Loco Lighting Concerns

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I'm quite comfortable with decoder installations, have done a few dozen up to this point, including older locos where isolating the motor is required. I've left a number of locos unlit for lack of understanding how to properly light DCC equipped locos.

I've done lots of reading and research, but still seem to be lacking. I know if I use 12V bulbs I'm okay, but I've got a number of locos with 1.5V bulbs that require the use of a resistor. I've read various things about how to calculate what size resistor to use, etc. Not being an electrical engineer I find myself stumped, and my several attempts at winging it have resulted in burned out bulbs or bulbs that do not light at all.

There is one project in particular that I am targeting: An Athean RTR GP35 in which I've installed an ESU Soundlok decoder. Have the decoder installed just fine. Tried using the stock 1.5V bulbs with a 1/4 w 560 ohm resistor, and the bulb refuses to light. I know the bulb is fine...same result with a 470 ohm resistor.

I'm starting to wonder if there's something specific to the ESU decoder that is causing me trouble. I plan on shifting my focus to another loco that has a hard wired decoder (not PnP).

I hate to keep scratching my head, frying bulbs, etc. I'm hoping someone out there can shed some light (pun intended!) on my situation. I will continue to search this and other forums, but as you might be able to tell I'm a little lost and frustrated.

Many thanks in advance for any advice, tips, etc.

Steve Woodward


In Training Down Under.
Hi Steve welcome to the forum. Do you have a multimeter? I'd suggest checking the voltage out of the decoder on the lighting wires and see what you get. I assume you have the lighting turned on? Your running a DCC system?

There maybe a need to set the decoder up to respond to the directional lighting too.


Thanks for the reply and the welcome!

I do have a multimeter, and will check the output voltage. I assume I should check it at the white and blue?

I did make sure lighting was turned on, and I tried it in both directions. I may have to investigate the specifics of the ESU decoder, I am only familiar with Digitrax decoders for the most part.

I am using a Digitrax Zephyr with a DT400 throttle.

Thanks again!
Well, I was able to get a bulb to light, but I could tell it was too bright so I didn't leave it on for long. With a 1/4w 560ohm resistor the reading was about 3 volts. Didn't blow the bulb but obviously I need a bigger resistor.

This causes some concern about heat. There are 4 bulbs, 2 in the front and 2 in the rear. Will 4 resistors out out too much heat?

At least there's some progress, will keep plugging away at it.


Coal Shoveler
I've heard 750-950ohm, 1/4watt resistors works for 1.5v bulbs. I think 1/4watt resistors won't put out as much heat as the 1/2watt ones.

Thanks for all the replies! I would prefer to use LEDs, but in this one particular case (the Athearn RTR GP35) I seem to be intent on making it work with the 1.5V incandescent bulbs. Maybe for more of a learning experience than anything else!

I will try higher ohm rated resistors in the meantime. And thanks Rex for the link to the resistor size chart.

Thanks again to all!


Wannabe Engineer
Don't worry about the resistors putting off heat, it takes a lot to make them warm up. I would maybe try a 1k ohm resistor to see if that makes the bulbs dim enough for you.
Yeah, I've just ordered quite an assortment of resistors, once they come in I'll start experimenting.

I think there's an opportunity for someone to provide pnp lighting solutions for DCC, most of use mere mortals are having a hard time with this!

Thanks again.


I think there's an opportunity for someone to provide pnp lighting solutions for DCC, most of use mere mortals are having a hard time with this!

TCS offers a PNP solution for 1.5V lights. The VR1.5 plugs into the auxiliary plug on newer T series decoders. Check it out here.
The small red decoder is the VR1.5 I've used this once and it works very well. Some other TCS decoders have 1.5V lighting pads built in. Like the A6X that fits Athearn Genesis units and allows you to use Athearn's 1.5V bulbs with no added resistors.

They also have this PDF file on what resistors to use.

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