resistors and bulb milliamp ratings

[Truck]

It may have been asked before, but I could not find a definit answer in the search engine.

A TCS T4X decoders light functions are rated at 100ma ea.

If I use two led's  rated at 3.2 volts at 20ma   and I use a 3.4K resistor for each, and a 3.5 v led at 30ma with a 1k resistor, even though they add up to 70ma combined do the resisters change the miliamp ratings when connected to the led's?

What I am debating is wether or not to connect the markers and the head lights together on one function output of the decoder or just run the extra wires to the tender and use the green or purple wires for the marker lights.

That will depend on the answer I get for the milliamp ratings.

  Thanks, Truck.

[rrinker]

 LEDs are current operated devices, and the rating is simply a MAXIMUM they can withstand. WHen you use resistors with LEDs, you cut down the current. So assuming a 12V output on the functions, using a 3.4K resistor will result in about 2.5ma through each LED. FOr the 3.5V LED with a 1K resistor, the current would be 8.5ma. So your total load on the decoder would be only 13.5ma.

Volts=current x resistence, or current = volts/resistence

volts = (decoder function output)-(voltage drop of LED from spec)

For incandescent bulbs, the current rating of the bulb IS the current, the resistors are used to drop the VOLTAGE to what the bulb is rated for. For LEDs, the resistor is used to dissipate the excess CURRENT so that no more than the rated amount flows through the LED. The voltage drop in an LED is fixed, the current drop in a light bulb is 'fixed' (it varies between cold and hot filament, but a lamp rated at 1.5 volts and 15ma draws 15ma when lit at 1.5 volts).

                                --Randy

 

[locoi1sa]

Truck.

 If your going through the trouble of wiring markers I would have them controlled by a separate function and not the headlights. This way when the train is in a siding only the markers would be on and the headlight dim. If you are really serious you can have the rear markers on one function and the front markers on another. Rear markers would be lit if running light or pushing a train. Front markers would be lit when the train is complete. Later in the steam era the front markers were color coded to indicate an extra either green or white. They would not be lit if it was a regular scheduled train only the rear markers would be lit on the last car or locomotive.

            Pete

[Truck]

Thanks, Randy & Pete

Both of your replies have been very helpful.