This calculator finds the resistor value to be used with an LED and a +12 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 = 12 Volt.
Example Calculation
The data sheet for a diode can be used to find its forward voltage and current.
For this diode, Vf = 2.2 V and we use I = 20 mA.
Use these values in the calculator above to find a resistor value of 490 Ohm. Approximately 200 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 12V and a forward voltage of 2.2V, a 490 ohm series resistor should be used to provide a forward current of 20 mA.