I've been using a 60 watt halogen bulb in a fixture over my lathe.
Suddenly the light is very dim, seems like 15 watts instead og 60. I
can understand a light burning out, a halogen uses a simple filament
right? Why would it fail dim? I've never seen any filament bulb fail
dim before.
I have seen a few halogens fail dim - generally 120V ones as opposed to
low voltage ones. But what hapens next is that after the first time being
switched off after failing dim, they don't light again at all.
Explanation in my experience:
The filament breaks and an arc forms in the gap in the filament. The
voltage drop in the arc reduces the filament current, and most of the
filament operates at reduced brightness, although the points where the arc
terminates will be brighter than normal (but small).
I have even seen a "stable burnout arc" in a non-halogen incandescent.
But in non-halogen incandescents, more usual - especialy if the filament
breaks when the current is higher than its usual steady-state value, as in
during a "cold start" - is the "unstable burnout arc". An "unstable
burnout arc" is one where the electric field (voltage gradiant) along the
filament is sufficient to conduct through arc plasma around or alongside
the filament. When that happens, the arc "blows up" and quickly becomes
in parallel with the whole filament. At that point, there is no filament
current to limit current through the arc, and the arc can draw hundreds of
amps until the next voltage zero crossing, an internal fuse wire in the
lamp blows, or you blow a fuse or pop a breaker. This is the familiar
"Bright Blue Flash".
But in an arc through argon (or krypton) (whether or not mixed with
nitrogen) at pressure anywhere near atmospheric with current a few amps or
a few tenths of an amp, the arc is surprisingly dim. Power input to the
arc goes mainly to electrode losses and heat conduction ("watt per
centimeter") losses, and whatever is left over does a good job of being
radiated in the near-infrared strong spectral lines of argon (or krypton).
In fact, a discharge through argon at pressure anywhere near
atmospheric pressure, in most cases, is almost as bright at 1 mA or a few
mA as it is at 100 mA or even a few hundred mA. Reason: Higher current
makes the arc more conductive, so the free electrons in the discharge
require less kinetic energy to maintain the discharge. And less energetic
electrons are less able to make argon radiate, especially at wavelengths
other than its main infrared wavelengths. (But in the several 10's or
hundreds of amps, the situation reverses somewhat - you need a higher
average kinetic energy of free electrons to push the current through and
then you get free electrons with enough kinetic energy to easily produce
some visible light.)
Back to halogen lamps - the gas pressure is higher, and halogen vapors
have a tendency to snag away free electrons, and at least one of these
makes burnout arcs less conductive and less able to spread beyond the gap
in a broken filament.
Maybe I said enough about burnout arcs...
- Don Klipstein (
[email protected])
