How do light switch security timers work?

A friend asked me to have a look at a Superswitch 2304 timer which stopped working when the bulb which it was switching blew, and replacing the bulb didn't fix it.

The answer, of course, was that when the bulb blew, so did an internal

3A fuse in the timer.

But, having had a look at the thing, I don't understand how it works. It replaces a conventional wall switch and only has 2 connections - live and switched live. Yet the thing itself needs power to operate the display/logic/relay, etc. - and it doesn't have a neutral connection. My first thought was that maybe it gets a connection from an earthed backbox - but the holding screws go through insulated sleeves, and don't connect to the circuit board.

My next thought was that it could get a neutral connection through the bulb (it apparently only works with tungsten bulbs). But surely, that would only work when the light was off. Once it had switched the light on, the live and switched live would be at the same voltage and wouldn't be able to power it to do any further timing/displaying/switching.

Clearly, I must be missing something! Anybody know what?

Reply to
Roger Mills
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A battery or supercapacitor.

Reply to
Graham.

Are you sure its not a simple bimetal strip in an enclosure in series with the bulb? I have one like that but in order for it to work with CFLs they have to be over 8 watt apparently, don't know about leds. Brian

Reply to
Brian Gaff

Roger Mills submitted this idea :

When the switch is off, there is potential/ voltage across the two terminals. That potential charges either a tiny battery, or a specialised capacitor so it has a power source when the light is on and there is no potential across the switch.

Most likely problem if the fuse has been blown by the lamp, is that the Triac has also been blown or possibly the relay, if it uses a relay for switching.

Reply to
Harry Bloomfield

Even when the lamp appears to be on the capacitor can be charged during the first few degrees of each cycle without any noticeable effect on the lamp brightness.

Reply to
Bob Minchin

According to the manual, it has an internal rechargeable battery that needs

2 hours with switch turned ON to charge from cold, and then gives 24 hour backup. When ON there must be a small voltage drop across the unit high enough to supply the charging current, and the display is run from the battery at all times. The 400W max bulb would take 1.7A, so if the voltage drop across the unit is say 1.5V to be able to charge an NmH cell, less than 3W is dissipated in the unit. The small voltage is what might be expected across a triac when conducting.
Reply to
Dave W

Yup you are right, it draws its power through the lamp (hence the filament lamp restriction). Its a very low level current draw and hence not enough to light the "off" lamp.

As to how its powered while the lamp is on, there are several ways it is done - depending a bit on the switch design. One option is to just keep a backup power source charged when the lamp is off. However that is sub optimal since you don't know how long the lamp will be on for, and whether there is sufficient "off" time to keep it charged. Better options will still allow it to draw power when the lamp is on, usually at the expense of dropping a small voltage in the switch.

Reply to
John Rumm

Thanks for all the replies.

I can't see anything which looks like a battery in it - but it does have a 6.3v 1000 uF capacitor (the blue one towards to bottom RH corner of

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which may well store some energy.

I don't know whether it would still work if I replaced the fuse - I didn't have a 3A BS646 fuse to hand - and my friend asked me to replace it with a simple on/off switch, so I haven't had an opportunity to test it.

Reply to
Roger Mills

replying to Roger Mills, Eamonn wrote: Iam not sure either but I have a Superswitch 2304 for about 15 years and now the LCD is unreadable so I cannot set the time. Do you remember how to reorder this switch as I have been very happy with it up to now, also my swith has 3 connection terminals at the back Regards Eamonn

Reply to
Eamonn

How old is this one I wonder. Brian

Reply to
Brian Gaff (Sofa 2)

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