Advice sought on d.c. switcher headlight conversions to LEDs

Model Railroader General Discussion (Model Railroader)
1

HI: Am old geezer, so thought I’d better ask this. Have 3 small HO d.c. loco roster for new in-progress l/o. Newest is Atlas Alco HH 600 Silver, just purchased and found out it has LED lights. Wish to convert other two to same lighting [ Spectrum GE 44 Ton and older Atlas S4 ]. Need advice on how to proceed, to avoid goofing things up [ maybe a web site referral? ]. Not interested in converting to DCC now [ that will come later ]. Sound is out due to hearing loss. Thank You in advance…Old Tom aka papasmurf in NH

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For the Atlas S4, I feel the easy way would be to install a NCE ATL-S4 decoder. It includes LED headlights. I have installed one, fairly easy to install, decent instructions included, and the lights are bright. You would then be ready when you take the leap to DCC. Out of the package it will run on DC.

The Spectrum 44-tonner I have no experience with, and cannot point you to any sort of solution.

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MANY THANKS WSOR! I will put this info in my pc’s HO file; follow when I convert my 3 locos to DCC. TTFN…Old Tom aka papasmurf in NH

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As WSOR 3801 has pointed out, currently available decoders will run on DC and you can install them now and go with a DCC system later. I have a spectrum 44 tonner (single motor version) that I added a TCS M1 decoder to and replaced the light bulbs with LEDs. I used 3mm warm white Leds with 1K ohm resisters and they have worked well for me. This isn’t a drop in decoder situation like the S4 so it will require some hard wiring. You may be able to hook up LEDs to the Spectrum light board without going through a decoder install but I haven’t looked closely enough at the board to say how easy it would be.

Joe

5

I have been working on converting Athearn blue box F units to run with LED head lights and was

stumped at first as to how to get varying DC voltage and low current to keep from burning out

the LED…Came across a company called Evan Designs that has a website:

ModelTrainSoftware.com.The packages contain the whole works for either DC or DCC

and LEDs come in different sizes and colors for what you may want. I am using kit U30 with

5 packs of preassembled rectified SMD parts under a black tape for insulation purposes.

For $20 no headaches for reversing an engine I needed F units that when backing

with no headlight on condition I just add a diode to it on one lead and test to see if stays

off in reverse.

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To run LED lights on a plain old DC system you have to deal with two problems.

  1. LED’s are polarity sensitive. They light for DC applied one way (“forward biased”) and stay dark when the polarity is reversed (“reverse bias”). They are only rated to withstand 6 volts reverse bias, so that full throttle (12 volts) will destroy them. The direction switch on you power pack works by reversing the polarity to the track to make the motors turn the other way.

  2. LEDS offer no useful resistance to the flow of current. They must have a resistor in series with them (or a fancier current limiting circuit). Most LED’s are rated to produce full brightness at 20 milliamps. Some of the very tiny ones may need the current limited to something less.

One solution is to use a full wave bridge rectifier to establish the polarity of the track power. Radio Shack used to carry them, cost a buck or two and are may be 1/4 inch square. With a 330 ohm current limiting resistor you are good to go. This gives max brightness at full throttle and the LED gets dimmer as you throttle down. It also keeps the LED on all the time, whjch I like. I used to add diodes to obtain directional lighting, but now I prefer the headlights to stay on all the time. It makes it easier to troubleshoot the “locomotive-won’t-move” problem. If the headlamp lights up the locomotive is getting power and the problem is mechanical.

Another solution is to merely protect the LED from reverse bias with a single protection diode, which can be in series or in parallel with the LED it protects. This gives you directional head lamps (if you get the polarity right way round). You want a little bigger current limiting resistor with this circuit , like 680 ohms, 'cause it doesn’t have the 1.4 volt drop caused by the full wave bridge.

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Good article on making constant LED lighting …

http://www.pollensoftware.com/railroad/index.html

Mark.

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Great article. Simple.