How it works
The voltage left after subtracting the total LED forward voltage from the supply falls across the resistor. Dividing it by the target current gives the required resistance.
R = (Vs − n × Vf) / IfP = If² × R
The calculator picks the closest E24 value above the calculated resistance, so the current does not exceed the target. The power rating is then chosen from 1/16 W to 1 W so that it is at least twice the actual dissipation.
Worked example
With Vs = 5 V, Vf = 2 V, one LED and If = 20 mA, R = 150 Ω and the dissipation is 60 mW. With the 2× margin rule, a 1/8 W resistor is used.
Design tips
- LED forward voltage depends on color and current. Use the value at the test current given in the LED datasheet.
- More LEDs in series reduce the resistor dissipation and improve efficiency. The supply voltage must still be greater than n × Vf.
- Battery voltage falls as it discharges. Calculating with the minimum voltage reduces the brightness variation.
Limitations
- LED forward voltage changes with temperature and manufacturing spread. Where the current matters, a constant-current source or LED driver IC is more stable.
- If the supply voltage is lower than the total LED voltage, the calculation is not possible.
Frequently asked questions
Can I connect several LEDs in parallel on one resistor?
Driving several LEDs in parallel from one resistor gives uneven currents. Use a separate resistor for each LED, or a constant-current source.
Why is the resistor power calculated at twice the dissipation?
Running a resistor at half its rating or less gives margin for heating and lifetime. This calculator applies that rule.