LED Series Resistor

An LED holds roughly its forward voltage V_f whatever the current, so the resistor must drop the rest of the supply: V_s − n × V_f for n LEDs in series. Ohm's law then gives R = (V_s − n × V_f) ÷ I_f and the resistor dissipates (V_s − n × V_f) × I_f. Each parallel string gets its own resistor of the same value; the supply delivers strings × I_f. If n × V_f reaches the supply voltage no resistor works, and the page says so.

V_f and I_f come from the LED's datasheet at the current you intend to run: they differ by colour and chemistry (a red indicator and a white power LED are volts apart), by part, and with temperature. This page does not assume a value for any colour — typical tables are guesses about a part you have not named. Use the datasheet, or measure V_f at the working current.

The exact value is rarely a stock part, so the page shows the nearest E24 (±5 %) or E12 (±10 %) preferred value on each side with the current it actually gives; the value above keeps the current at or under I_f. Because the resistor only sees V_s − n × V_f, a small drop makes the current sensitive to V_f: a 0.1 V shift changes the current by 0.1 ÷ (V_s − n × V_f). The page reports that figure and warns when it exceeds 10 % — a higher supply or fewer LEDs per string steadies the current.

The resistor's power rating and tolerance are the manufacturer's; derating to half the rating is common practice, not a rule applied here. Constant-current drivers, PWM dimming and high-power LEDs with thermal limits are outside a plain series resistor. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.