> I've come upon this interesting paragraph on piclist.com 
> (don't I already know what is written there? Well, no, in 
> fact now that there is about 3GB of text on the site, I find 
> more and more often that I have no clue were stuff came from. 
> Rather fun actually!) and I was curious about the unanswered 
> part of it: How does one know how much voltage to put through 
> the lamp to heat the filament without producing light?
> 
> "One feature of incandescent bulbs that you can exploit is 
> that when off, an incandescent filament is very low 
> resistance- much lower than when it is glowing. This allows 
> lamps to be used to protect against short circuits.


I've seen this in a few places.  Car lights was one, a blown bulb was
detected by a lack of current flow when switched on.  Presumably a short
circuit would be fould be lack of current flow caused by  blown fuse.
The stop lights were pre-heated by a small amount of current trickled
thru them.  Quite low, a couple of volts of so.  The theory is that
reducing the current onrush at switch on stops them blowing, and the
light itself comes on faster, so safety is improved (much like LEDs).

I also fixed a set of blinky chaser lights at a fish 'n' chip shop once.
There were 3 strings of lights, driven by 3 relays that were triggered
in sequence.  The system had 2 voltage sources, mains and a drop-down
transformer.  When ON, the relay fed mains (240v) to its string of
lights.  When OFF, it got the lower voltage from the transformer.
Simple, worked well.  So old I suspect it was originally ran the warning
lights on Noah's Ark.

The tranformer had failed (it also powered the relay circuit.  I suspect
it was a 24v unit, but I found a 30v one and put it in.  I played with a
variac and the bulb didn't seem to light up at that low a voltage.
Wouldn't matter if it did, as any light it produced would be washed out
by its neighbour.

Electric stoves are like this too, the element on low power can't be
seen to be glowing with the lights on, but can in the dark.

I've always wanted to set up an experiment to test bulb life.  3 bulbs,
on permanently one, one flashing, and one flashing but with pre-heating.
Set the flash cycle to a few seconds to allow the second one to cool
down.  I guess I already know the answer, but it would be interesting to
see if there were any major differences.

Tony

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