PC Hardware in a Nutshell, 3rd Edition

Main Power Switch Power Connectors

Desktop/AT and Desktop/BAT power supplies have a built-in paddle switch to turn power on and off. ATX and ATX-variant power supplies seldom have a physical power switch because they are turned on and off by the motherboard. Some ATX power supplies have a rocker switch on the back of the power supply that disconnects the power supply entirely from mains power. This can be useful because the alternative is disconnecting the power cable when you need to kill all power to the system, including +5V_SB, which is ordinarily always present.

Tower/AT and Tower/BAT power supplies have no built-in main power switch. Instead, they have four power leads that connect to a push-button or toggle switch on the case, as shown in Figure 26-4. These leads, which are usually white, black, blue, and brown, carry AC mains voltage to the power supply.

Figure 26-4. Typical connections for a Tower/AT or Tower/BAT power switch

Warning

Although these four wire colors are relatively standard, different switches require connecting them differently. These wires carry full mains voltage, which can kill you, so never work on them without first disconnecting the main power cable from the power supply. Connecting them improperly can also damage the power supply and the computer, so never use trial and error or guess about which wire goes where. Contact the case and power supply manufacturers ...