Automatic master/slave power switch...

[Nick]

WARNING:

This circuit is LIVE MAINS, so be very very careful. The risk is not just to you, but also to anyone else who uses this equipment or anything connected to it.

The responsibility is yours alone!

Many of us have systems that have several mains-powered boxes that have to be turned on & off together, but which may or may not have individual power switches. A case in point is my recent acquisition (written originally in 2011!) of a Quad 405-2 - fine if you are driving it off a switched outlet from a pre-amp, bit of a pain if you are not.

You can buy socket strips that have master-slave outlets where you connected the pre-amp/TV/whatever to the master outlet and the rest of your kit to the slave outlets, but I've found these unreliable and sometimes, if you have an SS pre-amp, not sensitive enough... the ones I've played with require a minimum of about 10W load to switch.

So, I did the following:

Based on an idea I'd seen before, I modelled the use of a zero-crossing solid-state relay (SSR) driven by the voltage drop across some a diode. There are two ways of getting sufficient voltage & current to drive the SSR - either use 3 diodes or use a small multiplier. The multiplier approach has the advantage of dissipating less heat under load at the cost of more of the larger components (capacitors) and the multiple-diode approach is the inverse of that.

   

I've gone initially for the multiple-diodes route as I've built my controller into a Bulgin 10A/6 way IEC 320 C13 outlet and only had about 1" x 1.5" of space to play with. I've also fused the unit at 5A as the SSR (an S202S02 - cheap-ish and good) is rated at 8A - 5A should be plenty. The snubber values are calculated for an inductive load (transformers) as per the SSR data sheets.

Both models work well, with the RMS values through the SSR LED being about right, so I drew it up in Eagle and built it:

   

The idea is really simple: When current is drawn by the master socket, a voltage drop of about 2.8V is developed across D1,D2 & D3 (3 x Vf). This is half wave rectified and used to turn on an SSR which in turn powers up the slave socket. D4 provides a return path for the other half of the cycle. D5 is a Schottky to limit forward voltage drop and thus limit losses (current though it is tiny). C1 & R1 are a snubber to provide some protection for the SSR if you are using an inductive load, e.g. an extractor

R3, C3 provide a voltage dropper for a "slave enabled" LED - D6 limits the reverse voltage over the LED to 0.6V (LEDs don't like reverse voltage) - R2 & D7 are the slave-enabled LED - all these components can be removed if you don't want the slave-enabled indicator. C1 & C3 should be X2 class...

The reason the snubber capacitor is a 10n X2/390R rather than something larger like 100n/100R is that Zc at 50Hz allows a good few mA through the snubber even when the switch is off - not good as it would be enough to light the "slave on" LED as well as leaking into the controlled kit. Using 10n provides good snubbing and low leakage.

When the master turns off, the slave turns off.

The whole thing is really simple and cheap and can be built into a power strip without much bother - I have a few of these around for various reasons; the one in the workshop has 3 masters and one slave - the slave is on the extractor, the masters are connected to various power tools. I also use them in the house to automatically turn off power amps when the preamp is turned off... You can split the strip in any ratio you like. The Sharp SSR are only about £3.50 at Farnell and are really neat devices - the whole thing probably costs less than £10 to build...

I really like this circuit