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really dim bulbs; Tungsten Halide lighting
-- [ From: human * EMC.Ver #2.5.02 ] --
At issue: whether running Halogen light intended for automotive (12V)
applications at roughly half voltage will result in harmful side effects, i.
e. shorter bulb life. Here's what I gathered from talking to a former
engineer at the lighting division of GTE Sylvania:
The "Halogen Cycle" in the incandescent Tungsten Halide lamp consists of the
random recovery of tungsten vapor deposited inside the quartz bulb by its
revaporization off the hot quartz into the halogen gas (typically Bromine or
Iodine) and redeposition on the Tungsten filament.
This provides one of the two primary advantage of this lighting system. By
keeping the inside of the bulb clean of Tungsten contamination, the light
output of the bulb stays very near its full level for the entire life of the
The second advantage, longer life, is due to the higher pressure inside the
bulb compared to a conventional incandescent lamp. This reduces the
vaporization of the Tungsten in the first place. Longer life is not due to
the redeposition, since that is random and does not occur where it would be
needed, i.e. the weak spot on the filament.
Applying this to our current problem, running the bulbs at six volts
continuously would tend to spoil the self cleaning advantage. However,
running them again at full voltage and thus full temperature would clean
them back up again and eliminate that potential problem. At the lower
voltage, less Tungsten would evaporate (sublimate) anyway, so there would be
less to worry about. As far as life of the bulb/filament, the Halogen Cycle
is not what provides the longer life, it is the pressure of the Halogen gas
lowering the vaporization rate.
A constant life of six volt operation would be much longer, but with
deteriorating light output.
Back and forth operation should also result in more hours of overall usage,
with no significant changes in the lifetime light output of the lamp.
Science? no. Accurate information? yes. We'll probably never run a decent
experiment to test this, we would have to do something like run one (or
fifty) bulb(s) at 12v while the other gets 6v and 12v and another gets 6v
While on the subject of bulb life, can we get a crude anecdotal comparison
going? Phil Payne has mentioned his lamp life expectancies, was it a few
months? Who out there running 100/80's has replaced enough bulbs to have a
good fix on how long they last? Igor, how about your 180/100's?
My varying mileage may be due to some of them being nautical miles.
7 Kelsey Road
Lee NH 03824
Used parts store: 80 5k, 80 5kt, 82 coupe, 87 5k