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Figure 10-12 Desktop PSU

The PC uses the 12.0-volt current to power motors on devices such as hard drives and CD-ROM drives, and it uses the 5.0-volt and 3.3-volt current for support of onboard electronics. Manufacturers may use these voltages any way they wish, however, and may deviate from these assumptions. Power supplies also come with standard connectors for the motherboard and interior devices.

Power to the Motherboard

Modern motherboards use a 20- or 24-pin P1 power connector. Some motherboards may require special 4-, 6-, or 8-pin connectors to supply extra power (Figure 10-13). We’ll talk about each of these connectors in the form factor standards discussion later in this chapter.

Figure 10-13 Motherboard power connectors

Power to Peripherals: Molex, Mini, and SATA

Many devices inside the PC require power. These include hard drives, floppy drives, optical-media drives, zip drives (for techs who enjoy retro computing), and fans. The typical PC power supply has up to three types of connectors that plug into peripherals: Molex, mini, and SATA.

Molex Connectors The most common type of power connection for devices that need 5 or 12 volts of power is the Molex connector (Figure 10-14). The Molex connector has notches, called chamfers, that guide its installation. The tricky part is that Molex connectors require a firm push to plug in properly, and a strong person can defeat the chamfers, plugging a Molex in upside down. Not a good thing. Always check for proper orientation before you push it in!

Figure 10-14 Molex connector

Mini Connectors All power supplies have a second type of connector, called a mini connector (Figure 10-15), that supplies 5 and 12 volts to peripherals, although only floppy disk drives in modern systems use this connector. Drive manufacturers adopted the mini as the standard connector on 3.5-inch floppy disk drives. Often these mini connectors are referred to as floppy power connectors.

Be extra careful when plugging in a mini connector! Whereas Molex connectors are difficult to plug in backward, you can insert a mini connector incorrectly with very little effort. As with a Molex connector, doing so will almost certainly destroy the floppy drive. Figure 10-16 depicts a correctly oriented mini connection, with the small ridge on the connector away from the body of the data socket.

Figure 10-15 Mini connector

Figure 10-16 Correct orientation of a mini connector

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CAUTION As with any power connector, plugging a mini connector into a device the wrong way will almost certainly destroy the device. Check twice before you plug one in!

SATA Power Connectors Serial ATA (SATA) drives need a special 15-pin SATA power connector (Figure 10-17). The larger pin count supports the SATA hot-swappable feature and 3.3 V, 5.0 V, and 12.0 V devices. SATA power connectors are L shaped, making it almost impossible to insert one incorrectly into a SATA drive. No other device on your computer uses the SATA power connector. For more information about SATA drives, see Chapter 11, “Hard Drive Technologies.”

Figure 10-17 SATA power connector

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NOTE SATA also supports a slimline connector that has a 6-pin power segment and a micro connector that has a 9-pin power segment.

Splitters and Adapters You may occasionally find yourself without enough connectors to power all of the devices inside your PC. In this case, you can purchase splitters to create more connections (see Figure 10-18). You might also run into the phenomenon of needing a SATA connector, but having only a spare Molex. Because the voltages on the wires are the same, a simple adapter will take care of the problem nicely.

Figure 10-18 Molex splitter

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NOTE It’s normal and common to have unused power connectors inside your PC case.

Testing DC

A common practice for techs troubleshooting a system is to test the DC voltages coming out of the power supply. Even with good AC, a bad power supply can fail to transform AC to DC at voltages needed by the motherboard and peripherals. Grab