LED resistor sizing and resistor power derating
An LED series resistor is simple, but the useful design check includes current, resistor dissipation, supply variation, and LED forward-voltage spread.
How do you choose an LED series resistor and resistor wattage?
Subtract the LED forward voltage from the supply voltage, divide the remaining voltage by the target LED current, then check the resistor power at the same operating point and select a practical rating with margin.
Model summary
- Resistor value: R = (Vsupply - Vf) / If.
- Resistor power: P = If^2 * R, or P = Vresistor * If.
- Worst-case current increases when supply voltage is high and LED forward voltage is low.
Worked example
For a 5 V supply, 2.0 V LED forward voltage, and 10 mA target current, R = (5 V - 2 V) / 10 mA = 300 Ohm.
Resistor power is 3 V * 10 mA = 30 mW.
A 60 mW or higher practical rating gives 2x margin before considering package derating and ambient temperature.
Practical derating note
A resistor wattage rating is not a promise that every small package can safely run hot in every layout. Check package, ambient, copper area, enclosure temperature, and manufacturer derating guidance.
Common mistakes
- Using typical LED forward voltage only, then being surprised by current spread.
- Choosing a resistor value without checking resistor power.
- Running indicator LEDs brighter than needed and wasting power or creating glare.
When the approximation breaks down
- The simple resistor model is for low-current single LED indicators and similar circuits. High-power LEDs usually need current regulation and thermal design.
- LED forward voltage varies with current, temperature, and device binning.