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Power-Supply Connectors

Power supplies employ several types of connectors; all are easy to identify and use. On the outside of the computer enclosure, a standard male AC plug and three-conductor wire (two power wires and a ground) draws current from a wall outlet, with a female connection entering the receptacle in the back of the power supply. On the inside are three types of connectors: the power main to the motherboard (which differ, as mentioned, in AT and ATX models) and two types of four-pin fittings to supply 5 volts and 3.3 volts of power to peripherals such as the floppy disk and hard disk drives. Let's take a close look at each in turn.

AT-Style Connections to the Motherboard

A pair of almost identical connectors, designated P8 and P9, link the power supply to the motherboard (see Figure 5.1). These connectors are seated into a row of six pins and matching plastic guides, or "teeth," on the motherboard. The P8 and P9 connectors must be placed in the proper orientation. The motherboard manual will show which fitting is for P8 and P9. If the connectors are not marked, make sure that the two black wires on each plug are side by side and that the orange wire (on P8) and the two red wires (on P9) are on the outside as you push them into place.

Figure 5.1 P8 and P9 connectors and motherboard fitting

The following table of power cables shows voltage values for each of the color-coded wires on P8 and P9. The ground wires are considered 0 volts; all voltage measurements are taken between the black wires and one of the colored wires.

Cable Color Supply In Tolerance
Yellow +12 10%
Blue -12 10%
Red +5 5%
White -5 5%
Black Ground N/A
Some computer makers employ proprietary power connections that require a special power supply. To install the new part, you will need to follow the instructions that are included with the computer.

Remember to install the P8 and P9 plugs so that the black wires are side by side. Installing them on the wrong receptacle can damage both the motherboard and the power supply. Figure 5.2 shows the P8 and P9 connectors and a motherboard.

Some power supplies have a third P-style connector. This is not used except on a very few motherboards and can be ignored on those where it is found . If you run into one, refer to the manual that came with the part for instructions on its requirements and installation.

Click to view at full size.

Figure 5.2 Connecting P8 and P9

ATX Motherboard Connections

The newer ATX main power connection is much easier to install. A single 20-wire plug is set into a fitted receptacle and secured with a catch on the side of the plug that snaps over the fitting. Figure 5.3 shows the parts being seated. A small, flat-tip screwdriver is a handy tool for easing the pressure on the catch to remove the plug. In some cases, it can be used during installation as well.

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Figure 5.3 Placing an ATX plug in its motherboard receptacle

Connections to Peripheral Hardware

Two standard types of connectors can connect to peripheral hardware:

  • Molex connector: This is the most commonly used power connector. It provides both 12-volt and 5-volt power. Hard disk drives, internal tape drives, CD-ROM drives, DVD (digital video disc) drives, and older 5.25-inch floppy disk drives all use this fitting. The Molex connector has two rounded corners and two sharp corners to ensure that it will be properly installed.
  • Figure 5.4 Molex connector (not to scale)

  • Mini connector: Most power supplies provide one or more "mini" connectors (see Figure 5.5). The mini, shown in Figure 5.4, is used primarily on 3.5-inch floppy-disk drives. It has four pin-outs and, usually, four wires. Most are fitted with keys that make it difficult, but not impossible, to install upside down. Be sure to orient the connector correctly; applying power with the connector reversed can damage or destroy the drive.
  • Figure 5.5 Mini connector (not to scale)

Two- and Three-Pin Mini Plugs

A less common type of power connector is used to connect the fan of a Pentium II or III processor to the motherboard for power, to connect a CD-ROM drive to a sound card, and to provide power for 3.5-inch floppy disk drives. These connectors have two or three wires which are usually red and black or red, yellow, and black.

Do not connect power-carrying mini plugs to audio or data devices such as a CD or a sound card, because you might damage or destroy those devices.

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