I have a Digitrax decoder I put in an Intermountain F3 and i think I blew the lightbulb. It worked at first but I attempted to resolder the wires and ruined one of them. The loco runs great just no light.
My question is; is there a link anybody knows of for this install and I purchased a pack of LEDs with resistors recomended by the Hobby shop. Do I need the resistors?
I use the golden-white 3-mm LEDs and a 1K resistor. Yes, you must have the resistor in series with the LED. Also, be aware that LEDs are polarized, so you need to have the leads going to the correct wires. As I recall from the one I did over the weekend, the blue (common) wire from the decoder goes to the longer of the two leads from the LED.
When I do any of these locomotive wiring jobs, I always pre-test my wiring configuration before I pull out the soldering iron. I’ve got a bunch of short alligator clip leads that I can use to connect all the components. That way I can test everything before I hard-wire anything.
Get yourself some shrink tubing for insulation, by the way. It’s neater, more reliable and generally smaller than using electrical tape.
Yes, an LED requires a resistor, and LEDs are polarized and must be connected correctly or they won’t work. Most LEDs require only 1.2 Volts DC at 15 to 30 Milliamps and a DCC decoder’s headlight circuit outputs 12 Volts DC. As a general rule, a 1,000 Ohm resistor connected to either lead of the LED should be okay, but if you think the LED should be brighter you can use a smaller resistance value. The longer of the two leads on the LED must be connected to the blue decoder wire (positive) and the short LED lead connects to the white (negative) decoder wire.
If you want to be more accurate, you can use Ohms law to calculate the value of the resistor if you know the exact voltage output of your DCC decoder, and the voltage and milliamp rating of the LED. The formula is R=E/I, or Resistance = Voltage divided by Current. For a 1.2 Volt LED that draws 15 milliamps connected to a decoder with a 12 Volt headlight output, you must first determine how much voltage needs to be dropped by the resistor. Subtract the required voltage (1.2) from the input voltage (12) to get a required voltage drop of 10.8. Then divide 10.8 by 15 milliamps (.015) to get a resistor value of 720 Ohms. Unfortunately, 720 Ohms is not a standard resistor value, so you should use the next higher value, which would be 820 Ohms in this case. A 1,000 Ohm resistor is the next step above 820, and will still probably be close enough.
Make sure when you are solderng the wires on the LED that you use some sort of heat sink (alligator clip etc). Clip it onto the LED lead you are soldering so it does not overheat and damage the LED. The wires will solder quicker if you tin them first. As stated you should clip the wires together first to make sure you have it right. If it doesn’t work just reverse the wires. It will not hurt the LED if it is wired backwards it just won’t work. I always check the LED after soldering the wires to it to make sure i didn’t fry another one[:D] Actually It really doesn’t matter what lead you put the resistor on. I buy my LEDs by the hundreds so I use them for everything on my layout includinig street ights, structure lights, control panels, Yard lights, and Loco lights. After you do one you will be an expert. If you buy your LEDs and resistors online you can get them significantly cheaper than at your LHS and certainly cheaper than Radio Shack. I typically pay less than 12 dollars for 100 LEDs and resistors can be bought for less than a buck for a hundred. Just don’t be too nervous, worst case you fry one and you try again. Its good soldering practice. I use any where from a 470 - 1k ohm resistor depending on size and color. But as stated I think you will find most LED will work just fine with a 1 k ohm resistor.
Terry[8D]