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Installing LEDs with DCC Decoders

by Don Fiehmann

Why use LEDs?

When installing lighting in a locomotive you have the choice of either lamps or LEDs. There are reasons to chose one over the other. Factors to consider are life, cost, size and ease of installation. Lamps have been with us for a long time. LEDs are newer and the technology of LEDs is changing for the better.

The first visible color LEDs were red followed by yellow and then green. In the last few years white LEDs have also become available. The first white LEDs had a blueish tint that did not look like an incandescent locomotive headlight. Recently the Golden-White, Yeloglo and Sunny-White LEDs have changed this. These new LEDs produce colors that are closer to that of an incandescent lamp. These new LEDs are now installed in some of the new locomotives, supplied with some decoders and available through many model railroad suppliers. The cost of LEDs is just a bit more than the cost of lamps, but well worth it over time. When installed correctly an LED will outlast the locomotive.

LEDs are available in different sizes. They run from the jumbo at 10 mm to a very small “chip” size that mounts on circuit boards. The common sizes are 3 mm and 5 mm. The 3 mm is the best for HO headlights. LEDs are also available in a range of colors: red, green, yellow, blue and white are the common colors. The LEDs that are made to resemble the yellow-white appearance of color of an incandescent lamp are the Golden-White, Sunny-White and Yeloglo. The Golden-White is made with an amber color epoxy that filters out some of the blue. OK when on but shows the amber color when off. The Sunny-White and Yeloglo LEDs have the incandescent appearance when lighted and clear when off.

LEDs vs. Lamps

The LED has many advantages over lamps. LEDs have a life expectancy rated in years and not hours as a lamp. LEDs are better at producing light. Lamps produce heat and some light, LEDs produce light and some heat! The light from a LED is much less sensitive to changes in voltage and current than lamps. The maximum current rating of most LEDs is 20 mA (milliamps). The light output from a LED is almost constant from about 5 mA to the maximum (about 20 mA).

LEDs need the correct polarity to work

The light from a lamp is not focused which is good for passenger cars and buildings. When in a locomotive bulbs put out some light through the headlight and in many cases end up also lighting the interior of the cab. LEDs produce a focused beam of light, this is why they work so well in the new LED flashlights, headlights, signals and panel indicators. In a darkened layout room an LED headlight can light up the area in front of the locomotive like a prototype headlight.

Wiring LEDs

When connecting an LED to a decoder a resistor is required to reduce the voltage. Unlike lamps that require you to select a value of resistor to match the lamp, selection for LEDs is easy. A 1 K (1000) Ohm 1/4 Watt resistor works fine with 10 to 14 volts on the rails. If using more then the standard 14 volts use a 1/2 watt 1 K resistor. If space is at a premium even a 1.5K Ohm 1/8 Watt resistor will work.

The LED is a diode and polarity must be observed when installed. Decoders have a common connection to power all of the function outputs. The common output is positive connection (usually blue) and the function outputs are a negative return connection. One resistor is needed for each LED connected to a function output. If you have two LEDs that will be controlled by the same function the LEDs can be wired in series and share the same resistor. When you solder wires to a LED it is a good idea to use a heat sink between the LED and the solder joint. A heat sink can be a long nose pliers with a rubber band around the handles. The rubber band will keep the pliers closed and trap the heat before it gets to the LED. Hint: If you are using heat shrink tubing on the wiring, slide the tubing over the wires before making the connection.

An LED does not generate much heat. This simplifies installation in a plastic body shell. The LED can be mounted directly to the plastic body shell without fear of damage to the shell due to heat from the LED.

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