Re: Can a halogen light be dimmed

[Charles Sullivan <cwsulliv@xxxxxxxxxxxx>]

On Sat, 07 Jan 2006 10:02:31 -0500, L.M.Rappaport wrote:

> Several manufacturers claim that halogen bulbs must be run at full
> temperature periodically to lengthen their lives and remove the carbon
> buildup which will form if they are run at reduced voltage.  I have no
> idea if that's true.

It's true.  The darkening you see on ordinary incandescent bulbs
after a while is tungsten which evaporates from the filament and
deposits on the glass envelope.  In a halogen bulb, the envelope
is quartz and reaches an operating temperature which would soften
or melt ordinary glass.  At this temperature a chemical reaction
takes place whereby the tungsten deposited on the quartz reacts
with the halogen gas in the envelope to form a gaseous tungsten
halide.  When this halide diffuses over to the incandescent filament
which is even hotter yet, it decomposes and redeposits tungsten
on the filament, preferentially at its hottest spots.  This allows
the filament in a halogen bulb to be operated at a higher temperature
than an ordinary bulb and thereby achieve greater efficiency
without shortening its life.

Operating a halogen bulb at reduced voltage lowers the temperatures
of both filament and quartz envelope and reduces (or can halt) the
reactions in the tungsten-halogen cycle.  The working temperature
range of a halogen bulb is controlled by the particular mixture of
halogens within the envelope.  Dimmable halogen bulbs are designed
to have a wider working temperature range, trading off against
other desireable characteristics like peak efficiency.

Regards,
Charles Sullivan