Xref: utzoo sci.electronics:16244 misc.consumers:25235
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!wuarchive!emory!att!pacbell.com!pacbell!osc!jgk
From: jgk@osc.COM (Joe Keane)
Newsgroups: sci.electronics,misc.consumers
Subject: Re: incandescent light bulb life extender
Summary: Use the rated voltage.
Keywords: lamp life, economics
Message-ID: <4078@osc.COM>
Date: 13 Dec 90 05:21:20 GMT
References: <5232@rsiatl.Dixie.Com> <4225@kitty.UUCP> <6213@videovax.tv.tek.com>
Reply-To: jgk@osc.COM (Joe Keane)
Followup-To: sci.electronics
Organization: Versant Object Technology, Menlo Park, CA
Lines: 27

I have some incandescent lamp curves in front of me, and they pretty much
agree with the formulas others have posted.  At 110% of rated voltage the
power consumed is 115% of normal, the light output is 140% of normal, and the
expected life is 40% of normal.  Conversely, at 90% of rated voltage the power
consumed is 85% of normal, the light output is 70% of normal, and the expected
life is 400% of normal.

You can see that it's easy to get a very large increase in life by dropping
the supply voltage a bit.  This fact is frequently re-discovered, as if no one
knows it and the light bulb companies are conspiring to make you buy more.
It's good to know if you only care about lamp life, say they're in a remote
location and hard to change.

The problem is the resulting loss in efficiency; notice that at 90% of rated
voltage the lumens per watt is down to about 80% of normal.  If you're
thinking economically, you should keep in mind that the cost of a light bulb
is much lower than the cost of the power it consumes during its lifetime.

The conclusion is that if you're dropping the supply voltage, or equivalently
using a higher rated voltage, you're making a mistake.  If you're happy with
the reduced light output, then you should use a lower wattage bulb with the
right voltage.  You'll use less power and also get better color.

Some sort of inrush current limiter and/or voltage regulator would be a good
idea, since start-up and high-voltage periods consume a disproportionate
amount of the lamp's life.  But that's a lot more complicated than a simple
rectifier.