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From: stevenw@disk.UUCP (Steven Weller)
Newsgroups: sci.electronics
Subject: Re: incandescent light bulb life extender
Message-ID: <4689@disk.UUCP>
Date: 7 Dec 90 14:13:41 GMT
References: <58335@brunix.UUCP> <1990Dec4.113531.10096@csc.canterbury.ac.nz>
Organization: Digital Information Systems of Ky (DISK), Louisville, Ky
Lines: 46

elec140@csc.canterbury.ac.nz writes:

>In article <58335@brunix.UUCP>, hleaves@ruby.vcu.edu (EAVES,HUGH) writes:
[Stuff about a rectifier in Radio Shack lightbulb extenders]

>I don't think it's anywhere near that complex. What destroys incandescent light
>bulbs is turning them on and off. A cold bulb filament is almost a dead short,
>hence the initial current thru a bulb when it's turned on is very high,
>although this quickly reduces down to the design value as the filament heats
>up. It is this initial high current that blows bulbs (notice that almost all
>bulbs go when you turn them on?). I imagine the discs are simply resistors,
>with a resistance much less than the steady state resistance of the light bulb
>(hence they will have negliable effect once the bulb filament is up to
>temperature), but greater than the initial resistance of a cold bulb which will
>limit the startup current.

>Basically they would provide a soft-start facility for light bulbs!

>--
>*********************************************************

>E.MAIL: c.kaiser@elec.canterbury.ac.nz
>*********************************************************

An interesting idea.  I had always assumed that they were rectifiers.
They simply halve the power of the bulb, thereby reducing the temperature
of the filament greatly and preventing destruction.  Rectifiers are
used because of the low power dissipation.

On the other hand, filaments on AC last longer than those on DC.  This
is because the crystal boundaries of the tungsten "creep" with the
current.  Impurities are added to the metal to 'pin' the crystals and
prevent this happening.  The creep eventually thins the filament and
causes not spots.  So rectifying the current could reduce the life.

The other alternative is a capacitor.  It reduces the power dissipated
in the bulb by phase-shifting the current.  I do not know whether the
size of a capacitor suitable for this purpose would be less than the
pacake these devices come in, however.

Does anyone know ?
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