Light Switch Multiplier - Page 1
Forum section: electronics / projects

author: ludzinc
author_rank: Unknown
timestamp: April 20, 2015, 11:23:18 am
content: 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!

author: LaserSteve
author_rank: Super Contributor
timestamp: April 20, 2015, 01:51:52 pm
content: The two resistors limit dissipation in the reverse clamps and soften the EMF spikes as well.Steve

author: electr_peter
author_rank: Unknown
timestamp: April 20, 2015, 03:42:14 pm
content: Can relays be driven in such a way? Only one half cycle is powered, other 8-10ms are unpowered. Seems to be a very brutal job for a relay and circuitry connected to it.

author: SeanB
author_rank: Super Contributor
timestamp: April 20, 2015, 05:43:18 pm
content: Done all the time in industrial controls, where you find a lot of 110VDC relays which are driven from 230VAC using a series diode. The relay pole has a shading ring on the top, which is a copper ring shorting part of the pole piece. This reduces chatter as it keeps the magnetic field above a level which holds the pole piece on during the off time. Only drawback is the release time is now going to be longer. The coil integrates the high voltage pulse so that the heating is the same as if it was 100VDC.

author: ludzinc
author_rank: Unknown
timestamp: April 20, 2015, 10:50:17 pm
content: InterestingHowever in this application the relays are marked as having 230V coils.What I don't know is the minimum pull in voltage of the relay.Time to find a datasheet!

author: bson
author_rank: Unknown
timestamp: April 21, 2015, 12:02:33 am
content: D6 and D7 are backflow shunts, to permit current to continue to flow through the coil during the cut-off phase. Apart from being required to keep the coil from building up huge voltage spikes they also help bridge the off phase with the backflow circulating. The two relays each get opposing halves of the AC through their opposing series diodes.