LED Basics
LED forward voltages by color, current sizing, and driver topology.
Reference
Forward voltage by color
| Color | V_f (typical) | Notes |
|---|---|---|
| Infrared | 1.2 V | GaAs (~940 nm) |
| Red | 1.8–2.1 V | AlGaAs, AlInGaP |
| Orange | 2.0–2.1 V | AlInGaP |
| Yellow | 2.1–2.2 V | |
| Green (pure) | 3.0–3.5 V | InGaN |
| Green (old) | 2.1 V | GaP — dim |
| Blue | 3.0–3.3 V | InGaN |
| White | 3.0–3.3 V | Blue + phosphor |
| UV | 3.3–4.0 V |
Sizing rules
- Resistor value
- R = (V_supply − V_f) / I_f
- Dissipation
- P_R = (V_supply − V_f) · I_f (size for at least 2×)
- Typical I_f
- 10–20 mA for indicator LEDs, 350 mA–1 A for lighting
- Never
- connect an LED directly across a voltage source — always limit current
Driver topology
- Resistor: simplest; efficient only when V_supply − V_f is small.
- Constant-current linear: wastes the difference as heat; fine at low power.
- Buck driver: 90 %+ efficient for higher current — standard for lighting.
- PWM dimming: preserves color; flicker must be >200 Hz to avoid visible artifacts.
- Series vs parallel: prefer series (same current through all). Parallel forces per-string resistor or driver.
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