What does 12v 36w mean on a bulb? (1 Viewer)

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Dumb question, I know.

But what exactly does it mean? Does it mean 36watts is what a bulb will consume at 12v?.. And does '12v' mean that the bulb will only work at 12v. What if you connect the bulb to a 24v supply?

Please excuse my ignorance.


Gold Member
At 24V it will glow much brighter, and for a much shorter time.

BTW, a 36w bulb with 12v will pass 3A. That's huge. This must be in a car.


Science Advisor
Gold Member
Yes, your bulb will draw 36 Watts (or Joules per second) when connected to a 12V power supply.
ok thanks..... so when it states '12v' it just means this bulb will work best (last longer) and is specifically designed for a 12v supply?

...And if it IS connected to 24v supply it will consume 72watts of power?


Staff Emeritus
Science Advisor
Gold Member
It's designed for 12V. It will probably burn itself out very rapidly if you double the voltage, as you'd be subjecting its filament to four times as much heat dissipation.

Power goes up with the square of the voltage (or the square of the current):

[itex]P = I^2 R = \frac{V^2}{R}[/itex]

If you double the voltage, you quadruple the power consumed.

- Warren


Science Advisor
Gold Member
However, reality is not always this simple. One of the phenomena explored in a later chapter is that of conductor resistance changing with temperature. In an incandescent lamp (the kind employing the principle of electric current heating a thin filament of wire to the point that it glows white-hot), the resistance of the filament wire will increase dramatically as it warms from room temperature to operating temperature. If we were to increase the supply voltage in a real lamp circuit, the resulting increase in current would cause the filament to increase temperature, which would in turn increase its resistance, thus preventing further increases in current without further increases in battery voltage.
http://www.allaboutcircuits.com/vol_1/chpt_2/5.html" [Broken]
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