LED Lighting Williams GG1

Yep, 470 ohm 1/2 watt is what I use for TMCC LED conversions.  Connect the resistors directly across the lighting outputs and then connect the LED in parallel with the resistor.

LED Resistor Value = (Applied DC Voltage - LED Forward Voltage Drop)/LED Operating Current

For example (made up numbers):

Applied DC Voltage = 20V

LED Forward Drop = 2V

LED Operating Current = 0.02A (20mA read 20 milliamps)

LED Resistor Value = (20 - 2)/0.02

=18/0.02

=900 Ohms

You may have to buy a slightly higher value like 910 ohms but the light output should be about the same.

Less current = dimmer LED

Too much current = burnt out LED/Short LED life

Peak AC Voltage = 1.414 * your pure sine wave transformer output

For example 1.414 * 20VAC = 28.28 Peak.

Although your Peak AC Voltage is not technically DC Voltage, this would be a good place to start for your DC Applied Voltage in the above calculation.

LED Forward Drop and LED Operating Current both vary for the color and type (size) of the LED.

I have been adding LED's to several Williams engines conventional (not TMCC)

I use the following they work great are cheap and simple wire hook up and do not need anything else. From Evans Designs

Work on AC or DC and up to 19 volts really bright;

http://www.modeltrainsoftware.com/bl-212.html

You might consider some of these add to the # boards or marker lights.

Just splice into the power leads before they go into the TMCC board.

On the conventional engines they light when the train starts moving, at low volts look great at what ever volts.

According to MR the correct one to use is a 390 ohm resister wired to ground. I already have the bayonet type LED bulbs that fit into the electrical socket and replace the incandescent bulb. 

The output of the TMCC R2LC is a half-wave DC that is effectively around 8-9 volts DC.  When you remove the 3V for the white LED drop, you have around 6V to drop in the resistor.  That would dictate a 300 or 330 ohm resistor, but I choose to go a bit larger and use a bit less LED current.  I also always include a diode in the headlight circuits as I've had reverse voltage spikes from the board take out several headlight LED bulbs.  The previously suggest 900 ohms won't hurt anything, but it's overkill.  Even powered directly from 18 volt track voltage, 900 ohms is a bit excessive, since you'd have to rectify the 18 VAC to drive the LED and thus end up with half-wave rectified voltage from the diode.

390 works fine as well, just draws a bit more current then the 470.  Since I get reliable operation with the 470 shunt resistors, I choose to use those.  I've installed LED's a ton of TMCC locomotives and never had any issues with the 470 ohm value.

What are the pro's and con's of using appropriately sized series resistors vs.  parallel resistors with LED's?

Well, the only reason for the load resistor we're talking about is to allow the triac on the TMCC R2LC board to sense current so it will fire and light the lights.  With a normal voltage source, there is no reason to have parallel resistors.  You still need the series current limiting device, be it a resistor or other circuit.

John, got it. Thanks.

You're welcome.

Went to local RS and they only had 470 ohm 1/2 watt resisters so that is what I will use. Do you connect the resister to one of the wires supplying power to the LED or connect it to ground.

Also I got a response from Evans which is a bit confusing since their products incorporate a resister and a bridge rectifier connected to the LED.

Here is their response to my question about installing their LED's in a TMCC engine.

Thank you for getting in touch.  It sounds like a great project.
The voltage is low where you connected.  So, why not go above the resistor?  Attach the LEDs right to the rail power coming in to your locos. 
You can use that existing screw base to hold up the light.  Some tell us they use a bit of play-dough pushed into the screw base.  Then push the light into that.  Then the light will be held nicely in position.

Seems to me that if you did that you would lose directional lighting!