LED resistor calculator

Type the supply voltage, the LED's forward voltage and the current you want, in that order: `9V, 2V, 20mA`. You get the exact resistance, the nearest standard value above it with the current that gives, and the power the resistor has to handle.

The forward voltage and current are on the LED's datasheet. If you don't have one, red LEDs are usually around 2 V and blue or white around 3 V, and 10 to 20 mA is typical for a small indicator.

An LED doesn't behave like a resistor. Once the voltage across it passes its forward voltage, the current climbs steeply with only a little more voltage, and with nothing to limit it the LED burns out. A resistor in series sets the current.

The resistor takes whatever voltage the LED doesn't. From 9 V with a 2 V LED, the resistor has 7 V across it, and for 20 mA it needs 7 ÷ 0.02 = 350 Ω. Resistors come in standard values, so the calculator picks the next one up in the E12 series, 390 Ω, which lets through about 18 mA. Going up keeps the current below what you asked for.

Nick Holonyak, working at General Electric's lab near Syracuse, New York, demonstrated the first LED to give off visible light on 9 October 1962. It glowed red, from a crystal of gallium arsenide phosphide he'd grown himself. Other colours came later, and each has its own forward voltage, which is why the datasheet figure matters.