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Locomotive Running Lights with DCC

Multiple unit lights

There are two flavours on Bachmann units: 158s and 166s have LEDs, 170s have bulbs that are diode controlled. They are arranged reversed in parallel so that track polarity that governs the direction of the unit also governs the indication of head or tail light.

If an unmodified unit were to be placed on the track on a DCC layout both red and white/yellow would be on simulateously because of the AC-like power at the track. There are lighting outputs on most decoders so modifications can be made for these to be used so that lights can be controlled and operate in the direction of operation of th unit.

The task is to convert the LED board from 2-wire to 3-wire. It's possible to remove the LEDs from the existing board and rewire, but in practice LEDs are so cheap it's easier to buy new LEDs and make a new board. On the 170s, cut the wires from the diode board and tease out the wires to the bulbs.

Remember that with DCC forward and reverse are relative to the unit, not the track, so make sure that you have decided which end of the unit is the front.

158 LED diagram

158 LED diagram

Output 1 (white wire) from the decoder can power the 'forward' direction headllights and tailights, output 2 (yellow wire) can power those for the reverse direction. The return from both sets of lights is connected to the blue wire on the decoder.

You will have three wires passing from power car to the trailer car: either run the wires between the cars, or use the special couplings from Roco that incorporate power connections. Another option is to use a trailer car decoder which is designed solely to work lights: it has no motor power output.

You must provide resistors to limit current flow through the LEDs on 158s and 166s, and it is advisable also to limit the voltage and current through the bulbs on the 170. Your 170 will have a voltage of between 0v and 12v across the lights when on a DC layout: more often than not a lower voltage than the maximum. However, under DCC the lights will burn bright even when the unit is stationary so your bulbs will have a very much shorter lifespan.

Understanding LEDs

A diode is an electronic component made of two pieces of silicon so that a voltage difference from anode to cathode allows a current to pass though it, but the reverse voltage does not. It‘s a one way valve like a letterbox flap: the pressure of the postman (the voltage) pushes letters (the current) into the house.

A Light Emitting DiodeLED – emits light whilst this is happening. Normally, a diode would control direction of current in a circuit, and the amount of current drawn would depend on other components. An LED used as an illumination device MUST have a balast resistor or it is simply a short circuit: a large current will flow and destroy the LED. The catch about a diode is that it uses an amount of voltage to allow it to pass the current - a typical LED uses 1.2 volts. A resistor must be provided to limit the current and that is the purpose of the resistor board in the 158s and 166s.

The value of the resistor you need in you circuit is calculated by application of Ohms Law:

Volts = Current (Amps) × Resistance (Ohms) — V = I×R

The diode in the circuit means that there is a voltage of:
12 - 1.2 = 10.8v

The current though a LED should be about 5 milliamps (0.02 Amps), so in the wiring example shown with four LEDs and one resistor, the value of the resistor needed (R) is given by:
R = V ÷I, or 10.8 ÷ .02 = 540 ohms. So any ¼ watt resistor of round about 500 ohms will do. Place your current limiting resistors on the yellow and white lines to the LEDs rather than the blue common return line as you may choose to use the other outputs from the decoder for other effects (coach lighting or smoke units for example) which also use the blue common return.

Page last modified: November 29 2011 12:19:38 PM

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