R = (Vsupply − n·Vf) / I. An LED is a diode, not a resistor —
its forward voltage barely moves while its current changes enormously, so the series resistor is
what actually sets the current. That is also why LEDs in parallel should each have their own
resistor: share one and the LED with the lowest Vf hogs the current and runs hot.
Forward voltage depends on colour and current — roughly 1.8–2.2 V for red, 3.0–3.4 V for blue
and white — and it falls as the die warms, so check the datasheet curve rather than trusting a
single number. Keep at least a volt or so across the resistor: with too little headroom, normal
part-to-part Vf spread swings the current wildly.
| Infrared | 1.2 – 1.6 V | Remote controls, optical sensors |
|---|---|---|
| Red | 1.8 – 2.2 V | AlGaInP |
| Amber / orange | 2.0 – 2.2 V | |
| Yellow | 2.1 – 2.4 V | |
| Green (traditional) | 2.0 – 2.4 V | Older GaP green |
| Green (pure, InGaN) | 3.0 – 3.4 V | Bright modern green behaves like blue |
| Blue | 3.0 – 3.4 V | InGaN |
| White | 3.0 – 3.4 V | Blue die with phosphor, so blue's Vf |
| UV | 3.3 – 3.8 V |
Forward voltage falls as the die warms and varies between production batches, so treat these as a starting point and use the datasheet curve for anything where brightness matters. The two greens differ because they are different semiconductors.