Providing Electricity: The Power Supply

The computer's power supply converts AC power from the domestic electrical service (power from a wall outlet) to the specific DC voltages that the CPU and other electronic circuits, and the motors of the disk drives, fans, and other mechanical devices, require to operate properly.

The power supply is usually located at the back of a tower or desktop case. Several sheet-metal screws hold the power supply to the case. The power supply often mounts on rails inside the case, next to a large hole in the back panel of the case that provides access to the AC power connector and the exhaust fan from the back of the computer. The back of the power supply might also include a small slide switch that selects the source voltage of either 117 VAC or 240 VAC, and a big master power switch that cuts off all AC power to the computer. Figure 4.2 shows the back of a desktop computer and identifies the power supply's location.

Figure 4.2: The power supply is located at the back of a computer's case.

Inside the computer, the power supply is a metal box with several bundles of wires connecting it to other components:

• The metal box is an essential part of the design because it keeps stray radio signals away from the rest of the computer and other nearby electronic equipment, and stray fingers away from lethal voltages inside the power supply.

• The power supply's DC outputs provide several voltages: +3.3 volts, +5 volts, −5 volts, +12 volts, and −12 volts, all relative to ground. These numbers are the specified values; in practice, the computer can work properly with voltage levels that are slightly higher or lower than the specification. The +3.3 v and +5 v outputs are used by the computer's electronic circuits; the motors in the disc drives, fans, and other mechanical components use +12 volts. The −5 v output was used by some older computers to provide power to floppy disk controllers and some other components, but it's now mostly obsolete. The −12 v output is used in some older serial port circuits, but it's not common in most modern computers. Both −5 v and −12 v are included to make new power supplies compatible with older components.

• The exact values of the power supply's output voltages are usually slightly more or less than the specified numbers. If the actual voltages are within an acceptable range (as shown in Table 4.1), the difference will not affect your computer's performance.

  Table 4.1: Power Supply Color Codes
  Open table as spreadsheet

  Nominal Voltage	Acceptable Range	Color of Wire
  +5 VDC	± 5%	Red
  −5 VDC	± 5%	White
  +12 VDC	± 5%	Yellow
  −12 VDC	± 5%	Blue
  +3.3 VDC	± 4%	Orange
  Ground or Common		Black
  Power Supply On indicator		Green

• The bundles of color-coded wires that come out of the power supply each have one or more standard power plugs that fit the power input sockets on the motherboard, the disk drives, the fans, and other components inside the computer's case. Each bundle of wires has a plastic connector at the end. Table 4.1 lists the industry-standard color codes for power supply wires.

Note 

Some power supplies, including those in Dell computers manufactured between 1996 and 2000, do not use the standard color codes.

The label on your computer's power supply (shown in Figure 4.3) includes these important items:

• The maximum number of watts and amps that the power supply can consume

• The individual output voltages

• The maximum current for each voltage

Figure 4.3: The label on the power supply shows its maximum input and output ratings.

A power supply rated at 250–300 watts or more is probably adequate for most home and office users. However a server with four or more disk drives, a game machine tricked out with the latest and most powerful CPU and graphics controllers, or a high-powered sound card can consume as much as 500 watts or more. As a rule of thumb, if you replace an existing power supply, use a new one rated at the same or a larger number of watts.