Hardwire versus using NMRA plug, wiring headlights to the decoder, LED diameter

Model Railroader Electronics and DCC
1

DCC-ready locomotives are easily converted to DCC with the NMRA 8 pin plug on the mobile decoder. Some say it is better to remove all factory installed wiring etc. and hardwire the decoder directly. My question: “what is gained by hardwiring over use of NMRA plug?”

In my DCC novice understanding, the NMRA plug was developed for several reasons for consistency across manufacturers and to make conversion easy for the user (very important given differences in knowledge, soldering experience, and willingness to take on the project). Also, the NMRA plug makes it easy to switch decoders if one wants to upgrade or needs to replace a faulty decoder.

Hardwiring ensures that all connections are made correctly and eliminates splices within individual wires. It does take more time and careful work to ensure good solder joints and soldering correct wire to the appropriate location.

All of my locomotives are DCC ready and easy conversions to mobile DCC. Sound is possible requires milling the weight for a couple of them. As a novice, is it worth effort to go full hardwire?

Electrical connections are pretty straight forward but there is one that I am unsure how to do. Probably very easy once ya know! Front and rear headlights have a white or yellow wire connected plus the blue common wire. Not sure how to solder the blue wire. Is there a common wire from each headlamp and both soldered to the blue wire connector on the decoder (assuming flat, uncovered decoder)? Does not seem right given that decoders can have four or more light functions connected to the common. And what about decoders that have a jst harness with 9 wires coming from the decoder. How does one t-splice in multiple light function connections to the common wire (look like t-splices in wiring diagrams)? Hopefully, my question makes sense, I’m a DCC novice so…

Final question, my P2k locomotives have 1.

2

I use the 8 pin NMRA connector exclusively on almost every locomotive, makes DCC life much easier. When I bought some Digitrax decoders that came with 9 pin JST connectors I wired them into a pair of Model Power E7s and to save space on four MDC Shays. My other 66 locomotives are 8 pin.

I sorta standardized on warm white 3mm LEDs, I still have a few locomotives using incandescent bulbs.

Mel

Modeling the early to mid 1950s SP in HO scale since 1951

My Model Railroad
http://melvineperry.blogspot.com/

Bakersfield, California

I’m beginning to realize that aging is not for wimps.

3

With an 8 pin connector, if you have a first start up problem- IS IT THE PLUG-OR THE DECODER? Hard wire and do a good job, you are 100 % sure it’s not the plug- On the installs that I do, the first thing to go is the OEM light board with the plug socket.For your lights using led’s is a good idea, they will last a lot longer than a bulb. 3MM will work for you. You need to use the blue wire to the positive lead of the LED with a resistor-(1K will work) in line- hook the white-or yellow to the other lead of the LED. You must use a resistor on each LED.

4

I wasn’t thinking on my earlier post. The original NMRA 8 pin connector will only work on older equipment, say from up to the 1960s. The newer locomotives have much more than headlight and backup lighting thus requiring more wires.

My newest locomotives are GP9s, SD9s and E7 series. I do have a pair of Krauss Maffeis from the 60s only because they’re different. I try to keep everything on my layout under 1955, I do have a 1957 Chevy and two 56 Chevys.

Mel

Modeling the early to mid 1950s SP in HO scale since 1951

My Model Railroad
http://melvineperry.blogspot.com/

Bakersfield, California

I’m beginning to realize that aging is not for wimps.

5

About the blue wire. It needs to run to every light function you plan on using. When I do it I usually do it like this:

The blue wire from the decoder gets cut short and I then get 2 pieces of blue wire and twist the ends together. Then solder the twisted ends and the wire from the decoder together. then heat shrink over that. This gives me on blue wire for the front and one blue wire for the back fo the engine.

Then I figure all the lights I want to use and pick the size of resistor for eachh function based on brightness. and solder these to the positive lead on the LED’s. Once that is done I take all the resistor leads from the front divide them into 2 groups add the Blue wire from ealrier to one of the groups and then twist the groups together and solder them. Then Twist the 2 soldered groups together end to end and solder that. Then cover with heat shrink this has to go on prior to soldering. Then repeat the process with the back. Usually its like 3 wires from one side of the final solder joint and a pair from the other. Not to hard the shorter you keep the wires the neater the appearance when done but a little more difficult to do. Always tin the wires prior to soldering and use a good electronic compatable flux.

Hope this helps/makes sense.

Ron

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Hello All,

Many factory-installed Printed Circuit Boards (PCBs) with NMRA 8-pin plugs not only have the circuitry for lights; incandescent or LED built-in, they also have chokes to prevent errant Radio Frequency (RF) signals on the board’s circuitry.

These chokes are required by European Union countries.

Because most locomotives are manufactured in China these are “included” in the PCBs for the North American Market.

As well-intentioned as these components are they can affect the performance of certain features of after-market decoders when using the NMRA 8-pin plug on the board.

Often these components are found on the motor rather than the PCB.

Here’s a thread on the subject…

Done with caps & resistors- -Uh…chokes, on motors…

The advantage of removing the O.E.M. PCB and hardwiring the decoder is these components built into the board are eliminated. Those attached to the motor need to be removed separately.

If the O.E.M. lights are LED, wired directly to the PCB with no resistors inline, then resistors need to be installed (as posted) with the aftermarket decoder, or a decoder that supports LED lighting must be used.

No matter how many accessory functions wires- -White, Yellow, Green, Violet, etc., you have they all use the single common (Blue wire) whether incandescent or LED.

When presented with multiple accessory functions many modelers simply “pig-tail” all the wires together to the single Blue wire from the decoder.

Other modelers, when given space in the locomotive, will fabricate a miniature terminal block, often with a strip of PCB.

The advantage to this method is each individual wire to the accessory is attached to the board rather than “pig-tailed” together.

If the need to repair, replace or add an additional common is easily done by soldering another wire to the board. Many after-market

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Mel, thanks for your replies! My p2k’s are converted by using a 9 pin JST connected to an 8 pin wiring harness. The JST plugs into the decoder and 8 pin harness plugs into the wired 8 hole connector on the locomotive. The latest version loksound v5 58420 decoder has an 8 pin harness which would be a quick plug in to my P2K’s eight pin connector and followed with speaker installation.

I am considering a TCS T1-LED mobile decoder that comes with a 9-wire harness for hardwiring. It has internal resistors for the front and rear headlights making LED conversion a snap! However, it would be easier to convert to sound later if I install the mobile decoder with an 8 pin harness + 9 wire JST. Decisions, decisions!

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Ron, jjdamnit, and woodone,

Thank you for the well-written descriptions! I’m a novice at DCC installation and have limited experience with soldering such small wires… Your writeups were most helpful!

The P2K’s do have a factory pcb that will be removed which leaves an 8 hole connector. You’re suggesting that I remove this as well rather than plug it into an 8 pin harness from the new decoder? I do understand they could be faulty but the locomotives are great runners now on DC and headlights work correctly… Perhaps the novice solution is to purchase both a 9 wire Jst and 9wire jst connected to an 8 pin harness. Try the latter first and if there is a problem, switch to hardwire with the 9 wire JST?

My P2K GP30’s only have front and rear headlights which simplifies wiring. A pigtail connection should be easy solution for the blue ground wire.

I am considering a TCS T1-LED decoder that has 1k resistors for front and rear headlights. The GP30’s are a second-generation diesel and I understand I should purchase “sunny white” LEDs, correct? The ones I see for sale on-line are all 3mm for sunny whites.

I want to avoid connecting the LED’s to the shell. So much easier to work on the locomotive to simply remove the shell and set aside without worrying about disturbing the wiring…

Thanks for your time and great advice!

9

I converted a number of HO DC locos to DCC with sound. In a couple I used a LokSound Select Direct, which was the circuit board replacement. That still required adding a speaker, of course.

In most cases I removed the original circuit board and bulb lighting, using the Select encapsulated decoder, with 8-pin plug. I removed the plug and hard wired in most cases. I learned to make my own speaker enclosures (0.040" styrene) with micro speakers. Since these speakers usually have improved sound with larger enclosures, removing the original circuit board often provided more flexibility for decoder and speaker placement. In some cases I used the smaller Select Micro decoder, good for 0.75A motor draw, to save more space when desired.

On lighting, I almost always convert bulbs to LEDs. I keep 5mm, 3mm and 0604 (pre-wired) on hand, plus 1k (and a few other size) resistors. If the bulb can be on the chassis, that is preferred, whereas bulbs attached to the shell (or lightpipes on the shell) make the bulb wiring (from the chassis) a bit long and tether the shell to the chassis re: maintenance (unless you install disconnects in the wiring). In many cases I shorten too long light pipes as useful. I do not hesitate to CA glue the LEDs to the shell, lightpipe, etc. One can use white glue, but you have to let it dry well until securely attached.

10

I would say use the plug if it gives you all you want.

Now, my engines have class lights, numberboards, and cab roof beacons so I need more function leads, so I do hardwire. But my grandfather taught me to solder back in 1961, so it’s no biggie for me.

11

I won’t repeat some of things that have already been stated. However, I will make one comment. If the 8-pin socket comes with the locomotive I usually try to take advantage of it. However, if the wires running to track power, motor, and lighting are attached to a PCB underneath with those flimsy plastic clips, I remove the clips and solder the wires directly to the PCB for a much more secure electrical connection.

Tom

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Keeping the DC board in place and using the 8-pin NMRA connector seems so easy, why not?

Well, in memory of our dear friend, Randy Rinker, here is what he always did when converting a DC loco to DCC . He always removed the DC board and skipped the 8-pin NMRA connector in favor of hardwiring the decoder in place. For a long time, I ignored his advice, and I paid the price more than once with fried decoders and/or blown function outputs and burned out incandescent bulbs.

Randy always replaced the factory bulbs with Sunny White or Golden White LEDs and put 1K ohm resistors on one leg of every LED unless the decoder had built-in resistors for this purpose.

Regarding wiring, the blue wire is always Common (+) and the (+) side of every LED is joined to the blue wire. I always add a resistor to the positive leg of each LED, but you can put the resistor on either leg.

You find the positive side of an LED by looking for the smaller of the two metal pieces (the ends of the legs) inside the LED enclosure. The larger metal piece is the negative side which connects to one of the function outputs for front headlight, Mars light, rear light, number boards, ditch lights.

Regarding the P2K loco, I use a 5mm LED for the Mars light and a 3mm LED for the headlight. I believe that Randy did the same thing. If the installation calls for an even smaller LED, you can always use a SMD LED, short for Surface Mounted Device. An SMD LED is a micro LED, smaller than 1mm.

Rich

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Electrically there can be no difference between using the NMRA plug and soldering direct. Same for the plastic clips that jam the wire strands onto what would be soldering pads.

Electricity isn’t a form of magic.

There’s a huge pile of urban myth in this hobby, I’ve noticed.

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Let’s not forget who installed the stranded wires onto the soldering pads in the first place and secured the wires with those little plastic clips - - the manufacturers!

Rich

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Can you name one as an example so we know where you are going with this?

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Thanks everyone! I appreciate reading your thoughts and advice! Helped me tremendously! Randy Rinker was my goto guy for many things and it was nice to see his name again and read his advice as given previously in the forum (thanks Rich).

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I’m not going anywhere with this.

We could start with overbuilt benchwork.

We could continue with strange ideas about DC reversing loops.

We could delve into using grease or oil or no lubricant in locomotive gear trains.

And the fairly weird obsession with voltage drops in a system that relies on lowering voltage to control and simulate locomotive movements.

But I refrain.

18

And you find out why when you need to remove one of those boards and put it back. It’s way easier to unclip those wires to remove the board than to unsolder them.

DC locomotives draw very little current. The bare wire inserted into the hole in the metal “soldering pad” secured by a reusable plastic clip is a sensible engineering solution to a simple problem.

19

Thank you for confirming that the notion of urban myths, or urban legends, do not play a role in model railroading.

Now, if there were a widely held belief that layouts cause cancer, or staring at LEDs causes blindness, or using wood to frame a layout leads to termite invasions, well, that would be a whole different story. [(-D]

Rich

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In a perfect world that might be true. But, for someone skilled in the basics of soldering, a hardwired decoder and soldered circuit board tabs will provide a more secure set of electrical circuits than a poorly installed 8-pin connector or a set of plastic clips. I speak from experience. So, electrically, there clearly can be a difference between using the NMRA plug, or plastic clips, and soldering direct.