24V light running on 12V

[Rossland]

I have a question for you electrical experts.

I have a number of incandescent courtesy lights that mostly light stairs at night. They are labelled 3watts at 24v, but are actually connected to 12v. The are not very bright, but they are supposed to be dim.

My question is will these bulbs still draw 3watts at 12v, or will they draw 1.5w since the voltage is half.

 

[tiltrider1]

Watts = amps x volts. You changed the volts but nothing else changed so yes 1.5 watts.

 

[Mischief Managed]

I have a question for you electrical experts.

I have a number of incandescent courtesy lights that mostly light stairs at night. They are labelled 3watts at 24v, but are actually connected to 12v. The are not very bright, but they are supposed to be dim.

My question is will these bulbs still draw 3watts at 12v, or will they draw 1.5w since the voltage is half.

At 12 volts they only use .75 watts.

 

[Mischief Managed]

Watts = amps x volts. You changed the volts but nothing else changed so yes 1.5 watts.

With half the volts, the amps cut in half too, so only 25% of the wattage, not half...

 

[C lectric]

It may be slightly higher than the straight line calculation. I agree mostly that if you halve the voltage on a resistance device you quarter the wattage.

With light bulbs, incandescent, though the very high temperature of the Tungsten filament also produces a higher resistance than when cool. That high temp is used to keep the current to spec.

When the voltage is halved in addition the temperature will drop, the resistance will also drop so the current will rise. It may not be a hill of beans in this case but personally I would simply insert the current function of your DMM in the circuit for one bulb and measure it.

Without actually measuring the filament temperature you cannot calculate the new temp so you would be guessing somewhat.

 

[Ski in NC]

Yep, on light bulbs and heating elements, resistance changes with temp. So the "half volts equals quarter watts" rule does not usually hold very well.

 

[FF]

If the question was is there any danger in this half voltage , the answer is NO.

 

[Rossland]

Thanks for the replies everyone. I was not concerned with any danger, since the bulbs barely get warm. But I was wondering if it was practical to replace these incandescent with LED's. The answer is no.

 

[Mischief Managed]

Thanks for the replies everyone. I was not concerned with any danger, since the bulbs barely get warm. But I was wondering if it was practical to replace these incandescent with LED's. The answer is no.

Agreed, and 24V bulbs at 12V will probably last longer than 12V LEDs too...