What is Interposing Relay in a PLC System?

In the automation sector, interposing relays are commonly utilized to isolate field and automation equipment ( IO modules). Interposing results are covered by NFPA 70e. Use a relay with an inverse EMP blussing diode to create a galvanic isolation between a control system (PLC) and external utilities with higher voltages.

The auxiliary relay known as an interposing relay is used to isolate two separate systems or devices. This could be due to the fact that they have different 0V references, voltages, and AC vs. DC.

Interposing Relay

We discuss the interposing relay with two cases, as discussed below:


Let’s say we wish to control a contactor with a 230 V AC coil voltage, but the PLC relay output voltage is 24 V DC. In this scenario, an interposing relay with a coil voltage of 24 V DC and a contact rating of 230 V AC is required.

As a result, the PLC relay will run the interposing relay first, and then we may easily operate the contactor using its auxiliary contacts.


Let’s say a PLC’s relay can only provide 1 A at 110 VAC, while the Controller that will be attached to the relay requires 3 A at 110 VAC.

As an interposing relay between the PLC relay and the Controller, an interposing relay having contacts rated for operation at 5 A(>3 A) at 110 VAC would be employed.

The interposing relay’s coil should require less voltage.


Electromechanical relays can be utilized as interposing devices between mismatched sensors, controllers, and/or control devices in addition to providing direct logic duties.

The following circuit schematic shows a very simple example of a relay used to interpose between mismatched components, where a fragile toggle switch is used to control a bank of high-power lights for an off-road vehicle.

The relay in this circuit has no logic function at all. It simply amplifies the signal transmitted by the dashboard toggle switch to send or stop electricity to the bank of high-watt lights.

To securely and reliably make and break the light circuit without the relay, a much heavier-duty toggle switch would have to be fitted in the dashboard of this vehicle.

The employment of a solenoid in the electric starting motor circuit for an internal combustion engine is another example of an interposing relay utilized in automotive applications.

The driver normally activates the start control switch by twisting a key located on the steering column or dashboard of the car. Meanwhile, the starting motor consumes hundreds of amps of power.

The solenoid relay that connects the key switch to the starting motor relocates the risk, allowing a relatively delicate key switch to safely activate the high-power motor.

A DC output proximity switch must trigger an input channel to a Programmable Logic Controller (PLC) rated for 120 volts in this industrial example of an interposing relay between mismatched components.

The proximity switch is used to drive one of the PLC input channels, hence the relay in this system does not execute any logic.

Because this specific PLC input requires 120 volts AC to activate, and our proximity switch works on 24 volts DC, attaching the proximity switch directly to one of the PLC’s input channels is not a viable choice.

Because of the voltage difference between the switch and the PLC input voltage, we must utilize the relay to interpose between the switch and the PLC.

When the proximity switch detects a nearby object, it activates its output, which energizes the relay coil. When the relay contact closes magnetically, it completes a circuit for 120 volts AC to reach PLC input channel 0 and energize it.

Without this diode, the proximity switch’s output transistor will be destroyed by the coil’s kickback voltage (which can reach hundreds of volts in potential).

It’s worth noting that this commutating diode appears to be connected backwards in terms of the polarity of the 24 volt DC power source, with the cathode facing the positive pole and the anode facing the negative pole.

When the proximity switch is turned off and the magnetic field of the relay coil falls, the diode only turns on when the polarity reverses (now acting as a source rather than as a load).

The diode gives the relay coil a continuous channel for its current while lowering a modest voltage (approximately 0.7 volts DC), dissipating the coil’s shunt.

Mismatched PLC outputs and control devices are also connected using interposing relays. The mismatch in this application could be in terms of voltage and/or current ratings.

The role of the relay in an output interposing circuit, like the input interposing circuit, is to be controlled by the PLC’s output channel and then transfer power to a field device that is incompatible with the PLC’s output.

An interposing relay connected to a PLC output channel is shown in the diagram below:

The transistor outputs of the PLC in this design can only tolerate 24 volts of DC at a modest current. Because the three-phase contactor coil requires 120 volts AC at low current levels to operate, the relay acts as a buffer between the PLC’s low-voltage and low-current output channel and the contactor’s coil’s comparatively high-voltage and high-current demands.

When the PLC de-energizes the relay coil, a commutating diode is used to dissipate the stored energy so that the subsequent kickback voltage does not destroy the PLC’s sensitive transistor output circuitry.


1. Interposing Relays are commonly used in Industrial Automation Systems to regulate motors and high-powered lights.

2. An interposing relay is a device that connects digital and analog circuits.

3. An interposing relay is a device that is employed as a driver in a digital control system, such as a motor driver, a light driver, or an electrical contactor driver.

4. Industrial protection systems and switchgear systems both use interposing relays. It’s used to connect low-voltage, low-current control equipment to high-voltage, high-current Circuit Breakers, for example.

5. Interposing Relays are used to regulate high-voltage, high-current isolators, actuators, and other low-voltage, low-current PLC systems, microcontrollers, and other devices.


1. It can provide strong isolation and separation between two distinct circuits, ensuring that no disturbance propagates from one to the other. Safety and security are also ensured.

2. It’s also suitable for usage in digital circuits.

3. It operates at a faster rate because it is a static device.

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