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It was time to upgrade the interior lights in my 2004 Volvo. I got some lamps from Ebay specified as LED Festoon, 80-85lm, 12V. As many others have experienced they kept on glowing faintly after the door was closed. When the ignition was turned off the lamps were completely off also, so there was no danger of draining the battery. Still it is not the way one expects lamps to behave.

One can get more expensive LED lamps which avoid this faint glow, “Canbus error free” seems to be the way to specify this. But mine were of the plain type, and the problem seems to be the leakage current in the FET switches that turn the lights on and off.

All it takes to solve it is a parallel resistor. Some have used 1k, others larger values. I just made some trials and found that 10k worked well. The advantage is that 10k will only dissipate 18 mW @13.5 Volts, while the 1k will dissipate ten times that, so I could use a small 1/4 W type. I soldered it on the back of the board as the image shows.

The reason for switching to LED is not really to save energy as the savings aren’t that great anyway. The whiter and brighter light is more important as you can see in the image with the LED to the left and the old incandescent lamp to the right.

While at it, I just had to do some reverse engineering of the LED lamps. There seems to be four parallel groups of three series-connected LEDs (the three in a row) giving a forward voltage of about 8.3 V. They are driven via a resistor of 120 ohms in series with what seems to be a bridge rectifier since the lamps don’t depend on being connected in a particular way with respect to polarity. They draw 18 mA @ 12V and 28 mA @ 13.5 V. This is not a very fancy way of constructing a LED lamp as there is no constant current regulation. The intensity will therefore vary with voltage, but hopefully it will work well here.