Path: utzoo!attcan!uunet!ogicse!zephyr.ens.tek.com!videovax!bill
From: bill@videovax.tv.tek.com (William K. McFadden)
Newsgroups: sci.electronics
Subject: Re: incandescent light bulb life extender
Message-ID: <6213@videovax.tv.tek.com>
Date: 11 Dec 90 00:38:27 GMT
References: <5232@rsiatl.Dixie.Com> <4225@kitty.UUCP>
Reply-To: bill@videovax.tv.tek.com (William K. McFadden)
Organization: Tektronix TV Measurement Systems, Beaverton OR
Lines: 52

I didn't really want to get involved in this discussion, as many
seem to know more about light bulbs than I, but let's look at some
tradeoffs.  Using the empirical formulas:

     LIFE = RATED_LIFE * (APPLIED_VOLTAGE/RATED_VOLTAGE)^-12
and
     LUMENS = RATED_LUMENS * (APPLIED_VOLTAGE/RATED_VOLTAGE)^3.5
and
     CURRENT = RATED_CURRENT * (APPLIED_VOLTAGE/RATED_VOLTAGE)^0.55

These formulas come from an internal publication by my company,
which I feel to be trustworthy.

Let's assume for the sake of argument that we have 100 watt 120V
and 130V bulbs with the same rated life.  This may not be a good
assumption, but it will allow us to look at operating cost as a
function of voltage rating.  Hence, running the 130V bulb on 120V
yields about 2.5 times the life and 3/4 the light output (in lumens)
of the 120V bulb.  In addition, the 130V bulb consumes 96% as much
current at 120V as it does at 130V.

In my region, electricity runs about $0.05 per KWH.  Assuming bulb
life is rated at 1000 hours, the 130V bulb will run about 2500
hours, drawing 96 watts and consuming 230 KWH at a cost of $11.50,
or $4.60 per 1000 hours of life.  Meanwhile, the 120V bulb runs
for 1000 hours, drawing 100 watts and consuming 100 KWH at a cost
of $5.00, or $5.00 per 1000 hours.

It is obvious that, unless the bulbs are very expensive, the purchase
price of the bulbs is a very small part of the total cost.  In
regions with costlier power, this is even more so. We can probably
go as far as to ignore the cost of buying the bulb.  Hence, over
its lifetime, the 130V bulb costs almost as much to operate as the
120V bulb and generates only 75% the light output.  Unless changing
bulbs is very inconvenient, most people would be better off with
a lower wattage (e.g., 75W) 120V bulb.  Lower wattage bulbs last
longer, too.

In areas with high line voltage, 130V bulbs may be the only choice.

BTW, Digi-Key has some "inrush current limiters" (NTC thermistors)
that are designed to handle currents over an amp and have "on"
resistances under an ohm.  Perhaps one of these would be more
suitable for light bulb current limiting than the thermistors that
have been mentioned so far.

Comments?
-- 
Bill McFadden    Tektronix, Inc.  P.O. Box 500  MS 58-639  Beaverton, OR  97077
bill@videovax.tv.tek.com,     {hplabs,uw-beaver,decvax}!tektronix!videovax!bill
Phone: (503) 627-6920       "The biggest difference between developing a missle
component and a toy is the 'cost constraint.'" -- John Anderson, Engineer, TI