Resistence of 5W bulb (Ohm's Law)

Tungsten filament lamps increase their resistance when running (white) hot by about 10 time. Try your meter on a domestic light bulb to check the resistance cold and work out what 100 Watt bulb should pass as current from 240 V supply. It should take about .42 amps at 240 volts to give 100 Watt consumption. That would require a resistance of about

Tony

Do not forget that the resistance goes up as the filament gets hot, usually a factor of about 10 times on a 12v bulb.

You have to use a meter setup that applies the 12v to the bulb and actually measures the current accurately, then you can use ohms law to determine what the hot resistance is going to be of the bulb.

David

Steve

That's what I thought but I have checked and double checked my meter and my usage of it and neither seems incorrect.

The resistance of filament bulbs has a strong dependance on their operating temperature. When you first switch on, there is a surge of current as the resistance is quite low, this then rapidly heats the filament, increasing resistance, which dampens the current growth, until an equilibrium is reach.

This current surge is can be damaging, which is often why a bulb that is flickered on and off will last fewer hours than one remainingly constantly on.

As has been pointed out, the cold resistance is very different from the running resistance - not surprising considering the temperature range. Very few - if any - materials will have a constant resistance with varying temperature.

You can make use of this feature. Wiring a headlamp bulb in series with a Hi-Fi loudspeaker will help prevent electrical damage at say a party. At moderate levels the very low resistance will have little effect on the sound. Increase the volume up to say around 50 watts from the amp and the bulb will light reducing the power into the speaker considerably. Good trick with teenagers when you go off on holiday. ;-)

Resistance is futile. You will now be assimilated.

:-)))

Andy Pandy To e-mail, address hopefully, self-explanatory !

That's about right for cold resistance

Is the "wattage" of a light bulb the light output? or combined heat and light?

does 90% of power in bulb being given out as heat sound about right?

-- Big Egg Hack to size. Bash to fit. Weld to join. Grind to shape. Paint to cover. My name is not "news". If you reply to that address, I won't get it

Combinede heat & light, 10% effecient's probably optimistic

Neither. It's the power consumption. But that power is converted into heat and light. Maybe also the tiniest amount of sound, given that no DC supply is pure. ;-)

No - it will depend on the type of bulb, but most in a car are much worse than this.