# DC light bulb on AC circuit

1. Feb 11, 2009

### pgoyer

Assuming equal voltage, will it burn-out an AC light bulb if I put it on a DC circuit?

2. Feb 11, 2009

### MATLABdude

What do you mean by equal voltage?

Equal RMS: probably not.

Equal peak: depends on how hot the filament gets

Equal average: won't turn on.

HINT: how is power calculated in an AC circuit?

3. Feb 11, 2009

### Averagesupernova

RMS volts, yes. Since a lightbulb is basically a resistor it should be fine. RMS AC is the same voltage that it takes to make the same amount of heat in a resistor driven with the same DC voltage.

4. Feb 11, 2009

### nyoo

Ohm's Law: volts (E) = amps (I) * ohms (R)
Watts Law: watts (P) = amps (I) * volts (E).
A useful derivation is: P = (I^2) * R = current squared * resistance

Let's try a 100-watt lightbulb. And 120 VAC.

AC: 100 watts = .833^2 amps * 144 ohms
120 volts = .833 amps * 144 ohms

I believe your previous correspondent, that a lightbulb is a resistor. So, maybe we know from above that a 100-watt lightbulb is a 144-ohm resistor. Let's try with a 12VDC car battery.

DC: 12 volts = .0833 amps * 144 ohms
1 watts = .0833^2 amps * 144 ohms

Does this show that the lightbulb will not burn out? Does it also show that the bulb won't dissipate enough power to glow?

5. Feb 11, 2009

### Averagesupernova

Who said anything about using the light bulb on 12 volts? The question was whether it was AC or DC.