How to Use ULN2803A: Examples, Pinouts, and Specs

The ULN2803A is a high-voltage, high-current Darlington transistor array designed to simplify the interface between low-power digital circuits and high-power loads. It consists of eight NPN Darlington pairs, each capable of driving loads up to 500mA and withstanding voltages up to 50V. The component is equipped with built-in freewheeling diodes to protect against voltage spikes, making it ideal for inductive loads such as relays, solenoids, and motors.

• Driving relays, solenoids, and stepper motors
• LED matrix and display control
• Interfacing microcontrollers with high-power devices
• Industrial automation and robotics
• Home appliances and automotive systems

The ULN2803A is a versatile component with the following key specifications:

The ULN2803A is available in an 18-pin package. The pinout is as follows:

1. Connect the Inputs: Connect the input pins (1-8) to the control signals from a microcontroller or other digital logic device. Ensure the input voltage is within the specified range (5V to 12V).
2. Connect the Outputs: Connect the output pins (10-17) to the load terminals. Each output pin corresponds to its respective input pin.
3. Freewheeling Diodes: Connect the COM pin (pin 9) to the positive supply voltage of the load. This ensures the built-in diodes protect against voltage spikes from inductive loads.
4. Ground Connection: Connect the GND pin (pin 18) to the ground of the circuit.
5. Power the Circuit: Ensure the load voltage and current do not exceed the maximum ratings of the ULN2803A.

• Current Limitation: Do not exceed 500mA per channel or the total power dissipation limit of the IC.
• Heat Management: If driving multiple high-current loads, consider adding a heatsink or improving ventilation to prevent overheating.
• Inductive Loads: Always connect the COM pin to the load's positive supply to utilize the internal freewheeling diodes.
• Input Voltage: Ensure the input voltage is compatible with the control signals from your microcontroller (e.g., 5V for Arduino).

The following example demonstrates how to use the ULN2803A to control a 12V relay with an Arduino UNO.

• Connect Arduino digital pin 2 to ULN2803A input pin 1.
• Connect ULN2803A output pin 10 to one terminal of the relay coil.
• Connect the other terminal of the relay coil to a 12V power supply.
• Connect the COM pin (pin 9) to the 12V power supply.
• Connect the GND pin (pin 18) to the Arduino GND.

// Example code to control a relay using ULN2803A and Arduino UNO

#define RELAY_PIN 2  // Arduino pin connected to ULN2803A input pin 1

void setup() {
  pinMode(RELAY_PIN, OUTPUT); // Set the relay control pin as output
}

void loop() {
  digitalWrite(RELAY_PIN, HIGH); // Turn the relay ON
  delay(1000);                   // Wait for 1 second
  digitalWrite(RELAY_PIN, LOW);  // Turn the relay OFF
  delay(1000);                   // Wait for 1 second
}

1. Relay or Load Not Activating

   • Cause: Insufficient input voltage or current.
   • Solution: Ensure the input voltage is within the specified range (5V to 12V). Verify the microcontroller's output current is sufficient to drive the ULN2803A inputs.

2. Overheating

   • Cause: Exceeding the maximum current or power dissipation limits.
   • Solution: Reduce the load current or use fewer active channels simultaneously. Improve heat dissipation with a heatsink or better ventilation.

3. Voltage Spikes Damaging the Circuit

   • Cause: Inductive loads generating back EMF.
   • Solution: Ensure the COM pin is connected to the load's positive supply to utilize the internal freewheeling diodes.

4. Incorrect Output Behavior

   • Cause: Miswiring or incorrect input signals.
   • Solution: Double-check all connections and ensure the input signals are correctly configured.

Q1: Can the ULN2803A drive LEDs directly?
A1: Yes, the ULN2803A can drive LEDs directly. However, you must use appropriate current-limiting resistors to prevent overdriving the LEDs.

Q2: Can I use the ULN2803A with a 3.3V microcontroller?
A2: The ULN2803A is designed for 5V to 12V input signals. If using a 3.3V microcontroller, consider using a level shifter or a different driver IC compatible with 3.3V logic.

Q3: What is the maximum total current the ULN2803A can handle?
A3: While each channel can handle up to 500mA, the total current should not exceed the power dissipation limit of 2.25W. Distribute the load evenly across channels to avoid overheating.