Computer Hardware:
     • Tools, Static & Cleaning
     • Form Factor
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     • Motherboard
     • Processor (CPU)
     • Power Supply Unit
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     • Sound Card
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     • Building a Computer
     • Overclocking

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Common PC Problems:
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     • Windows Shortcuts
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     • Number Base Converter

PSU Wiring & Testing

The Power Supply Unit produces three main voltages for the computer :- +3.3V (orange wire), +5V (red wire), and +12V (yellow wire). The Ground wire is coloured black.

Below is a diagram of both the 20 pin and 24 pin ATX Main Power Connector:

Power Supply Unit ATX wiring diagram.

The 'Power On' (PWR_OK) and 'Power Good' (PS_ON#) connections have special functions. When the 'Power On' (PWR_OK) is connected to Ground then the motherboard signals the ATX power supply to turn on. The 'Power Good' (PS_ON#) is an output from the PSU that indicates that the output has stabilized and is now ready to use.

Testing ATX Power Supply Unit

You can buy a 'Power Supply Tester' which will allow you to connect the 20 pin or 24 pin ATX Main Power Connector as well as other power connectors and it will automatically display if the voltages are good.

Power Supply Unit Tester.

You can also test a PSU manually by using a standard 'multimeter' and by following these instructions:

1. Switch off your computer and unplug the power cable from the mains socket.

2. Open the computer case and follow anti-static procedures.

3. Disconnect all power connectors from the motherboard, drives, and possibly graphics card and group them together ready for testing.

4. locate the 20 pin or 24 pin ATX Main Power Connector and short out pins 13 and 14 (on a 20 pin connector) or 15 and 16 (on a 24 pin connector) using a small piece of wire. This will allow the PSU to work despite not being plugged into the motherboard (see diagram above).

5. Plug the PSU into the mains socket and switch on the PSU. (If the PSU has a voltage selector then make sure it is set to the correct voltage for your region).

6. Switch on your multimeter and turn the dial to DC Volts and select 20V (or a voltage over 12V depending on the multimeter).

7. Connect the black (negative) probe of the multimeter to one of the black ground wires of a socket and then using the diagram above check all the wires on each connector with the red probe of the multimeter to make sure they have the correct voltages. (+3.3V (orange wire), +5V (red wire), +12V (yellow wire), -5V (White wire), and -12V (Blue wire)).

8. You will notice that the multi-meter voltage readings will be slightly different to what they should be but this will be okay as long as they are within the correct tolerance levels. (See the voltage tolerance levels table below). If any of the voltages are outside the tolerance level then the PSU should be replaced.

9. If the voltages of all the wires are within the correct tolerances then the PSU is working correctly and you can switch off the PSU and unplug it from the mains socket. Remember to remove the small piece of wire from the ATX Main Power Connector.

10. Reconnect all the power connectors to the motherboard, drives, and graphics card. Close the case.

11. Plug the PSU into the mains socket and switch on the PSU. Your computer should now boot up correctly.

12. It is also a good idea to test your PSU under load by running a taxing 3D game or using a benchmarking program for an hour or so.

ATX Power Supply Unit (PSU) Voltage Tolerance levels:
Wire ColourVoltageToleranceMinimum VoltageMaximum Voltage
The white -5V wire may or may not be used on your PSU.
For example - the yellow wire must have a reading of between +11.4V and +12.6V to be good.

Please be careful when handling and testing Power Supply Units. Never open up the actual PSU as it can be dangerous even when unplugged, and never poke anything inside a PSU. If the PSU does not work then simply replace the complete unit.

1. PSU 2. PSU Connectors 3. PSU Upgrade
4. PSU Wiring & Testing