I didn't realize I had started such an active thread.:-)  A
couple of comments:  Someone in N.Z. suggested putting a diode in
series with a lamp and a couple of others described the diode buttons
that fit in the lamp socket.  In one of my past lives, I was an
electronics technician with the Audio Visual Department at Oklahoma
State University and one of our staple fix-it jobs was to revive a
certain name brand of overhead projector that, for some reason, was
designed with a diode in series with the projection lamp.  I think
they even called it an 85-volt bulb but it's been 15 years so some of
my memory may have been dimmed by the sands of time.

	Anyway, the diode sometimes burned out to a short and put the
full mains voltage across the bulb.  The energy of a half-wave
rectifier is .7 the energy of full-wave sinusoidal AC so the bulb ran
somewhat brighter but not for long.  Those bulbs were usually in the
$20-$30 range so the customers used to get pretty peeved when they
replace the first bulb, thinking its time had run out only to find the
next bulb's time ran out a few minutes later.

	We'd plug in a new diode and sell them yet another $20 bulb
and the projector was whole once again until that diode overheated and
fused in to a fancy jumper once again.  I never understood the value
of that useless bit of over-engineering at all.  Plenty of models of
overhead projectors ran on similar bulbs except they were optimized
for 120-volt service so no diode was needed.  It would have made sense
if they were 120-volt bulbs and one could throw a switch to un-short a
diode in series with it for slightly dimmer (= longer bulb life)
operation.

	The Kodak Slide projectors did have a Dim position on their
power switch, but it was a ceramic tubular wire-wound resistor in
series with the bulb, not a diode.

	Someone on this list also suggested putting a full-wave bridge
ahead of the lamp and a filter capacitor across the output.  Another
lister responded that it would make the lamp brighter which is quite
true.  Just as a half-wave rectifier supplies .7 of the energy of the
full sine-wave AC, the filtered bridge will supply the peak, not RMS
voltage of the AC wave.  This is the square root of 2 or about ~1.4
times the RMS voltage.  On a 240-volt lamp, this will be hitting it
with a steady 340 volts assuming that the filter capacitor is large
enough  to fully charge.

	The bulb will run brighter, whiter and die much
sooner just like those ill-fated projector bulbs did.

	A bit of useless trivia is in order.  A lot of projector bulbs
have what is known as an iodized quartz filament.  They actually must
run at a certain range of voltage for optimum bulb life.  I don't
totally understand what is happening, but these type bulbs have a
process at which the filament vaporizes when run at low voltage but
recovers somewhat at the optimum temperature.  The lamps even have a
collection grid in them which is not an electrical component, but a
chemical one that is involved with this process.

	No matter how one cuts it, those bulbs are like rase cars.
They run extremely hot and die young with expected lives of a few tens
of hours at best.

	It's not uncommon for 16-millimeter movie projectors to use a
30-volt lamp since the filament can be made smaller which makes the
optics a little easier to produce.  Those systems have a monster
transformer and a few of them even have a Dim option on the lamp
switch which selects a slightly lower-voltage tap on the transformer.

Martin McCormick WB5AGZ  Stillwater, OK 
OSU Information Technology Division Network Operations Group
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