Siemens S5-135U/155U - Manual (Page 43)

3-17
System Manual
C79000-G8576-C199-06
3.3.4 Special Measures for Interference-Free Operation
As a rule, inductances such as contactor or relay coils controlled by
SIMATIC S5 do not require external quenching elements in the circuit,
because the quenching elements are already integrated in the modules.
Inductances should only be fitted with quenching elements
S
when SIMATIC S5 output currents can be switched off by additionally
fitted contacts, such as relay contacts. In this case the integrated
quenching elements in the modules are no longer effective;
S
if they are not controlled by SIMATIC S5 modules.
You can place freewheel diodes, varistors or RC networks in circuit with
inductances.
Circuitry for DC-Operated
Inductances
With Diode With Zener Diode With Varistor With RC Network
Circuitry for AC-Operated
Inductances
Figure 3-6 Quenching Circuits for Inductances
Fitting Quenching
Elements to
Inductances
Installation Guidelines