Overview of Power Supplies
A standard power supply draws power from a local, alternating current (AC) source (usually a wall outlet) and converts it to either 3.3 or 5 volts direct current (DC), for on-board electronics, and 12 volts DC for motors and hard drives. In all cases, it delivers both positive and negative DC to the computer. Power supplies must "condition" the power, smoothing out any radical changes in its quality. Many homes and offices have power that fluctuates far more than the delicate parts of a PC can tolerate and survive. Most PC power supplies also provide the system's cooling and processor fans that keep the machine from overheating.
If the computer's power supply is providing reliable, clean power and its own cooling fan works, all is well. If the power supply or its fan should fail or cause erratic behavior by the PC, the power supply must be replaced. (While it is possible to remove and replace a power-supply fan, the low cost of a power supply makes it more practical to replace the power supply itself.)
Many newer supplies have a universal input that will accept either 110 VAC (volts alternating current), 60 Hz (U.S. standard power), or 220 VAC, 50 Hz (European/Asian standard). When replacing a power supply, there are three things to consider: physical size, wattage, and connectors. This tutorial covers the basics of power supplies.
NOTE
A hertz is a measure of unit frequency: one cycle per second equals one hertz. A kilohertz (Kz) is 1000 cycles per second; a megahertz (Mz) is a million cycles per second.