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Purposes of a Power Supply

The power from a wall outlet is high-voltage AC. The type of power computers need is low-voltage DC. All computer parts (the electronic chips on the motherboard and adapters, the electronics on the drives, and the motors in the hard drive and optical drive) need DC power to operate. Power supplies in general come in two types: linear and switching. Computers use switching power supplies. The main functions of a power supply include the following:

  • Convert AC to DC
  • Provide DC voltage to the motherboard, adapters, and peripheral devices
  • Provide cooling and facilitate air flow through the case

One purpose of a power supply is to convert AC to DC so the computer has proper power to run its components. An ATX power supply does not connect to the front panel switch as the old AT-style power supplies did. With the ATX power supply, a connection from the front panel switch to the motherboard simply provides a 5-volt signal that allows the motherboard to tell the power supply to turn on. This 5-volt signal allows ATX power supplies to support ACPI, which is covered later in the chapter, and also lets the motherboard and operating system control the power supply. Figures 4.24 and 4.25 show the front panel connections to the motherboard on two different computers.

Figure 4.24

Figure 4.24. Front panel connections to a motherboard

Figure 4.25

Figure 4.25. Another view of front panel connections

Another purpose of a power supply is to distribute proper DC voltage to each component. Several cables with connectors come out of the power supply. With ATX motherboards, there is only a 20- or 24-pin connector used to connect power to the motherboard. The power connector inserts only one way into the motherboard connector. Figure 4.26 shows an ATX connector being inserted into a motherboard.

Figure 4.26

Figure 4.26. Installing an ATX power connector on a motherboard

Another purpose for a power supply is to provide cooling for the computer. The power supply’s fan circulates air throughout the computer. Most computer cases have air vents on one side, on both sides, or in the rear of the computer. The ATX-style power supply blows air inside the case instead of out the back. This is known as reverse flow cooling. The air blows over the processor and memory to keep them cool. This type of power supply keeps the inside of the computer cleaner than older styles.

Electronic components generate a great deal of heat but are designed to withstand fairly high temperatures. Auxiliary fans can be purchased to help cool the internal components of a computer. Some cases have an extra mount and cutout for an auxiliary fan. Some auxiliary fans mount in adapter slots or drive bays.

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