Siemens SIMATIC ET 200S Manual page 555

Wiring of DC-operated coils
DC-operated coils are wired with diodes and Zener diodes as illustrated in the
following figure.
Figure F-2
A circuit with diodes/Zener diodes has the following features:
• Switching overvoltages can be completely avoided.
A Zener diode has higher circuit interruption voltage.
• Long switch-off delay (6 to 9 times longer than without a protective circuit).
A Zener diode switches off quicker than a diode circuit
Wiring of AC-operated coils
AC-operated coils are wired with varistors or RC elements as illustrated in the
figure.
Figure F-3
A circuit with a varistor has the following features:
• The amplitude of the switching overvoltage is limited but not dampened.
• The gradient of the overvoltage remains the same.
• The switch-off delay is short.
A circuit with RC elements has the following properties:
• The amplitude and gradient of the switching overvoltage are reduced.
• The switch-off delay is short.
ET 200S Distributed I/O System
EWA-4NEB780602402-09
With diode
+
-
Wiring of DC-operated coils
With varistor
~
~
Wiring of AC-operated coils
With Zener diode
+
-
With RC element
~
~
F-3