On 10/03/2012 05:12 AM, Dave Plowman (News) wrote:
In article
,
hr(bob) wrote:
He is right, the stresses involved in the turn-on of the bulb each
time is equal to several hours of continuous running. If you cycle a
bulb on and off every few seconds, the total on time before the bulb
fails will be only a few hundered hours for a 1000 hour rated bulb,
It would be a strange way to rate the life of a lamp - on constantly,
since this pretty well never happens.
Do you find the 'flasher' lamps on your car failing more quickly than
similar lamps which don't flash?
I don't know of any data that supports this common idea, but I'd be
interested in reading about it if anybody's actually done the experiment
carefully. Electromigration is a smaller effect in an AC bulb, since
the leading order effect cancels.
I suspect that the notion that cycling is hard on bulbs comes from the
way that the bulb often fails at turn-on, when the thinnest hot spot
vapourizes before the rest of the filament has a chance to come up to
temperature and reduce the inrush current.
The tungsten in the lamp is run within a few hundred kelvins of its
melting point, so it's always in the fully annealed state, which ought
to mean that there are no metal fatigue mechanisms operating, just
material migration due to sublimation.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net