Light bulb ohms. What the heck is going on here?

When you hit that filament with 120 volts its resistance "instantly" goes real high to limit the current to the wattage of the bulb.

Measure the resistance when hot, it varies with temperature. Be very quick ;-)

best way.. an amp meter with bulb powered ON...
why do you think is took years to develop the light bulb..

vacuum, material to withstand start up power and last from high heat.

so YES to light a bulb it does take 3-4 amps.. for 5 microseconds.
get a digital oscilloscope and you can see the ramp down of current at startup.


I Know, No help.

What they said- a bulb is the ultimate Positive Temperature Coeffecient Resistor- very low resistance while cold, resistance rises fast as it heats. Back in the 70's and 80's speaker manufacturers used 1141 bulbs in line with tweeters to limit current :)

Voltage = Current x Resistance
Wattage = Voltage x Current

Combine the above
Wattage = Voltage (Current x Resistance) x Current
or
Wattage = Current x Current x Resistance

If you know Wattage and Resistance and want to find Current

Current = square root of (Wattage / Resistance)

Current = Square root of (40/32) = ~1.12 amps drawn

Current = Square root of (60/17) = ~1.85 amps drawn

So, it makes sense that a 60 watt bulb draws more amps. Looks like your measurements make sense.


Don

I worked as an electrician in the field of traffic signal systems. In the city of Los Angeles there are 4500 intersections that have TS controls and use about 350,000 light bulbs(lamps). As you can guess there are a lot of failures most of witch are caused by powering on the individual lamps at full voltage. Hitting the filament with 4 amps(+/-) creates a lot of damage and is where lamps usually fail.

In the eighties we decided that the technology was advanced enough that using a "Soft Start" switch was feasible and would pay for it's self in a short period of time with lower failure rates.

Today there are about 125,000 solid state switches that are used to turn on the lights at signalized intersections in L. A. City.

Very basically what happens is that the 5vdc switch is energized and it starts applying ac voltage from 0 volts rising very rapidly to 120 volts and it starts a "0" degrees on the AC cycle. I'll be honest, I can't for the life of me remember how the decrease in failure rate using a "Soft Start" solid state switch saves in power consumption and lamp failure over the use of coil and armature relay switches.

Another bit of useless information brought to you courtesy of a stuck at home RV owner, stuck in a S&B doing Honey does and refurbishing said S&B.And you're welcome.

When the filament heats up, the resisitance changes a lot.

Thank you all for the replies. Ok this does make sense. I actually did measure the bulb hot in a second test, but I only found about 5 more ohms. Obviously I wasnt fast enough. Clearly the rise in temperatture really ramps up resistance. Thanks for the insight on traffic lights!

In Canada in the colder provinces, municipalities are having trouble because some cities have changed traffic signal lights to led lamps to save energy, but now face the problem of the lights being covered with snow and no one can see them. They dont get as warm anymore and melt the snow. Someone didnt think that one through in the relentless quest to terminate incandescence.

Get a Kil-a-Watt meter.

JamesJudasPriest wrote:
In Canada in the colder provinces, municipalities are having trouble because some cities have changed traffic signal lights to led lamps to save energy, but now face the problem of the lights being covered with snow and no one can see them. They dont get as warm anymore and melt the snow. Someone didnt think that one through in the relentless quest to terminate incandescence.

Obviously they need to parallel those led lamps with incandescent lamps to provide some heat. Maybe they could dig the old ones out of the trash can.