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From: Terry King
Subject: Re: Halogen bulb drive RE:DC vs AC
Date: Mon, 30 Sep 2002 23:13:16 -0400
References: <4LPj9.email@example.com> <firstname.lastname@example.org>
Organization: Waits River Valley School
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> If this were a significant factor then all dc driven filaments would fail
> at one end - news to me if they do.
I did some research on this several years ago on small halogen lamps
used for illumination in a semiconductor film thickness
analyzer at IBM.
There IS a effect that is more pronounced at DC: the 'Edison effect'
in which electrons emitted from the negative end of the filament are
accelerated to the positive end and bombard the filament there. Basically
a tungsten-filament diode.
I recall a commercial one in a battery charger my Dad had in 1950; the
rectifier was called a "Tungar Rectifier"
and had a bright tungsten filament and a round carbon anode about a inch
in diameter very close to the filament. If overloaded (Don't ask... I
was only 10 years old..) the anode got red hot.
Anyway, my tests showed that a higher-voltage lamp (24V) had more effect
than a 12V lamp (No surprise..) AND repeated cycling on and off every
few seconds had a significant negative effect on life. The filaments
narrowed down visibly at the postive end. My conjecture was that the
temperature cycling caused molecular-level cracking in the filament
surface that was then bombarded immediately by electrons from the other
end of the lamp.
Bottom line: Don't cycle the lamp off until not needed for 60
seconds, and replace the $12 lamp every 6 months, on a $20,000 tool.
Regards, Terry King ...In The Woods In Vermont
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