ULN2803ADW Darlington Transistor Array: Features, Pinout, Circuit and Applications

The ULN2803ADW is an 8-channel high-voltage, high-current Darlington transistor array designed specifically for switching inductive loads. Each of its eight NPN Darlington pairs is rated for a maximum collector current of 500 mA, with integrated common-cathode clamp diodes to protect against inductive load transients. Ideal for relay, lamp, solenoid, and display drivers, this device simplifies interfacing with TTL or CMOS logic thanks to built-in 2.7 kΩ series base resistors.

Key Features:

• Eight independent Darlington transistor pairs
• High-voltage outputs: 50 V
• Maximum current per output: 500 mA
• Internal output clamp diodes for inductive load protection
• Compatible with TTL and 5-V CMOS logic inputs
• Outputs can be paralleled for increased current

Specifications:

• Output Voltage Rating: 50 V (maximum)
• Output Current Rating: 500 mA per channel
• Built-in Base Resistor: 2.7 kΩ
• Integrated Output Clamp Diodes
• Logic Compatibility: TTL, CMOS (5 V)
• Package Type: SOIC-18 (DW package)

ULN2803ADW Pinout:

Pin Descriptions:

• Input pins (1B–8B): Connect directly to TTL or 5-V CMOS logic signals; integrated 2.7 kΩ series resistor included.
• Output pins (1C–8C): Open-collector Darlington transistor outputs; connect to load, switching it to ground.
• COM pin: Common diode clamp connection; must be tied to the positive supply line for inductive load protection.
• GND pin: Common system reference ground connection.

Pin Name	Pin Number	Description
1B	1	Channel 1 logic input
2B	2	Channel 2 logic input
3B	3	Channel 3 logic input
4B	4	Channel 4 logic input
5B	5	Channel 5 logic input
6B	6	Channel 6 logic input
7B	7	Channel 7 logic input
8B	8	Channel 8 logic input
GND	9	Common ground reference pin
COM	10	Common connection for internal clamp diodes; connect to load positive supply
8C	11	Channel 8 output (Darlington transistor collector)
7C	12	Channel 7 output (Darlington transistor collector)
6C	13	Channel 6 output (Darlington transistor collector)
5C	14	Channel 5 output (Darlington transistor collector)
4C	15	Channel 4 output (Darlington transistor collector)
3C	16	Channel 3 output (Darlington transistor collector)
2C	17	Channel 2 output (Darlington transistor collector)
1C	18	Channel 1 output (Darlington transistor collector)

Typical Applications:

• Relay driver circuits
• LED and gas-discharge display drivers
• Lamp and solenoid drivers
• Logic buffers and line drivers
• Motor control interfaces

Advantages of Using ULN2803ADW:

• Simplifies circuit design by reducing component count
• Robust handling of inductive loads with built-in protection
• Easy interfacing with standard logic devices (TTL, CMOS)

ULN2803ADW Relay Driver Circuit

This circuit utilizes the ULN2803ADW as a low-side driver to control a 12 V relay coil (K1). The activation process is clearly separated into the following stages:

• Input Signal Stage:
  The control signal (ACTIVATE_RELAY) from a microcontroller or logic device passes through a current-limiting resistor (R59) to drive the internal LED of the optocoupler (PC817).

• Isolation and Level Translation:
  When the optocoupler LED is activated, its internal transistor (pins 3 and 4) turns on, pulling the input pin (1B) of the ULN2803ADW close to ground (GNDA). This action activates the corresponding output transistor inside the ULN2803ADW.

• Relay Activation:
  With the ULN2803ADW output transistor (1C, pin 18) conducting, the relay coil is energized, connecting the coil from +12 V to ground, thus activating relay K1.

• Voltage Regulation and Pull-Up:
  The regulated 5 V (5V1) provides a stable voltage supply to the optocoupler transistor's collector. The pull-up resistor (R58) ensures the ULN2803ADW input pin remains at a defined high level when the optocoupler transistor is off, preventing unintended relay activation.

• Protection and Safety:
  The COM pin (pin 10) of the ULN2803ADW connects directly to the +12 V supply, activating internal clamp diodes to protect the chip from voltage spikes generated by the relay coil upon deactivation.

Conclusion:
The ULN2803ADW provides a reliable and efficient solution for controlling high-current, inductive loads directly from low-level logic signals. Its integrated protection features and straightforward design make it a valuable component in diverse electronic applications.