LED Current Limiting Resistor

LEDs (Light Emitting Diodes) must always be operated with a resistor in series to limit the current to a safe level. Most LEDs can only tolerate a current of about 30mA maximum. To calculate the resistor value required the forward voltage (V_f) and current (I_f)of the LED must be known. These can be found from the suppliers catalogue. For standard LEDs V_f is about 2V and I_f is about 20mA.

The resistor value can be calculated as follows:

R = (V_s - V_f)  / I_f

= (Supply Voltage - Forward Voltage) / Forward Current

Note that I_f is in Amps, NOT Milliamps! (20mA = 0.02A)