Power Supply (PSU)

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A Power Supply Unit (often shortened to PSU) is a device that converts AC power to DC power for your desktop computer. Modern PC power supplies use the ATX standard which dictates the form factor and voltages. Since the insides of power supplies contain lethal AC voltages tinkering around in a PSU should be done with extreme caution.

Connectors[edit | edit source]

24 PIN Connector[edit | edit source]

The 24 PIN connector provides all necessary power to the motherboard. Older motherboards may have a 20 pin connector instead, also some PSUs may have the 24 PIN connector split into a 20+4 version for backward compatibility.

CPU Power[edit | edit source]

The CPU power connector provides power for the CPU. The connector itself is usually a 4+4 pin type connector and the motherboard may have only a 4 pin header, higher-end motherboards usually have a full 8 pin header.

PCI-E Power[edit | edit source]

The PCI-E power connector provides extra power for PCI Express expansion cards, most commonly graphics cards. It may come in 6 pin, 6+2 pin (for backward compatibility), or 8 pin versions. A single 6 pin can supply up to 75W, while the 8 pin version can supply 150W.

SATA Power[edit | edit source]

The SATA Power connector provides power for SATA-based hard drives, solid-state drives, and Hybrid drives.

MOLEX Power[edit | edit source]

The MOLEX Power was used for floppy drives, DVD drives and older peripheral cards. In modern systems, it is frequently not used.

Faulty PSU symptoms[edit | edit source]

PSU faultiness can exhibit a wide range of symptoms, below is a list:

No. Issue Information
1. PC will not power on The computer does not power on or powers on briefly with fans spinning. Other components failing can also cause this issue.

Causes[edit | edit source]

  • PSU wattage is insufficient for the installed components. To troubleshoot this refer to troubleshooting step No. 3 and 4 (To be added).
  • The PSU has failed and needs replacing or board-level repair.
  • Another component may have failed.
2. Sudden power-off The computer will suddenly power off. This can happen at any moment but most commonly it happens when the PSU is under load.

This symptom most likely means the PSUs overload protection has kicked in and shut it off to prevent damage or catastrophic failure.

3. Blue screen of death
4. Component failure

Common troubleshooting steps[edit | edit source]

Here are some common troubleshooting steps you can do if you believe your PSU is not working or is faulty.

No. Test name Steps Result
1. Power-on test Take the 24 pin connector while the PSU is plugged in and turned on.

Then using an unfolded paper clip create a short between the green wire (power-on) and black wire (ground).

If the PSU fan starts spinning the PSU, has turned on and is working. If not, continue to test 2.
2. Measuring voltages This test requires a multimeter. While the PSU is plugged in and turned on, use an unfolded paper clip to create

a short between the green wire (power-on) and black wire (ground) on the 24 pin connector. Then set the multimeter to measure DC voltage, attach the negative lead to one of the black pins and start measuring these wires using the positive lead:

  • Yellow - 12 Volts
  • Red - 5 Volts
  • Orange - 3.3 Volts
If all the voltages are within a 5% tolerance of the listed values then the PSU is working normally. If the fan does not spin it is most likely dead.
3. Load Test
For this test, you must download and install FurMark and Prime95. In FurMark select your monitor's resolution and set Anti-aliasing to 8X MSAA. In Prime95 a window like shown in the image should show up, click OK and the stress test will start. Launch both of these tests simultaneously for 24 hours. Your computer will not be usable while running these tests.

These programs will test the stability of your computer and if the PSU can handle running the components in your system at full load. If your computer is still running after the 24 hour period then you have passed the test.
4. Remove components To be added To be added