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DCC Functions and Effects

"This little light of mine, I'm gonna let it shine..."

Oh wait, my little girl is not talking about LED's, is she.  But, in this How To #9, I am.

Lighting is a very powerful feature of DCC and should be taken full advantage of.

LED's have become the light of choice for most modelers and for good reason.  They are brighter than incandescent bulbs. They will last thousands of hours longer than incandescent bulbs.  They operate at a significantly lower temperature than incandescent bulbs.  And most important, LED's are really much easier to install on DCC-equipped engines, rolling stock and layouts when compared to incandescent bulbs.

With respect to model railroading and specifically DCC decoder installations, LEDs are connected to a function output to provide on/off control and to add special lighting effects to locomotives and rolling stock that were virtually impossible in years past.

It's a crying shame to waste a function output so lets discuss how to "light 'em up!"

LED Color
The biggest complaint and argument presented for not using an LED is color.  A locomotive headlight has a nice golden white hue to it.  Here is a picture I took to use as a reference for color-matching LED's.
CSX GP38-2 #2506 westbound on the Crawfordsville Sub working a cut at the SDI steel mill in Pittsboro, IN.

There are now several options available for matching this color.  The LED color is often referred to as Warm White or Sunny White.  LED color is measured by the dominant wavelength.  The type of Warm White LED's I use and offer for sale have a dominant wavelength of 3500mm.  Here is a I picture I took of this type of LED installed in a pair of N scale Atlas Dash 8-40C locos.  I am pretty happy with the color match.

A pair of N Scale Atlas CSX Dash 8-40C's with upgraded SBS4DCC Warm White LEDs and add-on Ditch Lights on display on the C&SW RR

LED's come in many other colors as well.  Red, green, yellow and blue are particularly useful for modeling applications.  Most reds, yellows and blues are pleasing to the eye.  Green can be tricky however, especially for signaling.  But again, wavelength is the key.  A good "signal" green wavelength is around 520mm.

LED Types

LED's came in many shapes and sizes.  Some of the most popular sizes used in model railroading are:

Surface Mount Diodes (SMD) - Surface Mount Type (SMT)

The size of the of the most common types are:

0401 - 1.0 mm x 0.3 mm x 0.3 mm
0402 - 1.0 mm x 0.5 mm x 0.3 mm
0603 - 1.6 mm x 0.8 mm x 0.6 mm
0805 - 2.0 mm x 1.3 mm x 1.1 mm
1206 - 3.2 mm x 1.6 mm x 1.1 mm
2-PLCC 1210 - 3.5 mm x 2.8 mm x 1.9 mm
6-PLCC 5050 - 6.0 mm x 5.0 mm x 2.5 mm

Representative samples of the 0603, 0805, and 1206 types are pictured here.  Notice their size compared to the President's ear.

Radial and Axial Lead Types

Some of the most popular sizes used in model railroading are:

1.8 mm - Axial Lead with 1.8 mm lens
2 mm - Radial Lead "Tower" type with 2 mm lens
3 mm - Radial Lead "T1" type with 3 mm lens
5 mm - Radial Lead "T1-3/4" type with 5 mm lens

There are and endless variety of other sizes too but these fill most modeling needs.

LED's can also be packaged so that they have two or even three colors emitted from a single unit.  These are referred to as Bi-Color and Tri-Color LED's.

LED "Dropping" Resistors

Most model railroad applications of LEDs require a "dropping" resistor.

LEDs are rated by Forward Voltage at max milli-amps (fV@mA).  This is the maximum voltage that can be safely applied to the LED to produce maximum brightness and the amount of power it will consume at that brightness.

Forward Voltage is the spec we need to worry about.  Milli-Amps simply tells us how much of our available power the device will consume.  

Forward Voltage is adjusted by adding a "dropping resistor" to reduce the input voltage to a safe working level.

For example, the rated output of your command station is 14.5V at the rail.  The LED is rated at 3.2fV.  We need to reduce the voltage by 11.3V to prevent damage to the light.

The exact formula looks something like:

R = (VS - VF) / I


= Resistor Value (Ohms)

VS = Supply Voltage (Volts)
VF = LED Forward Voltage (Volts)
I = LED Current (Amps)

In model railroad applications, maximum brightness is not necessary.  Safe operation on any layout, maximum life for minimal maintenance and replication of the headlight effect are the most desirable characteristics.

Generally speaking, a 1000 ohm resistor will protect the LED on any Z, N, or HO layout.  This is my default size as it accomplishes all of me primary goals.

A 1k resistor will protect a 3.2fV LED up to about 21V Supply Voltage.  At 21 volts, we could match the scale speed of the record setting 385 mph run of the French TGV!

Adding a higher-value resistor will make the LED dimmer.  This is effective for marker and passenger car lights where dimmer is usually better.  I usually keep 1k, 5k and 10k resistors available for adjusting brightness.  The only limit to adding resistance is when the LED will no longer produce light.

The by-product of adding resistance is heat however.  It is important to size the resistor properly so as not to generate excessive heat that might damage your shell or create other problems.

Again, the exact formula looks something like:

P = I² × R


= Power (Watts) 

 = LED Current (Amps)

= Resistor Value (Ohms)

Doing the math, that 1000 ohm resistor will disipate 0.128 Watts of power when used with a 3.2fV LED at 14.5 Vs.  

Generally speaking, a 1/8 or 1/4 watt resistor will protect the LED on any Z, N, or HO layout.  This is my default size as it accomplishes all of me primary goals.  The larger the value, the cooler the resistor will operate.

Of course, the easy way to experiment with this is to use any one of a thousand online calculators to see the effect of resistance on current and watts.  

Here is a link to one of my favorite resistor calculators:  

The Best Current Limiting LED Calculator

So, bottom line... This is why I supply a 1000 ohm 1/8 or 1/4 watt resistor with all of the LEDs I offer.  It is a safe value for model railroading applications.  A higher ohm rating will make the LED operate dimmer.  A higher watt rating will make the Resistor operate cooler.

LED Wiring Basics

LED's can be wired up using new wire, scraps of excess decoder wire, magnet wire, or even by using the existing circuit boards already in the locomotive.

The magnet wire has become a very popular choice among DCC decoder installers and model railroaders alike because of its size.  Although soldering magnet wire looks tough, don't let the size of the wire deceive you.  It does require a steady hand but you would not believe the size of the iron point I use to do this.  It looks like my foot about to step on an ant.

Please see How To Series #1 - How To Solder Magnet Wire To Surface Mount LEDs for a more information about this technique.

Of course, you always get these pre-wired from if you don't have the time or desire to try it.
Using the existing board is one of the best techniques since the board is securely mounted in the frame and it makes a nice platform to support the LED.  It also creates a convenient place to connect the track power inputs of the decoder.

The trick with and most difficult aspect of this technique is to be sure to inspect every trace on the existing board to understand it's purpose and connection points.  It is usually necessary to isolate several traces by cutting the board to prevent damage to the decoder.

A neat product available online at that auction site (credit to my old buddy Lou V. for a sample of this) is two-conductor flexible PC board.  The product is incredibly thin and flexible and has two traces perfectly spaced for use with surface mount LEDs.  It is one of the most versatile products I have come across because is can fit anywhere.  I uses a piece of Kapton tape to insulate the exposed traces.
Before doing any wiring, review the specs for your decoder to know the rating of the function output. 

Most decoders supply track voltage to the voltage common (+) wire.  Some decoder are now available that can supply regulated 5V or even 1.5V power to the outputs.  This reduced voltage can also be useful for using incandescent bulbs and servo motors and such.

The function output rating of most decoders is high enough to support installing multiple LEDs on each output.

The key is to recognize that each function output will support multiple LEDs.  Just remember that the decoder controls the output, not the LED.  All LEDs wired to an output will operate the same way.

See How To #10 - DCC Functions and Effects for more on this subject.

Wiring a single LED to a decoder is pretty straight forward.  Connect the common (blue) wire to the anode (a) and the function (white, yellow, etc.) wire to the cathode (k).  Be sure to include a dropping resistor in the circuit.  It really doesn't matter which side of the LED it is on.   I like to put the resistor on the cathode (-) side.  There is typically a separate wire for each cathode so it serves as a nice "double check" to make sure I did not forget it.

Remember, within reason, you can't hurt the LED by wiring it backwards.  The first thing to do if you turn it on and do not get any light is reverse the wires.  If it still doesn't light up then you have a different problem.

If you are unsure which 
Remember, within reason, you can't hurt the LED by wiring it backwards.  The first thing to do if you turn it on and do not get any light is reverse the wires.  If it still doesn't light up then you have a different problem.

If you are unsure which 
LED with resistor connected to a DCC Decoder
Multiple LEDs on connected to a single function can be wired in Series or in Parallel.

Multiple LEDs on connected to a single function can be wired in Series or in Parallel.

Multiple LEDs on connected to a single function can be wired in Series or in Parallel.

Multiple LEDs on connected to a single function can be wired in Series or in Parallel.

Multiple LEDs on connected to a single function can be wired in Series or in Parallel.

Wiring in series is always the preferred method because an equal amount of current passes through each LED.  This means that all of the LEDs will glow at the same brightness.  Generally, there is a limit of three or maybe four LEDs per rung when wired in series because there is not enough voltage to drive the circuit. 
Multiple LED's with resistor in Series connected to a DCC Decoder
Wiring in parallel is sometimes necessary.  An example is an installation that uses multiple Bi-Color LEDs that have a common cathode like used for installing swiss-prototype lighting.  When it is necessary to wire in parallel, I use a resistor for each LED.
Multiple LED's with resistor in Parallel connected to a DCC Decoder
One really neat trick I use from time to time is to connect multiple functions to a single LED.

I like to do this when doing installs on E and F units to generate multiple effects from a single light fixture.  Using a Bi-Color Red/Warm White LED, I can get the following effects from a single decoder and light fixture, assuming the decoder has enough function outputs to support all of the effects:

1. Dimmable On/Off White
2. Dimmable On/Off Red
3. MARs White
4. MARs Red

Some decoders like the ESU LokPilot and LokSound v4.0 support this same set of effects with a single function output so the wiring (and subsequent space required in an N scale installation) is greatly reduced.

Be sure to use a 1k resistor for each function.
LED with resistor connected to multiple DCC Decoder Functions

It is also possible to add an "anti-flicker" capacitor to your lighting circuit without making any board-level modifications to the decoder.  Just remember that you have to do this to each function so space can be a big issue.  

A capacitor of at least 220mF will make a big difference in performance.

I have done this without the diode on the common wire but it is a good idea to include it to protect the decoder.  Caute procedere.  A 1N4007 diode is fine for this.

This is generally more useful on rolling stock applications than locomotives. 
LED with resistor and "anti-flicker" capacitor connected to a DCC Decoder

LED Installation

Installing the LEDs in the shell is probably the hardest part of the process.

Whenever possible, I mount the LED to the frame.  This has several advantages.  When the LEDs are mounted in the shell, you always have excess wire to deal with.  It also makes the shell permanently attached to the frame which makes general maintenance much harder.  You can use micro connectors to solve this but they require a lot of space and add a lot of expense to the cost of the installation.

The easiest way to do this is to use the existing circuit board as noted earlier.  If that is not an option, you just have to get creative since there is no clear-cut, simple, or repeatable method that can be used in every application.

I often find that it is necessary to mount the LED directly to the shell.  When this is my option, I typically try to use the existing light pipe if there is one present.  I simply cut the pipe down to the minimal required length and glue the LED to pipe with CA.  

If there is no light pipe present, I simply position the LED so that it is centered over the light fixture and glue it to the shell.  I typically use Microscale Micro Kristal Kleer or similar product to make a lens in the light fixture.  Sometime, I place a drop of the Kristal Kleer on the shell first then embed the LED in KK.  When dry, the KK acts as a diffuser to direct the light to the desired place.  You can also use scraps of old light pipes to accomplish the same goal.

When the glue and/or Kristal Kleer is dry, I coat the entire assembly with liquid electrical tape.  The liquid tape does several things.  It insulates the LED electrically.  More importantly, it acts as a "boot" to cover the assembly and hold the LED securely in place.  The best characteristic of the liquid electrical tape is that it is a very thick, and very opaque substance so it blocks out any stray light very effectively.

So that's that.

There is a lot of technical detail to selecting and installing LEDs.  Don't get bogged down in the details of the engineering and design.  

Just focus on the basics of resistor selection and mounting and your locomotive headlights will be simply brilliant.

  • Use warm-white or bright-white depending on your preference... It's your loco!

  • Select the best size and shape for the application

  • Use a minimum 1k ohm 1/8 watt dropping resistor.  Use 1/4 watt whenever possible.

  • Wire multiple LEDs in series whenever possible, in parallel only when necessary.

  • Mount to the frame if possible and to the shell when necessary.

  • Use Kristal Kleer and Liquid Electrical Tape as mounting aids.

Be sure to visit the store to see our full line of LED Lighting products today!