# How to determine required voltage of a lightbulb

I have a bulb here that has no markings except it says 25 watts, I just wanted to know what voltage does it need to power up. Our standard voltage here is 220 V, I tried plugging it on a 110 V transformer and it powered up.

Is there any way I could determine if it could take 220 V?

• A 220 volt bulb operating on 110 volts should be very dim, and yellowish, I think. If it appeared to be normal brightness, then it is probably a 110 volt bulb. Commented Feb 28, 2014 at 4:46
• Measure the current at 110 V. If it's around 230 mA, then it's a 110 V bulb. Commented Feb 28, 2014 at 13:35

If you already hooked it up to $110$ Volts and it did not blow up, then you should now measure the actual voltage being supplied and measure the actual current going through the bulb at the measured voltage.

$$\text{Power in Watts} = \text{Volts} \times \text{Current}$$

So if you measure the Voltage and Current and multiply those two quantities together, you will get the Wattage at $110$V nominal.. If it's near enough to $25$ Watts then you can assume it's a $110$ Volt lamp.

Method 1:

If the filament is glowing bright yellow you are probably operating it at its rated voltage. If the filament is glowing a dark orange you are probably operating it at less than its rated voltage.

Method 2:

Measure the power consumption using a kill-a-watt or similar device.

If it is drawing about 25W at 110V, that means it is rated for 25W @ 110V.

It it was rated for 220V you would expect it to only draw approx 6W @ 110V (P=V^2/R, R is constant)

Method 3:

Measure the resistance of the light bulb using a multimeter.

If it is rated for 110V:

R = V^2 / P

R = 110^2 / 25

R = 484 Ohms

If it is rated for 220V:

R = V^2 / P

R = 220^2 / 25

R = 1936 Ohms

These calculations do not account for:

• The difference between AC resistance and DC resistance; or
• The temperature of the filament.

But should be close enough to tell you whether the bulb is rated for 110V or 220V.

• Don't use method 3, filament resistance is highly dependent on temperature. Commented Feb 28, 2014 at 7:05