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What is The Difference Between a Relay and a PLC?

A PLC control system and a relay control system are comprised of an input, output, and control section. A relay refers to a mechanical switch that operates using electromagnetic signals. It is used to connect several circuits controlled by one signal. A programmable logic controller (PLC) is an example of a physical real-time system that produces output results in response to input conditions within a limited period.

Early PLCs resembled a schematic diagram of relay logic and were programmed in ladder logic.

Instrument technician on the job check wiring on PLC cabinet
However, modern PLCs can be programmed in several ways such as programming languages from relay-derived ladder logic. This article looks at some of the differences between a relay and a PLC and the way each of them works to execute certain functions.

What is Relay?

A relay is an electro-mechanical device comprised of a relay coil and a relay contact. It works by turning electric circuits on and off to create logic operations that provide process control to the system. They can use a network of electric circuits to perform logical operations when wired in combination. A relay coil is used to produce an electromagnetic field that attracts metal while a relay contact is a switch that turns an electric circuit on and off. The purpose of the relay coil is to alter the state of the relay contacts using an electrical signal. The fundamental elements of relay logic circuits include a power supply, relay components, and connection wires.

What is a PLC?

A PLC is a programmable device that operates on the same hard wires. It differs from a relay logic that is comprised of a network of hardwired electrical devices. Although both systems can perform logic computation, a relay logic does it using electric circuits while a PLC does it using a microprocessor. However, the wiring in a PLC is greatly reduced because the only devices that require hardwiring are the input and output devices. The ladder logic program that is stored within the PLC memory replaces the control relays used to form the logic functions and control functionality.

How do PLCs Differ from Relays?

  • Remote Control

PLCs can be used to turn on things from a distance using the same wiring. Although a relay can also be used for this purpose, it requires a lot of wiring to achieve the same task as a PLC. You can run a pair of fiber optics and control several circuits remotely with a PLC system. However, with relays, each of these circuits needs to be wired back to the control room and a switch thus may require periodic testing and maintenance.

  • Interlocks

A PLC can prevent operators from performing dangerous or illegal actions. Although it is easy to implement interlocks in relays, they often change when the project goal changes or the system is reconfigured. Changing interlocks on a relay system can be quite difficult because it needs rewiring and updating schematics. A PLC is more flexible and easier to use with interlocks since the system can be upgraded without rewiring.

  • Process Control

Process control refers to closed-loop control of a process. For instance, regulating a heater to maintain a tank at a specific temperature. It is quite difficult for a relay-based system to perform this action because it would require additional circuitry such as amplifiers, signal conditioners, comparators, and probably a digital process control device. On a PLC system, you can connect analog sensors directly to the PLC to provide signal conditioning, calibration, and scaling.

  • Sequencing

Sequencing refers to automated actions that take place after a particular event or a specific time delay. Both digital and analog timer relays are not only expensive but also require several manual adjustments to ensure the timed sequence works correctly. Although they may work just fine for a well-defined sequence, things get more complicated when you try to implement a branching or complex sequence to a relay. On the contrary, it is easy to update timers in PLCs because they cost less and only require a little time to update the code with the help of a software engineer and technician.

  • Monitoring

A PLC can collect information of the time between events such as recording pressures and temperatures during an event and the duration a pump has been running. It is hardly possible to achieve this with a relay-based system because it is expensive and requires constant modification and maintenance once the control system changes. It is easy to write software that records the available data to the PLC without the help of additional hardware.

A PLC-based system is better than a relay system as it can perform several tasks that require constant improvement or modification. Although you can build a relay control system to accomplish these tasks, you may find yourself spending more time and money once you start controlling more devices or complex processes.

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