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LED Current Limiting Resistor

Calculate the correct current limiting resistor value for LEDs in series, given supply voltage, LED forward voltage, and desired current.

Red/Yellow/Orange � 2.0V, Blue/White/Green � 3.0�3.5V, IR � 1.2V. Check datasheet.
Standard LEDs: 20 mA max. High-efficiency: 10 mA. High-power: 350 mA�3 A.
For series connection, forward voltages add up. Ensure supply > total Vf.

Results

Resistance Required150 O
Power Dissipated in Resistor0.060 W

📖What is it?

A current-limiting resistor protects LEDs from excess current by dropping the voltage difference between the supply and the LED forward voltage. Without it, LEDs quickly burn out. R = (Vsupply - Vf) / I. For series LEDs, total Vf = Vf � number of LEDs.

🎯How to use

Enter your supply voltage, the LED forward voltage from the datasheet (or estimate: red/yellow � 2V, blue/white � 3.3V), desired current in mA, and the number of LEDs in series. Round up the calculated resistance to the nearest standard E24/E12 value.

💡Example scenario

5V supply, red LED (Vf = 2V), 20 mA, 1 LED: R = (5-2)/0.02 = 150 O. Power = (5-2) � 0.02 = 0.06 W. Use a 1/4W resistor. For 3 LEDs in series: totalVf = 6V � exceeds supply! Reduce series count or increase voltage.

🏆Pro tip

Always round up to the next standard resistor value (not down) to keep current within the LED rating. For power LEDs (>100 mA), use a constant-current driver instead of a resistor � resistor-based control wastes power and varies with temperature. Add 20�30% headroom to resistor power rating (e.g., use 1/4W for 0.06W dissipation).