In my parents house, to avoid ripping up the bathroom wall tiles, around 25 years ago the local sparky installed a 'switch multiplier'. This allowed the single switch in the wall the be replaced with a dual switch, so that you could control a light and fan on one circuit, and a heat lamp on another.
Of course, the unit has now died (relay burnt out) and while a replacement is being sourced and fitted (Clipsal 780K1) I took the old one apart to see if I could work out how it worked.
First of all, the switches in the wall plate are fitted with series diodes, one 'reverse' biased and the other 'forward' biased so that the multiplier can detect which switch is closed, based on the polarity of the pulse it receives.
The Multiplier itself has two relays, one for each load circuit and the control circuitry enables each relay as required.
I've attached a schematic of the system (having poblems uploading mutitple pics - will post photos of the unit later).photos of the unit, and I managed to strip the foam of the back so I could reverse engineer the circuit. I've also attached my schematic - feel free to correct me if there are any errors on there. I've omitted the load switching on the schematic to give clarity to the control circuit.
Now I believe that I understand it's operation in principle - For example if S1 is closed, D2 conducts positive pulses of the 230V AC, D4 also conducts these pulses and K1 closes.
Similar for D3 / D5 / K2 on the negative cycle.
But, this is what confuses me:
1. What role does D6 / R8 and D7 / R9 play? EMF spike protection only? Then why the resistors?
2. If you feed 230V coils half wave rectified power, are you not then running them at lower voltages, and if so is there a risk of overheating AC coils (I've had expereince with AC solenoids not liking being fed DC).
I look forward to your comments - and yes I'm playing it safe and will have a new switch multiplier installed rather then risk burning the house down with a DIY repair!