Siemens Simatic ET 200S Operating Instructions Manual page 315

Diode/Zener diode circuits have the following properties:
• Switch off surge voltage can be totally avoided.
• High switch-off delay (6 to 9 times higher than without protective circuit).
Wiring of DC-operated coils
AC-operated coils are wired with varistors or RC elements as illustrated in the following
diagram.
①
②
A circuit with varistors has the following properties:
• The amplitude of the switch off surge voltage is limited, but not dampened.
• The gradient of the overvoltage remains the same.
• The switch-off delay is small.
The properties of RC circuits are:
• Amplitude and gradient of the switch-off power surge are reduced.
• The switch-off delay is small.
ET 200S
Operating Instructions, 12/2005, A5E00515771-03
Zener diode has higher switch off voltage.
Zener diode switches off faster than diode circuit
1
with varistor
with RC element
F.1 Special Measures for Interference-Free Operation
2
Interference-free operation
F-3