Electrical and electronics
LED series resistor calculator
Find the series resistor for one LED or a string of LEDs from the supply voltage, forward voltage and current, with the next standard value and its power rating.
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Showing the worked example. Results update as you type.
Result
- Forward voltage varies between parts and with temperature: red ≈ 1.8–2.2 V, green/blue/white ≈ 2.8–3.6 V. Check the datasheet value at your current.
Method and limits
The resistor drops whatever voltage the LEDs do not: R = (Vs − n Vf) / I for n LEDs in series at current I. Its dissipation is P = I² R. The tool picks the next standard value above the exact result, so the current stays at or below your target, and suggests a resistor rated for at least twice the dissipation.
Worked example: 5 V supply, one red LED at 2 V and 20 mA: R = (5 − 2) / 0.020 = 150 Ω, dissipating 60 mW, so a ¼ W (or ⅛ W) resistor is ample.
Limits
Forward voltage varies between parts, with temperature and with current, so the real current differs from the target by a few per cent. Strings in parallel each need their own resistor. For high-power LEDs, or a supply barely above the string voltage, use a constant-current driver instead.
References
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