Electronics Toolbox

LED Series Resistor Calculator

This calculator gives the values for current limiting resistors for LEDs.




R1 =

Power =

Voltage Divider Circuit

To calculate the correct resistor value for an LED, you need to ensure that the LED receives the appropriate current to operate without being damaged. This involves knowing the LED's forward voltage \( (V_f) \) and desired current \( (I_f) \), as well as the supply voltage \( (V_{supply}) \). Here's a step-by-step guide to calculating the correct resistor value:

Steps to Calculate the Resistor Value

  1. Determine the Supply Voltage \( (V_{supply}) \): This is the voltage of the power source you'll use to power the LED.

  2. Find the Forward Voltage \( (V_f) \): This is the voltage drop across the LED when it is forward-biased. It is typically provided in the LED's datasheet. Common values are:

    • Red LEDs: ~2.0V
    • Green LEDs: ~2.0-3.0V
    • Blue LEDs: ~3.0-3.5V
    • White LEDs: ~3.0-3.5V

  3. Determine the Desired Forward Current \((I_f)\): This is the current you want to flow through the LED, usually specified in the LED's datasheet. Common values range from 10mA to 20mA for standard LEDs.

  4. Use Ohm's Law to Calculate the Resistor Value: \( R = \frac{V_{supply} - V_f}{I_f} \)

Example Calculation

Let's go through an example calculation.

  • Supply Voltage \((V_{supply})\): 9V
  • Forward Voltage of LED \((V_f)\): 2V
  • Desired Forward Current \((I_f)\): 20mA (0.020A)

  • Subtract the Forward Voltage from the Supply Voltage: \( V_{R} = V_{supply} - V_f = 9V - 2V = 7V \)

  • Calculate the Resistor Value Using Ohm's Law: \( R = \frac{V_{R}}{I_f} = \frac{7V}{0.020A} = 350\Omega \)

So, you would use a 350-ohm resistor to limit the current through the LED to 20mA.

Choosing the Nearest Standard Resistor Value

Resistors come in standard values, so you might not find a 350-ohm resistor exactly. In such cases, you can choose the nearest standard resistor value, which is 360 ohms. This slight increase in resistance will slightly reduce the current through the LED, which is generally safe for the LED's operation.

Power Rating of the Resistor

Additionally, it's important to check the power rating of the resistor to ensure it can handle the power dissipation. The power dissipated by the resistor is given by:

\( P = V_{R} \times I_f = 7V \times 0.020A = 0.14W \)

A 1/4 watt (0.25W) resistor would be suitable in this case, as it is higher than the calculated power dissipation.

Summary

  1. Determine the supply voltage \((V_{supply})\).
  2. Find the forward voltage \((V_f)\) of the LED.
  3. Determine the desired forward current \((I_f)\).
  4. Use the formula \(R = \frac{V_{supply} - V_f}{I_f}\) to calculate the resistor value.
  5. Choose the nearest standard resistor value.
  6. Ensure the resistor's power rating is adequate.

By following these steps, you can select an appropriate resistor to safely and effectively drive your LED.