In a message dated 6/23/99 10:07:59 AM Pacific Daylight Time,
shumaker@xxxxxxxxxxxxxxx writes:
<<
to beat this one to death - I would have to agree with George. What
usually burns out a bulb is the surge of current when you first turn
it on, not the change in temperature. The resistance is much lower when
the filament is cold than when it is hot, hence it pulls more current
when you first turn it on. If there is a weak spot, it will burn out
there. The filament is weaker when hot, so the quicker it cools, the
less likely it is to be damaged. The rate it cools or heats shouldn't
matter, as it is current that blows the bulb. My thoughts on the matter.
>>
I haven't checked many slide projector circuit designs but other
devices with heated filaments like overhead projectors,TV sets etc reduce
the cold inrush current shock by including a thermally sensitive resistor in
series with the filament. These have high resitance when cold and low
resistance when warmed up this greatly improves lamp/filament life by
reducing warmup peak current. Probably most modern projectors which run off
24Volt lamps inherently have some current limit as the 24v transformer is too
small to provide the almost five times normal inrush current and so limits
the current.
I have had a number of projector lamps fail by the quartz glass
cracking at the base pins. This would argue for not cooling rapidly to
minimize stress in the leads as things cool differentially.
An interesting aspect of the Halogen lamps is that they need to run
the envelope at very high temperature as well as the filament to prevent
boiled off metal from condensing on the envelope and blackening it rather
than being recycled back to the filament.
Regards
Tim Hughes
Hi100@xxxxxxx
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