Introduction to Industrial Automation

Figure 6.52  The insertion of a switching element in the circuit of a digital output in a series 

requiring the placement of a surge suppressor.

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  Introduction to Industrial Automation

Surge

suppressor

L1

L2

11

22

33

44

1

2

3

4

L1

L2

Inductive

load 

Output module

L1

L2

1

2

3

4

L1

L2

Figure 6.53  The insertion of an external switch parallel to the digital output of the PLC requir-

ing the placement of a surge suppressor.

C

11

22

33

44

1

2

3

4

Input module

L1

L2

Surge

suppressor

Inductive

load

C

1

2

3

4

Input module

L1

L2

Figure 6.54  When a button or switch is common to a PLC (such as an input device) and a con-

ventional automation circuit, then protection via a surge suppressor is required.

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Basic Operating Principles of PLCs 

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261

whenever it is needed, regardless of the PLC’s logic. In this case, transient protection is also needed 

and the insertion of a surge suppressor is required.

Finally, Figure 6.54 presents an input device (push button) that, except for the activation of the 

input 4 of the PLC, also causes the activation of an external inductive load. Although the opera-

tion of the latter is independent of the PLC’s logic, it may cause electrical noise or interference, 

and consequently a transient protection is required by the utilization of a surge suppressor. In all 

the previous cases, the typical form of the equivalent circuit of a surge suppressor is an RC series 

circuit with elements of R = 1 K

Ω

 and C = 0.5 

μ

F.