LED Series Resistor Calculator

Why does an LED need a resistor?

The voltage across an LED stays almost constant even when the current changes a lot. Connected straight to a voltage source, nothing limits the current and the LED burns out. A resistor in series takes up the difference between the supply and the LED voltage and sets the current.

Formula

R = (V_s − n × V_f) / I

V_s is the supply voltage, V_f the forward voltage of one LED, n the number of LEDs in series and I the desired current. The power dissipated in the resistor is the voltage across it times the current.

Example

One LED with a forward voltage of 2 V runs at 20 mA from 12 V. R = (12 − 2) / 0.02 = 500 Ω. That is not a standard value. The next E12 value up is 560 Ω: the current becomes 10 / 560 = 17.9 mA and the resistor dissipates 0.18 W. A quarter watt part would run close to its limit, a half watt part has comfortable margin.

Why the next value up?

A resistor below the calculated value pushes the current above the target. The next value up lowers it slightly, the change in brightness is rarely visible and the LED stays on the safe side. Standard values come from the IEC 60063 E12 and E24 series.

Limits

Take the forward voltage from the datasheet: it depends on colour, current and temperature. When the supply is close to the LED voltage a resistor gives poor regulation. Power LEDs for lighting need a constant current driver.