I am installing 2 SMD LEDs in an older Stewart/Kato unit with a Soundtraxx mobile decoder (not sound equipped) with the harness. I wired them both to the front light function output and at first, they would both light up but now for some reason, only one LED works. I reset the decoder and I tested the LED that wouldn't light after de-soldering it from the harness and it works just fine. Each uses their own 1/4 watt 1K resistor. How can I resolve this so I don't end up buying a full board decoder for the sake of LEDs?
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Two LEDs in parallel like that should be fine. I don't know why it would work one day then stop, unless a wire came loose. With a 1K resistor on each one, the combined current (two LED/resistor circuits in parallel is 2x the current of a single one) is still below the limit for the function output. You don't have one wired backwards, do you?
You may find you need to use more than a 1K resistor, not for current limiting, but because the surface mount LEDs are generally quite bright.
--Randy
Modeling the Reading Railroad in the 1950's
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humm odd try desoldering the one that works and see if the other comes on , if so try subbing with another LED and see if that cures the problem.
Since the failed LED still lit after you desoldered it from the harness, at least you know that you didn't inadvertently burn out the LED. Desolder the other LED and start over again. Attach one LED to the function output without soldering it and see if it stays lit. Then add back the other LED without soldering it. Be sure that they both stay lit be resoldering all of the connections.
Rich
Alton Junction