Siemens SIMATIC S7-400 Hardware And Installation Manual page 82

Wiring the S7-400
Suppression for DC-Operated Coils
DC-operated coils are interconnected to diodes or zener diodes.
With diode
+
-
Figure 4-7
Suppression with Diodes / Zener Diodes
Suppression with diodes or zener diodes exhibits the following characteristics:
Switching overvoltages can be avoided entirely. A zener diode has a higher
turn-off voltage.
High turn-off delay (6 to 9 times higher than without suppressor circuits)
The zener diode switches off more quickly than a diode circuit.
Suppression with AC-Operated Coils
AC-operated coils are suppressed with varistors or RC elements.
With varistor
~
~
Figure 4-8
Suppression with a Varistor
Suppression with a varistor exhibits the following characteristics:
The amplitude of the switching overvoltage is limited but not attenuated.
The steepness of the surge voltage remains the same.
Low turn-off delay.
Suppression with RC Elements
Suppression with an RC element exhibits the following characteristics:
The amplitude and steepness of the switching overvoltage are reduced.
Low turn-off delay.
4-20
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Suppression for DC-Operated Coils
Suppression with AC-Operated Coils
S7-400 and M7-400 Programmable Controllers, Installation Manual
With zener diode
+
-
With RC element
~
~
C79000-G7076-C424-01