Types of the PLC

In this article, I will cover the topic on types of PLC available in the market. In the previous article, you get to know about What is PLC and what component makes up the PLC. It is important to know every single detail before proceeding to the next topic. Click on red colour text to know more.


There are two types of PLC available in the market,


1. Compact PLC

2. Modular PLC


Let understand each one in brief.


Compact PLC:

  • Compact PLC contains all the module in a single case.

  • So, you can not expand the module in case of a new component installed in the field.

  • SMPS has come inbuilt convert incoming power of 230 VAC to 24 VDC which PLC requires to operate.

  • Fixed number of input and output is used decided by the manufacturer.

  • Such PLCs are used on a very small scale of application where expansion doesn't require.

  • Following is the image of Allen-Bradley compact type of the PLC. As you can see you can not expand any I/Os. If you are looking to learn in great detail then click here.

Modular PLC:

  • In modular PLC you can add expansion module,

  • It does not contain inbuilt SMPS. you must have to use a separate one to convert supply from AC to DC.

  • Whenever you buy a modular PLC then buy PLC with an extra 50% to 60% room because when you are adding a new system to your existing plant you can add easily when you have space available in your PLC.

  • In case of any faulty module, you can replace that single faulty module instead of replacing whole PLC that reduce a cost and time.

  • Modular PLCs are used in a medium and large application.

  • Following is the image of modular PLC of Siemens s7-300 model.


Transistor type PLC:

  • Transistor type of PLC is used where fast action, positioning and counting is required.

  • In servo application, transistor type PLC is used to record pulses.

  • Care must be taken before choosing PLC based on application. If you commit a mistake in selecting the PLC, it going to cost you a lot.

  • Transistor output is a solid-state device which can handle small to medium amount of the current.

Relay type PLC:

  • In relay type of PLC, outputs are mechanical contacts.

  • Relay type PLC can use with both AC and DC supply.

  • Mostly used in all application except high-speed application.

  • Relay outputs are used to control current up to 2 Amps.

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