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USB-Controlled Power Switch

Build this and automatically power up all your PC's peripherals when you start the computer. It works via the PC's USB port.

By Jim Rowe

Most of the first generation of personal computers had an ‘IEC’-type 240V outlet on the back of the box, which provided power switched by the PC’s own power switch.

This allowed you to control the power to the computer’s monitor, printer and other peripherals simply by plugging in a power distribution board to this outlet and plugging the peripheral power cords into the distribution board outlets.

The power switch on the front or side of the PC then controlled everything, which was very neat and convenient.

Click for larger image
Here's the completed project, mounted (in this case) inside a Kambrook KPB6 Powerboard. One outlet is sacrificed in this version to accommodate the USB-UP PC board and a label is fitted over the unused outlet.
Click for larger image
The alternative power board from Jaycar, the Powertech MS4031. It is more expensive than the Kambrook but does not need any "surgery" to fit the USB-UP PC board inside (so you retain all six outlets) and also has very worthwhile surge/spike protection built in.

Unfortunately this handy switched power outlet disappeared from later models, presumably because it became harder to implement when PC manufacturers changed over to software-controlled power supplies.

So, with most newer PCs, if you wanted to control everything with a single switch, you’ve been forced to use a power distribution board with its own master power switch.

There is a way to get true single-switch operation, though, if you’re using a recent model PC with at least one USB port (and that means just about any PC made in the last few years or so).

This is to control the power fed to the peripherals using an electronic switch triggered by low-voltage DC from the PC itself, via its USB port. The electronic switch then turns the peripherals on when the PC is turned on, and turns them off when it’s turned off.

The electronic switch needs to be optically isolated, so there’s no risk of 240VAC getting back into the low voltage circuitry of the computer via the USB port. But a high-voltage opto-coupler neatly solves that problem.

In this article we’ll show you how to build a USB-controlled electronic power switch right inside a low-cost power distribution board, for maximum safety and convenience.

PLEASE NOTE:

This project involves opening and modifying a mains powerboard. Do not attempt this project unless you are experienced in mains wiring and construction. Contact with the mains can cause severe injury or death. Never work on a power board with the plug in an outlet, let alone turned on.

ALSO: While the original powerboard is rated at 10A, 240V (2400W) the modifications made limit the total loading to around 700-750W, or 3A. This limit should be more than adequate for the intended application: switching computer peripherals.

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