LED Resistor Calculator
Calculate the current-limiting resistor for an LED circuit.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Enter the supply voltage, the LED forward voltage, and the desired LED current. The calculator shows the required resistance and the power the resistor must dissipate.
About LED Resistor
LEDs require a current-limiting resistor to prevent damage from excessive current. The required resistance equals the voltage drop across the resistor (supply voltage minus LED forward voltage) divided by the desired current. The power dissipated by the resistor equals the current squared times the resistance. Always choose a resistor with a power rating above the calculated dissipation for reliability.
Frequently Asked Questions
Why do LEDs need a current-limiting resistor?
LEDs have a very low internal resistance. Without a resistor, excessive current flows and destroys the LED. The resistor limits current to a safe level.
How do I find the forward voltage of my LED?
Check the LED datasheet. Typical values: red LEDs about 1.8-2.0 V, green about 2.0-2.2 V, blue and white about 3.0-3.4 V.
What current should I use?
Standard LEDs typically run at 20 mA (0.02 A). High-brightness LEDs may use more. Always check the datasheet for the maximum and recommended current.