On Tue, 09 May 2006 22:54:58 +0100, Harry Bloomfield
wrote:

T i m brought next idea :
Anyone aware of a an 'emergency' light that (ideally that functions
like a std 5' flouro) that would:

Come on when turned on (no!g). ;-)

Stay on when 'on' and the power fails.

A standard 'Maintained' emergency light fitting would do both of the
above.

Ok, I've got a 18" one now so I can play a bit ..

Not to come on when 'off' when the power fails (tricky one huh) ;-)

A rather dangerous way to do it, but a relay wired between the 'always
live' and the switch wire would only operate when the light switch was
off and could be used to inhibit the discharge of the battery via the
light - a normally closed contact pair in the +ve battery lead.

Ok, ok .. I getcha ..

Problem
is that it should not interrupt the charging circuit of the battery -
now if we add a diode across those contacts (around 50v 3amp), diode
arrow pointing to the +ve on the battery that would allow it to charge.

Ok ..

Problem- The diode would drop around 0.7v - so the charge would be less
than optimum except when the lights are turned on.

These charger are normally not that accurate are they Harry, neither
constant voltage nor current and just relying on the 'softness' of the
cct to stop overcharge?

A suitably chosen
value of resistor in series with the diode, high enough that under
short discharge conditions it presented almost no load on the battery,
but under charge helped overcome the 0.7v drop of the diode perhaps(?).

Hmm ..

The higher the voltage of the battery, the less the voltage drop would
matter. The diode would be shorted out if the power failed, so there
would be full output under power fail conditions.

Ok, thanks for that and food for thought.

I might just install it as_is and put up with the odd time when the
power fails and I'm not in the garage (so wouldn't need the light).

Are there more fancy versions with opto sensors that stop the light
coming on in daylight (even if the power fails)?

All the best ..

T i m