LED Resistor

R = (Vs − Vled) / Iled

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Result
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Formula

R = (Vsource - VLED) / ILED

Description

LEDs are current-driven devices that require a series resistor to limit the current flowing through them. Without a resistor, the LED draws excessive current and burns out almost instantly because its forward voltage is nearly constant regardless of current. The resistor drops the difference between the supply voltage and the LED's forward voltage, converting excess energy to heat. Different LED colors have different forward voltages due to their semiconductor bandgap: red LEDs are typically 1.8-2.0V, green 2.0-2.2V, blue and white 3.0-3.4V.

Variables

  • Vsource — Supply voltage (V)
  • VLED — LED forward voltage drop (V)
  • ILED — Desired LED forward current (A)
  • R — Required series resistance (Ω)

Practical Notes

Standard indicator LEDs typically run at 20mA (0.02A) for full brightness, but 5-10mA is often sufficient and extends lifespan. High-power LEDs may need 350mA or more and require proper thermal management. Always round up to the next standard resistor value to stay within safe current limits.

Related Formulas

Series Resistance Parallel Resistance Resistor Power Derating Resistive Attenuator Resistor Tolerance Range Resistor Thermal Noise (Johnson–Nyquist)

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