LED Resistor Calculator — Series Resistor for LEDs
Enter your supply voltage, LED colour (or custom forward voltage) and desired current to get the exact series resistor needed, its power dissipation, and the nearest E24 standard value.
V
LED colour
mA
Nearest standard (E24): 1000 Ω
- 1
Voltage drop across resistor
Vs − Vf = 5 − 2 = 3 V - 2
Forward current in amperes
20 mA ÷ 1000 = 0,02 A - 3
Series resistance R = (Vs − Vf) ÷ If
3 ÷ 0,02 = 150 ΩRound up to the nearest E24 standard resistor value shown above.
Wie funktioniert dieser Rechner?
R = (Supply Voltage − LED Forward Voltage) / Forward Current. For a red LED (Vf = 2 V) on 5 V at 20 mA: R = (5−2) / 0.02 = 150 Ω. Resistor dissipates P = (Vs−Vf) × If watts. Round up to the nearest standard (E24) resistor value. Always check the LED datasheet for exact Vf.
Formel
How this is calculated
Every LED requires a current-limiting series resistor to prevent the LED from drawing too much current and burning out. Without one, even a small excess voltage drives exponentially more current through the LED's p-n junction. The required resistance is simply the voltage "leftover" after the LED takes its share (Vs − Vf), divided by the desired forward current (If): R = (Vs − Vf) / If.
Forward voltage (Vf) is a property of the LED's semiconductor material, not its physical size. Red and yellow LEDs typically use AlGaAs or GaAsP with Vf around 2.0–2.1 V; blue, white and UV LEDs use InGaN with Vf around 3.0–3.4 V; IR LEDs (GaAs) sit around 1.2–1.8 V. These are typical values — consult your specific LED's datasheet for precision. Standard small-signal LEDs operate at 20 mA; high-power LEDs used for lighting can require hundreds of milliamps.
The calculated resistance is then rounded to the nearest E24 standard resistor series value, which spans values in a 24-step-per-decade sequence. For power dissipation, always pick a resistor rated for at least twice the computed wattage to allow headroom. If the supply voltage is lower than or equal to Vf, no resistor will make the circuit work — a higher voltage supply is needed.
Häufige Fragen
The LED will draw more current than its rated maximum. This heats the junction and reduces LED lifetime, potentially burning it out immediately. Always use at least the calculated resistance; using a slightly larger value is safer (slightly dimmer but longer-lived).
Yes. Add up the forward voltages of all LEDs (e.g. three red LEDs: Vf = 3 × 2.0 = 6 V), then use that sum in the formula: R = (Vs − ΣVf) / If. The supply must be greater than ΣVf. In series all LEDs carry the same current.
Vf is determined by the bandgap energy of the semiconductor — the energy needed to move an electron across the p-n junction, which is emitted as a photon. Higher energy photons (shorter wavelength = blue/UV) require a larger bandgap and therefore a higher forward voltage than lower energy photons (longer wavelength = red/IR).
Auch bekannt als
TG we-Calculate Editorial Team. (2026). LED Resistor Calculator — Series Resistor for LEDs [Online calculator]. TG we-Calculate. https://we-calculate.com/de/calculator/led-calculator
TG we-Calculate Editorial Team. "LED Resistor Calculator — Series Resistor for LEDs." TG we-Calculate. 2026. https://we-calculate.com/de/calculator/led-calculator.
TG we-Calculate Editorial Team, "LED Resistor Calculator — Series Resistor for LEDs," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/de/calculator/led-calculator
@misc{wecalculate_led_calculator, title = {LED Resistor Calculator — Series Resistor for LEDs}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/de/calculator/led-calculator}}, year = {2026}, note = {TG we-Calculate} }
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