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# What's the 220 V equivalent of a 120 V 40 W lightbulb?October 13, 2010 7:17 PM   Subscribe

What's the 220 V equivalent of a 120 V 40 W lightbulb?

I have a lava lamp, bought in the US 10 years ago. The 120 V 40 W lightbulb burned out, and I need to buy a new one in Chile, where we use 220 V. The lamp base says "use only 40 WATT A 15 appliance bulb".
I think the central issue here is the heat the bulb generates.

What 220 V lightbulb should I get?
posted by signal to technology (14 answers total)

40W refers to the power draw of the bulb and is directly proportional to the heat created. Therefor a 40W 220v bulb will put out the same heat as a 120V 40W bulb. It'll just pull half the amps. So any local 40W bulb with the correct form factor should work.
posted by Mitheral at 7:20 PM on October 13, 2010

A 240V 40W bulb. What matters is the total power used by the bulb, which is 40W. It'll draw half the current at double the voltage compared to a 120V bulb, meaning the power will be the same.
posted by zsazsa at 7:20 PM on October 13, 2010

Watts are watts, mostly. The bulbs will be the same. Get the 40W. The bulb rated for 220 volts will actually last longer there.
posted by Burhanistan at 7:23 PM on October 13, 2010

OK, thanks all.
posted by signal at 7:26 PM on October 13, 2010

The above answers are wrong -- potentially dangerously so. A light bulb with a fixed resistance operated at twice the voltage will produce four times as much power: P = V2/R. At best, this is likely to burn out the bulb quickly, and at worst it could create a fire hazard.

The actual situation is more complicated because the filament's resistance increases with temperature, but it will definitely not draw "half the current". There are some more details here.
posted by teraflop at 8:15 PM on October 13, 2010

You should be OK with a 220V 40W bulb. If you used your 110V 40W bulb at 220V, you would have a little less than four times the power (and heat). This may be why your bulb burned out (and this could be dangerous). If you use a 220V 40W bulb at 110V if you go back to the US, it will be very dim.

To be clear, the dangerous situation that teraflop writes about only applies if you use a 110V bulb at 220V. Using a 220V bulb at 220V is not a problem at all (though I suppose it is theoretically possible that some part of the lamp is not rated for 220V and could fail).
posted by ssg at 8:34 PM on October 13, 2010

Your link doesn't apply teraflop. signal's lamp was designed for the US market and therefor has a rating sticker that assumes 120V line voltage. signal wants to use the lamp in Chile which has 220V at standard wall sockets.

Therefor signal is wanting to use a 220V bulb in an appliance plugged into 220V outlet. A 40W 220V bulb powered by 220V line voltage is going to consume the same power as 40W 120V bulb powered by 120V line voltage (the aforementioned 40W, 95% of which will be released as heat). That is what 40W means, at least for a simple resistive load like an incandescent light bulb. As a side effect because the power consumption is the same but the voltage has doubled (more or less) the current draw will be half as much.

There is the teeny tiny oh so remote of slightest of possibilities that the wire in the lamp isn't rated for a potential of 220V but considering the safety factors inherent in electrical wiring and the very low power consumption of the lamp I wouldn't worry about that at all even if the wire is only rated for 110V. Which it probably isn't. Most appliance wire in the US is rated for around 600V.

PS: A15 is the standard
posted by Mitheral at 8:35 PM on October 13, 2010

Dang it PS: A15 is the standard for the physical bulb base and small glass size of the bulb you are looking for.
posted by Mitheral at 8:37 PM on October 13, 2010

The above answers are wrong -- potentially dangerously so

A light bulb with a fixed resistance operated at twice the voltage will produce four times as much power.

This statement is correct, but your reasoning is wrong. What leads you to think that a 40W 220V bulb will have the same resistance as a 40W 120V bulb?

40W is 40W, for the OP's purposes, nothing else matters.
posted by HiroProtagonist at 9:02 PM on October 13, 2010

I see, it looks like I misinterpreted the question. Sorry if I overreacted.
posted by teraflop at 9:09 PM on October 13, 2010

A15 is the standard for the physical bulb base and small glass size of the bulb you are looking for.

More specifically, the 'A' refers to the shape (standard light-bulb shape) and the '15' means it's 15/8" or one and seven-eighths inches in diameter. The base used on A15 lamps (and all other common screw-in bulbs in the US) is called a 'medium base.' Hopefully Chile uses that same base for their 220v bulbs; if they don't, you'll have to go to a hardware store and get a replacement socket.
posted by jon1270 at 2:43 AM on October 14, 2010

I assume the lamp still has a 110 volt plug on it, and you're using some kind of power converter to make it go. In that case, the only option is to find another 110v 40watt bulb.

If you rewire it for 220, you'll want to make sure the base for the bulb fits the 220v 40 watt bulbs.
posted by MikeWarot at 7:33 AM on October 14, 2010

Mikewarot's answer is wrong, unless the lava lamp has some kind of active transformer circuitry in it. If it's just a positive and negative wire connected to the plug (14AWG to 18AWG) linked to the positive and negative terminals of the lightbulb, you can cut off the USA 110V plug and put whatever is needed in Chile on it.

Only if the lamp has active circuitry in it that specifically requires a 120V source will you need a stepdown transformer. I am in a 230V country right now and we use exactly the same threaded screw type lightbulb sockets that are used in the USA.
posted by thewalrus at 7:43 AM on October 14, 2010

A Watts a Volt-Ampere the whole world round.
posted by bonehead at 8:04 AM on October 14, 2010

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