LED Series Resistor Calculator
Enter supply, LED Vf, count, and target current. Picks E-series resistor (E24/E12), shows actual current, resistor power, total input, efficiency, and optional Vf spread impact.
For multiple parallel strings, use one resistor per string (this calculator assumes that).
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How to Use the LED Series Resistor Calculator
Design the series resistor for one or more LEDs in series, choose an E-series value (E24/E12), and check current, power rating, efficiency, and the effect of LED forward-voltage spread and parallel strings.
1) Enter the inputs
- VS — supply voltage.
- Vf (typ) — forward voltage of one LED (from datasheet at your current).
- LEDs in series (N) — count per string.
- ILED — target current for one string.
- Parallel strings — number of identical strings (each with its own resistor).
- Power margin × — safety factor for resistor wattage (e.g., 2×).
2) Choose E-series & rounding
- E24 gives finer steps than E12.
- Round up is safest → slightly lower current.
- Nearest matches closest preferred value.
- Round down increases current → use only if you’ve checked power and LED limits.
3) Optional: Vf min/max
- Enter Vf,min and Vf,max from the datasheet to see the current range due to LED variation.
- Output shows I range (Vf min/max) using the selected resistor value.
4) Read the outputs
- Headroom VR — voltage across the resistor at the target current.
- R (exact) and R (selected) — ideal vs chosen E-series value.
- ILED (actual) — current with the selected resistor.
- PR per string and R wattage (rec.) — resistor dissipation and recommended rating with your margin.
- η (efficiency) — LED power vs input power.
- Max LEDs (≈) — rough upper bound ⌊VS/Vf⌋ (ignores margin).
Quick Examples
Single string, basic design
Given: VS = 12 V, Vf = 2.0 V, N = 3, ILED = 20 mA
V_R = VS − N·Vf = 12 − 6 = 6 V
R_exact = V_R / I = 6 / 0.02 = 300 Ω
E24 (round up) → 301 Ω
I_actual ≈ 6 / 301 = 19.9 mA
P_R = I²·R ≈ (0.0199²)·301 ≈ 0.119 W
Wattage (×2 margin) ≈ 0.24 W → use ≥ 0.25 W resistorTwo parallel strings (each with its own R)
Strings = 2 → total current ≈ 2 · I_actual ≈ 39.8 mA
LED power (both strings) ≈ 2 · (N·Vf·I_actual) ≈ 2 · (6 · 0.0199) ≈ 0.239 WVf spread (datasheet min/max)
Vf,min = 1.8 V, Vf,max = 2.2 V
With R_selected = 301 Ω:
I_min ≈ (12 − 3·2.2) / 301 = 5.4 / 301 = 17.9 mA
I_max ≈ (12 − 3·1.8) / 301 = 6.6 / 301 = 21.9 mAInsufficient headroom
If VS ≤ N·Vf then V_R ≤ 0 → increase VS or reduce N.Pre-Run Checklist
- VS > N·Vf so there’s headroom across the resistor.
- Target ILED within LED’s rated current & thermal limits.
- Choose E-series & rounding (prefer round up).
- Resistor wattage with margin ≥ PR.
- For multiple strings, each string has its own resistor.
- Consider Vf variation with temperature and binning.
Equations Reference
- VR = VS − N·Vf
- Rexact = VR / ILED
- Iactual = VR / Rselected
- PR = Iactual2 · Rselected
- PLED,total = Nstrings · N · Vf · Iactual
- η = PLED / (VS · Iin)
FAQ & Tips
Why separate resistors per string?
LEDs don’t share current evenly. A single resistor for multiple parallel strings can cause one string to hog current.
What if current is too sensitive to VS ripple?
Increase R (round up), reduce N, or use a constant-current driver or buck LED driver.
Can I dim with PWM?
Yes—ensure resistor power and LED average current are within ratings at the chosen duty cycle.
Tolerance?
Include resistor tolerance (e.g., ±1%/±5%) and LED bin tolerance in your current margin.
Mini Workflow
1) Enter VS, Vf(typ), N, ILED, strings, power margin → pick E-series & rounding
2) Read R(exact) and choose R(selected); verify I(actual) and P_R
3) Check efficiency & total input current; adjust for thermal or battery life
4) (Optional) Add Vf min/max → verify current spread
5) Finalize resistor wattage with safety margin and build