Multimeter question?

Question

I have a 60W bulb that says 130V on it. I'm assuming that means that when 130V is applied to it, the bulb consumes 60W of power (shouldn't it be 120V, though?). Rearranging the equations V = IR and P = IV to get the resistance in terms of voltage and power you get R = V²/P. 130V²/60W ≈ 280Ω.

So here's the problem. When I turn my multimeter to the 2000Ω setting and touch the two wires to the light bulb, one to the bottom and one to the side, I get a readout of 22.

Answer 1

You must need to multiply by 100, read the instructions, then it would be 2200 very close to 2000.

It depends on the bulb also. Compact Florescent(CF) are very efficient , for same wattage, they use about 1/4 the energy, again read the info on the bulb package.

Answer 2

Your multimeter is reading the filiment resistance when it is cold. A better way to measure it would be to use the multimeter to measure the current the bulb is consuming while turned on and calculate the resistance with ohms law.

Another problem is that you rmultimeter is measuring the resistance at DC, while really the bulb operates at 60Hz. This won't make a big difference, though.

Answer 3

1. Some incandescent bulbs are designed for 130 volts so that you get a lot more life at 120 volts, but at less light.

2. light bulbs have a resistance that is very temperature dependent. They have a low resistance when cold that goes up as the bulb heats up.

3. I suspect you are actually measuring the 22 ohm cold resistance of the bulb.

4. if you actually need to measure the resistance when hot, do it indirectly. Measure the AC current and the voltage and divide.

Answer 4

I think the 22 is actually 220 ohm.

Answer 5

Your calculations are correct; the resistance is about 280 ohms when the filament is hot.  Normally the resistance is much lower when it is cold, which means you get a higher current when you first turn on the light.  That's why bulbs usually burn out when they are turned on, instead of after they have been on for a while.  I think that the 130 V is just a maximum rating, and 60 W is the actual power you get at a standard voltage of 115 to 120 volts.  The ohmmeter reading sounds about right; the resistance of lamp filaments is very nonlinear.  By the way, the fact that the bulb is normally used on AC doesn't affect things very much.  On the other hand, if you check the resistance of a transformer with the meter, you will also get a much lower reading than you expect, because the inductance of the transformer creates a lot more impedance for AC than the DC resistance measured by the meter.

Answer 6

Most conductive materials change specific resistance with changes in temperature.

The resistance-change factor per degree Celsius of temperature change is called the temperature coefficient of resistance.