Difference between Relay and PLC

Both relay and PLC are devices that are used in the electrical and electronic control systems for the automation of processes.

  • A control system that is designed by using relays is known as relay logic control (RLC).

  • Similarly, a control system that is designed by using a microcontroller and other solid state switching devices is known as programmable logic control (PLC).

In this article, we will discuss all the major differences between relay and PLC by considering different parameters such as basic definition, components, function, memory, I/O, applications, etc. But before discussing the differences, let's start with some basics of Relay and PLC so that it becomes easier to understand the differences between them.

What is a Relay?

An electrically operated switch which is used for switching a high power circuit with the help of a low power circuit is known as relay. Hence, in simple words, a relay is nothing but an electrically operated switch.

A typical relay consists of a set of input terminals and a set of operating contact terminals. Where, the input terminals are provided for inputting a single or multiple control signals.

Based on construction and design, there are two types of relays namely electromagnetic relays and solid-state relays. An electromagnetic relay consists of a coil wound around a soft iron core, where the iron core provides a path of low reluctance for the flow of magnetic flux. It also consists of a movable armature and a set of contacts. On the other hand, the solid-state relays do not consists of any moving part, they use semiconductor devices instead of electromagnets.

The relays are used in such applications where it is required to control the operation of an electrical or electronic circuit by a low power electric signal or where a number of circuits have to be controlled by one electric signal. Some practical applications of relays are in electronic circuits, home appliances, signalling and control in railways, motor control circuits, power generation and transmission systems, etc.

What is a PLC?

PLC stands for Programmable Logic Controller. A PLC is basically a microcontroller device that is used for automation of processes in industries.

A typical PLC consists of input module, processing element, program (PLC software), power supply and output module. A programmable logic controller (PLC) receives input signal from the input devices such as sensors and processes these input signals based on the instructions to produce the desired outputs.

A mechanism called HMI (Human Machine Interface) is used to interact with the PLC in real time. The HMI provides an interface on a digital screen through which a user can input the instructions in the PLC and can change the system settings.

A PLC is programmed by using different types of programming language such as block diagram, ladder logic, structural programming, etc. to perform a specific task. In practice, the PLCs come in two versions namely Compact PLC and Modular PLC. The PLCs are the control devices that provide more flexibility and robustness in the control systems. The PLCs are used in different industries such as gas and oil, power production, petrochemical, biomedical, cement manufacturing, etc.

Difference between Relay and PLC

Both relay and PLC are the devices that are used for industrial automation, however there are several differences between them that are given in the following table −

Basis of Difference Relay Programmable Logic Controller (PLC)
Definition A relay is an electrically operated switch that is used to turn ON / OFF a high power circuit by using a low power signal. A PLC or programmable logic controller is a semiconductor based switching device which is used to control the industrially processed automatically.
Primary function The main function of a relay is to control the switching operations in the circuit. The main functions of a PLC are monitoring and controlling of the circuit.
Types The relays are broadly classified into two types namely electromagnetic relays and solid state relays. The PLCs are also classified into two types namely compact PLC and modular PLC.

An electromagnetic relay consists of an electromagnet, an armature, set of input terminals and set of control terminals.

While, a solid-state relay consists of semiconductor based mechanism for switching of the circuit.

A typical PLC consists of an input module, a processing element, application (PLC program), power supply and output module.
Operation In case of relay, when a low power control signal is provided at the input terminals, the armature of the relay moves under the influence of magnetic-field of electromagnet to turn on and off the circuit. When input signal is received from the input devices like sensors, the processing device makes decision and releases an output signal through the output module.
Nature of operating signal Relays operate on the analog signals. PLC operates on the digital signals.
Name of control system The control system designed by using relays is known as relay logic control. It is also called hard-wire control system. The control system in which PLC is used is known as programmable logic control.
Programming Relays do not require programming to work. PLCs are programmed to work. There are different programming languages available for PLC programming.
Modification in circuit design
It is comparatively difficult to modify the circuit design of a relay circuit. The circuit design of a PLC can be easily modified.
Memory Relay does not have memory. PLC has a memory module to store the program.
Fault determination It is difficult to detect the fault in a relay circuit. It is very easy to determine the fault in a PLC based circuit by using software.
Flexibility Relay provides a less flexible control system. The flexibility of a PLC base system is comparatively more.
Input output capability A relay has relatively less I/O capability. PLC has more I/O capability.
Response time The response time of a typical relay is around 10 ms or less. The response time of a PLC is around 50 ms or above.
Advanced functionality A relay performs only fault detection function. And it does not have any other advanced functions. PLC can also be programmed to perform several advanced functions like timer, counter, etc.
Applications Relays are used in motor control circuits, lighting controls, traffic control system, power system protection, home appliances, automotive, etc. PLCs are used in cement manufacturing, transportation systems, paper production, glass production, temperature control systems, smart grid for fault detection, power generation and transmission, smart traffic control systems, elevators and lifts, fire detection system, etc.


The most significant difference that you should note here is that a Relay is a simple electrically operated switch, while a PLC is a smart programmable switching device. Both are extensively used in different electrical and electronic systems in different industries.