This calculator finds the **resistor value** to be used with an LED and a **+24 Volt **supply. It also provides **the power dissipation** in the resistor – an important number for resistor selection.

**LED Resistor Calculator**

To find the resistor, enter the following for the LED

- Forward Voltage (
**Vf**) - Current (
**I**)

## LED Resistor **Formula**

**R = (V-Vf)/I**

In this case **V = 24 Volt**.

**Example Calculation**

The data sheet for a diode can be used to find its forward voltage and current.

For this diode, **Vf = 2.1 V** and we use** I = 10 mA**.

Use these values in the calculator above to find a resistor value of **2.19 kOhm**. Approximately **220 mW of power** is dissipated in the resistor.

Referring to this datasheet,only the 1206 size resistor rated at **250 mW** can be used. The others are rated lower and hence would be damaged by the amount of current through the circuit.

**Summary**

For a supply voltage of **24V** and a forward voltage of 2.1V, a **2.19 kOhm** series resistor should be used to provide a forward current of 10 mA.