Choose a resistor for a series LED circuit

In a low-voltage DC circuit, subtract the total LED forward voltage from the supply voltage. The remainder is the voltage across the resistor. Divide it by the current limit in amps.

Calculate a resistor and wattage →

Work a 5 V example

Use a 5 V supply with ±5% tolerance and one LED whose forward voltage ranges from 2.0 to 2.2 V. At the highest supply, the resistor can see 5.25 − 2.0 = 3.25 V. For a 20 mA current limit, the resistance at that corner must be at least 162.5 Ω.

Allow for the resistor tolerance

A resistor rated at ±5% can be as low as 95% of its nominal value. The nominal value therefore needs to be at least 162.5 ÷ 0.95 ≈ 171.1 Ω. The next E24 value is 180 Ω. It can fall to 171 Ω, so the upper current estimate is about 19.0 mA.

Check power and brightness

At the high-current corner the resistor dissipates 3.25² ÷ 171 ≈ 0.062 W. Check both power derating and the current rating in the component datasheets. At low supply voltage a long LED string may have too little voltage left across the resistor. For parallel strings, give each branch its own resistor.

DigiKey’s series LED resistor calculation · NXP’s LED drive circuit analysis.