"Roger Mills" wrote in message
...
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?

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.
--
Dave W