Allen-Bradley 1746-OB8 24V DC Digital Output Module

Description

Product Introduction & Engineering Value

When fast, solid-state switching is needed without the mechanical contact wear of relay cards, maintenance teams turn to transistor switching. The Allen-Bradley 1746-OB8 provides eight high-current sourcing outputs engineered to drive 24V DC industrial loads inside legacy SLC 500 chassis.

Because transistor outputs bypass mechanical armatures, the module delivers sub-millisecond response times for high-cycle applications. Plant engineers rely on the 1746-OB8 for direct replacement in critical machine interlocks where relay contact welding would otherwise lead to unscheduled downtime.

Technical Specifications

• Number of Outputs: 8 sourcing channels
• Operating Voltage Range: 10 to 30V DC
• Continuous Current per Channel: 1.0 A at 30°C (derated to 0.50 A at 60°C)
• Continuous Current per Module: 5.0 A total max
• Surge Current: 3.0 A for 10 ms (repeatable every 2 seconds)
• Off-State Leakage Current: 1.0 mA max
• On-State Voltage Drop: 1.2 V DC max at 1.0 A
• Signal Delay (Off to On / On to Off): 0.1 ms max / 1.0 ms max
• Backplane Current (5V DC): 135 mA
• Backplane Isolation: 1500V AC field-to-backplane

Field Application & The “Trench” Experience

A high-speed palletizing cell at a beverage bottling plant began throwing intermittent fault codes when a mechanical relay output card stuck closed, keeping a pneumatic pusher arm extended into the main conveyor line. Swapping the failing relay card for a solid-state 1746-OB8 resolved the mechanical sticking issue instantly. The module took over driving the 24V DC directional solenoid valves directly, effortlessly handling the continuous cycle rate of 40 actuations per minute.

• Automotive Stamping Indexers: Driving 24V DC pilot solenoids for pneumatic clamp assemblies in high-cycle welding cells.
• Packaging Machinery Labelers: Controlling solid-state print head triggers and high-speed box drop gates.
• Chemical Batching Panels: Actuating 24V DC intrinsically safe relay coils that interface with field explosion-proof solenoid valves.

1746-OB8

1746-OB8

Transparency SOP: QA & Testing

1. Visual & Board Component Audit: Inspecting backplane edge connector gold fingers, verifying fuse integrity, and checking output transistors for thermal stress marks.
2. Backplane Current Verification: Mounting the module in a live SLC 1746 rack to monitor 5V DC logic rail power consumption against factory limits.
3. 8-Point Full Load Testing: Connecting individual 24V DC lamp loads and power resistors to all eight output terminals simultaneously; toggling outputs on/off at maximum switching frequency via RSLogix 500 test routines.
4. Leakage & Voltage Drop Check: Measuring off-state leakage current and on-state voltage drop under full 1.0A load using precision digital multimeters to verify transistor saturation health.

The Veteran’s Tech Trap Guide

• ⚠️ Sourcing vs. Sinking Pitfall: The 1746-OB8 is a sourcing module (VDC connected to terminal supply, switching positive voltage out to the field load). If your existing field field sensors or slave devices expect a sinking drive (1746-OV8), installing an OB8 will cause a complete wiring incompatibility, shorting your field supply if forced.
• PRO TIP: Inductive Kickback Protection: Although the module includes internal surge protection, driving un-suppressed inductive loads (large solenoid coils, mechanical brakes) will cause premature transistor failure due to voltage spikes. Always wire a reverse-biased flyback diode across inductive DC field loads directly at the coil terminals.
• ⚠️ Current Derating at Higher Ambient Temps: Total module output current is rated for 5.0A max at lower enclosure temperatures, but drops significantly as ambient panel heat reaches 60°C. If all eight outputs carry 1.0A continuous loads in a hot cabinet, thermal shutdown or trace damage can occur—distribute high-current loads across multiple cards when operating near maximum thermal limits.

Dynamic FAQ

Q: Does the 1746-OB8 feature onboard fusing?

A: Yes, the module contains internal fuses to protect board traces against catastrophic dead shorts. However, proper external branch fusing for individual field output runs is still recommended.

Q: Can I use the 1746-OB8 to switch AC voltages?

A: No. The 1746-OB8 uses DC solid-state transistors and operates strictly on 10 to 30V DC. Attempting to pass AC voltage will damage the internal semiconductors. For AC switching, use a 1746-OA8 or 1746-OW8 relay module.

Q: What is the status display layout on the front bezel?

A: The front panel includes eight red status LEDs (numbered 0 through 7) that illuminate whenever an individual output point is commanded ON by the PLC logic.

Q: Are replacement modules supplied with terminal blocks?

A: Yes. Every unit ships complete with a fully inspected, removable 10-point terminal block (1746-RTB) ready for field conductor termination.