Know and Understand the Difference between PLC and Relay Control

Difference between PLC and Relay control
3 mins read

PLC (Programmable Logic Controller) control and relay control are the two most popular controls in the world of automation.

Although they both have comparable objectives, they function according to quite distinct philosophies. We shall examine the difference between PLC and relay control in this post, as well as their advantages, disadvantages, and common uses.


Top Differences Between PLC and Relay Control


PLC Control

Relay Control


A PLC is a digitally operating apparatus that can perform logic operations using input and output devices, CPU, and field devices.

Relays are electromagnetic devices. When you give voltage to the coil, the coil energizes and the contacts get ON/OFF just like a switch.

Main Function

PLCs are used in process plants to control and monitor the operation.

Relays are used to control simple circuits. Monitoring the operation using a relay is a difficult task.


PLCs need to get programmed. Ladder programming is the most popular programming language for PLCs.

Relays cannot be programmed. Relays need to get hardwired to get operated.


PLC system includes components like I/O Modules, communication modules, power supply, CPU, and some special function modules (for encoder connections).

The relay system includes wires and a physical relay.

Internal Construction

PLCs are solid-state devices.

A relay consists of a coil, normally open, and normally close contacts.


PLCs come with internal memory and are also expandable if needed.

Relays do not have memory.


With PLCs troubleshooting is easy. You need to check the program and find out the fault. However, skilled professionals are required to understand the ladder program.

Relay logic is easy to understand and does not require any extra skill to understand the relay logic. But when it comes to troubleshooting it is very hard with relay logic due to a lot of wires.

Change in Logic

PLCs allow the user program to change. It is very easy to change the logic without changing any kind of wiring.

Changing relay logic is a tedious task. The reason is, that you need to change the wiring every time if you want to change the logic.

Response Time

The response time of PLC ranges from 21 milliseconds to 50 milliseconds. The response time depends on the ladder program stored inside the PLC and the number of I/Os it needs to handle for the entire plant. PLCs do work of hundreds of relays at a time. So, in response time also PLCs win the race.

The response time of a typical relay is 10 milliseconds.


PLCs can perform arithmetic operations and have inbuilt timers and counters. Additionally, PLCs can also transfer the data.

Relays cannot perform arithmetic operations. Separate timers and counters are required. Cannot transfer the data.


PLCs are mainly of two types - Compact PLCs and modular type of PLCs.

Relays are mainly of two types - Electromagnetic relays and solid-state relays.

Understanding the difference between PLC and Relay Control

PLC Control


As per NEMA, PLC is a digitally operating electronics apparatus. It is having integrated memory system, a processor, and an operating system. PLCs can be programmed as per process requirements. Users can choose ladder logic language to program the PLC to perform complex process tasks.

You can establish connections of field devices such as pushbuttons, limit switches, sensors, flowmeters, temperature transmitters, control valves, etc. With the help of these inputs and outputs PLCs can handle complicated automation tasks and have a wide range of applications.

Advantages of PLC Control:


PLCs are made to be flexible. You can program the PLC for a variety of applications. They are appropriate for a variety of applications since they can be programmed to carry out a wide range of functions.

Complex Logic Handling:

PLCs are excellent at handling intricate logic operations. It has built-in timers and counters. They can run loops, conditional statements, jump functions, and arithmetic computations, enabling complex control schemes.


PLCs are compact. Although it does complex functions the size is small. It requires less space to get accommodated inside the Electric Panel.

Modularity and Scalability

Because PLC systems are modular, expansion and customization are simple. Because of this, they are appropriate for applications where the control needs may alter over time.

Data Processing and Communication

PLCs can process data and connect with other devices and systems, making it possible to integrate them with data collecting or higher-level control systems.

Disadvantages of PLC control:

Cost of the PLC

Relay control systems are often less expensive as compared to PLC control systems. The price includes the hardware, programming tools, and programming knowledge.

Required Programming Skills

Skilled engineers are required to program the PLC, for effective software creation, modification, and troubleshooting.

Applications of PLC

The list is huge but to mention a few you can use PLCs in –

Crane application

Conveyor system

Process plant

Chemical industry

Steel industry

Power plant



Tire manufacturing companies, etc.

Relay Control


A more established technique for industrial automation is relay control. To regulate the operation of machines or processes, electromechanical relays are used. There is a coil in the relay which needs an electrical signal. Once the signal is received the coil energizes and changes the NO and NC contacts. A relay can handle two separate voltage potentials.

Advantages of Relay Control


Relay control systems are comparatively easy to understand and use. Technicians can easily understand the logic since they do not require programming experience.

Less Cost:

Relay systems can be a cost-effective alternative for relatively simple control needs. They cost less to initially set up and have fewer components.

Disadvantages of Relay Control

Limited Logic Handling:

Relays have a limited capacity for performing intricate logic operations. They work well for straightforward on/off control jobs.

Less Flexibility:

Relay-based control systems can be more difficult to modify than PLCs. The reason is relay logic has a greater number of wires. If you want to change the logic, you need to change the wiring and rewire it as per the logic requirement.

Lack of advanced features:

If you want to transfer the data, relays cannot do that. They cannot process data and send it to other devices via communication protocols. Also, relays cannot do complex mathematical calculations like PLCs.

Applications of Relay Control

Relays can be used in combination with PLCs. You can use a relay where complex process control is not required.

Some examples of relay control are –

  • Simple ON/OFF control of motors.
  • Simple lighting control like street lights.
  • With PLC relay can be used to energize the starter circuit of the motor.
  • Electronics cards of home appliances, etc.


Both PLC control and relay control have certain advantages and work well for many industrial automation applications. The exact needs of the application, such as complexity, costs, and the need for additional functionality, will determine which option is best. Ultimately, the best control system will be chosen after a detailed analysis of the project’s requirements.



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