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What Resistor for RGB LED?

Published in Electronics 3 mins read

The appropriate resistors for an RGB LED depend on the LED's forward voltage for each color and the desired current. Based on the provided information, you'll need different resistors for each color, as the forward voltages differ.

Resistor Values for RGB LED (Assuming 150mA Current)

Here's a breakdown of the resistor values, power ratings, and calculations:

Red LED

  • Forward Voltage (Vf): 2.2V

  • Desired Current (I): 150mA (0.15A)

  • Supply Voltage (Vs): Assuming 5V (typical for Arduino/microcontroller - adjust if different)

    Calculation:

    1. Voltage Drop Across Resistor (Vr): Vs - Vf = 5V - 2.2V = 2.8V
    2. Resistance (R): R = Vr / I = 2.8V / 0.15A = 18.67 ohms (Use nearest standard value of 22 ohms)
    3. Power Dissipation (P): P = Vr I = 2.8V 0.15A = 0.42W (Use a resistor with a power rating of at least 0.5W, preferably 1W for safety).

  • Resistor: 22 ohm, 1 Watt

Green and Blue LEDs

  • Forward Voltage (Vf): 3.5V

  • Desired Current (I): 150mA (0.15A)

  • Supply Voltage (Vs): Assuming 5V (typical for Arduino/microcontroller - adjust if different)

    Calculation:

    1. Voltage Drop Across Resistor (Vr): Vs - Vf = 5V - 3.5V = 1.5V
    2. Resistance (R): R = Vr / I = 1.5V / 0.15A = 10 ohms
    3. Power Dissipation (P): P = Vr I = 1.5V 0.15A = 0.225W (Use a resistor with a power rating of at least 0.25W, preferably 0.5W for safety).

  • Resistor: 10 ohm, 0.5 Watt

Summary Table

Color Forward Voltage (Vf) Desired Current (I) Resistor Value (R) Power Rating (P)
Red 2.2V 150mA 22 ohm 1W
Green 3.5V 150mA 10 ohm 0.5W
Blue 3.5V 150mA 10 ohm 0.5W

Important Considerations:

  • Forward Voltage Variance: The forward voltage can vary between different RGB LEDs, even of the same type. Always check the datasheet for your specific LED. If the data sheet isn't available, start with higher resistor values (e.g., 220 ohms) and decrease them until you achieve the desired brightness, while carefully monitoring the current.
  • Current Limiting: These resistor values are designed to limit the current to approximately 150mA. Exceeding the maximum forward current can damage or destroy the LED.
  • Supply Voltage: The calculations above assume a 5V supply. If you are using a different voltage, you will need to recalculate the resistor values.
  • Experimentation: LED brightness is subjective. You may need to adjust the resistor values slightly to achieve your desired effect. Always start with higher resistance values and gradually decrease them.
  • Series Resistors: Each resistor for each color should be in series with the corresponding LED pin.
  • Power Rating: Choose resistors with power ratings significantly higher than the calculated power dissipation. This ensures that the resistors do not overheat.

By using appropriate resistors for each color in your RGB LED, you can ensure proper operation and longevity.

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