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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: incandescent light bulb life extender
Summary: Lamp characterstics, extending lamp life & a safety warning
Message-ID: <4225@kitty.UUCP>
Date: 7 Dec 90 05:24:46 GMT
References: <5232@rsiatl.Dixie.Com>
Organization: Recognition Research Corp., Clarence, NY
Lines: 112

In article <5232@rsiatl.Dixie.Com>, jgd@Dixie.Com (John G. DeArmond) 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)

	An exponent of 13 is more common.

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

	I'm not certain what "efficiency" means here, but perhaps the
original author is referring to lumens per watt.  Using this definition,
"efficiency" varies close to the *square* of the voltage.

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

	Not exactly...

	It's *much* more significant than that.  For a typical incandescent
vacuum lamp of 10 lumens per watt, or for a typical gas-filled incandescent
lamp of 16 lumens per watt, lumens vary as voltage is raised to an exponent
of 3.4.

	Such exponents are derived from extensive empirical data collected
over the years by manufacturers of incandescent lamps, and are not the
result of any theoretical calculations.

> 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.
> ...
> I mount the thermistors in the same compartment with the bulb.

	While the above installation technique may function as intended,
readers should be aware that such an "aftermarket" installation: (1) most
likely voids the UL approval on the lighting fixture in question; (2) is
a National Electrical Code (NEC) violation; (3) most likely violates local
building codes because of (1) and (2); and (4) may result in some difficulty
with insurance coverage in the event of a fire.

	Permissible wiring within lighting fixtures is tightly controlled
and specified by the NEC due to elevated temperatures found within such
fixtures.  I know of no lamp life extender which is UL-approved for the
series wiring installation (as described above) within a lighting fixture,
and based upon my knowledge of UL "philosophy", I doubt that such a device
could ever meet UL approval.

	UL does approve the various button-type devices which insert within
the lamp socket for lamp life extension, with such devices utilizing diodes
or thermistors.

	I would strongly urge readers to utilize such button-type devices
instead of the above wiring suggestion.  Why risk a fire?

	How could one possibly have a fire, you may ask?  Simple.  Consider
a light bulb whose filament opens, with a short filament section drooping
against a support wire, thereby creating a *much* lower resistance.  While
such lamp faults usually burn open within the lamp envelope, a virtual
short circuit can also result.  Most readers have probably seen the effects
of a burned out lamp which is then jostled while under power.  Consider
now that we have such a low resistance fault drawing *much* more current
than a normal lamp, and consider the effects of such overcurrent on the
thermistor.  It is quite possible that the thermistor could burn under
such circumstances.  The button-type devices are deemed safe because
they are not only enclosed within the metal screw area of the lamp socket,
but are further insulated by the dielectric surrounding the socket (often
a ceramic material).

> 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. ...
> 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.

	The 16-ohm device is actually a Panasonic P/N PNT-126, and has
a maximum rating of 0.6 watts.  Its R25/R50 deg C resistance ratio is 2.3,
which means that its resistance is still about 7 ohms at 50 deg C.  I
would find it difficult to believe that at 85 deg C ("60 degees C over
ambient") its resistance would be as little as 1 ohm.  2.5 to 3 ohms
seems more likely, which means we could be dissipating at least 2.5
watts.  That's *FOUR* times the *maximum* rating of the thermistor!

	Also, the above thermistor series is primarily intended as a
temperature measuring element, and not for any significant power dissipation
in any protective circuit.  That's why its only rated at 0.6 watts.

	In my opinion, a much better, simpler and lower-cost alternative
to any type of thermistor device (even the button-type) is to find a
wholesale electrical supply firm which will sell you lamps rated for
130 volts (instead of the usual 120 volts).  Such 130 volt lamps are
also available rated for low-temperature and "rough duty" service, and
those suckers will usually last for years in a home environment.  I have
one above my kitchen sink (a bitch to replace because of a curtain rod)
that has been running for almost *4* years with a *lot* of on-off cycles.

	Another very robust lamp available from a wholesale electrical
distributor is rated for traffic signal use.  The only disadvantage of
this type of lamp which limits its application for home use is that it
is available only in a clear envelope (i.e., not frosted). 

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
VOICE: 716/688-1231   {boulder, rutgers, watmath}!ub!kitty!larry
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