Incandescent light bulbs and Ohm's law

[xtian]

I have a 40 watt incandescent light bulb. It measures 26 ohms DC resistance. This does not work with Ohm's law. Why? (Should measure 360R to dissipate 40 watts at 120 volts.)

[xtian]

Because: Google it.

http://www.allaboutcircuits.com/vol_1/chpt_2/6.html

It's a non-linear conductor.

[ampgeek]

The electrical resistance of metals typically increase with temperature.

In don't think that an order of magnitude increase is unreasonable to expect given the temperature of the filament.

Cheers,
Dave O.

[ampgeek]

Bingo!

[Leo_Gnardo]

Because: Google it.

http://www.allaboutcircuits.com/vol_1/chpt_2/6.html

It's a non-linear conductor.

Why the light-bulb "limiter" works so well. If current stays low, the lamp doesn't light .

[JMFahey]

I have a 40 watt incandescent light bulb. It measures 26 ohms DC resistance[COLD]. This does not work with Ohm's law. Why? (Should measure 360R[HOT] to dissipate 40 watts at 120 volts.)

But ... that much difference?

The temperature difference is huge.

Lamp filaments are the few things we are in daily contact with which work white hot (not orange or red hot) which ,means they are as hot as they can go without melting.

And they are not made of standard metals such as iron/copper/aluminum/lead but tungsten, one of the highest meltingn point metals in current use.

The higher the temperature increase, the higher the resistance variation.

[pops]

Also they are so hot that if they were not in a vacuum, but had oxygen they would burn up quickly.

[sluckey]

This is also the reason that a light bulb will blow when you first turn the switch on, rather than when you turn the switch off.

[Structo]

I think the reason the two numbers don't match up is due to the energy loss in the heat of the filament.

[Structo]

This is also the reason that a light bulb will blow when you first turn the switch on, rather than when you turn the switch off.

When you energize current to the light bulb, the filament is shocked by the sudden current flow (induction), putting stress on the filament which can break due to the mechanical shock that takes place.

[roberto]

http://hypertextbook.com/facts/2004/DeannaStewart.shtml

[JMFahey]

Thanks Roberto

From one table in your link, Tungsten at 2700 degrees Kelvin (about what lamp filaments rise to) resistance is over 20X that of ambient temperature.

[rp]

http://www.centennialbulb.org/

http://www.centennialbulb.org/photos.htm

[Structo]

Also they are so hot that if they were not in a vacuum, but had oxygen they would burn up quickly.

I thought they used Argon in light bulbs?

[xk49w]

Leslie 122 reverb amp uses incandescent to limit drive into the tank, paralleled with a 16 ohm speaker voice coil. Light leaks from under the chassis when cranking it!