Allen-Bradley 1756-OB8 ControlLogix 24VDC 8-Pt Output

Description

Product Introduction

The Allen-Bradley 1756-OB8 is an 8-point high-current DC output module built for the ControlLogix platform. Slotting into any local or remote 1756 I/O chassis, it bridges 24V DC field devices—such as heavy solenoid valves, large contactor coils, and hydraulic proportional drivers—directly to the controller without needing intermediate interposing relays.

Unlike standard high-density cards like the 1756-OB16, which top out at 0.5A per channel, the 1756-OB8 handles a massive 2.0 Amps per individual point. Built-in electronic fusing protects each channel circuit against short circuits and overcurrent spikes, saving cabinet space and wiring labor by eliminating external terminal-block fuses.

Key Technical Specifications

Application Scenarios & The “Trench” Experience

A hydraulic stamping press stops cold on the main line. The diagnostic panel points to a blown output point driving the primary main-manifold solenoid valve. The technician checks the cabinet and finds a low-density 0.5A output card melted because an aging solenoid coil pulled 1.4A on inrush. Interposing relays were never wired in due to tight enclosure limits. Swapping the burned module out for a heavy-duty 1756-OB8 gives you the 2.0A channel headroom needed to drive those high-draw coils directly without roasting the board again.

Target Application Scenarios

• Hydraulic & Pneumatic Manifolds: Drives heavy-duty 24V DC directional control valves pulling upwards of 1.5A steady-state.
• Motor Control Centers (MCC): Triggers larger NEMA size 1 and size 2 24V DC contactor coils directly without interposing relays.
• Industrial Brake Assemblies: Controls fast-acting electromagnetic holding brakes on conveyor systems and hoist drums.
• High-Draw Indicator Stacks & Horns: Switches high-wattage acoustic alarms and industrial beacon arrays.

Field Case Study

A steel rolling mill experienced repeated output card failures on its crop shear station due to inductive kickback from large 24V DC hydraulic valves. The existing 16-point output module couldn’t clear the thermal stress. We supplied a bench-tested 1756-OB8 card. The maintenance team landed the 8 heavy-draw field leads onto a 1756-TBNH terminal block, updated the I/O configuration in Studio 5000, and brought the shear line back online. The module’s onboard electronic fusing absorbed the switching spikes, eliminating weekly downtime calls.

1756-OB8

1756-OB8

Transparency SOP: Quality Assurance & Testing

We don’t just ship boxes; we test them on actual Allen-Bradley racks. Every 1756-OB8 module moving through our facility undergoes a strict 5-step verification process:

1. Inbound Inspection: Inspecting backplane edge connector pins, housing latches, front LED windows, and checking for shorted trace discoloration.
2. Live Rig Testing: Inserting the card into an active 1756 chassis powered by a 1756-PA75 power supply to verify backplane boot routines and status LED sequencing.
3. Full-Load Channel Testing: Wiring all 8 channel outputs to a 2.0A resistive load bank running at 24V DC to verify sourcing output voltage, thermal dissipation, and latch stability.
4. Electronic Fuse Simulation: Momentarily shorting each channel to ground to verify that the electronic fusing trips instantly, lights the channel fault status LED, and resets cleanly upon command.
5. Final QC & Anti-Static Packaging: Neutralizing static charges at an ESD workstation, fitting protective edge covers, and sealing the unit in static shielding bags with fresh desiccant.

The Veteran’s Tech Trap Guide (Crucial Value-Add)

⚠️ Watch Out for Total Module Amperage: While each point on the 1756-OB8 can handle 2.0 Amps individually, the whole card is capped at 13.0 Amps total across all 8 points at 60°C. If you turn on all 8 channels simultaneously at 2.0A each, you will draw 16.0A and trip the module’s thermal overload protection. Calculate your total continuous load!

❗ Don’t Forget Inductive Suppression: Even though the 1756-OB8 has built-in electronic protection, driving heavy inductive loads (solenoids, brakes, relays) without a flyback diode or RC surge suppressor across the field coil will cause premature channel degradation. Put a suppression diode right at the coil terminals. Trust me.

⚠️ RTB Terminal Block Selection: The 1756-OB8 does not come with a terminal block in the box. You must order a 20-pin Removable Terminal Block separately—either the 1756-TBNH (screw clamp) or 1756-TBSH (spring clamp). Standard 36-pin RTBs (like 1756-TBCH) will not fit this card housing.

Frequently Asked Questions (FAQ for Conversion)

Q: Can I use a 1756-OB8 to replace a 1756-OB16E module?

A: Yes, provided you only need 8 channels or add a second card. The 1756-OB8 offers higher per-point current capacity (2.0A vs. 1.0A on the OB16E). You will need to update the module catalog number in your Studio 5000 / RSLogix 5000 project I/O tree and reassign your tag addresses.

Q: Does the 1756-OB8 require mechanical fuses?

A: No. The 1756-OB8 features built-in electronic fusing. If a short circuit occurs in the field, the card detects the overcurrent condition and shuts down the affected channel to protect the internal circuitry.

Q: How do I reset a tripped electronic fuse on a channel?

A: You can clear the electronic fuse fault either by cycling 24V DC field power to the module or by toggling the channel output state off and back on through your Studio 5000 logic application.

Q: Is the 1756-OB8 hot-swappable in a live ControlLogix rack?

A: Yes, it supports RIUP (Removal and Insertion Under Power). You can pull and replace the card without shutting down the chassis or disturbing adjacent I/O modules. However, ensure field power to the RTB is safely isolated before pulling wiring.

Q: What warranty comes with this module?

A: We back all New Surplus and Refurbished 1756-OB8 digital output modules with a full 12-month operational warranty. If the module develops hardware issues within one year under standard operating conditions, we issue an immediate replacement or full refund.