ULN2803ADW

The ULN2803ADW from Texas Instruments is an 8-channel NPN Darlington transistor array designed for interfacing low-level logic circuits to high-voltage/high-current loads. Each of the eight Darlington pairs consists of two NPN transistors in a cascade configuration, providing a current gain exceeding 1000. This allows a few milliamps of input current from a 3.3V or 5V MCU to switch loads up to 500mA at 50V. The open-collector outputs sink current to ground, making the device suitable for driving relays, solenoid valves, stepper motors, and LED arrays. Each channel includes an internal flyback/clamp diode connected to the common COM pin, which protects the output transistors from the inductive kickback generated when switching inductive loads. The COM pin must be connected to the load supply voltage to provide a return path for the flyback current. Channels can be paralleled for higher current capability. The DW suffix denotes the SOIC-18 wide-body surface-mount package.

Each of the eight channels in the ULN2803ADW consists of an NPN Darlington pair with a 2.7kΩ input resistor and a built-in flyback diode. When a logic HIGH (typically 3.3V or 5V) is applied to the input pin, current flows through the 2.7kΩ resistor into the base of the first transistor. The first transistor amplifies this base current and drives the base of the second transistor, which provides additional current gain. The total Darlington gain exceeds 1000, so a 1.8mA input current (5V input / 2.7kΩ) can sink over 350mA at the output. The output is an open-collector NPN Darlington that pulls the load to ground when activated. The saturation voltage is typically 1.1V at 200mA and 1.6V max at 350mA. When driving an inductive load (relay coil, motor winding), the collapsing magnetic field generates a flyback voltage spike when the Darlington turns off. The internal clamp diode (cathode to COM, anode to output) provides a safe current path for this spike, as long as the COM pin is connected to the positive supply of the load. Without the COM connection, the flyback spike can exceed the 50V rating and destroy the output transistor. Multiple channels can be paralleled by connecting their inputs and outputs together to increase current capability, but the total package dissipation must be managed.

• Relay Arrays: 8 channels directly drive 8 relay coils without external transistors or diodes; COM connects to relay supply
• Stepper Motors: 2 channels per winding (bipolar) or 4 channels (unipolar) drive small stepper motors at up to 24V
• MCU I/O Expansion: Darlington gain >1000 allows direct connection from 3.3V MCU GPIO to 24V/500mA loads
• LED Arrays: Open-collector outputs sink up to 500mA per channel for high-brightness LED indicators or displays
• Industrial PLC: 8 isolated outputs on one IC replace 8 discrete transistor+diode combinations, simplifying board layout

ULN2803ADW is an 8-channel Darlington driver from TI in SOIC-18 for 5V logic. The ULN2803A provides the same in DIP-18 for through-hole. The ULN2801A offers 6.5V/500µA input for 6-15V CMOS. The ULN2802A is designed for 14-25V PMOS logic. The ULN2804A accepts 6-15V CMOS input levels. The MC1413B from onsemi provides equivalent 8-channel driver in SOIC-18. The TBD62783A from Toshiba offers 8-channel source driver (high-side) alternative. For 4-channel, ULN2003A provides 7 drivers in DIP-16.