Adding SMD's to Function Outputs on Decoders

[tehachapifan]

Going to try to add some 3014 SMD's to the extra function outputs on my Digitrax DN163A decoder for the MP15AC. I have learned that resistors must be added and where the function pads are, but I'm not sure which side of the SMD goes to the function output or the common pad. The SMD's have a shorter wire on the cathode side. I would rather get it right the first time rather than experiment, if someone happens to already know. Thanks!

[C855B]

Easiest way to visualize it is they go on each function output.

Think in terms of each LED needing its own resistor. It doesn't matter which side of the LED the resistor goes on, it just has to have one.

[tehachapifan]

Thanks, Mike. I would not have guessed that. I recall it making a difference when I would add LED's to standard DC light board outputs (would work one way but not the other).

On a related note, what is a good way to insulate the exposed joints after adding a resistor to the SMD's circuit with out adding much bulk? Seems this could be a potential short source once the shell is replaced and everything gets compressed.

[C855B]

Pete(ski) says he has good results by "painting" the exposed bits with cyanoacrylate ("Crazy") glue. Two coats of the normal thin stuff should be sufficient.

[tehachapifan]

Seems like a simple enough solution!

[jdcolombo]

Let me try to clarify Mike's original response, because I think it was misunderstood.

It doesn't matter which side of the LED you add a resistor to.  It can be either the cathode or anode.

However, it DOES matter how you wire the LED to a decoder's function outputs.   In general, the decoder's blue wire is the positive common, and it must be wired to the positive (anode) side of the LED.  The function pad/wire (which we unfortunately refer to as the function "output" as though it were the positive side of the wiring, when it's actually the negative side) should go to the negative (cathode) side of the LED.   If you wire it in reverse, the LED will not light up.

I use 1K, 1/8w standard carbon round resistors, which are quite small, for nearly all my LED installations; you can get fancy and try to calculate an exact resistance for the amount of current you want to supply to your LED's, but I find that 1K works for about everything.   If you are really pressed for space, another way to do this is to get a small piece of circuit board material, make a cut through the copper cladding (be careful to cut only the cladding, not the whole board!), and then solder SMT 1K resistors to the piece of circuit board so that they "straddle" the cut.  Now you can cut away one or two of these soldered resistors on their circuit board piece to use as your dropping resistor(s) for the LED(s).

John C.

[tehachapifan]

OK, thanks for the clarification, John! That's the info I needed.

[peteski]

Russ, you mention SMD (surface mounted device) where I think you meant LED (light emitting diode). Or a SMD LED.   

[peteski]

If you are really pressed for space, another way to do this is to get a small piece of circuit board material, make a cut through the copper cladding (be careful to cut only the cladding, not the whole board!), and then solder SMT 1K resistors to the piece of circuit board so that they "straddle" the cut.  Now you can cut away one or two of these soldered resistors on their circuit board piece to use as your dropping resistor(s) for the LED(s).

John C.

And if you are really strapped for space you can solder the thin wire (usually magnet wire with enamel insulation) directly to the SMD resistor. Then you can take some small heat shrink and use it to cover that assembly.  Or coat the assembly with several layers of CA glue and accelerator (to set the glue quickly so it forms a coating).

[tehachapifan]

Russ, you mention SMD (surface mounted device) where I think you meant LED (light emitting diode). Or a SMD LED.   

Yes, I meant SMD LED's.

So... I've been testing the function outputs on this decoder (the DTX MP15DC decoder) and I'm getting some weird behavior I can't figure out. If I program CV49 (the onboard forward light) with a light effect (i.e Mars [002]), nothing happens with the forward light at all but the rear light will display the effect when the loco is in REV. Also, when I try the function pads with a test LED (with resistor) is still only get effects when the loco is in REV. I have not been able to produce effects of any type with the loco in FWD. Any ideas? This stuff is going to make me crazy(er)!