will a resistor do the trick?

Guybo

Torque Junkie
Points
52
Location
Tampa, FL, USA
Car
13 GenCoupe 2.0T
The dome light in my car has 2 positions. Door- which comes on when the doors are open and the ignition is off, and which turns off when the car is driving with doors closed. Theres also full manual which is lights off unless I hit the button.

With the OEM incandescent lights installed, it works like it should. With doors closed, set to Door mode, the dome lights go out when I drive. But recently I got red led dome lights. Now when I drive with doors closed the dome lights stay on (slightly less bright than when I hit the Manual button however).

I have read that some cars keep a little power going to the dome lights all the time, but not enough to light up an incandescent. Could that be enough to light up an led? Would a resistor hooked up in line on the power wire to the dome light be enough to stop this? If so, what size or how can I determine the size?
 
It's worth measuring this before faffing around with it. Get a volt meter and see what happens when all is off. Bear in mind that some cars enter a low power state for 5-10 mins after ignition turned off before shutting everything down. Audi's run the fuel pump when the drivers door is opened and all internal power circuits are open for a while after the ignition is switched off. My cigar lighter will charge things for a short while only after I turn off, this caught me out as I was expecting a fully charged phone when I returned a few hours later!
 
Gah. Thanks anyway guys. I installed a resistor inline II went by old-git's suggestion but could only find a 300 ohm 1/4 watt) and all it did was make the light dimmer- both while the lights are supposed to be on and while they are supposed to be off too. There isn't enough of a difference between full on and supposed-to-be-off to work.
 
CANBUS wiring can give results that we don't like. A series connected current limiting resistor is required with ANY LED and these clearly are there already in place (within the LED 'bulb') otherwise they'd have burned out instantly.

Series resistors won't fix this.

A reversed biased Zener diode connected in series plus a shunt (parallel) resistor across the LED should fix this.

Zener diodes behave like standard diodes in forward bias, with a typical 0.6v drop across the device. In reverse bias they are effectively open circuit until breakdown (avalanche) voltage is reached whereupon they conduct fully. You will have to play with the shunt resistor values to determine a suitable one. A DVM is helpful but please bear in mind that semiconductors do NOT obey Ohm's law.
 
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