PLC vs Relay Logic Controller: In this post, we are going to discuss the differences between PLC vs Relay Logic Controller. Before the differences of PLC vs Relay Logic Controller, the basic definition, advantages & disadvantages of PLC and Relay Logic Controller are discussed below.

Definition of PLC

A Programmable Logic Controller (PLC) is a user-friendly microprocessor-based circuit that is used to conduct multiple and complex control operations.

A PLC is essentially microprocessor or microcontroller-based. It has multiple inputs and multiple outputs. It functions according to the software program.

PLC monitors crucial process parameters and controls the process operations accordingly.

The PLC replaces much-complicated wiring with a single circuit. It can select the on/off states of output devices. Analog output devices can also be controlled by PLCs.

The input to the PLC can also be taken from analog devices or discrete (on/off) devices.

The PLCs have tremendous computing capacity but at the same time, their size is very compact.

Advantages of PLC

1. PLCs are highly flexible.
2. They have compact size and reduced cost.
3. PLCs are accurate, high speed and reliable.
4. PLCs can be reprogrammed easily and quickly.
5. PLCs can handle large number of inputs and outputs.
6. Data storage and logging is easier since PLCs can be easily interfaced to computers.
7. Programming of PLCs is easier.

Disadvantages of PLC

1. For small operations or one or two functions, PLCs are not cost effective.
2. PLCs use electronic devices and microprocessors. They fail some times in adverse environmental conditions.
3. PLCs are not fail safe.
4. If the operation is never altered, PLCs prove to be costly.

Relay Logic Controllers

The relay sequencer is nothing but a relay logic controller in which the logical sequence of events is controlled by the proper operation of relays. There are many types of relays used in such a controller which are control relays, time delay relays, etc. The pushbuttons and switches are the input devices that control the operation of relays while the outputs are closed contacts that operate lights, motors, solenoids, etc.

The main aspect of such a relay logic controller is that each rung of the ladder diagram is evaluated simultaneously and continuously. If any input device changes its state anywhere in any rung, its effect on all the rungs is immediate.

In such a relay logic controller there is a relay logic panel consisting of necessary wiring to execute the logic of the ladder diagram. Once the wiring is completed, a relay sequencer is said to be programmed.

Disadvantages of Relay Logic Controller

1. If the logic is to be changed then it is necessary to rewire the panel, partly or completely.
2. The rewiring job is complicated and time consuming.
3. Possibility of hardware i.e. wiring failure always exists.
4. The patch panels may be used. In this method all relay contacts are brought out on panel through sockets and then plugs can be used to make the necessary connections. Due to this, logic can be modified if required. But this makes the system complicated and costly.

Comparison of PLC with Relay Logic Controller

1. The computer based PLC is most suitable compared to relay logic as input and output variables have only two states and hence are binary variables. This is most suited for PLC.
2. Most of the control relays which are physically required in relay logic get replaced by software commands. This reduces the hardware.
3. As the hardware is reduced, the problems related to hardware like wiring failure also get reduced considerably due to PLC.
4. If there is any change in logic, it is not necessary in PLC to replace hardware or wiring. It is required to change the software which is very easy. Thus Programmed sequence can be easily altered in PLC.
5. The special actions such as time delays, counters, registers can be easily achieved through the software in PLC.
6.  The programming cost in relay logic is high due to complicated wiring. But programming cost in PLC is low.
7. The space required for PLC is less compared to relay logic controller.
8. Due to modern technology, special semiconductor solid state devices are developed which can control high power actions with the help of low level commands from a computer.
9. The relay logic controller is not reusable as PLC.
10. The relay logic controller does not have arithmetic capability and memory as against this PLC has strong arithmetic capability and large memory space.

Difference between PLC and relay logic controller

S.No.	PLC-based control panel	Relay logic panel
1	A Single microprocessor-based circuit is control operations.	A large number of relays are used to control the operations.
2	Wiring is on the printed circuit board which is minimum.	It consists of complex wiring.
3	The operation can be changed by simply changing the software program	To change operation, complete the wiring needs to be replaced.
4	Replacement time and cost are negligible.	Replacement time and cost are very high.
5	PLC programs are simple to analyze and understand.	Relay logic is very difficult to understand.
6	Size is compact and reliability is high.	Size is large and hence reliability is reduced.
7	It is portable easily.	It is not easily portable.
8	Highly flexible and adaptable.	Flexibility is negligible and less adaptive
9	Data storage and logging are easy since PLCs can be easily connected to Computers.	Data storage and logging is difficult since relay logic panels cannot be easily interfaced to computers
10	The occurrence of an event is recognized immediately and action is taken.	PLCs use the scanning principle. Hence the occurrence of an event is not recognized until the next scan. Hence action is not immediately taken.

Insulated Gate Bipolar Transistor (IGBT)