1. PLC and Relay System
Electronics and
Instrumentation
Maintenance Department

2. Relay System
What is relay ?
How it works ?

3. How it works ?
Current flowing through the coil of
the relay creates a magnetic field
which attracts a lever and changes
the switch position

4. How it works ?

5. Small relay logic
OUTPUT
A
B
0
0
0
0
0
1
0
1
0
1
1
1
V+
OUTPUT
A
B

7. DISADVANTAGES OF RELAY
CIRCUTS
 Number of hardware will increase with
increase of logic complexity
 Relay energizing and de-energizing
takes time
 High failure rate
 Slow performance
 Bulky design difficult to manage
 Difficult troubleshooting

8. Drawbacks
of Relays are
overcome by
PLC’s.

9. PLC
What is a Programmable
Logic Controller (PLC) ?
How PLC works ?
What is the need of PLC ?
Hardware involved with
PLC
Advantages of PLC

10. What is Programmable Logic Controller (PLC)
?
CPU
Inputs
Outputs
A PROGRAMMABLE LOGIC CONTROLLER is a solid state
control system that continuously monitors the status of
devices connected as inputs. Based upon a user written
program, stored in memory, it controls the status of devices
connected as outputs.

11. PLC with Distributed I/O
Getting Complex
CPU
Inputs
Outputs
Remote
Outputs
Inputs

12. What is a PLC with Distributed Control?
Getting Really Complex
CPU
Inputs
CPU
Outputs
Outputs
Inputs
Remote
Outputs
Inputs

13. INSIDE PLC
Communications
Port
I
n
p
u
t
Isolation
Barrier
C
i
r
c
u
i
t
s
Central
Processor
Unit
(CPU)
MEMORY
data
program
Low Voltage
AC Power Supply
or
DC Power Supply
O
u
t
p
u
t
C
i
r
c
u
i
t
s
Isolation
Barrier
CR
High
Voltage

14. How PLC works?
 The CPU contains an “Executive” program that tells
the PLC how to:
 Execute the control Instructions
 User’s Program
 Communicate with other devices
 Other PLCs, Programming devices, I/O devices, etc.
 Perform Housekeeping activities
 Diagnostics, etc
 This program is stored in “nonvolatile” memory
 Meaning that the program will not be lost if power is
removed

15. How PLC works
START
 Four Steps in the PLC Operations
 Input Scan
 Scan the state of the Inputs
 Program Scan
 Processes the program logic
 Output Scan
 Energize/de-energize the outputs
 Housekeeping
 This step includes communications, Internal
Diagnostics, etc.
Housekeeping
Output
Scan
 The steps are continually repeated - processed in a
loop
Input
Scan
Program
Scan

16. PLC

18. Need of PLC
 To eliminate the high costs associated
with inflexible, relay-controlled systems.
 To Measure input with high speed.
 Fast Control (Command and
Regulation).
 Robust Protection.
 Diagnosis.

19. Advantages of PLC
 Compact in size.
 Less wiring (Wiring between devices
and relay contacts are done in the PLC
program.)
 Easy to implement complex logic.
 Faster action and controlling.
 Easy troubleshooting.
 Easy to implement change in logic.

20. Leading Brands Of PLC
 AMERICAN
1.
2.
3.
4.
5.
6.
7.
Allen Bradley
Gould Modicon
Texas Instruments
General Electric
Westinghouse
Cutter Hammer
Square D
 EUROPEAN 1. Siemens
2. Klockner & Mouller
3. Festo
4. Telemechanique
 JAPANESE
1.
2.
3.
4.
Toshiba
Omron
Fanuc
Mitsubishi

21. Thank you