Siemens CPU 312C

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S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
12-28 A5E00105492-03
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The figure below shows DC-operated coils equipped with diode or Zener diode
circuit.
with diode
with Zener diode
+
-
+
-
Figure 12-9 Circuit for coils operated with DC voltage
Diode/Zener diode circuits have the following characteristics:
• Opening surge voltage can be totally avoided.
The Zener diode has a higher switch-off voltage capacity.
• High switch-off delay (6 to 9 times higher than without protective circuit).
The Zener diode switches off faster than a diode circuit.
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The figure shows coils operated with AC voltage and varistor or RC circuit.
with varistor
with RC element
~
~
~
~
Figure 12-10 Circuit for coils operated with AC voltage
The characteristics of varistor circuits are:
• The amplitude of the opening surge is limited rather than attenuated.
• The surge rise-ratio remains the same
• Short off-delay.
The characteristics of RC circuits are:
• Amplitude and steepness of the opening surge are reduced.
• Short off-delay.