On 27/04/2017 12:41, John Rumm wrote:
On 26/04/2017 23:31, Roger Mills wrote:
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?


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.



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
https://app.box.com/s/no8ccg03hf7yr2f8odqlfw9r03fael5z) 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.
--
Cheers,
Roger
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