Working of Interposing Relays in PLCs

Why is it needed to use Interposing Relay?

Interposing relays are used between mismatched sensors, controllers, and/or control devices. To control the action of high power circuits, we cannot drag the high power lines to the control panel, as it is costly and dangerous. So interposing relays are used to control the status of high power lines.

Interposing relay use in PLC system:

Interposing relay system protects the PLC panel from overcurrent in field affecting internally the PLC. Directly connecting the proximity switch to one of the input channels of the PLC is not a practical option.

In this particular PLC shown above input requires 120 volts AC to activate, and our proximity switch operates on 24 volts DC. The mismatch between switch voltage and PLC input voltage requires us to use the relay to “interpose” between the switch and PLC. When the proximity switch senses an object nearby, its output activates, which in turn energizes the relay coil. When the relay contact magnetically closes, it completes a circuit for 120 volts AC to reach input channel 0 on the PLC, thereby energizing it.

A commutating diode in parallel with the relay coil, the purpose being to dissipate the coil’s stored energy upon de-energization when the proximity switch turns off. Without this diode in place, the coil’s “kickback” voltage will destroy the proximity switch’s output transistor.

The same effect can happen at PLC output circuit. Transistor outputs can only handle 24 volts DC, and at fairly low current, which is insufficient for the working of many devices.

Here in the example shown above three-phase contactor3 coil requires 120 volts AC at modest current levels to function, and so the relay interposes between the PLC’s low-voltage and low-current output channel and the relatively high-voltage and high-current demands of the contactor’s coil.

Once again we see the use of a commutating diode to dissipate the relay coil’s stored energy whenever the PLC de-energizes it, so that the resulting “kickback” voltage does not damage the fragile transistor output circuitry within the PLC.

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