As most of you know, Atlas has offered some wonderful diesels models; some in our favorite railroad schemes. I model Seaboard Coast Line and the predecessors Atlantic Coast Line and Seaboard Air Line. Two of the Atlas offerings, EMD SD35 and its passenger sister, SDP35 were owned by SAL, ACL and acquired by SCL post merger. They are great models, but one major distinctive feature is missing. The SAL (and the one ACL ) SDP35 units were outfitted with a MARS light assembly between the number boards as shown below:
I was wondering if this is backwards? I always thought the blue wire is the common and the negative side of the LED is attached to the blue. The positive side is where the resistor and the function wire is supposed to go. Am I right?
Decoder functions (white wire, yellow wire, etc) are current sinks - the output transistor connects the wire to ground, so the function color are the negative lead in any sort of circuit where polarity matters. The blue wire is a common positive from the voltage regulator.
It sounds backwards to a non-electronics person but there are several advantages to using current sinks with a positive common. First of all, for the same amount of current capacity, the sink uses a smaller transistor. Second, when the functions are current sinks, you can attach more than one function to the same circuit with no damage to the decoder. Say you wanted a red light that lights up when the train is in reverse OR flashes on demand by pressing a different function. You can hook the yellow lead to both the reverse light AND this read light, and also hook the F2 wire to the red light. So when you go in reverse, the red light goes on. Or when you activate F2, the red light flashes.
Thanks for the clarification. I read the literature wrong that came with my TCS decoders. I did install LEDs in a loco and everything turned out great. It was a fairly easy project.
