# How to check voltage and wattage of an unmarked incandescent bulb?

I've salvaged 160 incandescent bulbs from fairy lights. They were connected in 80 bulbs in series then connected in parallel. They were plugged into 220V.

How do I measure the voltage and wattage of an individual bulb? I have a multimeter and ammeter available if that helps.

• What do you mean "in series connected parallelly"? Are you talking 80S2P? Apr 19, 2021 at 6:17
• 220/80 = 2.75V. Perhaps 3V bulbs. Feed one with 3V and measure the current. Multiply current by voltage and you have the power (=wattage). Apr 19, 2021 at 8:23
• @hearth Please excuse my ignorance I dont know what 80S2P means. Sounds like (80 in series) + parallel + (80 in series) i dont know if i make sense. Apr 21, 2021 at 3:02
• 80S2P means two parallel strings of 80 bulbs in series. Apr 21, 2021 at 5:21
• Thank you @KristoferA it is working nonstop with maximum brightness at 2.75v and it hasnt blown since. Apr 24, 2021 at 22:31

220V across 80 lamps in series gives 2.75V. The chances are they are 3V lamps (as that would work up to 240V without constantly blowing the lamps).

There won't be any way to tell the wattage, other than connecting one lamp to a 3V supply and measuring the current.

An incandescent bulb is like a resistor, so the more voltage that is applied the more current is sinked and therefore power that is beign dissipated. There will be a maximum power that it can withstand before braking. If you want to find this out, connect one bulb to a power supply and measure voltage and current. Start off at 0V and increase the voltage slowly until the bulb start lighting. Then you can keep on going until the bulb explodes. There you found it maximum limit. I would recommend not applying more than 70% of that voltage continuosly to the rest of the bulbs.

Incandescent fairy lights are sometimes known as mini-Christmas lights in the US were they have 120 volt electrical outlets. To get an idea of the power, measure the resistance of the unknown mini Christmas incandescent bulbs with an ohm meter and compare them to bulbs of know wattage.

There are 4 mini Christmas incandescent bulbs that I know something about (there are more than 4; there are at least 3 different 2.5 volt bulbs of different wattage.) There is the 2.5 volt, 0.170 amp, 0.42 Watt that has a resistance of ~1.34 ohms at zero voltage; a 3.5 volt, 0.125 amp, 0.44 Watt that has a resistance of ~2.5 ohms at zero voltage; there is a 6.0 volt, 0.080 amp, 0.48 Watt that has a resistance of ~6.7 ohms at zero voltage and there is the 12 volt, 0.080 amp, 0.96 Watt that has a resistance of ~13.5 ohms at zero voltage.

The 2.5 volt bulb is usually found in a series chain of 50 bulbs, the 3.5 volt bulb in a 35 bulb chain, the 6 volt in a 20 bulb chain, and the 12 volt in a 10 bulb chain. As you can see the voltage multiplied by the number of bulbs is equal to ~120 volts in series supplied by the power outlet.

The Vermont site has a more complete list of mini incandescent Christmas bulbs.