PC mains switch

Posted on May 24, 2012

This project couldn't be any simpler if it tried. Using the PC's internal 12V supply to operate a relay, it is 100% reliable. If the PC is on, the peripherals are on and vice versa - it can't be otherwise (unless the relay fails). Other solutions have been suggested in magazines and the like, often using a USB port. The problem is that some PCs don't switch off the 5V supply to USB devices even when the PC is off. The auxiliary supply within the ATX power supply unit maintains a low power 5V supply all the time, and this is sometimes used for the USB ports to charge cordless mouses (meece?) or other devices.

PC mains switch
Click here to download the full size of the above Circuit.

I use my unit to switch off the power to my monitor and PC sound system, and the one I built for SWMBO (she who must be obeyed) shuts down her modem and printer. Any peripheral device can be controlled provided the total current rating for the power board is not exceeded. Naturally enough, additional units can be used if you need to switch off a lot of equipment - the relay loading on the 12V supply is minimal, so several relays can be used if needed. There is nothing special about the PC wiring, but it is a very good idea to make sure that the 3.5mm socket is insulated from the panel. Although the polarity is not important, it is strongly recommended that the black lead (earth / ground) connects to the jack sleeve (the threaded section that goes through the panel). The +12V supply connects to the tip. If you happen to get it wrong somehow, the insulation will prevent a short circuit of the 12V supply. If the supply is shorted, it will probably cause an instant supply shut-down and possible data loss. Please Note Although not shown in the PC wiring loom above, it is essential that you use a 1A in-line fuse in the +12V supply lead. I have been advised by a reader that many PC power supplies have no current limiting for the 12V supplies, and an accidental short-circuit can cause extremely high current to flow. This will cheerfully burn the insulation off the wiring, and may also damage the power supply. As an...

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