Back to Basics - The Power Supply Unit

The power supply unit, or PSU, is what provides power at various different voltages to the motherboard and other components inside the system case. The size and type of connections attached to the wiring loom comply with the ATX standard so that, physically at least, any PSU can be mounted in any ATX system case. The PSU is always mounted at the top rear of the system case and will normally have at least one fan that serves two functions: it vents warm air from the system case that arises from the CPU heatsink; it cools the PSU itself, which can generate quite a bit of heat when a full load is being pulled on it.

When the PSU is mounted in the system case the back panel of the PSU protrudes to reveal the standard mains kettle lead connector and, on some models, an on-off switch. There is usually also a voltage selector switch which should always be set at 230V for the Irish supply. Mounting and dismounting the PSU in the system case is via four screws on the back panel.

There are two connectors that connect directly to the motherboard - these are the ATX power connector and the 12V power connector (see pictures). Both these connectors are keyed, which means that they cannot be inserted incorrectly into the motherboard sockets. The PSU provides power at a number of different voltages: +12V, -12V, +5V, -5V and +3.3V each of which is supplied to the motherboard via the ATX connector. The accompanying pin-out diagram displays the voltages associated with each pin of the ATX connector.

In addition to the ATX connector and 12V power connector, both of which are plugged directly into the motherboard, the PSU has two other types of connector (well it actually has more, but the others are outside the scope of this article) and these are the 12V Molex connector, which is used to bring power to the optical drives and hard disc drives, and the 12V floppy disc connector, both of which can be seen in the accompanying photo.

The output of a PSU is measured in Watts and PC manufacturers have had a tendency to put in PSUs with low-ish power ratings (150-200W). That's not a problem until you start adding extra components to your PC, which increases the overall power consumption. This can happen if you add an extra hard disc drive, or install a more up to date graphics card. If you exceed the overall power rating of your PSU you may find that the PC randomly crashes or reboots. The problem can be readily rectified by upgrading the PSU to say 250-300W. You should be aware that you can now buy PSUs rated at up to 700W - but bigger is not always better. It's been estimated that even the most heavily loaded PC running full bore will not exceed 300W, so going for a PSU that delivers over twice that amount of power may just result in extra heat being generated, requiring extra fans for cooling, resulting in a noisier PC.

To finish up this series I'll bring you on a tour of the innards of one of PC Medic's PCs that will show all of the items mentioned in our Back to Basics articles in situ in a real PC.