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Why
is it so important that tungsten halogen lamps receive between 10.8
and 12.0 volts?
By Steve Parrott
Before we get into the daunting technical answer, rest assured that
it’s not unlike a game of tennis. If you can wade through the
science, I’ll reward you with the tennis at the end. Let me
introduce you to “The Tungsten/Halogen Regenerative Cycle”.
Tungsten halogen lamps get their name from the metal tungsten that’s
used for the filament wire and the halogen gas (actually iodine, bromine
or a combination) that surrounds it.
As a current passes through the wire, it heats up, the tungsten atoms
vibrate wildly and electrons and photons are released creating light.
Without the halogen gas these excited tungsten atoms would quickly
evaporate making the filament wire thinner and thinner until it couldn’t
handle the current and breaks.
Also, (without the halogen gas) the evaporating tungsten atoms would
deposit on the inner walls of the lamp, blackening them and reducing
the light output.
The presence of halogen (very reactive gases)
slows both the evaporation of tungsten atoms and the blackening
of the walls. It does this in a very clever way. When the tungsten
atoms leave the filament they react with the halogen in a cooler
area of the lamp and form a gaseous tungsten/halogen compound.
This compound floats around inside the lamp until it eventually
reaches the hot filament and deposits the tungsten metal there,
thus extending its life. This is the Tungsten/Halogen Regenerative
Cycle.
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The Tungsten/Halogen Cycle |
This Cycle is very much dependent on temperature. The halogen only
picks up the tungsten in a certain low temperature range. And, it
deposits it in a certain high temperature range. If the temperature
(directly related to voltage) goes too high then the halogen gas won’t
react with tungsten and the tungsten will deposit on the glass and
the filament becomes thinner. If the temperature goes too low then
the halogen picks up the tungsten but won’t deposit it, instead
it (the gaseous compound) condenses on the glass and the filament
becomes thinner. In both cases the lamp life is reduced.
Now for the tennis, imagine Joe Tungsten with a bucket of tennis balls,
hitting them across the net to Jim Halogen. This back and forth hitting
can go for a long time as long as Jim can comfortably run back and
forth hitting the balls. If the air temperature drops, Jim’s
hands are freezing and he can’t hit the ball. If the temperature
goes too high, Jim drops from exhaustion and the balls sail past.
In both cases, the bucket quickly empties. In the same way, halogen
gases have a certain ideal temperature range where they work well.
And the temperature is determined by the voltage.
To
summarize, both high and low voltages reduce lamp life. If you don’t
have one already, I suggest you invest in a good voltmeter!
P.S. Most voltmeters, even the expensive ones, are calibrated
in the factory to have their greatest accuracy in the 120 volt range.
In the low voltage range, these voltmeters could be off by as much
as one half a volt. The CAST Volt/Amp Meter (Model CMETER) is calibrated
for greatest accuracy in the 12 volt range. If your jobs are plagued
by short lamp life, incorrect voltage measurement could be the cause. |
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