Introduction to Industrial Automation

current causes their fault activation.

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Basic Operating Principles of PLCs 

◾

 

257

impedance Z of the input to substantially absorb the leakage current. The notations shown in 

the figure are the following:

V = Nominal operating voltage of the input module (PLC)

V

L

 = Maximum voltage under which the input remains OFF (PLC)

I

R

 = Current flow through the resistance R

I

L

 = Current flow through the input when it is under voltage V

L

Z = Input impedance (PLC)

R = Input resistance adjustment (PLC)

I

leak

 = Leakage current of the photocell (Photocell)

The notations concerning the (PLC) or (Photocell) refer to the technical characteristics of the cor-

responding device. The rest of the presented variables will be calculated in what follows. It is assumed 

that from the technical characteristics of the digital input module, the following values can be extracted:

 

◾

Maximum allowable voltage in the OFF mode, V

L

 = 60 V

 

◾

Input impedance Z = 40 K

Ω

 

◾

Nominal operating voltage of the input stage V = 230 V

Then, the maximum allowable leakage current of the input can be calculated as I

L

= V

L

/Z = 

1.5 mA.

In the case that the leakage current of an input device is greater than 1.5 mA, this will cause a 

faulty activation to the input of the PLC and thus it is necessary to insert the resistor R as shown 

in Figure 6.51a (everything related to the impedance adjustment has been marked in gray). From 

the technical characteristics of the photocell, it is assumed that I

leak

 = 12 mA (quite larger than the 

permitted 1.5 mA). Then the resistor R, which as mentioned above interferes to absorb the leakage 

current of the input, while preventing the current to flow to the input, can be calculated as:

 

R V /I

V / I

I

K

L R

L

leak

L

=

=

−

=

(

)

.

5 71

Ω

which is the maximum permissible value. However, since the resistor R will be under a voltage 

230 V when the photocell is energized (ON state of the input device), and should have the equiva-

lent power (for 230 V and not for 60 V) or:

 

P

V I

V /R

W

R

R

=

=

=

2

9 26

.

that is the minimum value. Therefore, the impedance adjustment will be achieved with a resis-

tance of approximately 5 K

Ω

 and 10 W.