Siemens SIMATIC Manual page 62

Digital modules
3.5 How to protect digital modules from inductive overvoltage
Configuration of coils operated with direct current
Coils operated with direct current are shown in the following diagram and configured with
diodes or Z diodes.
①
②
Figure 3-2
Properties of the diode/Z diode circuit:
● Cut-off currents can be avoided. Z diode withstands a higher cut-off voltage.
● High cut-off delay (6 to 9 times higher compared to non-protective cid´rcuits).
Faster cut-off of the Zener diode compared to the diode circuit.
Connection of AC-operated coils
The operation of AC coils with varistors or RC elements is shown in the diagram.
①
②
Figure 3-3
Properties of a circuit with varistor:
● The amplitude of the cut-off current is limited but not attenuated.
● The steepness of the overvoltage remains the same.
● Low cut-off delay.
Properties of acircuit with RC elements:
● Reduction of the amplitude and steepness of the cut-off current.
● Low cut-off delay.
62
with diode
with Z diode
Configuration of coils operated with direct current
with varistor
with RC element
Connection of AC-operated coils
S7-300 Module data
Manual, 06/2017, A5E00105505-AJ