Embedded Calculators & Part Finder
Size a value with 64 free calculators, then find the real component that fits — in stock, at the best price. For MCU, power, RF & firmware. No account.
LED Current Limiting Resistor
Find the nearest E24 standard resistor value and power rating to safely drive your LEDs.
LEDs (Light Emitting Diodes) require a series resistor to limit the current, preventing them from burning out due to thermal runaway.
Formulas:
- Single LED:
R = (Vs - Vf) / If - Series LEDs:
R = (Vs - (N * Vf)) / If - Parallel LEDs:
R = (Vs - Vf) / (N * If)(Note: matching Vf is required)
Usage: Input your supply voltage, LED forward voltage, and target current. The tool suggests the closest standard E24 resistor and recommends a safe power rating.
When you need it: Any time you drive an LED from a rail higher than its forward voltage — a 5 V MCU pin, a 12 V supply, or a battery. The LED's forward voltage (Vf) is roughly fixed, so the series resistor is what actually sets the current, and therefore the brightness and lifetime. Skip it and the LED draws uncontrolled current and dies from thermal runaway.
Worked example: 5 V supply, a red LED with Vf = 2.0 V, target 15 mA → R = (5 − 2.0) / 0.015 = 200 Ω. The nearest E24 value is 200 Ω. Power dissipated is I²R = 0.015² × 200 ≈ 45 mW, so a common 1/8 W (125 mW) resistor is more than enough.
Tips & gotchas:
- Vf depends on colour: red ≈ 1.8–2.2 V, green ≈ 2.0–3.0 V, blue/white ≈ 3.0–3.4 V. Use the datasheet Vf at your operating current.
- Round the resistor up to the next standard value — that keeps you under the target current rather than over it.
- Modern indicator LEDs are already bright at 1–5 mA; you rarely need 20 mA. Lower current saves power and heat.
- Series LEDs need a supply above N×Vf; parallel LEDs need matched Vf or an individual resistor per LED.