LED Series Resistor Calculator

Formula Used

The series resistor limits current by dropping the remaining supply voltage:

• V_LED,total = N × V_F
• V_R = V_S − V_headroom − V_LED,total
• R = V_R / I
• P = I² × R

Tolerance current range uses worst-case combinations: high supply with low LED drop and low resistor gives max current, and the reverse gives min current.

How to Use This Calculator

1. Enter your supply voltage and the LED forward voltage.
2. Set how many LEDs are in series and your desired current.
3. Optionally add headroom voltage for wiring or driver losses.
4. Choose tolerances and a resistor series to match your parts.
5. Press Calculate and review ideal, standard, and power results.
6. Use Download CSV or Download PDF to save outputs.

Example Data Table

These examples assume zero headroom and show ideal resistor values. Your standard choice may differ.

Design Purpose of a Series Resistor

LEDs are current‑driven devices, so a series resistor is the simplest way to set a safe operating current from a fixed supply. This calculator turns your electrical targets into a practical resistor choice, then reports power dissipation and expected current spread for real components. It also suggests standard values so your parts list stays straightforward.

Core Equation and Unit Consistency

The resistor drops the remaining voltage after the LED string’s forward voltage is satisfied. The calculation uses R = (V_S − V_headroom − N×V_F) / I. Keep volts in V and current in amperes; a 20 mA target is 0.020 A.

Selecting Forward Voltage with Realistic Data

Forward voltage depends on color, construction, and temperature. Typical values are about 1.8–2.2 V (red), 2.0–2.4 V (green), and 2.8–3.4 V (blue/white). Datasheets specify V_F at a stated current, so match the value to your intended operating point.

Choosing Current for Brightness and Lifetime

Indicator LEDs often look good at 2–10 mA, while many small signal LEDs are commonly used at 10–25 mA. Higher current raises brightness but also increases heat and accelerates lumen depreciation over time. If you are unsure, start lower and step up gradually.

Multi‑LED Strings and Headroom Margin

When LEDs are in series, their forward voltages add. If the leftover resistor voltage becomes very small, current becomes sensitive to supply ripple and LED V_F variation. Reserving modest headroom for wiring, connectors, or upstream losses improves stability and repeatability.

Power Dissipation and Resistor Rating

Power is computed as P = I²R (equivalently P = V_R×I). Choose a resistor with comfortable margin; using at least a 2× rating is a practical rule for lower temperature rise and better long‑term stability, especially in warm enclosures.

Standard E‑Series Values and Tolerance Spread

Ideal resistance rarely matches a stocked value, so the calculator rounds to common E‑series options. Picking the next higher standard value slightly reduces current, which is usually safer. Tolerance matters: ±5% parts can noticeably shift current, while ±1% parts tighten the range.

Verification Steps and When to Use Drivers

After wiring, measure supply voltage under load, then measure resistor voltage and compute I = V/R to confirm performance. If your supply varies widely (batteries, automotive rails) or you drive high‑power LEDs, a constant‑current driver improves efficiency and brightness stability, but the resistor method remains excellent for quick builds.

FAQs

1) What if the calculator shows negative or zero resistor voltage?

That means the supply voltage is not high enough for the chosen LED string plus headroom. Reduce the number of series LEDs, choose a lower Vf LED, lower headroom, or increase the supply voltage.

2) Should I pick the nearest or the next higher standard resistor value?

For most LED work, the next higher value is safer because it reduces current. Use the nearest value only if you have verified brightness and temperature, and the power rating comfortably exceeds calculated dissipation.

3) How do I select a resistor power rating?

Compute resistor power, then apply margin. A common approach is choosing a resistor rated at least twice the calculated power. In tight spaces or high ambient temperatures, use a larger margin or a physically larger resistor.

4) Why does LED current change between units of the same color?

LEDs have forward‑voltage variation and Vf shifts with temperature. With a fixed resistor, those changes alter the resistor voltage and therefore current. Tighter resistor tolerance and more resistor voltage drop (more headroom) reduce the spread.

5) Can I run LEDs directly from a battery with only a resistor?

Yes, if the battery voltage range is narrow and you design for the highest battery voltage. If the battery varies widely, brightness will change. For better stability and efficiency, use a constant‑current driver.

6) Is PWM dimming compatible with a series resistor?

Yes. Set the resistor for the desired peak current, then dim by adjusting PWM duty cycle. Keep the PWM frequency high enough to avoid visible flicker and ensure the LED’s average current stays within its rating.

7) What should I check if the LED is too dim or too bright?

Confirm polarity, wiring, and the actual supply voltage under load. Measure the resistor voltage drop and compute current using I = V/R. Adjust resistance upward to reduce brightness, or downward to increase it within safe limits.