Calculate LED voltage, resistor, current, or source voltage from three inputs in a series circuit using VLED = VS − I×R for LED circuits.

One LED and a series resistor at a known DC operating point. LED forward voltage depends on current, temperature and LED type; this equation does not predict it from the LED alone.

LED Series Circuit Formula

At a known DC operating point, source voltage equals the LED voltage plus the voltage drop across its series resistor. LED forward voltage itself depends on LED type, current and temperature.

VLED = VS - I R
  • Vs = source voltage in volts
  • VLED = LED voltage in volts
  • I = series current in amperes
  • R = series resistance in ohms

Choose the quantity to find and enter the other three known operating-point values.

VS = VLED + I R
R = (VS - VLED) / I
I = (VS - VLED) / R

The LED voltage and source voltage modes use the series voltage balance. Resistance mode divides the resistor voltage drop by current; current mode divides that drop by resistance. A zero denominator cannot determine a unique inverse result.

Voltage inputs use V or mV, current uses A or mA, and resistance uses Ω, kΩ or MΩ. Inputs convert to V, A and Ω internally; select the desired result unit.

Common Series Circuit Conditions

A forward-operating LED circuit requires a suitable source voltage and current limiting. These conditions describe the simple series model.

Relationship Resistor Drop Current Result Interpretation
VS > VLED Positive Positive for positive resistance Usable model; verify component ratings
VS < VLED Negative Not a forward-current operating point Rejected for this forward DC model
VS = VLED Zero Zero for positive resistance Boundary only; does not establish LED operation

Voltage Unit Conversion Reference

Unit Equivalent in Volts Example
1 V 1 V 5 V = 5 V
1 mV 0.001 V 2000 mV = 2 V

Example Problems

Example 1: Find LED voltage

A 5 V source supplies 20 mA through a 150 Ω series resistor and an LED.

VLED = VS - I R
VLED = 5 - 0.020(150) = 2 V

The LED voltage at this operating point is 2 V.

Example 2: Find series resistance

A 5 V source and an LED with 2 V forward voltage are intended to operate at 20 mA.

R = (VS - VLED) / I
R = (5 - 2) / 0.020 = 150 Ω

The model gives 150 Ω, dissipating 0.06 W. Choose a suitable resistor power rating with allowance for tolerances.

FAQ

What is source voltage in an LED circuit?

Source voltage is the DC supply across the LED and series resistor together. It must cover the LED voltage plus the resistor voltage drop.

Does this calculator predict an LED’s forward voltage?

It infers voltage from a specified operating-point current, source and resistor. It does not model an LED’s current–voltage curve; use the LED datasheet at the intended current and temperature.

Why is current limiting needed?

An LED is not an ohmic resistor. Use an appropriate resistor or current-regulated driver and verify current, voltage and power ratings. Zero resistance provides no current limiting, and zero-denominator inverse calculations are indeterminate.