Lighting for HO Athearn GP-35 : LUSENET : Wiring for DCC : One Thread

Sorry if this has been answered already (I scanned thru and read everything I saw which -MIGHT- have fit and didn't find an exact answer:

So -- I have just made the transition to DCC. I have one FT-A unit with a factory decoder. It runs perfectly. I also have an Athearn GP-35 which is probably seven or eight years old.

For this loco, I bought and installed an NCE-D13SR decoder. The loco RUNS properly (tho it's tough to start sometimes, but I haven't really played around with any of the CV settings to dial it in properly...) but tonight I tried my hand at installing some Miniatronics lights to replace the big ugly factory light.

I used the Model # 18-701-10 incandescent lamps. They are 1.5 Volt, 40mA bulbs which are a direct fit for the two lights located between the front number boards.

My questions are thus:

1. How do I get the "DIMMING" function (F1 on my Prodigy Advance) to work?

My decoder has a green wire for "F1" but I dunno where to stick it...

2. At a stand-still, the lights are beautiful and bright. But when you apply power to the motor, the lights flicker and go very dim.

Is this a problem with the green wire I mentioned before? Or is it in the choice of resistor?

With 2 40mA bulbs I'm drawing 80mA of current. The NCE manual suggests a 150 ohm resistor in this case, which is what I installed -- a single resistor with the bulbs paralled off of it.

Would this work better with two 300 ohm resistors -- one for each bulb? Or should I lay in a parallel set of diodes, as some other site suggested, to try to better regulate the voltage?

In any case, it seems like something just isn't working right, and after typing this up I get the feeling it has more to do with that green wire than I previously had thought possible.

Thanks in advance for any help.

ryan adam

-- ryan adam (, January 07, 2005



I can't help you with the Prodigy or the NCE decoder. I can only help with the generic DCC issues.

Since you are using 1.5V bulbs, things will run a bit cooler if you put the bulbs in series instead of parallel. That will put your current draw back at 40mA. Use 1 300 ohm resitors in series with the 2 lamps. This isn't a big deal, but since you have to rewire things any way, you might as well do it this way.

You never mentioned the blue wire. Use the blue wire instead of connecting the lamps to the locomotive's chassis. Therefore, wire everything up in series.

Here's a little "diagram":

green wire ---- resistor ---- 1st lamp ----- 2nd lamp ---- blue wire

Note: If the number board is too bright, feel free to use a higher value you resistor. This will dim the board. Use any value higher than 300 that suits you.

-- Allan Gartner (, January 07, 2005.

The two lamps are both front headlights?? In either case, the blue wire supplies power (+ polarity) to any and all light function outputs. The second wire intended to/for each lamp circuit is: The white wire controls the front headlight(s). The yellow wire controls the rear headlight(s). The green wire controls whatever you have programmed to the F1 output of the decoder. Every function output on the decoder is programmable to respond to other specific DCC function commands, with or without special effects. Decoder function output #1 (F1) doesn't necessarily correspond to DCC function command F1. Read the manual carefully.

Not sure what the F1 button does on your Prodigy unit, or what you expect to happen when you press it.

re - your lamp wiring and resistor selection is OK, but be aware that the resistor is dissipating (wasting heat) about 0.08 x 0.08 x 150 =~1 watt. It will get hot! Use a 2 or 3 watt resistor if you can.

In future you would be better off supplying a separate 300 ohm 1 watt resistor for each lamp, and then wiring them in parallel to the function output. (0.04 x 0.04 x 300 = 0.5watts. should use a 1 w resistor) Reasons - 1) Resistor heat is spread out on the surface area of two resistors... Lower temperatures. 2) When/If one lamp burns out the other lamp will continue to function. [When you wire two bulbs together with a single resistor, if one bulb burns out the remaining lamp sees ~2X voltage/current and also quickly burns out.]


-- Don Vollrath (, January 07, 2005.

Thanks for the tips guys.

To clarify some things -- I am using the blue wire and not chassis ground. I have Blue Wire ---> 150 ohm resistor --> (light + light). On the other side I have White Wire ---> (light + light).

I'll try running the lights in series to see if the lower current draw provides a more even light output.

The two lights I'm using fit into the two molded "holes" for lights in the center of the number boards - that's why I'm using two bulbs for one function.

Once I get this working, I plan to use another pair for the molded holes in the rear of the loco. I'm still not sure what configuration this particular road number had as far as front headlights, so I either need to fill the molded holes on the hood or put some lights there, too...

Anyhow - when I mentioned the F1 function for the green wire, I was referring to the way my Bachmann FT-A unit (which has a factory installed decoder) acts.

On this unit, If I activate F0 the headlight goes on or off - if the loco is going forward. In reverse, I have no light.

If I press F1 while the headlight is on, it DIMS.

Now - since this function is effectively taking voltage AWAY from the circuit, I dunno how to make that work on my NCE decoder. It does have a whole host of options for the lighting CV so maybe I just need to select something other than the default.

Thanks again for the quick replies.

ryan adam

-- ryan adam (, January 07, 2005.

Now I'm just talkin' to myself...

But in case anyone else comes across a similar problem, this is what I discovered:

1. The NCE decoder uses F4 and/or F8 to provide headlight dimming. Programming the proper CV value into the headlight function gives me what I want.

2. One of my "just-get-it-working-and-make-it-pretty-later" steps was the source of my erratic voltage. Instead of finding a proper place to install the negative pickup wire, I managed to get a cold-join on a part of the pot-metal frame.

While this was good enough to make the loco run (kinda) it wasn't good enough to provide full functionality.

When I discovered that pushing DOWN on the contact point with my finger made the light work better, I decided to try something else. I relocated this wire directly onto the negative side of the truck, rather than using any part of the frame, and I now have bright lights no matter what the speed of the loco. It also goes at slow speeds just fine, which it wouldn't do with the bad pickup.

Thanks again for the suggestions - I did as Allan suggested and wired the bulbs in series. This way I should get less heat & protect the 2nd bulb if the first goes out all at the same time while keeping extra components to a miniumum.


-- ryan adam (, January 07, 2005.

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