case mod 101: the ATX power switch demystified

A lot of case mod projects involve the power switch. And so a lot of modders have questions that involve the basics of how a PC power switch works and what sort of mods can be made to it. In this article, we’ll explore the theory and practice of your computer’s power switch, and learn a little bit about the ATX standard along the way. We’ll also discuss the five things to keep in mind when doing basic power switch mods.

Ancient History: the AT power switch

Before the ATX specification was developed, the power switch was a frequent cause of failure. It served to interrupt the mains power to the power supply, so it carried the full current load of the computer at mains voltage. So besides being failure-prone, it was relatively dangerous to work on. You could misconnect an AT power switch and cause a dangerous short circuit. Plus, there was no way for the PC to shut off power, since the switch was in the mains side.

Modern Times: the ATX power switch

The ATX specification fixed the worst problems of the AT power supply. It took the power switch out of the mains side, so that safety and reliability improved dramatically. It made the power supply switchable from the DC side, so that software shutdown was now possible. This had the side effect of allowing many new switch styles, since the switch no longer had to carry the full current load of the PC, it could be a mini or sub-mini pushbutton, and could be easily located just about anywhere on the case.

So lets look a little closer at how the PC is powered on. The PSU is powered from the mains supply. Some PSUs have their own shutoff switch, which allows interrupting the mains supply for whatever reason. As long as the PSU is powered from the mains, it is also providing your motherboard with a small amount of power from a connector known as the +5VSB (pin 9 of the motherboard connector, connected to a purple wire), or five volt standby. Even though your system is powered down, you are still drawing a small amount of current from this. The circuits that control the power switch are among the things using that power. When you press the power button, you momentarily close a set of contacts. The motherboard circuity reacts to that signal by sending a signal to the PSU. The signal is called the PS_ON# (pin 16 of the motherboard connector, connected to a green wire), and the setting of a LOW voltage (0V) on this connector tells your PSU to deliver full power to your system. Many of you are probably familiar with the trick of “hot-wiring” a PSU by connecting a pin with a black wire to the one with the green wire. In doing this you are forcing 0V on PS_ON#, which starts the power supply. When your computer does this, the circuit holding that pin low is controlled by software, so this is how a soft shutdown works (as well as things like wake-on-modem or wake-on-LAN).

When the PSU is delivering full system power, it’s also required to hold a HIGH (+5V) signal on a connector called PWR_OK (pin 8 of the motherboard connector, connected to a grey wire), which signals to the system that voltages are within tolerances dictated by the specification. These three signals, +5VSB, PS_ON#, and PWR_OK, make up the “housekeeping” connections to your PSU. They allow the management of power by the system, and together solve the problems with the AT spec.

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