[net.invest] bulb savers

keithe (12/01/82)

I remember the old, old 16mm movie projectors having a four-position
(rotary) power switch - two offs and two ons. The reason for two ons is
that the sound-track exciter lamp was run on dc (ac would put hum in
the sound), and the two positions ran the current through the bulb in
opposite directions. Thus, over enough on/off cycles, the bulb would
have the exciter current run through it in each direction equally.
When I asked (someone) "why" I remember being told that it was to
increase the bulb life; for some reason running the current only one
way thru the bulb apparently reduced the filament life. Now, this may
have been on a differently-constructed bulb than a standard light bulb,
but it still makes me wonder about putting a rectifier in series with
one.

keith ericson

kk9w (12/06/82)

The problem with DC lamps is that the filaments are subject to a process
called DC notching.  This is a result of the molecules of the filament being
pulled along with the DC current, causing thin places or notches in the
filament.  Naturally, this process will eventually break the filament if
it is allowed to progress, hence the preference for ac lights or two positions
on the DC lamp power supply.

Dave Andersen pur-ee!kk9w