Replacement Power Supply Form Factors and Connectors

When you shop for a power supply, you also need to make sure it can connect to your motherboard. There are two major types of power connectors on motherboards:

• 12-pin, modeled after the original IBM PC power connector and used on Baby-AT and LPX motherboards (see Figure 5.5)

  Figure 5.5. Twin 6-pin power connectors are used on LPX and other power supply types that attach to motherboards with a 12-pin connector.

  

• 20-pin, used by all motherboards in the ATX family (see Figure 5.6) and by the NLX riser card (which provides power to the NLX motherboard)

  Figure 5.6. The ATX power supply has a single 20-pin power connector. The extra connectors (compared to the LPX design in Figure 5.5) provide support for 3.3V and for software- or keyboard-controlled power down. Many of these power supplies don't have an external power switch for that reason.

  

caution

Another difference between ATX and LPX connectors is that the ATX connector uses specially shaped holes and a locking clip for positive keying, whereas the LPX 12-pin connector requires the user to properly align the black wires of connectors P8 and P9 to make a correct connection. If the LPX power supply wires are not oriented correctly, the motherboard could be damaged when the power is turned on.

Figure 5.7 lists the pinouts for these connectors. Note that only 5V and 12V DC power levels are supported by 12-pin (LPX) power supplies, whereas ATX power supplies also support 3.3V DC power levels.

Most systems that still use the 12-pin power supply connector use a type of power supply referred to as Slimline , LPX, or PS/2. Slimline power supplies have a power switch that connects directly to the power switch on the front of the case. Systems that use the 20-pin power supply with a full- size ATX motherboard (refer to Chapter 4, "The Motherboard and CPU") use ATX power supplies. However, systems that use the smaller Micro-ATX and Flex-ATX motherboards typically use an SFX (small form factor) power supply. SFX power supplies might have internal or external cooling fans, and can have the AC power connector on the short side (most versions) or the long side (used by some eMachine and HP computers that use micro-tower cases). ATX and SFX power supplies do not use a cable-mounted power switch, although some ATX power supplies have a built-in on-off switch.

Figure 5.8 shows how these power supplies compare physically to each other.

Figure 5.8. Rear view of typical LPX (top), ATX (middle), and SFX-L (bottom) power supplies.

If your wattage calculations or your tests (covered later in this chapter) agree that it's time to replace the power supply, make sure the replacement will

• Have the same power supply connectors as the original.

• Have the same form factor (shape, size, and switch location).

• Have the same or higher wattage rating; a higher wattage rating is highly desirable.

• Support any special features required by your CPU and motherboard. For example, motherboards that support the Pentium 4 processor use a 4-pin connection known as the ATX-12V connector as well as the standard 20-pin ATX connector. Some motherboards use a 6-pin connector known as the auxiliary connector as well as the normal 12-pin or 20-pin connector. See Figure 5.9.

  Figure 5.9. ATX-12V (left) and auxiliary (right) power connectors.

  

caution

Most Dell computers built from September 1998 through 2000 use a proprietary version of the 20-pin ATX power supply connector. Dell's version doesn't have any 3.3V wires, so if you plug a standard power supply into a Dell PC that uses the proprietary version, or use a regular motherboard as an upgrade for a model that has the proprietary power supply, stand by for smoke and fire! See "Avoiding Power Supply Hazards," later in this chapter, for all the gory details.

To ensure reliability, a power supply should have a UL (Underwriters Laboratory) or CE (European) rating; these are the leading international standards associations for powered devices. Many very low-cost power supplies lack either rating and can produce erratic voltage levels and might represent a significant high-voltage hazard .

It's also desirable to select a power supply that has enough Molex (large four-wire) and floppy drive (small four-wire) connectors to support the drives and case fans you plan to use in the system (see Figure 5.10). If your power supply doesn't have enough connectors, you can add Y-splitters to divide one power lead into two, but these can short out and can also reduce your power supply's efficiency.

Figure 5.10. Molex (left) and small (right) drive power connectors.