Introduction to Industrial Automation

6.10.8   Output Impedance Adjustment

In Figure 6.51b, the connection of an output device, with a very high impedance (Z) in a digital 

output of the PLC, is presented. The symbols presented in this figure have the same meaning as 

the case of Figure 6.51a, except for the voltage V

L

, which now expresses the maximum allowable 

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  Introduction to Industrial Automation

voltage for which the output device remains in the OFF state. The technical characteristics of the 

digital output module is assumed to have the following settings:

 

◾

Leakage current in the OFF state of output, I

leak

 = 8 mA

 

◾

Minimum current of the load (Z) at the ON state of the output, I

ON 

= 60 mA.

Therefore, the output of the PLC shows a leakage current of 8 mA when it is in the OFF state. 

Every output device that can operate with a current smaller or equal to 8 mA will require the 

insertion of an adjusting resistor to ensure that it will remain OFF when the output of the PLC is 

deactivated.

On the other hand, since the output device should absorb a current of at least 60 mA, to ensure 

that the output will remain ON, every output device that does not require an operating current 

of more than 60 mA will need to be adjusted with a resistor in a parallel connection in order to 

absorb the extra current from the output. Thus, there are two conditions that need to be met, 

with the second one to ensure the satisfaction of the first one. In this case, the calculation of the 

resistance R will be based on the second condition.

For example, it is assumed that the output of the PLC is connected to the input of another 

PLC, with the same above characteristics for “communication” purposes (1 bit information trans-

mission from the first PLC to the second). Then it is derived that:

 

I

V /Z

mA

mA

L

L

=

=

<

1 5

8

.

If no adjustment is made, the output device—that is, the input of the second PLC—will be 

activated, while the output of the first PLC is in the OFF state due to the leakage current. In order 

not to have the operating current I

ON

 exactly on the limit of the 60m, the resistor R is chosen to 

absorb all the I

ON

 current. Thus, in this case it is derived that:

 

I

I

R V/I

K and P

V I

W

R

ON

ON

R

ON

=

=

=

=

=

,

.

,

.

3 83

13 8

Ω

Thus, with the selection of a 3.8 K

Ω

 and 15 W resistance, it is ensured that the current is I

ON

 > 

60 mA. At this point it should also be checked whether, in the OFF state, the voltage at the ends 

of the load—that is, at the input of the 2nd PLC, denoted by 

′

V

L

 is 

′ <

V

V

L

60

. Thus:

 

R

R Z

K and V

I R

V

V

total

L

leak

total

=

=

′ =

=

<



3 47

27 76

60

.

.

Ω

Instead of simple resistance, which is usually greater than the 5 W and therefore produces 

heat in the ON state (230 V), a combination of R, C series elements can be used that have the 

advantage of producing much less heat can be used. The RC combination should of course have 

the corresponding impedance that has been calculated for the simple resistance, and its calculation 

will be based on the relationships,

 

Z

R

fC

C

C

=

+

=

2

2

1

2

X

X

,

,

π

 

R

C

  

where f is the network’s frequency and C the capacitance of the capacitor.

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Basic Operating Principles of PLCs 

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