wire. The multimeter reading should show +12 volts. Then move the red probe to the red wire and the reading should show +5 volts. This means the power supply is good and you may go to step three. However, if there are no readings or your results are different, your power supply will need replacing.

  3. 3

    Locate the power supply in the rear of the computer, where the external power cord from the wall plugs into the machine. On the inside of the computer you will find the back of the power supply sticking out, with a bundle of cable leading to the motherboard. Follow the bundle of cable to a two-piece connector on the motherboard; these connectors will be labelled P8 and P9. Unplug this two-piece connector, but first take note how the plug is connected so you can plug it in again the same way.

  4. 4

    Find a black wire on the power supply connector labelled P8 and insert the black probe into the pin opening, then insert the red probe into the pin opening of the red wire. The multimeter reading should be +5 volts. Next, leave the black probe where it is and move the red probe to the yellow wire. The multimeter should now read +12 volts. Again, leave the black probe where it is and switch the red probe one more time to the blue wire. The multimeter should read -12 volts. If there are no readings or your results are different, replace the power supply.

  5. 5

    Move the black probe to a black wire on the power supply connector labelled P9, and move the red probe to the white wire of P9. The multimeter reading should show -5 volts. Leave the black probe where it is and move the red probe to the three red wires on P9, and the reading should show +5 volts for each one. If there are no readings, or if you get different results, replace the power supply.

Tips and warnings

  • Any reading that is off by a small percentage is OK. For example, if the multimeter says +5.02 instead of +5 there is no problem. However, if any readings are off by a large margin, e.g. if it should read -5 but says -8 instead, replace the power supply.
  • Working with electrical power supplies can be dangerous. If you are not completely confident in your ability to diagnose power problems, consult a professional.

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