How to Figure Out What Resistor to Use for LEDs

A selection of LED devices.

A light-emitting diode (LED) is a semiconductor device that emits light waves when forward biased. The diode must be protected by a resistor to limit the flow of current to a specified level. Some LEDs have a resistor built in. The forward voltage across the diode (Vf) at the specified forward current (lf) is used to calculate the value of the resistor required..

Instructions

1. Calculate the value of the series resistor (r) using this equation:

   r = ( Vt - Vf ) / If

2. Set the value of each component within the equation:

   Total circuit voltage Vt = 5 Volt

   Forward voltage Vf = 2 Volt

   Forward current If = 5 / 10^3 Amp

3. Solve the equation for r:

   r = ( 5 - 2 ) / 5 / 10^3

   r = 3 x 10^3 / 5

   r = 600 ohm

Tips & Warnings

• A value of Vf = 2 Volts is correct for red LEDs. Other colors will have a higher value

References

• Light Emitting Diodes (LEDs)