Sizing Current-Limiting Resistors for LEDs
Why an LED needs a series resistor, how to choose forward voltage and current, and how to land on a real E12 value.
Why an LED cannot run without a resistor
An LED is a diode, and its current rises very steeply once the voltage passes its forward threshold. Connect one straight across a supply even slightly above that threshold and the current runs away, overheating the junction and burning it out in moments. A series resistor fixes this by absorbing the difference between the supply and the LED's forward voltage, turning a fragile voltage-driven device into a predictable current-driven one. That is the whole job of the current-limiting resistor this tool sizes.
Choosing forward voltage and current
Two datasheet numbers set the answer: the forward voltage Vf and the target current. Forward voltage varies by colour, roughly 1.8 to 2.2 V for red, and 3.0 to 3.4 V for blue and white, because bluer light needs more energy per photon. The current sets brightness, and 20 mA is a common full-brightness figure for standard 5 mm indicators, though many modern LEDs look bright at 5 to 10 mA. Entering the real datasheet values, rather than round guesses, keeps the LED inside its safe operating range.
Landing on a value you can buy
The exact formula rarely gives a stocked resistor, so the tool snaps the result to the nearest E12 preferred value. E12 spaces twelve values per decade on a logarithmic scale, so 1.0, 1.2, 1.5, 1.8, 2.2 and so on up to 8.2, matching 10 percent tolerance parts. A calculated 500 ohm therefore rounds to the nearest E12 neighbour, 470 ohm, and a blue LED needing 90 ohm rounds to 82 ohm. When in doubt, stepping to the next value up trades a touch of brightness for extra safety margin on the LED.
Power rating and staying cool
The resistor converts the leftover voltage into heat, and that dissipation is P = I^2 x R, equal to the voltage across the resistor times the current. A 5 V circuit at 20 mA wastes only about 0.06 W, so a standard 0.25 W resistor runs cool. But raise the supply to 12 V and the same LED at 20 mA now burns about 0.2 W, closer to a quarter-watt part's limit, so a 0.5 W resistor is the safer choice. Always compare the power figure to the resistor's rating, not just its resistance, before building the circuit.