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Since the filament of an incandescent light bulb gets as hot as it
does, the thermal coefficient of the tungsten is a factor to be considered.
If you figure out what the resistance of a 100 Watt bulb must be to draw
the correct current for your local mains power and then read a cold bulb
with your Ohm meter, you will see that the resistance is very low and nowhere
close to what your calculation said it should be.  This means that the
resistance of the bulb will drop as it dims which will increase the current
over what one would expect with a fixed resistor of some kind.  This also
means that there is a serge of current when power is applied and
the filament is not yet hot.  This is why switching circuits for
incandescent lamps are rated for higher amperages than if those same switches
are used for other types of loads.  The bulb can act as a sort of poor man's
constant current source if one isn't too picky about accuracy.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group
Martin McCormick wrote:
>
>         Since the filament of an incandescent light bulb gets as hot as it
> does, the thermal coefficient of the tungsten is a factor to be considered.
> If you figure out what the resistance of a 100 Watt bulb must be to draw
> the correct current for your local mains power and then read a cold bulb
> with your Ohm meter, you will see that the resistance is very low and nowhere
> close to what your calculation said it should be.  This means that the
> resistance of the bulb will drop as it dims which will increase the current
> over what one would expect with a fixed resistor of some kind.  This also
> means that there is a serge of current when power is applied and
> the filament is not yet hot.  This is why switching circuits for
> incandescent lamps are rated for higher amperages than if those same switches
> are used for other types of loads.  The bulb can act as a sort of poor man's
> constant current source if one isn't too picky about accuracy.
>
> Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
> OSU Center for Computing and Information Services Data Communications Group

Also very good in series with loads that require large inrush
but small holding current
Relays and soleniod's are some of the best examples

--
Peter Cousens
email: petercousens.her.forthnet.gr
snailmail: Peter Cousens, karteros, Heraklion, Crete, 75100, Greece,
phone: + 3081 380534,    +3081 324450   voice/fax

After Bill Gates announced to the world that he was Microsoft,
his wife was asked to comment. She said that as his wife, she
had been the first to notice this problem
In a message dated 97-01-03 18:28:47 EST, you write:

>This means that the
>resistance of the bulb will drop as it dims which will increase the current
>over what one would expect with a fixed resistor of some kind.  This also
>means that there is a serge of current when power is applied and
>the filament is not yet hot.

This is why many machine control user interfaces that use incandescent bulbs
never turn them off completely.  In the off state there is a small current
provided that heats the bulb to just begin glowing.  In the on state the
added current provided brightens the bulb.  This was not a problem when
machine control was done with relays but as soon as they tried to use
transistor outputs from computer systems they started popping transistors.

Mark A. Corio
Rochester MicroSystems, Inc.