Power Supplies
A power supply provides electrical power to the computer components. It draws power from a local, alternating current (AC) source such as a wall outlet and converts it to either 3.3 or 5 volts direct current (DC) for on-board electronic components, and 12 volts DC for motors and hard disk drives. In addition, most Pentium 4 motherboards include voltage regulator modules to power the CPU, which runs on 12V power. The power supply delivers both positive and negative direct current to the computer. Power supplies must also "condition" the power input, i.e., it must smooth out any radical changes in the quality of the electrical supply as many homes and offices have power that fluctuates far more than the delicate computer components can tolerate. Most power supplies have a universal input that will accept either 110 volts alternating current (VAC), 60 hertz (Hz) (U.S. standard power), or 220 VAC, 50 Hz (European, Asian, Southern African standard).
If the power supply or its fan should fail or cause the computer to behave erratically, the power supply must be replaced. When replacing a power supply, there are three things to consider: physical size, wattage, and connectors.
Power supplies are generally available in two varieties: AT power supplies and ATX power supplies, although the ATX has an ATX24 derivative with a 24 pin connector, used by many server Motherboards before release of the Pentium 4 processor; and an ATX12V derivative with an additional 4-pin 12 volt connector. The two varieties of power supplies are based on the types of motherboard connections they support but do have a few other differences.
- The ATX design is preferable as the on-off power control circuit on ATX boards is built into the motherboard and not in the power supply as in AT power supplies.
- AT power supplies connect to the motherboard through a pair of six-wire connectors while ATX-style power supplies connect through either a single 20-pin connector or a 20-pin connector and a 4-pin ATX12V connector for Pentium 4 Motherboards. The exception is the ATX24 which has a single 24-pin connector rather than a 20-pin connector.
- Generally all older Pentium-based computers and 486-based and earlier computers use AT supplies while almost all Pentium II and Pentium III-based computers use ATX power supplies. All Pentium 4- based computers require an ATX12V power supply with a 4-pin ATX12V connector in addition to the normal 20-pin connector.
A few motherboards and power supplies provide both AT and ATX fittings and switch support. These are rare, but provide more options should you have to repair such a system. Generally, you should use ATX power supplies if possible.
When replacing a power supply, the main issues to be aware of are how much wattage the computer needs to power its components and how many peripheral connectors are required. The power supply must produce at least enough energy to operate all the computer's components at the same time. You can determine a computer's power consumption by adding the power requirements, measured in watts, for all the devices in the unit. This will provide you with the power supply the computer will consume to operate; however, a computer's operating consumption is always lower than the power required at startup when hard disk drives and other heavy feeders simultaneously compete for the available startup power. Most general-use computers require 130 watts while running and about 200-205 watts when at startup.
Servers and highperformance workstations often have an abundance of random access memory (RAM), multiple drives, SCSI (Small Computer System Interface) adapters, and power-hungry video cards, along with one or more network cards. These systems often demand power supplies of 350-500 watts.