Siemens Simatic S7-300 CPU 31xC Series Installation And Operating Instructions Manual page 239

Circuit for coils operated with DC voltage
The figure below shows DC-operated coils equipped with diode or Zener diode circuit.
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Diode/Zener diode circuits have the following characteristics:
• Opening surge voltage can be totally avoided.
• High switch-off delay (6 to 9 times higher than without protective circuit).
Circuit for coils operated with AC voltage
The figure shows coils operated with AC voltage and varistor or RC circuit.
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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.
S7-300, CPU 31xC and CPU 31x: Installation
Operating Instructions, Edition 08/2004, A5E00105492-05
with diode
The Zener diode has a higher switch-off voltage capacity.
The Zener diode switches off faster than a diode circuit.
with varistor circuit
A.3 Lightning and surge voltage protection
with Zener diode
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with RC circuit
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Appendix
A-29