Understanding the Role of Interposing Relays
In industrial automation, electrical isolation and signal matching are critical when connecting different control devices. An interposing relay acts as an intermediary between systems that operate at different voltage levels, current capacities, or grounding references.
This relay ensures safe and reliable communication between a Programmable Logic Controller (PLC) and field devices such as contactors, motors, or sensors.
Why Interposing Relays Are Used in PLC Systems
In a typical PLC control system, the controller may output 24 V DC signals, while the field device—like a motor contactor—may require 120 V or 230 V AC.
Directly connecting these devices would result in damage or malfunction. Therefore, an interposing relay is installed between the PLC and the field device to match electrical levels and protect components.
The relay’s coil operates on the PLC’s low-voltage output, while its contacts handle higher-voltage circuits safely and effectively.
Example 1: Driving a Contactor from a PLC Output
Consider a scenario where a PLC output provides 24 V DC, but the contactor coil requires 230 V AC. The solution is to use an interposing relay with a 24 V DC coil and 230 V AC contact rating.
When the PLC energizes its output, it activates the relay coil. The relay contacts then close and allow the 230 V AC circuit to energize the contactor coil.
This method isolates the two circuits, ensuring the PLC only handles low-voltage logic signals while the relay manages the high-voltage load.
Example 2: Handling Higher Current Loads
Sometimes the issue is not voltage but current capacity. For example, if a PLC relay can handle only 1 A at 110 V AC, but the connected load requires 3 A, an interposing relay provides the necessary interface.
In this case, the interposing relay’s contacts are rated above 3 A, and its coil operates within the PLC’s limits.
As a result, the PLC safely controls higher-current devices without overloading its internal relays or output modules.
Signal Amplification and Isolation Functions
An interposing relay does not perform logic functions. Instead, it amplifies electrical signals and provides galvanic isolation between circuits.
For instance, a small signal from a low-power switch can trigger the relay coil, which then switches a much larger load.
This concept is common in both automotive and industrial applications, where low-power controls manage high-power devices safely.
Automotive Analogy: The Starter Solenoid
A familiar example is the starter solenoid in a vehicle. The ignition key switch sends a small current to the solenoid, which then activates the high-current starter motor.
Without this interposing device, the key switch would need to handle hundreds of amps, making it impractical and unsafe.
Similarly, in industrial systems, the interposing relay protects the PLC from exposure to high power while enabling it to control demanding loads.
Interposing Relays for Input Signal Conversion
Interposing relays are also used on the input side of PLC systems.
For example, a 24 V DC proximity sensor might need to trigger a 120 V AC PLC input. Direct connection would not work due to voltage mismatch.
When the sensor detects an object, it energizes the relay coil at 24 V DC. The relay’s contacts then close the 120 V AC circuit, activating the PLC input channel.
This ensures proper signal transfer and electrical isolation between the sensor and the controller.
Importance of Commutating Diodes
When using DC-powered relay coils, it is essential to include a commutating diode in parallel with the coil.
The diode dissipates the coil’s stored energy when the circuit turns off, preventing a high-voltage “kickback” that could damage the PLC output transistor or sensor driver.
The diode must be installed in reverse polarity relative to the power supply so that it conducts only during coil de-energization.
Output-Side Interposing Relays in PLC Applications
On the output side, an interposing relay allows a PLC to control devices that exceed its voltage or current limits.
For instance, a 24 V DC transistor output from a PLC can energize a relay coil, whose contacts then control a 120 V AC contactor coil.
This configuration provides both electrical isolation and safe operation, ensuring that the PLC remains protected from electrical surges and overloads.
Best Practices for Implementing Interposing Relays
When selecting or wiring an interposing relay, engineers should consider coil voltage, contact ratings, and switching frequency.
Using industrial-grade relays from trusted brands such as Omron, Phoenix Contact, or Schneider Electric ensures long-term reliability in harsh environments.
It is also important to include protective diodes, fuses, and proper grounding to enhance system safety and longevity.
Expert Insights: Why Interposing Relays Remain Relevant
Despite advances in solid-state interfaces and smart I/O modules, interposing relays continue to play an essential role in industrial automation.
They offer flexibility, easy troubleshooting, and proven reliability—qualities still unmatched in many modern systems.
As control systems integrate with IIoT platforms and edge devices, these relays ensure compatibility across mixed-voltage architectures, especially in retrofitted or hybrid automation setups.
Application Scenario
In a manufacturing plant, PLCs often control motors, pumps, and solenoids with different voltage requirements. Interposing relays act as the bridge, allowing a single PLC to safely interface with diverse devices.
For example, a 24 V DC PLC output may operate relays that switch 120 V AC contactors, while another set of relays adapts input signals from 48 V DC sensors.
This modular approach simplifies maintenance, standardizes wiring, and protects sensitive control electronics from electrical stress.
Key Takeaways
Interposing relays isolate, protect, and adapt signals between mismatched control devices.
They enable safe operation across varying voltage and current levels while extending the life of PLC components.
Although simple in design, their role in maintaining electrical safety and control reliability remains fundamental to industrial automation systems.