Master/Slave Switch

In this age of
enlightenment any sort of relationship that could be described as
master/slave would be questionable but for the purposes of this circuit
it gives a good idea of how it functions. The circuit senses mains
current supplied to a ‘master’ device and switches ‘slave’ equipment on
or off. This feature is useful in a typical hi-fi or home computer
environment where several peripheral devices can all be switched on or
off together. A solid-state relay from Sharp is an ideal switching
element in this application; a built-in zero crossing detector ensures
that switching only occurs when the mains voltage passes through zero
and any resultant interference is kept to an absolute minimum.

All of the triac drive circuitry (including optical coupling) is
integrated on-chip so there are very few external components and no
additional power supply necessary. This makes the finished design very
compact. Diodes D1, D2, D3 and D4 perform the current sensing function
and produce a voltage on C2 when the master equipment is switched on. A
Schottky diode is used for D5 to reduce forward voltage losses to a
minimum. The circuit is quite sensitive and will successfully switch the
slave even when the master equipment draws very little mains current.
The RC network formed by R1 and C1 provides some protection for the
solid-state relay against mains-borne voltage transients.

This circuit is connected to the mains. it is important to be aware that
the chip has lethal voltages on its pins and all appropriate safety
guidelines must be adhered to! This includes the LED, for safety it must be fitted behind a transparent plexiglass shield.
Author: Karl Köckeis – Copyright: Elektor July-August 2004

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