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Complete CompTIA A+ Guide to PCs: Disassembly and Power

May 24, 2013

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Power Supply Form Factors

Just as motherboards come in different shapes and sizes, so do power supplies. Today’s power supply form factors are ATX, ATX12V v1.x, ATX12V v2.x, and micro-ATX. Other form factors include LFX12V (low profile), SFX12V (small form factor), EPS12V (used with server motherboards and has an extra 8-pin connector), CFX12V (compact form factor), SFX12V (small form factor), TFX12V (thin form factor), WTX12V (workstation form factor for high-end workstations and select servers), and FlexATX (smaller systems that have no more than three expansion slots). Intel, AMD, and video card manufacturers certify specific power supplies that work with their processors and video cards. A computer manufacturer can also have a proprietary power supply form factor that is not compatible with different computer models or other vendors’ machines. Laptop power supplies are commonly proprietary.

The ATX12V version 2 standard has a 24-pin motherboard connector instead of a 20-pin version 1 connector. This did away with the need for the extra 6-pin auxiliary connector. In addition, version 2 power supplies have a SATA power connector. Some 24-pin motherboard connectors accept the 20-pin power supply connector. Table 4.2 lists the possible ATX power supply connectors.

Table 4.2. ATX power supply connectors

Connector	Notes	Voltage(s)
`24-pin`	`Main ATX power connector to the motherboard`	`+3.3, +5, +12, –12`
20-pin	Main power connector to the motherboard	+3.3, +5, –5, +12, –12
`15-pin`	`SATA connector`	`+3, +5, +12`
8-pin	12V for CPU used with an ATX12V v1 power supply	+12
`8-pin`	`PCIe video; connects to a PCIe video adapter. Note that some connectors are 6+2-pin meaning they accept either the 6- or 8-pin cable.`	`+12`
6-pin	PCIe video; connects to PCIe video adapter	+12
`6-pin`	`Sometimes labeled as AUX; connects to the motherboard if it has a connector`	`+3.3, +5`
4-pin Molex	Connects to peripheral devices such as hard drives and CD/DVD drives	+5, +12
`4-pin Berg`	`Connects to peripheral devices such as the floppy drive`	`+5, +12`
4-pin	Sometimes labeled as AUX or 12V; connects to the motherboard for CPU	+12
`3-pin`	`Used to monitor fan speed`	`N/A`

Figure 4.19 shows a few ATX power supply connectors. Figure 4.20 shows more ATX power supply connectors.

Figure 4.19. Common power supply connectors

Figure 4.20. ATX power supply connectors

Figure 4.21 illustrates the compatibility between the ATX 20- and 24-pin motherboard connector standards. Notice in Figure 4.21 that the power cable is only one connector, notched so the cable inserts into the connector one way only. This is a much better design than older power supplies, where two connectors were used and could be reversed. Also, notice that a power good signal (labeled PWR_OK in Figure 4.21) goes to the motherboard. When the computer is turned on, part of POST is to allow the power supply to run a test on each of the voltage levels. The voltage levels must be correct before any other devices are tested and allowed to initialize. If the power is okay, a power good signal is sent to the motherboard. If the power good signal is not sent from the power supply, a timer chip on the motherboard resets the CPU. Once a power good signal is sent, the CPU begins executing software from the BIOS. Figure 4.21 also shows the +5vsb connection to provide standby power for features such as Wake on LAN or Wake on Ring (covered later in this chapter).

Figure 4.21. ATX 24- and 20-pin motherboard connectivity

A high-quality power supply delays sending the power good signal until all of the power supply’s voltages have a chance to stabilize. Some cheap power supplies do not delay the power good signal. Other cheap power supplies do not provide the power good circuitry, but instead, tie 5 volts to the signal (which sends a power good signal even when it is not there).

The number and quantity of connectors available on a power supply depends on the power supply manufacturer. If a device requires a Berg connector and the only one available is a Molex, a Molex-to-Berg connector converter can be purchased. If a SATA device needs a power connection, a Molex-to-SATA converter is available. Figure 4.22 shows a Molex-to-SATA converter and a Molex-to-Berg converter.

Figure 4.22. Molex-to-SATA and Molex-to-Berg converters

Power supply connectors can connect to any device; there is not a specific connector for the hard drive, the optical drive, and so on. If there are not enough connectors from the power supply for the number of devices installed in a computer, a Y power connector can be purchased at a computer or electronics store. The Y connector adapts a single Molex connector to two Molex connectors for two devices. Verify that the power supply can output enough power to handle the extra device being installed. Figure 4.23 shows a Y power connector.

Figure 4.23. Y Molex connector

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