Siemens Automation System S7-400 - Manual (Page 199)

Assembling and Installing Systems
A-31
Automation System S7-400 Hardware and Installation
A5E00850741-01
Suppression for DC-Operated Coils
DC-operated coils are interconnected to diodes or Zener diodes.
+
-
+
-
With diode With Zener diode
Figure A-9 Suppression for DC-Operated Coils
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 With RC element
~
~
~
~
Figure A-10 Suppression with AC-Operated Coils
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 an RC element exhibits the following characteristics:
• The amplitude and steepness of the switching overvoltage are reduced.
• Low turn-off delay.