> >It's been my general experience that a globe powered up from cold with a
> >sealing leak will produce yellowish white internal smoke (an oxide of
> >Tungsten?) and coat the bulb with a white layer.

That agrees with my experience.  Color varies, probably to due to
varying filament alloy and or secret sauce.

> >A friend  in the auto servicing trade opined that the inner dark coating
> >sounded like a high temperature fi;ament failure while operating.
The classic "burned out" bulb fails when the filament erodes through
use and finally breaks.  This happens most often at turn on, where
the thermal strains and the cold inrush current are high.

One of two things can happen when the filament breaks.  Often, it
just falls open.  Sometimes, though, a metal plasma arc forms briefly
between the open ends of the filament before it blasts the ends far
enough apart to break the circuit.  This is actually MORE likely in
lamps which have very good mechanical restraint on the filament, such
as those intended for high reliability with vibration.

This second failure mode causes black to shiny-black residue on the
inside of the envelope.  It also causes solid state lamp drivers to
emit some smoke of their own on occasion due to the huge current
carried in the arc.  To build a really bullet-proof lamp driver, its
best to current limit the supply.  This has two effects: it "soft
starts" the lamp by limiting the cold inrush, and it protects the
driver against this "metal arc mode" lamp failure (and shorted
wiring!).

Regards,

Barry.
------------
Barry King, KA1NLH
NRG Systems "Measuring the Wind's Energy"
http://www.nrgsystems.com
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