Difference between Relays and PLCs

Relays are electro-mechanical switches that have coil and two types of contacts that is NO & NC. But a Programmable Logic Controller, PLC is a mini computer which can take decision based on the programme and its input & output. Thus a PLC can perform many functions with the same hard wiring. A simple architecture of PLC is shown in figure below.

But for performing many functions using Relays we need to have so many hard wiring. Basic functions of both Relay and PLC is same that is controlling electro-mechanical systems in the industries. Simple tasks can be performed by both efficiently for example running / stopping  an electric motor after sensing an overload or fault. But job of PLC is extended to some more functions that is required for more accuracy / transparency in the production and less human interference in the system to avoid delay in the production.

Relay are electro-mechanical switches that have coil and two types of contacts that is NO & NC. When NO contact is ON it will pass the current to the load, when NC contact is ON it will stop passing the current to the load. NO keeps the circuit open and NC keeps the circuit closed. If they are ON they do opposite job. When current passes through coil of relay it makes the contacts ON.

For better understanding, we consider an example and see how Relay and PLC work.

Consider a task in an industry. An object to be moved from one place to another place. That would be done generally by Conveyor system. A electric motor would be coupled with conveyor belt so that when motor will run, conveyor will move and any object placed on that conveyor will be shifted from one place to another. To automate the task we mount a sensor at starting edge of the conveyor belt so that when object is placed on it, it will be sensed and conveyor will run.

Now here how relay will perform this task?

Sensor will be wired with coil of Relay and Power Supply. NO contact of relay will be wired to the electric motor. When sensor senses objects it makes the current from Power Supply to pass to the coil by which coil gets energized and it makes NO contact NC as the Relay picks up. NC contact closes the circuit of Power Supply to motor by which motor runs and conveyor moves.

How PLC will do this task?

PLC has both NO and NC contacts. But unlike Relay PLC is a solid state industrial computer which has a CPU & inputs / outputs. The logic we make above to run a conveyor system by doing some wiring to Relay, sensor, motor etc. can be made in PLC but it would be done by some programming language in the software dedicated for PLC, then it will be downloaded to the CPU of PLC. Connect the sensor to input of PLC and output to the motor. The input where we connect the sensor can be used as NO or NC contact in the PLC programming. Now the linking of sensor input to directly to motor output can be done by writing some code or more commonly by graphical diagram based program for example ladder diagram or Function block diagram.

This was simple task where one NO contact was used but in the industries each machines will have hundreds of NO, NC and even thousand in bigger machines. If we develop the control logic through Relay maintaining such big numbers of contacts are very tough job. Whereas those NO & NC are housed in a rather small PLC if we use PLC. Therefore we get many advantages in the control system by having less wiring.

Along with above basic job some modules can be incorporated into PLC that can sense analog signal, also can help to communicate/ transfer data from one PLC to another PLC / HMI / SCADA / DCS and many more.


PLC – Programmable Logic Controller

HMI – Human Machine Interface

SCADA – Supervisory Control and Data Acquisition

DCS – Distributed Control System

Leave a Comment