Headlamp bulb power/current

peter999

One of my headlamps was playing up.
Started working again, maybe dodgy contact.

I checked resistance of some standard 12V 60/55W spare headlamp bulbs on multimeter.

The resistance of the 2 filaments (for Dim/High beam) seems to 0.3 ohm.

Does that make sense ??

As with basic physics Current I=V/R, would give I=12/0.3 =40 Amps

Then Power P=I^2xR = 40^2x0.3 = 480 Watts !!

What am I doing wrong as power should be around 60/55 Watts ??

peter999

vaio

the resistance increases dramatically when the filaments heat up

forgotmyname

Watts / Volts = Amps.

60W bulb / 12v = 5 Amps.

Im sure resistance would vary a lot depending on the material used for the bulb. Thats why your maths dont work.

Stephen_Leak

vaio wrote: »

the resistance increases dramatically when the filaments heat up

And this is why bulbs usually blow, when you switch them on.

For a dodgy contact, I'd reach for a little bit of wet & dry abrasive paper and a squirt of WD40, not my multimeter.

Notmyrealname

peter999 wrote: »

What am I doing wrong as power should be around 60/55 Watts ??

peter999

You're overthinking it.

Power = Current x Voltage. We know the power so Power/Voltage = Current. So it is 60/12=5A.

Mankysteve

vaio wrote: »

the resistance increases dramatically when the filaments heat up

R(T) = R(T0) * [ 1 + a(T - T0) ]

T0 = reference temperature (deg Celsius) T = temperature of interest (deg Celsius) R(T0) = resistance at reference temperature (ohm) R(T) = resistance at temperature of interest (ohm) a = temperature coefficient of resistivity (1/deg Celsius)

give_them_FA

You forgot to add, Mankysteve, that as the vehicle's speed approaches that of light, you won't be able to see where you're going any more!:rotfl::rotfl::rotfl:

forgotmyname

give_them_FA wrote: »

You forgot to add, Mankysteve, that as the vehicle's speed approaches that of light, you won't be able to see where you're going any more!:rotfl::rotfl::rotfl:

True but if he goes a bit faster he can see where he has been.

:beer: