Product Overview

The Allen-Bradley 1771-OB is a high-capacity discrete DC output module engineered for the legacy PLC-5® platform. This module serves as a power driver interface, providing eight independent output channels specifically designed to handle high-current DC loads. By delivering up to 1.5 amps per point, the 1771-OB eliminates the need for interposing relays in many applications, allowing the PLC to directly command heavy-duty industrial components such as large DC solenoids, motor starters, and hydraulic valves.

Operating within the universal 1771 I/O chassis, the module leverages the rugged backplane for logic-side power while isolating field-side DC power to maintain system integrity. Its "driver" designation reflects its ability to provide significant inrush current support, ensuring that inductive loads are energized quickly and reliably without damaging the module's internal solid-state circuitry.

Control Architecture and Field Performance

The 1771-OB is distinguished by its electrical robustness and ability to thrive in electrically noisy and thermally demanding environments.

• High-Current Driver Design: Each of the 8 sourcing outputs is rated for a continuous 1.5A at 12–24V DC, with a total module capacity of 10 amps, providing a concentrated power solution in a single slot.

• Low On-State Voltage Drop: Optimized internal switching reduces heat dissipation and ensures that the maximum available voltage reaches the field device, improving the efficiency of DC actuators.

• Opto-Electrical Isolation: The module features comprehensive opto-isolation between the backplane logic and the field wiring, protecting the PLC-5 processor from voltage spikes and ground loops.

• Visual Diagnostics: A series of front-panel status LEDs provide real-time feedback for each output point, significantly reducing the time required for loop testing and field fault isolation.

Technical Specifications

• Manufacturer: Allen-Bradley (Rockwell Automation)

• Model Number: 1771-OB

• Module Type: Digital DC Output Driver

• Number of Outputs: 8 Points (Sourcing)

• Operating Voltage: 12–24V DC

• Continuous Current per Point: 1.5A Max

• Surge Current: 3A for 10ms (Repeatable every 2 seconds)

• Backplane Current Draw: 250 mA @ 5V DC

• Power Dissipation: 12.0 Watts (Max)

• Operating Temperature: 0°C to 60°C (32°F to 140°F)

• Weight: 0.9 kg (1.98 lbs)

• Dimensions: 3.5 x 13.0 x 14.5 cm

Industrial Implementation and Safety

• Relay Replacement: The 1771-OB is frequently used to modernize panels by removing bulky external relays, thereby reducing cabinet heat and improving switching speeds.

• Inductive Load Handling: Specifically designed to manage the back-EMF generated by DC coils, though the use of external suppression (diodes) is still recommended for maximum contact life.

• Chassis Compatibility: Fully compatible with any 1771 Universal I/O chassis (1771-A1B through 1771-A4B) and various PLC-5 processors.

• Maintenance Protocol: Features a removable wiring arm (1771-WA) that allows for module replacement without disconnecting individual field wires, minimizing downtime during service intervals.