Overview

The ULN2803A is an 8-channel high-voltage, high-current NPN Darlington transistor array IC manufactured by Texas Instruments, STMicroelectronics, and Toshiba. Featuring 8 independent open-collector Darlington pairs with common emitters, each channel is capable of sinking up to 500mA continuous collector current (600mA peak) and sustaining up to 50V off-state output voltage.

Each input includes an internal $2.7\text{ k}\Omega$ series base resistor, allowing direct connection to $5\text{V}$ TTL or CMOS outputs (Arduino, Raspberry Pi, 74HC logic). Integral suppression (flyback) clamp diodes are connected to a shared Common (COM) pin to safely absorb inductive kickback spikes when driving relays, solenoids, unipolar stepper motors, and DC motor loads.

Quick reference

Pinout (DIP-18 Package)

             ┌───┴───┐
          1B 1│ 1   18│ 1C
          2B 2│       │17 2C
          3B 3│       │16 3C
          4B 4│       │15 4C
          5B 5│ULN2803│14 5C
          6B 6│       │13 6C
          7B 7│       │12 7C
          8B 8│       │11 8C
         GND 9│       │10 COM
             └───────┘

Internal Channel Schematics

  Input 1B..8B ───[2.7kΩ Resistor]───┬───[Base Q1]
                                     │
                                   [Zener]   [Output 1C..8C]
                                     │              │
                                     └───[Base Q2]──┤
                                                    ├───►|─── COM (Pin 10)
                                                    │   Flyback
                                                   GND (Pin 9)

Specifications

Typical Applications

8-Relay Bank Drive Circuit

  +12V Relay Power Rail ───────────────────────────────┬───────────────── [Pin 10: COM]
                                                        │
                                                 (+) [ 12V RELAY COIL ] (-)
                                                        │
  5V MCU Pin ──────────────► [Pin 1: 1B]   [Pin 18: 1C] ┘
                              ULN2803A
                              [Pin 9: GND] ────────────────────────────── GND

• When MCU Pin is HIGH (5V): Transistor turns ON, sinking relay current to GND (Relay energizes).
• When MCU Pin is LOW (0V): Transistor turns OFF, internal clamp diode (Pin 10 COM) safely discharges inductive coil energy back to +12V.

Common mistakes

• Forgetting to connect Pin 10 (COM) to the inductive load supply ($V+$): If COM is left unconnected when driving relays, solenoids, or motors, the internal flyback diodes cannot discharge inductive voltage spikes, destroying the Darlington output transistors.
• Assuming 500mA per channel across ALL 8 channels simultaneously: Total package power dissipation ($P_D$) is limited to approx $1.5\text{ W}$ for DIP-18. Sinking $500\text{mA}$ per channel across all 8 channels simultaneously ($8 \times 0.5\text{A} \times 1.1\text{V} = 4.4\text{W}$) will overheat and destroy the IC. Parallel channels or reduce duty cycle for high-current loads.
• Expecting positive voltage output: Outputs are open-collector current sinks, not current sources. Loads must be wired between the supply voltage ($V+$) and the output pins (1C–8C).

Notes

• ULN2803A vs ULN2003A: ULN2803A contains 8 channels in an 18-pin DIP package; ULN2003A contains 7 channels in a 16-pin DIP package.
• ULN2803A vs ULN2804A: ULN2803A has $2.7\text{k}\Omega$ resistors for 5V TTL/CMOS; ULN2804A has $10.5\text{k}\Omega$ resistors for 6V–15V CMOS.