LED Resistor Calculator

**LED resistor formula:** R = (V_supply − V_forward × n) / I_LED Where: - R = resistor value (Ω) - V_supply = supply voltage (V) - V_forward = LED forward voltage (V) - n = number of LEDs in series - I_LED = desired LED current (A; use mA × 0.001) **Power dissipation in resistor:** P = (V_supply − V_forward × n) × I_LED P = V_resistor × I_LED P = I_LED² × R Choose resistor power rating ≥ 2× calculated for safety margin. **Worked example: red LED on 5 V** V_supply = 5 V V_forward = 2.0 V (red LED) I_LED = 20 mA = 0.020 A R = (5 − 2) / 0.020 = 150 Ω P_resistor = 3 × 0.020 = 0.060 W = 60 mW Use 150 Ω, 1/4 W (250 mW) resistor — well within rating. **Typical LED forward voltages:** | LED Color | V_f (typical) | |---|---| | IR (940 nm) | 1.2 V | | Red | 1.8-2.2 V | | Orange/Amber | 2.0-2.2 V | | Yellow | 2.0-2.2 V | | Green (traditional) | 2.0-2.4 V | | Green (high-brightness) | 3.0-3.5 V | | Blue | 3.0-3.5 V | | White (phosphor blue) | 3.0-3.5 V | | UV | 3.3-3.7 V | | Deep UV | 4.5-7.0 V | **Typical LED currents:** | Type | Current | |---|---| | Small indicator | 1-5 mA | | Standard 5mm | 20 mA | | High-brightness 5mm | 30 mA | | High-power (1W) | 350 mA | | High-power (3W) | 700 mA | | 10W LED | ~1 A | | Lumileds Luxeon | up to 4 A | **Standard resistor values (E12 series):** Closest values: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 (and ×10, ×100, etc.). For our 150 Ω example, 150 Ω is an exact E12 value. If not in E12, choose next higher (always round UP to limit current safely). **Series string of LEDs:** Multiple LEDs in series share the same current; their forward voltages add. Example: 3 white LEDs (V_f = 3.2 V each) in series on 12 V supply: V_resistor = 12 − (3 × 3.2) = 12 − 9.6 = 2.4 V R = 2.4 / 0.020 = 120 Ω P = 2.4 × 0.020 = 48 mW **Parallel LEDs (not recommended without individual resistors):** LEDs in parallel without individual resistors will not share current equally due to V_f mismatch. One LED will hog current and may burn out. Always use one resistor per LED branch, or use a constant-current driver. **High-power LED drivers:** For LEDs above ~50 mA, simple resistors waste too much power and don't regulate current well. Use: - **Linear current regulators**: simple, cheap (e.g., LM317, NSI50010YT1G). - **Switching current regulators**: efficient (e.g., LM3414, AL8805). - **Constant-current LED drivers**: for high-power LED arrays. **Color and brightness:** Brightness in lumens depends on: - LED efficiency (lm/W). - Current (more current = more brightness, with limits). - Wavelength (eye most sensitive at 555 nm green). Typical efficiency: 100-200 lm/W for modern white LEDs. A 1W high-power LED produces 100-200 lumens — equivalent to a small candle to a flashlight. **LED PWM dimming:** For variable brightness, use PWM (pulse-width modulation) rather than reducing current. PWM at >1 kHz appears as smooth dimming, more efficient than reducing current. **Diode equation (advanced):** LED behavior follows the Shockley equation: I = I_s × (e^(V/(nV_T)) − 1) Where I_s = saturation current, n = ideality factor (~2 for LEDs), V_T = thermal voltage (~26 mV at 300 K). In practice: most analysis assumes constant V_f when current is in normal range. **Common LED resistor values for 5 V supply:** | LED Color | V_f | R for 20 mA | |---|---|---| | Red | 2.0 | 150 Ω | | Green (high-brightness) | 3.0 | 100 Ω | | Blue | 3.2 | 90 Ω | | White | 3.2 | 90 Ω | For 12 V supply: | LED Color | V_f | R for 20 mA | |---|---|---| | Red | 2.0 | 500 Ω | | Green | 3.0 | 450 Ω | | Blue/White | 3.2 | 440 Ω | **Safe maximum current:** Datasheet I_max (absolute maximum) − don't exceed. Recommended I_typical for normal operation. Running near I_max reduces lifespan dramatically. For long life, run at 50-70% of I_max.