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How To Test A Power Supply Unit

A faulty PSU (power supply unit) can be the reason for all kinds of computer issues. Some examples:

  • The computer is completely dead (not powering up at all)
  • The fans are spinning but the computer doesn’t boot up (black screen)
  • The system does not run stably (random rebooting or freezing)
  • Single components are not working properly

If your computer has one of those symptoms or any similar issue, it’s always a good idea to check the PSU because a faulty PSU does not only prevent your computer from running properly. It can also damage your hardware components. So in this little how-to I will show you how to check your computer’s PSU.

Preceding Information

There are many different PSUs with different connectors out there. The standard form factor for computer cases, mainboards and PSUs is called ATX but there are different versions of ATX. As for the power supply connectors these are the most important differences:

  • Newer PSUs usually have a 24-pin main power connector (“the big one” that is attached to the mainboard), whilst the older ones only have 20 pins. In my examplary pictures I’m using an old PSU with a 20-pin main power connector. Basically it’s built the same – deducting pin 11, 12, 23 and 24 if compared to the 24-pin schematic diagram (see further down or in this PDF file).
  • Most PSUs have an additional P4 (4-pin) connector which is also attached to the mainboard to provide the CPU with extra power, newer ones might have a P8 (8-pin) connector.
  • There might be several 4-pin and/or 6-pin connectors to provide the video card with extra power.
  • Finally there are the Molex 8981 (4-pin disk drive power connectors) and S-ATA connectors – and maybe one for a floppy drive.
  • Some PSUs also have an AUX connector for extra 3.3V and 5V power.

Keep in mind: the following tests are not 100% diagnostically conclusive because the PSU might behave differently if electrical load is attached.

Warning: the following procedures might damage your PSU or connected devices. Use at your own risk!

Method 1 – Using A Special Testing Device

If you have a tester like this, things are pretty easy. You just have to attach the main power connector and then (one by one individually) the other connectors. The green LEDs indicate whether the voltages are fine or not. This is good for a quick test but if there’s something wrong you can’t figure out which pins are defective and what voltages they actually provide.

Method 2 – Using A Multimeter

With a multimeter you can test every single pin individually and see what voltage is on it.

In order to power up the PSU without having it connected to the computer, you will have to connect the PS_ON pin with one of the GROUND pins (check the schematic diagram below or this PDF file). You can do that with a regular paper clip:

Then you have to set up the multimeter correctly. Attach the black cable to the ground jack (COM) and the red one to the voltage-measuring jack (V). The turn-switch has to be set to 20V DC voltage (V –, not V ~):

Now you’re ready to go. Connect the black cable’s tip to one of the GROUND pins and the red cable’s tip successively to all of the other pins that provide power:

In this manner you can check all the connectors. The offsets should be +/- 5% at a max. You should get a new PSU if you have detected dead pins or if the measured values are out of the valid range. If you want to, you can use the tables in the PDF file to document your measured values. If you have different connectors just ask Google for the pin configuration 😉

3 Comments.[ Leave a comment ]

  1. I love your wordpress design, where do you download it through?

  2. It’s the zBench theme. You can download it at http://wordpress.org/extend/themes/zbench

  3. Thank you for providing this information. It was very helpful and prevented me from buying a new PSU needlessly.

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