L.E.D. voltage drop resistor

Your dropping resistor value can be calculated with a simple calculator.

The formulae:

Resistor drop  = ( System voltage - LED voltage)  / LED current.

Spelled out:

The value of the resistor drop (current limiting resistor) is equal to the charging alternator voltage and not the battery voltage minus the L.E.D. voltage and then divided by the L.E.D. current will give you the required resistor value in ohms.

You can check for the charging voltage at the battery with a volt meter while the vehicle is running.

In your case you would input into your calculator in the following format; assuming you have 14 volt charging.

(14 volt - 1.85 volt) / 0.020 amp =  607.5 ohm resistor value

Hope this is of any use to you and good luck.

Francisco

I X E = P    SO....   1.8V X .020 A = 0.036W or 36mW   

a 1/4W would be more than enough, good luck

Thats the power dissipated by the LED

The power burned off on the resistor is (using 14V like previous) is (14-1.85)*0.02=0.243 W, a little under a 1/4 W so it might get warm, you may want to use more than one LED or a 1/2 W resistor...