Differences between PLC and programmable smart logic relay


Welcome back to my blog! Today I would like to focus on the differences between PLCs and programmable relays. I hope you will enjoy it!


Surprisingly, the topic is not as complex as it may seem at first glance 🙂 In the past, control systems were implemented with relay elements. They were often drawn on sheets of different formats, often with random dimensions. That is why at that very stage some simplified and standardized solutions were sought after 🙂


The first control systems were built on the basis of electro-mechanical systems, i.e. on the basis of different types of relays, timers and counters. Their main drawback was the fact that such systems simply took up a lot of space. In addition, these systems were highly unreliable. Further problems included varying and, at the same time, significant propagation times of individual elements, contact oscillations, low number of relay contacts, problems connected with uneven wear and tear of relay contacts.


Introduction of PLCs along with programmable relays reduced the cost of designing and building control systems. The time needed to deploy the system has decreased, service has been simplified, and system reliability has increased.


What is a PLC?



Programmable Logic Controllers (PLCs) are microprocessor systems (industrial computers) that are under the control of a real-time operating system and have been designed to:


  • Collect measurements from input modules, from digital and analog units, sensors and measuring devices
  • Execute user programs containing encoded control and data processing algorithms by means of data about processes or machines obtained before 
  • Generate control signals relevant to the calculation results of those programs and pass them through the output modules to the components and actuators


In addition, they have the capability to:


  • Transmit data by means of modules and communication links
  • Carry out program and hardware diagnostics


In other words, these controllers constitute, in fact, the central unit which receives and processes signals sent from the environment and sends them on to individual receivers. The logic behind it makes them easy to interact with the environment.


Among the most popular manufacturers of PLCs, the following will definitely stand out:


  • Mitsubishi
  • Allen-Bradley
  • Omron
  • Schneider
  • ABB


What is a programmable smart logic relay?



Programmable transmitters can be defined as a group of one of the smallest compact controllers serving up to a dozen of digital inputs/outputs, typically of relay type (rarely analogue ones), often integrated with a small display. They are not very flexible, but, at the same time, they are much cheaper than PLCs.


A programmable relay can be specified as a universal switch and control device used for home and industrial applications. The most popular programmer on the market is definitely LOGO! by SIEMENS but that does not mean that it is the best there is. Brands such as TECO, Easy, NEED, Omron, etc. should also be taken into consideration. 


What exactly do we use them for? A relay gives us the ability to control everyday devices such as lighting, blinds, shutters, radiators and air conditioners, etc. Programmable relays can also be seen in the industry, e.g. during the construction of control and automation systems. Their application in the industrial use is dependent on legal admissions and standards.


In a nutshell, programmable relays constitute a truncated version of the PLCs. It is possible to use them in order to perform simple functions, but the limitations mentioned above restrict them from being used in more complex processes. Relays are mainly set for the management of binary inputs and outputs, as well as for simple logical functions. In addition, it is possible to use analogue inputs and perform operations on them, but rather in small quantities. The number of digital inputs and outputs used is not as impressive as one would have hoped for, even in the most advanced models.


Finally, one should keep in mind that the price of an extended version of a programmable controller will be similar to the price of a simple PLC. 

Well, that’s all for now. I hope you enjoyed it and that I gave you some food for thought!

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