Introduction to Industrial Automation

Figure 7.50  Implementation of the conventional ON-delay starting circuit in a PLC. 

(a) Conventional circuit, (b) implementation with PLC and (c) program.

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Basic Programming Principles of PLCs 

◾

 

319

In a similar way, a timer can be programmed to implement any other time function of 

Figure 7.48. As an example, Figure 7.51 shows the classic automation circuit, the I/O connec-

tions to the PLC and the LAD program for the operation of a machine (relay C) with a time 

delay for stopping. When the sensor contact S

1

 closes, the OFF-delay timer is activated, which 

immediately closes the delay contact T, and therefore the relay C is energized. When, at some 

time instant, the contact of S

1

 becomes open, the timer is immediately deactivated, but the 

timer’s switching contact T opens with a time delay T and hence the relay C will be deactivated 

with the same delay (20 s in the program). The corresponding Boolean language program is the 

following:

A

I1.0

L

‘20 sec’

SF 

T10

A

I1.3

R

T10

A

T10

=

Q0.2

=

M3.7

BE

Example: For a machine (relay C) it is desired, from the moment that an operating condi-

tion will be satisfied, to operate for time T

2

, to stop for time T

1

 and for this to be repeated 

automatically until the operating condition ceases. The required time function—ON for T

2

, 

OFF for T

1

—is implemented with the conventional automation circuit of Figure 7.52a. The 

described time function follows the auxiliary relay d, as shown in Figure 7.52b, while the sen-

sor S

1

 plays the role of the operating condition. For the connection of the I/O devices shown 

PLC

Inputs

Outputs

0 V

C

+24 V DC

0 V

+24 V DC

Q0.2

I1.0

Reset

I1.3

Sensor

N

(a)

(b)

(c)

C

S

1

R

T

T

LAD

I1.0

Q0.2

I1.3

T10

S

20 sec

R

T12

M3.7

TV

SF

Q

Sensor

S

1