⚡ Electronics#LED#저항#회로
LED Resistor Calculation: Circuit Basics
5 min read · Last updated: 2026-05-08
Why LEDs need a resistor
LEDs (Light-Emitting Diodes) are current-sensitive devices. Connecting them directly to a power source causes excess current to destroy them immediately. A series resistor limits the current to a safe level.
Current-limiting resistor formula
R = (V_supply − V_f) / I_f
- V_supply: supply voltage (e.g., 5 V, 3.3 V)
- V_f: LED forward voltage
- I_f: LED forward (operating) current in amperes
- R: required resistance in ohms (Ω)
Typical LED characteristics
| LED color | Forward voltage (V_f) | Typical current |
|---|---|---|
| Red | 1.8–2.2 V | 20 mA |
| Yellow | 2.0–2.2 V | 20 mA |
| Green | 2.0–3.5 V | 20 mA |
| Blue | 3.0–3.5 V | 20 mA |
| White | 3.0–3.5 V | 20 mA |
| Infrared (IR) | 1.2–1.6 V | 20–100 mA |
Worked examples
5 V supply, red LED (V_f = 2.0 V), 20 mA:
R = (5 − 2.0) / 0.020 = 150 Ω
Choose the nearest standard value: 150 Ω, or 180 Ω for slightly more protection.
3.3 V supply, blue LED (V_f = 3.3 V), 10 mA:
R = (3.3 − 3.3) / 0.010 = 0 Ω — theoretically no resistor needed, but 33–68 Ω is recommended for protection.
Practical tips
- Always choose a standard resistor value (E12/E24 series) equal to or slightly above your calculated value.
- To increase brightness, increase current, but never exceed the LED's maximum rated current.
- Resistor power dissipation:
P = (V_supply − V_f)² / R. Use a 0.5 W resistor if P exceeds 0.25 W.
Key takeaways
- Formula:
R = (V_supply − V_f) / I_f - Convert mA to A before substituting (20 mA = 0.02 A).
- Pick a standard resistor at or slightly above the calculated value.