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.
