Why Do LEDs Need a Current-Limiting Resistor?
An LED (Light Emitting Diode) is not a simple resistive load. Above its forward voltage threshold it behaves like a near-short circuit — its dynamic resistance can be as low as 1–5 Ω. Without a series resistor, even a small excess of supply voltage drives an enormous current, destroying the junction in milliseconds.
A current-limiting resistor absorbs the voltage difference between the supply and the LED forward voltage, converting it to heat at a safe and controlled current level. The formula is simply Ohm's Law:
R = (Vs − Vf) / If where If is in amperes
Always choose a resistor whose power rating is at least twice the calculated dissipation (P = I² × R) to keep the component cool and within its long-term rating.
Common LED Forward Voltage Reference
| LED Color | Vf Range (V) | Typical Vf (V) | Typical If (mA) |
|---|---|---|---|
| Red | 1.8 – 2.2 | 2.0 | 20 |
| Orange | 2.0 – 2.2 | 2.1 | 20 |
| Yellow | 2.0 – 2.2 | 2.1 | 20 |
| Green (standard) | 1.9 – 2.4 | 2.1 | 20 |
| Green (bright / HP) | 2.9 – 3.5 | 3.2 | 20 |
| Blue | 2.7 – 3.6 | 3.2 | 20 |
| White | 2.7 – 3.6 | 3.2 | 20 |
| UV (400 nm) | 3.1 – 3.8 | 3.4 | 20 |
| IR (850 nm) | 1.2 – 1.7 | 1.5 | 20–100 |
Standard Resistor Values (E12 Series)
The E12 series contains 12 values per decade. Use the nearest value equal to or higher than the calculated resistance to avoid over-driving the LED.
Example: if you calculate 73 Ω, the nearest E12 value ≥ 73 Ω is 82 Ω (82 × 10⁰). This slightly reduces current below the target — safe for the LED.