LED Resistor Calculator

Resistor value and power for LEDs in series or parallel strings.

Inputs
Single LED (5V, red) 3× series (12V, white) 2 parallel strings (9V, blue)
Notes
Formula R = (Vs − NsVf) / If; power PR = If2R.
Standard values Choose the next higher standard value (E12/E24) and ensure adequate wattage (≥2×).
Wiring Diagram
Results
Series resistor per string R = – Ω (nearest: – Ω)
Resistor power PR ≈ – W
Total current Itotal ≈ – A, Total power Ptotal ≈ – W
Tip: Prefer one resistor per parallel string for balanced currents.
About & Learning
What this is: A calculator for current‑limiting resistors used with LEDs in series or parallel strings.
Why it’s used: LEDs are not resistive loads; a resistor sets a safe current to prevent thermal runaway and brightness variation.
Design tips: Provide headroom (V_s slightly above N_s·V_f), pick the next higher standard value for R, and choose a resistor with ≥2× power margin.
Series vs Parallel Comparison
AspectSeriesParallel (per string)
CurrentSame through all LEDsSeparate resistor per string keeps currents balanced
Voltage requirementV_s ≥ N_s·V_f + marginV_s ≥ V_f + margin (per LED)
Resistor valueR = (V_s − N_s V_f)/I_fR = (V_s − V_f)/I_f (per string)
PowerP_R = I_f² RP_R = I_f² R (per string); total power sums over strings
ProsSingle resistor, matched currentEasy to add strings; resilient to LED variation
ConsNeeds higher supply voltageMore resistors; total current grows with strings
Note: Avoid a single resistor for multiple parallel LEDs without matching; device variation can cause current imbalance.

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About This Tool & Methodology

Computes series resistor values for LEDs using SI units from supply voltage, LED forward voltage, and desired current. Reinforces Ohm’s law and practical ranges.

Learning Outcomes

  • Apply Ohm’s law to LED current limiting.
  • Understand headroom and power dissipation.
  • Practice unit consistency and safe design margins.

Authorship

  • Author: Anish Nath — Follow on X
  • Last updated: 2025-11-19

Trust & Privacy

  • Runs locally in your browser.
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