Bitsyncmaster
09-16-2011, 04:36 AM
Just an informative post of the electrical noise produced by an incandescent bulb turning on. Not a common problem but there have been a few caused by brake lights turning on and the noise upsetting some new electronic circuits. LEDs do not have this condition.
I just replaced one of my dome lights with LEDs and decided to measure the resistance of the OEM incandescent bulb when cold. Pretty easy to do with a digital multimeter. Measure with the test leads. Keep the meter on until the reading stabilizes (2.33 ohms). Then do the same with just the test leads shorted (0.79 ohms). The result is bulb resistance minus the test lead resistance (1.54 ohms).
So if your car voltage is 14.5 volts (engine running) you will get a 9.4 amp current spike when that one bulb turns on. The current quickly drops to the normal 480 ma because the tungsten filament resistance increased with heat.
One head light cold I read 0.41 ohms......
It's a good thing the current spike is so short that component ratings do not have to account for the spike. Also the wire resistance of the circuit limits the actual voltage the bulb gets during this cold start.
I just replaced one of my dome lights with LEDs and decided to measure the resistance of the OEM incandescent bulb when cold. Pretty easy to do with a digital multimeter. Measure with the test leads. Keep the meter on until the reading stabilizes (2.33 ohms). Then do the same with just the test leads shorted (0.79 ohms). The result is bulb resistance minus the test lead resistance (1.54 ohms).
So if your car voltage is 14.5 volts (engine running) you will get a 9.4 amp current spike when that one bulb turns on. The current quickly drops to the normal 480 ma because the tungsten filament resistance increased with heat.
One head light cold I read 0.41 ohms......
It's a good thing the current spike is so short that component ratings do not have to account for the spike. Also the wire resistance of the circuit limits the actual voltage the bulb gets during this cold start.