Find the right current-limiting resistor for any LED: supply voltage, LED forward voltage and desired current in — exact ohms, nearest E24 value and wattage rating out.
| Input | Result |
|---|---|
| 12 V, red 2.0 V, 20 mA | 500 Ω → use 510 Ω, 0.200 W → 1/2 W |
| 5 V, white 3.2 V, 20 mA | 90 Ω → use 91 Ω, 0.036 W → 1/8 W |
| 9 V, 2.1 V, 15 mA | 460 Ω → use 470 Ω, 0.103 W → 1/4 W |
The forward voltage Vf is set by the semiconductor material, which also sets the color. Use these typical ranges when the datasheet is not at hand — measure or look up the exact part for precision work:
| LED color | Typical Vf |
|---|---|
| Infrared | 1.2 – 1.6 V |
| Red | 1.8 – 2.2 V |
| Yellow / Green (standard) | 2.0 – 2.2 V |
| Blue / White / True green | 3.0 – 3.4 V |
Standard indicator LEDs run at 20 mA; high-brightness parts may want 10 mA or 30 mA — check the datasheet. Several LEDs in series add their forward voltages: three red LEDs on 12 V use Vf = 3 × 2.0 = 6.0 V, leaving (12 − 6) ÷ 0.02 = 300 Ω.
An LED is a diode: past its forward voltage, current rises almost vertically with voltage. Connected straight to a supply it draws far more than its rating and burns out in seconds. The resistor absorbs the leftover voltage (V_supply − V_f) and pins the current to I = (V_supply − V_f) ÷ R.
Standard 5 mm indicator LEDs are rated 20 mA and are already bright at 10–15 mA. Running slightly under rating extends life and barely changes brightness. High-power LEDs (1 W+) are driven by constant-current drivers, not plain resistors.
Dissipation is P = (V_supply − V_f) × I. Pick a part rated at least twice that so it runs cool: 0.2 W of heat means a 1/2 W resistor. Common through-hole ratings are 1/8 W, 1/4 W, 1/2 W and 1 W.
In series, yes: add the forward voltages and use the same formula — three 2.0 V reds on 12 V leave 6 V for the resistor, giving 300 Ω at 20 mA. In parallel, no: each LED needs its own resistor, because small V_f differences make one LED hog the current.
The LED simply will not light — a white 3.2 V LED on a 3 V coin cell stays dark. You need a higher supply, a lower-V_f LED color, or a boost driver circuit.