Here's our Hardware Tip for.. May 7, 1999
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The basic function of the power supply is to convert the type of electrical power available at the wall socket to that which is usable by the computer circuitry. The power supply in a conventional desktop system is designed to convert the 120-volt, 60Hz, AC current into something the computer can use, specifically, +5v and +12v DC current, and +3.3v as well on some systems. Usually, the digital electronic components and circuits in the system (motherboard, adapter cards, and disk drive logic boards) use the 3.3v or +5v power, and the motors (disk drive motors and any fans) use the +12v power. The power supply must ensure a good, steady supply of DC current so that the system can operate properly.
Power supplies are rated in watts. A PC requires sufficient wattage for the machine to run properly. The average desktop PC with two hard drives and a CD-ROM drive will need an average of 115 to 130 watts while running, and up to 200 watts when booting up. Play it safe and buy 200-230 watt power supplies. They are by far the most common wattage, and will give you plenty of extra power for booting up, as well additional functionality in the future.
Power supplies are available in a variety of shapes and sizes. Although the sizes are very standard, the names for the sizes are not. For most desktop or mini-tower PCs, the power supply is an "AT mini' power supply in the 200-250 watt rating. To save time and repeat visits to your friendly neighborhood electronics shop, do yourself a favor: remove the suspect power supply and take it in with you to guarantee that you choose the correct replacement.