When turning a computer
on and off, various peripherals (such as printers, screen, scanner,
etc.) often have to be turned on and off as well. By using the 5-V
supply voltage from the USB interface on the
PC, all these peripherals can easily be switched on and off at the same
time as the PC. This principle can also be used with other appliances
that have a USB interface (such as modern TVs and radios). This so-called ‘USB-standby-killer’ can be realized with just 5 components. The USB
output voltage provides for the activation of the triac opto-driver
(MOC3043) which has zero-crossing detection. This, in turn, drives the TRIAC, type BT126.
The circuit shown is used by the author for switching loads with a
total power of about 150 W and is protected with a 1-A fuse. The circuit
can easily handle much larger loads however. In that case and/or when
using a very inductive load a so-called snubber network is required
across the triac. The value of the fuse will also need to be changed as
appropriate. The circuit can easily be built into a mains multi-way
powerboard. Make sure you have good isolation between the USB and mains sections.