case mod 101: the ATX power switch demystified

ATX connector pinout diagram
ATX connector pinout diagram

So when you want to go about modifying your power switch, understanding this information is important. It will allow you to pick a proper switch for the purpose. Here’s the five things you need to keep in mind about the ATX power switch:
First, the switch makes momentary contact. This means that a toggle-type switch will generally not work as a replacement power switch unless it’s also momentary. If you try to use a toggle, the motherboard circuitry will interpret the closed switch as a request to power down.
Second, the switch is normally open, also known as push-to-make. When you select a momentary switch, be aware some are normally closed, or push-to-break.
Third, the current carried by the ATX power switch is very low, so you may select almost any switch you like without regard to the current rating. This also means you can extend the wires to your power switch using inexpensive hookup wire.
Fourth, the voltage carried is no more than 5V, so you don’t need to pay attention to the voltage rating.
Fifth, the minimum switch you need is SPST (single-pole, single-throw), but any of the other switch types can be connected to act as a SPST switch, simply by selecting the correct two leads.

That covers the basics, and even though this is a 101 article, I’m going to address a couple of other common questions about power switches.

Q: Can I use an ignition-style, or key switch?
A: Absolutely, as long as it meets the requirement of being momentary, it will work as an ATX power switch.

Q: I’d like to lock the kids out, or keep them from powering the machine off while I’m using it. What sort of switch is best?
A: You can use a non-momentary key switch in series with the power button. When the key switch is closed, the power button will work normally. When the key switch is open (and presumably the key removed), the power button will neither power on or power off your system. Soft shutdown will work like normal however.

Q: Can I use a military-style covered toggle switch?
A: Usually, the flip covers are separate from the switches. So if you can buy a momentary toggle switch that fits your flip cover, you can use that directly. Several online retailers will sell you this combination. However, you may also choose to use the toggle as a “arming” switch for the real power button. For this trick, place the toggle in series with the real power button, so that it interrupts the connection made by the power button (you may interrupt either wire to the power switch). This is especially effective if you have a lighted power button, since you can “arm” the power button more visibly by having the toggle light up the power button at the same time. That would require a DPST or DPDT toggle to close both the switch and the light circuit simultaneously.



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