Circuit Lab
v1.0.0

LED Resistor Calculator

Supply Voltage

Min: 4.75 V | Max: 5.25 V

LED Parameters

ℹ️ Presets are typical for indicator LEDs at 3 mA. Always verify in datasheet. For power LEDs or higher currents, use Custom.

Target Current

Default: 3 mA. Valid range: 0.01–30 mA (10 µA – 30 mA)

Resistor Series

Select which E-series to search for resistor values:

Power & Headroom

Typical: 1.5× (67%), Conservative: 2.0× (50% max power rating)

Minimum voltage across resistor (default: 0.5 V)

PWM Mode (Optional)

Enter parameters and calculate to see results

LED Resistor Sizing

Calculate the current-limiting resistor for a single LED with worst-case analysis across supply voltage and LED forward voltage tolerances. Select from preset LED colors or enter custom values.

Basic Calculation

The nominal resistance is calculated from Ohm's law:

Rnom = (Vsup − Vf) / ILED

• Vsup = Supply voltage [V]

• Vf = LED forward voltage [V]

• ILED = Target LED current [A] (default 3 mA)

• Rnom = Required resistance [Ω]

E-Series Selection

The calculator finds standard resistor values from enabled E-series (E6/E12/E24/E48/E96) that bracket the nominal resistance:

Rdown ≤ Rnom ≤ Rup

Worst-Case Analysis

For each candidate resistor, the calculator computes LED current across all corners:

Imin = (Vsup,min − Vf,max) / R

Ityp = (Vsup,typ − Vf,typ) / R

Imax = (Vsup,max − Vf,min) / R

Power Rating

Resistor power dissipation is calculated with a safety margin (default 2×):

Pmax = Imax² × R

Prating = Pmax × margin

PWM Mode (Optional)

When PWM is enabled, the resistor is sized for peak current, and average values are calculated as:

Iavg = duty × Ipeak

Pavg = duty × Ipeak² × R

Where:

VsupV_{sup}= Supply voltageV
VfV_f= LED forward voltageV
ILEDI_{LED}= LED currentA (mA in UI)
RR= Current-limiting resistanceΩ
PP= Power dissipationW
dutyduty= PWM duty cycle—

📘 Usage Notes

  • • This calculator is for SINGLE LED only; series strings and parallel branches require different analysis
  • • LED presets are typical values for indicator LEDs at ~20 mA; always verify with datasheet
  • • For power LEDs (>100 mA), use Custom preset and consult manufacturer specifications
  • • Worst-case analysis ensures LED operates within current limits across all supply/temperature conditions
  • • Headroom warning triggers when voltage across resistor is too low (<0.5 V default)
  • • Temperature coefficient (dVf/dT) is typically negative for LEDs (Vf decreases at higher T)
  • • PWM mode sizes resistor for peak current; ensure LED can handle instantaneous peak
  • • Standard power ratings: 1/16, 1/10, 1/8, 1/4, 1/2, 1, 2, 3, 5, 10 W
  • • E-series selection affects available resistor values; E24/E48 recommended for precision
  • • When in doubt, choose higher resistance (lower current) to protect LED