1794-OW8 Allen Bradley Flex I/O 8 Channel Isolated Relay Module

1794-OW8 Isolated Relay Output Architecture

The 1794-OW8 is an 8-channel isolated electromechanical relay output module belonging to the Allen-Bradley Flex I/O Series produced by Rockwell Automation. Built for switching mixed AC and DC power loads, this module contains 8 independent Form A (normally open) dry contacts with channel-to-channel galvanic isolation rated at 250V continuous. Operating on an external 24 Vdc supply (19.2 to 31.2 Vdc range at 125 mA), it draws a minimal 35 mA at 5 Vdc from the Flexbus backplane. The module snaps onto standard terminal base units including 1794-TB2, 1794-TB3, 1794-TB3S, 1794-TBN, and 1794-TBNF, utilizing a mechanical keying setting at Keyswitch Position 9. Internal RC snubber circuits across each relay contact protect switching elements from inductive voltage spikes, while eight yellow front-panel LEDs display real-time output coil status.

Technical Specifications

Target Equipment & Application Scenarios

The 1794-OW8 provides galvanically isolated relay contacts capable of routing intermixed AC and DC voltages on a single slot. Key target machinery and system integration scenarios include:

• Pilot lights, stacks, and alarm annunciator panels operating across non-uniform voltage lines
• Interposition control for motor starter coil energization in MCC (Motor Control Center) buckets
• Solenoid valve control in hydraulic power units (HPU) and pneumatic manifolds
• Interfacing PLC outputs with third-party boiler control circuits or security interlock loops
• Fractional horsepower AC/DC actuators requiring complete dry-contact electrical separation

Diagnostic & Monitoring Capabilities

Integrated hardware indicators and contact protection circuits ensure high signal reliability during field operations:

• Channel Output Indicators: Eight yellow status LEDs positioned along the front panel display the real-time drive status of each relay coil logic input.
• 250V Galvanic Channel Isolation: Each relay contact set is electrically separated from adjacent contacts, backplane power, and user logic to prevent voltage cross-over.
• Built-In RC Snubber Protection: Internal snubber circuits wired across each contact tip absorb high-voltage inductive kickback spikes generated when switching solenoids or contactors.

Packaging, Fast Delivery & Warranty Terms

• Product Condition: Brand New, Factory Sealed Packaging
• Warranty Coverage: Full 12 Months Replacement Warranty
• Order Handling: Dispatches within 24–48 hours upon payment clearance
• Global Logistics Partners: Express delivery worldwide via FedEx, UPS, or DHL

Global PLC Spare HUB's Insight

The 1794-OW8 is one of the most flexible modules in the Flex I/O lineup due to its true dry-contact isolation, allowing engineers to switch 120V AC, 240V AC, and 24V DC from different power buses on the same physical module. However, pay close attention to external supply stability; always supply a clean, unswitched +24V DC line to the module's power terminals. Voltage dips on the 24V DC auxiliary supply can cause minor variations in coil energization timing, which may momentarily force a single contact to bear full inrush load during group switching operations. Additionally, when driving highly inductive loads, installing external MOV surge suppressors directly across field coils will preserve internal relay contacts beyond the 100,000-cycle minimum design threshold.

Frequently Asked Questions (FAQ)

Q: What field symptom indicates that the internal RC snubber circuit is interacting with a low-power AC load?

A: Low-current AC loads (such as high-impedance LED indicators or small relays) may stay partially energized or flicker when the contact opens due to the 1.0 mA max off-state leakage current passing through the snubber circuit; placing a parallel bleeder resistor across the load resolves this issue.

Q: Can a 24V DC load and a 230V AC load be wired simultaneously on adjacent channels of the same module?

A: Yes. Each relay output is individually isolated with a 250V continuous basic insulation rating, allowing different supply voltages and power sources to be safely switched across different channels on the same module.

Q: How can output short-circuit protection be added when wiring load circuits to the module base?

A: Mount the module on a 1794-TBNF fused terminal base unit, which incorporates fuse holders for 3.0 A, 250 Vac slow-blow fuses (Littelfuse 239003) directly in series with each relay output channel.

Q: Why is there a maximum switching frequency limit of 0.3 Hz (1 operation every 3 seconds) specified for rated loads?

A: Electromechanical relay contacts experience arc heating and mechanical bounce (1.2 ms mean bounce time) during state changes. Restricting full-load switching frequency to 0.3 Hz prevents thermal buildup on contact tips and preserves contact life.

Q: Selection Guide: When should I choose the 1794-OW8 relay module instead of a solid-state DC output module like the 1794-OB16?

A: Select the 1794-OW8 when your application requires dry-contact isolation, mixed AC/DC switching on a single module, or zero-voltage-drop contacts. Choose solid-state modules like the 1794-OB16 for high-frequency switching applications where rapid cycling would wear out mechanical contacts.

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