LEDs require a current-limiting resistor to operate at the ideal operating point and ensure a long lifespan. Here, you’ll find all the formulas for their calculation. Additionally, you’ll find a simple online calculator to determine the LED current-limiting resistor using your LED parameters.

## Limiting LED Voltage and Current

A Light Emitting Diode (LED) is a semiconductor component that has a **forward voltage**. The forward voltage depends on the wavelength and thus the color of the respective LED.

Here are some forward voltage values for standard LEDs:

- Red: 1.8 V
- Yellow: 2.0 V
- Green: 2.2 V
- Blue: 3.6 V

However, many circuits operate at a voltage of **5 V** or **12 V**, which is significantly higher than the LED’s forward voltage. Therefore, a current-limiting resistor is required to reduce the excessive voltage.

Additionally, the current-limiting resistor also provides **current limiting** because the internal resistance of the LED decreases after the LED starts to emit light. Without a current-limiting resistor, the current would increase significantly at that moment, potentially damaging the LED.

## Calculation of the LED Current-Limiting Resistor

The LED current-limiting resistor must be sized so that the difference between the supply voltage (of the circuit) and the forward voltage can drop on it.

### Resistor Formula

The basic formula for calculating the resistor according to Ohm’s law is:

$$ R = \frac{U_R}{I} $$

Where:

$R$ is the resistor value

$U_R$ is the voltage across the resistor

$I$ is the LED operating current

To determine the voltage across the resistor, subtract the LED forward voltage $U_F$ from the supply voltage $U$:

$$ U_R = U – U_F $$

Combining these equations, the resistor formula becomes:

$$ R = \frac{U – U_F}{I} $$

### LED Current-Limiting Resistor Calculator

With this online tool, you can easily calculate the current-limiting resistor for one or more LEDs, both in series and in parallel. Simply input the number of LEDs, the supply voltage, LED forward voltage, and LED current.

### Power Dissipation of the LED Resistor

Another important aspect is the power dissipation $P$ of the resistor, measured in Watts (W). It is calculated as the product of the voltage across the resistor and the LED operating current:

$$ P = U_R \cdot I $$

In practice, it is advisable to dimension the resistor with **2 to 10 times the power** of the calculated value to ensure that the resistor does not get too hot.