LED Resistor Formula
R = (Vsupply − Vforward) ÷ Iforward. Forward voltage (Vf) is typically 1.8–2.2V for red and yellow LEDs, and 2.8–3.5V for blue, green, and white LEDs. Most indicator LEDs run at 10–20mA. Example: 5V supply, red LED (Vf = 2.0V), 20mA → R = (5 − 2) ÷ 0.020 = 150Ω.
Why the Resistor Is Critical
LEDs have near-zero forward resistance above their threshold voltage. Without a resistor, even a 0.1V increase above Vf can instantly double or triple current, burning out the diode. The resistor limits current to a safe value regardless of small supply fluctuations. Never omit it—even when the supply only slightly exceeds Vf.
Power Dissipation
The resistor dissipates P = I² × R watts. For a 150Ω resistor at 20mA: P = 0.020² × 150 = 0.06W. A standard ¼W resistor handles this comfortably. For LEDs above 50mA or high duty-cycle applications, use a ½W or 1W rated resistor to prevent heat buildup and premature failure.
Frequently Asked Questions
Why do LEDs need resistors?
LEDs are current-controlled devices with a nonlinear voltage–current relationship. Without a series resistor, even a small overvoltage causes exponentially more current to flow, quickly burning out the LED. A series resistor limits current to the LED’s rated forward current (typically 20mA for standard LEDs), ensuring stable, long-term operation.
Can I put multiple LEDs in series?
Yes. Add up all the forward voltage drops (e.g., 3 × 2.0V = 6.0V) and subtract the total from the supply voltage before calculating the resistor value. Verify your supply exceeds the total forward voltage. In parallel configurations, use individual resistors per LED — a shared resistor cannot compensate for LED-to-LED variation in forward voltage.