I wiring up 3 warm white 3mm LED's for the front lights on my BR81 and I have them wired in series. I originally started with a 1K resistor and the light worked fine but were very bright and too bright to look realistic to my taste. So I connected a 25K potentiometer in series with the LED's and turned it to add resistance until I thought the light looked correct. I disconnected the potentiometer making sure not to turn it and measured the resistance and it was approximately 17K.

If I use a 18K resistor (closest value) will it cause any issues with the decoder or other problems I may have overlooked?

Thanks, Chuck

Hi!
When I replace yellow LEDs with white LEDs in a Märklin Hobby loco with a dumb decoder then I usually add 10 kOhm in series with the 5k6 the loco already has.
I also tried adding 15k and 20k and it always was bright enough - with a total of 20k6 or 25k6.

For smart decoders I do not add resistors (keeping the factory-installed 5k6) and dim the light in the decoder.

So if you use 10k or 15 then you can still dim the lights in the decoder - assuming the decoder is not dumb like a 60760.