YOKOGAWA ALR121-S50 – Compact limit alarm relay for reliable process monitoring and trip functions

The YOKOGAWA ALR121-S50 is a slim DIN-rail alarm module designed to watch a single analog signal and drive relay outputs when your process crosses set thresholds. From my experience, this type of unit is typically used to protect pumps, trigger tank overfill alarms, generate temperature trips, or raise interlocks when a 4–20 mA or 1–5 V loop drifts out of a safe range. The “121” designation commonly indicates 1 input with 2 independent relay setpoints, and the S50 is a factory option code. It’s a simple block you drop next to your PLC/DCS to add clean, isolated alarm contacts without reworking the entire control strategy.

Order Placement Process and Guarantees

• Warranty: 365 days
• Lead time: 1 week for in-stock items; no more than one month at the latest
• Payment: 50% advance payment; full payment prior to delivery
• Express options: FedEx, UPS, DHL

Key Features

• Single analog input monitoring – Typically accepts a process loop (4–20 mA or 1–5 V) for alarm supervision.
• Two independent alarm relays – High/low or dual-threshold configuration for flexible trip logic.
• Isolated dry contacts – Clean SPDT outputs for PLC/DCS digital inputs, annunciators, or interlock circuits.
• DIN-rail form factor – Slim body fits crowded cabinets; wiring is straightforward in most cases.
• Front indicators and adjustability – You might notice simple status LEDs and accessible setpoint adjustments on this family, which speeds up commissioning.
• Industrial temperature range – Built to live in standard control enclosures without special conditioning.
• Failsafe-friendly design – In many cases, you can wire it for de-energize-to-trip strategies preferred by safety teams.

Technical Specifications

Application Fields

You’ll usually see the ALR121-S50 in places where a clean alarm or trip is all that’s needed without revising PLC logic:

• Water & wastewater: high-tank level alarm, pump dry-run protection via discharge pressure
• Food & beverage: temperature over/under alarm on pasteurization loops
• Oil & gas / chemicals: interlock contact on differential pressure or flow deviation
• Power generation / boiler rooms: makeup water conductivity or drum level deviation alarms
• OEM skids: quick add-on alarm block for 4–20 mA transmitters without extra PLC cards

Advantages & Value

• Reliability – Yokogawa hardware tends to be robust; alarm relays like this are usually set-and-forget for years.
• Compatibility – Works with standard 4–20 mA and 1–5 V loops; relay contacts land on any PLC/DCS DI card.
• Panel space savings – Slim DIN-rail footprint simplifies retrofits and crowded cabinets.
• Cost control – Avoids buying extra I/O modules when you only need a discrete alarm contact.
• Supportable – Straightforward setup; maintenance teams appreciate the clear status indication.

Installation & Maintenance

• Cabinet & environment – Mount on IEC TH35 DIN rail in a ventilated control panel. Keep ambient within the stated operating range and away from vibration or corrosive gases.
• Power & wiring – Use a clean 24 V DC supply. Land shielded signal cables; ground the shield at one end only to reduce noise. Separate power and signal wiring where feasible.
• Setpoints & logic – Configure high/low thresholds and, if available, hysteresis and delay to avoid nuisance trips. Many teams prefer de-energize-to-trip for safety.
• Routine checks – Function-test relays during scheduled maintenance. A yearly verification with a loop calibrator typically keeps it compliant.
• Cleaning & updates – Light dusting only; avoid solvents. If a firmware or option update exists for this option code, schedule it during planned downtime.

Quality & Certifications

• Certifications – CE and RoHS are typical for this class; UL/CSA recognition may apply depending on the option code.
• Manufacturer quality system – Produced under ISO 9001 quality management at Yokogawa facilities.
• Warranty – 365-day warranty coverage as stated above.

Related and Supporting Components

• 24 V DC DIN-rail power supply modules (sized with headroom for inrush and cabinet expansion)
• DIN-rail signal isolators or repeaters for long cable runs or noisy environments
• Surge protectors on field lines to improve relay contact life in harsh electrical areas
• Panel accessories: labeled terminals, fuse holders, and a compact annunciator if local alarm visibility is needed

One thing I appreciate is how quickly teams can drop a unit like the ALR121-S50 into an existing loop to solve a real nuisance alarm problem without touching the PLC codebase. If you’re matching an installed base, sharing the exact option string from the nameplate helps confirm input scaling and relay form before shipment.