LED Resistor Calculator

Why LEDs Need a Resistor An LED (Light Emitting Diode) is not a resistor: it has a fixed forward voltage drop (Vf) and will draw unlimited current without a current-limiting resistor. Too much current burns the LED instantly. Too little current and the LED is dim. The resistor limits current to the LED’s rated operating current (typically 20 mA for standard LEDs).

The Formula R = (V_supply − V_f) / I_LED Where: V_supply = supply voltage (e.g., 3.3V, 5V, 9V, 12V) V_f = LED forward voltage (voltage drop across the LED at operating current) I_LED = desired LED current (typically 10–30 mA) R = resistor value in ohms

Typical LED Forward Voltages Red LED: 1.8–2.2 V (typical 2.0 V) Orange LED: 2.0–2.2 V (typical 2.1 V) Yellow LED: 2.0–2.2 V (typical 2.1 V) Green LED (standard): 2.0–2.4 V (typical 2.2 V) Blue LED: 3.0–3.5 V (typical 3.2 V) White LED: 3.0–3.5 V (typical 3.2 V — white LEDs use a blue chip with phosphor) UV LED: 3.2–3.8 V Infrared (IR) LED: 1.2–1.6 V (typical 1.4 V)

Typical Operating Current Standard 5mm LED: 20 mA (maximum), 10–15 mA for long life High-brightness LED: 30–50 mA Low-current LED: 2–5 mA (some indicator LEDs) Power LED: 350 mA, 700 mA, 1A, or higher (require dedicated drivers)

Resistor Power Rating Power dissipated in resistor: P = I² × R = (V_supply − V_f) × I Use a resistor rated at at least 2× the calculated power. Standard resistor ratings: 1/8 W, 1/4 W, 1/2 W, 1 W, 2 W. For most LED circuits at 20 mA, a 1/4W resistor is sufficient.

Multiple LEDs in Series Series connection: V_total = V_f1 + V_f2 + … + V_fn. Current is the same through all. Advantage: all LEDs have identical current and brightness. R = (V_supply − n × V_f) / I Maximum LEDs in series: n < V_supply / V_f.

Multiple LEDs in Parallel (not recommended) Parallel LEDs have mismatched Vf values — one LED draws more current and burns out. Better to use multiple series strings, each with its own resistor.

Why white LEDs share Vf with blue LEDs A white LED is actually a blue LED with a yellow phosphor coating. The blue chip emits blue light (Vf ≈ 3.2 V); the phosphor absorbs some of it and re-emits broad yellow light. The mix looks white to the eye. The forward voltage is set by the blue chip underneath, which is why white and blue LEDs have nearly identical Vf values. Royal-blue LEDs at very short wavelengths can climb to 3.8 V, and 4 V or above usually means you have a damaged junction.

Round UP, never down, when picking from the E12/E24 series The exact calculated R might be 145 Ω. The nearest E12 value is 150 Ω (round up) or 120 Ω (round down). Always pick the higher value. Rounding down sends more current through the LED than rated; rounding up runs the LED slightly dimmer but well within spec, often extending its lifetime.

When a resistor is the wrong answer For 5 mm indicator LEDs at 20 mA, a series resistor is fine. For high-brightness power LEDs at 350 mA and above, a resistor wastes most of the supply voltage as heat and needs frequent re-calibration as the LED warms. Use a dedicated constant-current LED driver IC (linear like the AL5809, or switching like the LM3414) which holds the LED current regulated regardless of supply voltage drift or LED temperature. Power LEDs also need a metal-core PCB or heatsink to keep junction temperature under 85°C.