Power Supply Unit (PSU) Basics including ATX Power Supply Pinouts.

Welcome to our power supply basics guide.

In this section we discuss what a power supply is and what it does, summarise the different voltages produced and detail the pinouts.

Power Supply Unit (PSU) Basics
  • Power Supply Overview
  • Power Supply Connectors
  • ATX Power Supply Pinouts
safety first
Power Supplies contain dangerous voltages and should only be opened by experienced and qualified engineers, there are no user servicable parts inside.
Always disconnect the PSU from the mains supply before removing the cover of the PC.

What is a power supply and what does it do?

A photo of an ATX Power Supply (PSU)
The power supply unit (PSU) in a PC regulates and delivers the power to the components in the case.

Standard power supplies turn the incoming 110V or 220V AC (Alternating Current) into various DC (Direct Current) voltages suitable for powering the computer's components.

Power supplies are quoted as having a certain power output specified in Watts, a standard power supply would typically be able to deliver around 350 Watts.
The more components (hard drives, CD/DVD drives, tape drives, ventilation fans, etc) you have in your PC the greater the power required from the power supply.

By using a PSU that delivers more power than required means it won't be running at full capacity, which can prolong life by reducing heat damage to the PSU's internal components during long periods of use.

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Always replace a power supply with an equivalent or superior power output (Wattage).

There are 3 types of power supply in common use:
  • AT Power Supply - still in use in older PCs.
  • ATX Power Supply - commonly in use today.
  • ATX-2 Power Supply - recently new standard.

The voltages produced by AT/ATX/ATX-2 power supplies are:
  • +3.3 Volts DC (ATX/ATX-2)
  • +5 Volts DC (AT/ATX/ATX-2)
  • -5 Volts DC (AT/ATX/ATX-2)
  • +5 Volts DC Standby (ATX/ATX-2)
  • +12 Volts DC (AT/ATX/ATX-2)
  • -12 Volts DC (AT/ATX/ATX-2)
A power supply can be easily changed and are generally not expensive, so if one fails (which is far from uncommon) then replacement is usually the most economic solution.

The power supply connectors

4 Pin Berg Connector
4 Pin Berg Connector
Used to connect the PSU to small form factor devices, such as 3.5" floppy drives.
available in: AT, ATX & ATX-2
4 Pin Molex Connector
4 Pin Molex Connector
This is used to power various components, including hard drives and optical drives.
available in: AT, ATX & ATX-2
20 Pin Molex ATX Power Connector
20 Pin Molex ATX Power Connector
This is used to power the motherboard in ATX systems.
available in: ATX( ATX-2 have four extra pins)
4 Pin Molex P4 12V Power Connector
4 Pin Molex P4 12V Power Connector
Used specifically for Pentium 4 Processor Motherboards.
available in: ATX (integrated into the power connector in ATX-2)
6 Pin AUX Connector
6 Pin AUX Connector
Provides +5V DC, and two connections of +3.3V.
available in: ATX/ATX-2

ATX Power Supply Pinouts

Below are pinout diagrams of the common connectors in ATX power supplies.
Diagram of ATX Power Supply Pinouts
note: The pinouts above relate to the connectors not the sockets.
note: To power up an ATX or ATX-2 PSU for testing, short pin 14 (PS_ON) with one of the grounds.
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