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The Power Supply Unit (PSU)

Often overlooked in its importance the PSU or Power Supply Unit performs the critical task of converting AC mains power, which is generally 100-120v or 220-240V depending on your location, to the lower DC voltages used by the internal components of a computer. Computer power supplies are rated based on their output power and a typical computer will have a PSU between 400 and 500 watts, though higher performance machines may require anything up to 1000 watts. If your computer has lots of additional cards and hard drives it may experience "brown outs" where the PSU cannot provide enough power to keep the machine running, often charactorised by the machine rebooting or freezing. In this instance a higher wattage PSU is desirable.

The PSU generally is an easily identifiable square metal box with a fan on the face that will be visible from the outside of the case and a bundle of wires on the side that is closest to where the mainboard of the computer would be. These wires have a variety of different connectors on them depending on the voltage and intended use-

  • The mainboard connector (or P1) is the connector that is plugged into the motherboard to provide it with power. This is a large connector with either 20 or 24 pins in the instance of an ATX power supply, the most commonly used standard of PSU's in PC's. Usually a 20 pin connector has an additional 4 pin plug on the PSU that when used in conjunction with the 20 pin connector can take the place of a 24 pin connector. It has a clip or tang that should be correctly aligned when connecting to the mainboard.
  • 4 Pin Molex connectors are generally used to provide power to peripherals such as hard disk drives, cd-rom drives and to provide additional power to high performance video cards.
  • 4 Pin Floppy drive or Mini-Connectors as they are sometimes called are used almost exclusively to power floppy disk drives.
  • 4 pin square auxillary power connectors are often required to provide additional power to the mainboard
  • Serial ATA power connectors are a flat L shaped connector to provide power to SATA hard disk drives and optical drives.
  • 6 Pin auxillary power connectors are found on some PSU's and are usually used to provide additional power to PCI express video cards.
Power Supply