Post by thread:Light Bulbs as Ballasts (was Re: Thnx for the Ni-c

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: spam_OUTpeterTakeThisOuTcousens.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. 200 Buell Road, Suite 9 Rochester, NY 14624 Tel: (716) 328-5850 --- Fax: (716) 328-1144 http://www.frontiernet.net/~rmi/ ***** Designing Electronics For Research & Industry *****