Product Overview

The Schneider Electric SR2B121JD is a Zelio Logic compact smart relay intended for small automation systems that need programmable control, basic analog measurement, and time-based operation. It operates from a 12 V DC supply and combines eight discrete inputs, four of which support compatible 0–10 V DC analog signals, with four relay outputs. An integrated LCD display and real-time clock allow the controller to handle both input-driven sequences and scheduled functions.

The 12 V DC supply makes the SR2B121JD suitable for control systems built around a compatible low-voltage DC source. Its discrete inputs can receive signals from suitable switches, sensors, and auxiliary contacts, while the analog-capable inputs can process voltage signals from compatible measuring devices. This mixed-input arrangement is useful where a small machine needs a limited amount of process monitoring alongside ordinary on/off control.

For example, a compact tank-control system could use float switches to indicate high and low levels while a compatible 0–10 V sensor supplies a continuous level-related signal. The smart relay can evaluate these conditions and control a pump contactor through a relay output. The remaining outputs can be assigned to alarms or auxiliary equipment according to the application program.

The built-in real-time clock supports time-dependent logic. A small ventilation or lighting system can combine scheduled operation with manual commands and equipment status signals. The controller can coordinate these functions within its programmed logic without requiring a separate timer for every operating condition.

The SR2B121JD includes an LCD display and navigation buttons for local access to supported device functions. Programs can be developed using Zelio Soft, with ladder logic and function block diagram programming available for supported applications. Ladder logic suits technicians familiar with conventional relay circuits, while function block diagrams can help organize comparisons, timing operations, counters, and other control functions.

Its four relay outputs switch compatible external control circuits. They can operate interface relays, contactor coils, and other loads within the applicable contact ratings. For motor-driven equipment, the smart relay normally provides the control command while a properly rated contactor and protective devices handle the motor’s power circuit.

Program data is retained in EEPROM memory for up to ten years at the specified reference temperature of 25°C. This helps preserve the stored application through a supply interruption. A separate copy of the approved program is still valuable for maintenance, troubleshooting, and controller replacement.

When selecting the SR2B121JD, the analog input arrangement deserves particular attention. The four analog-capable inputs accept compatible voltage signals, not direct 4–20 mA current signals. A current-output transmitter therefore requires a suitable signal-conditioning or conversion arrangement if it is to be used with these inputs.

The controller also has a limited number of onboard I/O points. Its 12-point configuration is suited to compact machines and utility panels, but applications requiring many field devices, extensive communications, or advanced motion control may need a larger controller.

The SR2B121JD is best suited to small automation tasks where a 12 V DC supply, basic voltage measurement, relay-based switching, and scheduled operation are all useful within one compact controller.

Technical Specifications

Product Features

• 12 V DC power supply: Suits compatible low-voltage DC control systems.
• Eight discrete inputs: Receives suitable on/off signals from field devices.
• Four analog-capable inputs: Supports compatible 0–10 V DC measurements.
• Four relay outputs: Switches external control circuits within the specified contact ratings.
• Built-in real-time clock: Allows supported programs to include scheduled operating periods.
• Integrated LCD display: Provides local access to supported device information and functions.
• Ladder logic and FBD programming: Supports different approaches to implementing the control sequence.
• EEPROM program retention: Preserves stored program data for up to ten years at 25°C.
• DIN rail or screw mounting: Supports installation in suitable control enclosures.
• Mixed-signal control: Combines discrete inputs, basic voltage measurement, and relay switching.

Working Principle

The SR2B121JD reads its discrete input states and compatible analog voltage signals, evaluates the stored control program, and operates its four relay outputs according to the programmed conditions. The real-time clock allows supported logic to incorporate scheduled operations.

12 V DC Supply → SR2B121JD → Program Execution → Relay Outputs → External Control Circuits

The eight discrete inputs receive signals from compatible field devices, such as push buttons, limit switches, float switches, and auxiliary contacts. These signals indicate operating commands or equipment states. The program uses them to determine whether a particular action is permitted.

Four inputs can also be used for compatible 0–10 V DC analog measurements. A suitable sensor can provide a voltage corresponding to a process variable such as level or position. The controller can compare the input value with programmed thresholds and use the result to activate an output or change the operating sequence.

For example, a pump-control program may start a pump when a level measurement crosses a configured threshold, provided the required interlocks are satisfied. A separate discrete input can indicate a manual stop command, while another can report the state of an external contactor. The controller evaluates these conditions together rather than relying on a single input.

The program can be created using ladder logic or function block diagram programming. Ladder logic represents conditions through contacts, coils, and supported logic elements. Function block diagrams use connected blocks to implement operations such as comparisons, timing, and counters.

When an output condition becomes true, the associated relay contacts change state and switch the external control circuit. A relay output may energize a contactor coil, which then switches the motor’s power supply. The smart relay’s contacts must remain within their electrical ratings; motor switching and protection must be handled by suitable external equipment.

The real-time clock enables supported programs to use time-based functions. A ventilation system, for example, can enable operation during programmed periods while retaining manual controls and equipment interlocks. The clock supports scheduling but does not replace process monitoring or required safety arrangements.

The analog inputs require compatible voltage signals. A 4–20 mA transmitter cannot be connected directly as though the controller had current-input capability. Where a current-output transmitter is used, suitable signal conversion is necessary to produce a compatible voltage signal. Input wiring, signal common connections, and the source’s voltage limits also affect measurement accuracy.

During commissioning, technicians should test the discrete inputs, compare analog readings against known signal values, and verify that each relay output responds to the intended program conditions. The clock settings should be checked where the application uses scheduled operation.

EEPROM memory retains the stored program for up to ten years at 25°C. After a power interruption, the machine’s restart behavior depends on the program and the states of the external circuits. The control arrangement should prevent unexpected movement or hazardous restart.

When an analog value appears incorrect, inspect the sensor output, signal wiring, and program comparisons. If an output does not operate, check the input conditions and logic before investigating the relay contacts and external load. This separates measurement faults from program and switching faults.

Role in the Industrial Control Architecture

The SR2B121JD serves as the local programmable controller between field sensors and external control circuits. It processes discrete signals, evaluates compatible analog measurements, applies clock-based logic, and switches its relay outputs.

Discrete Sensors and 0–10 V Signals → SR2B121JD → Relay Outputs → Contactors and Control Loads

The smart relay handles the programmed sequence, while external switching devices manage the controlled loads. The signal types, power-supply arrangement, output ratings, and required safety functions must all be accommodated in the final control architecture.

Industrial Applications

Installation and Maintenance

1. Verify the model reference: Confirm that the unit is SR2B121JD and that its 12-I/O arrangement matches the application.
2. Prepare the control enclosure: Provide suitable mounting, wiring access, and environmental conditions for the device.
3. Install the controller: Use a compatible 35 mm DIN rail or the supported screw-fixing arrangement.
4. Check the DC supply: Verify that the supply voltage is within the specified 10.4–14.4 V DC range.
5. Identify discrete input circuits: Label each field signal and confirm that its voltage and wiring arrangement are compatible.
6. Check analog signal wiring: Confirm that each analog source provides a compatible 0–10 V DC signal and that the common wiring is correct.
7. Wire the relay outputs: Follow the terminal diagram and observe the applicable contact and total output-current limits.
8. Develop the control program: Implement the required sequence using supported ladder logic or function block diagram programming.
9. Configure clock functions: Set the real-time clock and required schedule parameters where time-based operation is used.
10. Test the input channels: Verify discrete input states and compare analog readings with known signal values.
11. Verify output responses: Test the relay outputs, external contactors, interlocks, and power-restoration behavior under controlled conditions.
12. Maintain program records: Keep the approved application, I/O assignments, analog settings, clock configuration, and wiring diagrams.

If an analog reading is incorrect, inspect the signal source, wiring, and program interpretation. If a scheduled operation does not occur, check the clock configuration and the relevant logic conditions. Isolate the appropriate electrical supply before carrying out wiring or maintenance work.

Related Components

Recommended Related Models

Key Advantages

• Combines eight discrete inputs, four analog-capable inputs, and four relay outputs.
• Operates from a 12 V DC supply.
• Supports compatible 0–10 V DC process measurements.
• Includes a real-time clock for scheduled operating sequences.
• Provides an integrated LCD display for local interaction.
• Retains stored program data in EEPROM under the specified conditions.
• Suits compact automation systems that combine measurement and relay-based control.

Technical FAQs

What supply voltage does the SR2B121JD require?

The SR2B121JD operates from a nominal 12 V DC supply, with specified supply limits of 10.4–14.4 V DC. The power supply must remain within this range during normal operation and account for the requirements of any connected control equipment supplied from the same source.

How can the analog inputs be used with a process transmitter?

The four analog-capable inputs support compatible 0–10 V DC signals. A transmitter that produces a 4–20 mA output requires a suitable signal converter or conditioning arrangement before its measurement can be used by the smart relay.

What does the built-in real-time clock allow the controller to do?

The clock supports time-dependent program functions, such as scheduled operation of ventilation or lighting circuits. These functions can be combined with discrete inputs and interlocks to coordinate equipment operation.

What should be considered when selecting the SR2B121JD for a pump-control panel?

Check that the available control supply is compatible with 12 V DC, that the sensors provide suitable input signals, and that the four relay outputs can accommodate the required control circuits. The motor itself should be switched through appropriately rated power equipment with suitable electrical protection.