LED's & Resistors

[rlandry6]

I just installed a decoder in a frame from Southern Digital. Included in the package was a LED, that according to the instructions was to replace the rear light board(it's an Atlas diesel). The LED is wired direct, no resistor. The other original light board is retained and left intact except for cutting a trace to isolate track power. So my question is, what types of light bulbs can be used by direct hard wiring, what needs a resistor, and if it needs a resistor, how do you determine the value needed. Are there different types of LED's, some needing a resistor, and some not?
If this is going to be too long to explain here, a link to a plain english explanation would really be appreciated.

Bob

[TomDiehl]

LED's come in different voltage ratings. The resistor in series with it allows you to use it on a higher voltage than it's rated for, known as a dropping resistor because it "drops" the amount of voltage that's across the LED. The plain LED's you buy (not part of a retrofit kit like you talk about here) should have a voltage and a current (amps or milliamps) rating, (milliamps (mA) is 1/1000 of an amp). Subtract the voltage rating of the LED from the track voltage (12 Volts) The remainder will be the voltage across the resistor. Since the Amps (or milliamps) in the series circuit will be constant, the rating from the package will be used. Simply divide the voltage by the current to get the resistance.

The calculation is from a formula known as Ohms Law, Voltage equals current (amps) times resistance (ohms), usually stated as E=IxR

For example, your LED is rated at 3 volts, 10 mA (or .01 Amps) This makes your calculation easy. Your track voltage (12 volts) minus the LED voltage (3 volts) means the resistor has to drop 9 volts. So 9 volts divided by .01 Amps is 900 ohms, the rating of the resistor needed.

Since resistors don't come in exact resistance values that you'll calculate every time from this formula, just get the one closest in value, preferably a bit higher just in case your track voltage goes above 12 volts for some reason.