Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!emory!rsiatl!jgd
From: jgd@Dixie.Com (John G. DeArmond)
Newsgroups: sci.electronics
Subject: incandescent light bulb life extender
Message-ID: <5232@rsiatl.Dixie.Com>
Date: 6 Dec 90 06:25:25 GMT
Organization: Rapid Deployment Systems (making go-fast things and things that-go fast)
Lines: 46

skwu@boulder.Colorado.EDU (WU SHI-KUEI) writes:

>Spoken like a true representative of a light bulb maker!  Since the life of
>a bulb approximates:

>	Life = 1 / V ** 11  (the exponent varies between 10 and 12)

>while the efficiency (light output) drops almost linearly with reduced voltage,
>building the diode in would only reduce the sale of bulbs.

I believe light decreases as the square of the reduction in voltage but
your point is well made anyway.

Here is how to extend your bulb life indefinitely with practically (<1%)
decrease in output.  Place a negative tempco thermistor of the proper
value in the lamp lead.  The thermistor limits inrush and self-heats to
a low resistance value in about a second.   The inrush is what finally
gets the bulb.

I've been using them here for quite some time with an average line
voltage of 130.  My hall lights went from an average life of 2 weeks to
not having changed one since I put the thermistors in about 3 months 
ago.

Here are some part numbers, all available from DigiKey:

100 watt bulb		16 ohm cold		ptn126
150 watt PAR flood	8 ohm cold		ptn124
4 bulb fixture,
100 watt ea			5 ohm cold		kc024n-nd

Typical on resistance for the 16 ohm device is 1 ohm which means that it
dissipates about a watt with a 100 watt bulb.  I mount the thermistors
in the same compartment with the bulb.  This provides extra heat when
the bulb is lit which decreases the on resistance even more plus it 
provides plenty of air space around the part.   The part will rise
about 60 degrees C over ambient when in operation so you don't want
to insulate it or pack it in a closed box.

John

-- 
John De Armond, WD4OQC        | "Purveyors of speed to the Trade"  (tm)
Rapid Deployment System, Inc. |  Home of the Nidgets (tm)
Marietta, Ga                  | 
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