PC Hardware

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.

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.

Power-Supply Sizes

Power supplies are available in a few standard sizes and shapes. However, the names for power supplies are anything but standard. They are based on the types of case they will be used in and the types of motherboard connections they will support. This is because different styles of cases place items such as plug fittings, mounting screws, and fans in different places.

A few years ago, a new type of motherboard cable power fitting began to appear on the market. The older models are known as AT-style, and the newer ones are known as ATX. We cover both in this tutorial, because you will need to be able to work with either one. The ATX design simplifies the placement of connections, so there is little to worry about with case compatibility. The main issues to be aware of are how much wattage the PC needs to power its parts and how many connectors for peripherals are required. Generally speaking, older Pentium-based computers and all 486-based and earlier PCs used AT supplies; almost all Pentium II and later-based systems use ATX supplies. The ATX design is preferred for two reasons:

  • The on/off power control circuit (not the button) on ATX boards is built into the motherboard. On AT PCs, it comes from the power supply.

  • AT power supplies connect to the motherboard through a pair of six-wire connectors. ATX power supplies connect through a single 20-pin connector.

A few motherboards and power supplies provide both AT and ATX fittings and switch support. They are rare, but will open up more options if you have to repair such a system. Generally, you should use ATX for all replacements, if possible.

It's a good idea to compare the existing power supply to the new one. Make sure that they are physically the same size, have the same connectors, and that the new one has at least the same power rating. Some high-quality power supplies offer "silencer" fans, that are much quieter than most models.