WAGO-I/O-SYSTEM 750
Connect Devices
33
750-509 2DO 230V AC 0.3A/SSR
Manual
Version 1.1.0
6.3
Protective Circuits for Contacts of Relay Modules
Switching off inductive loads such as contactors and solenoid valves can generate
transients with voltage peaks of up to several thousand volts. Very often, these
transients exceed the permissible limits specified in the EMC standards. They
must therefore be limited by using external protective circuits.
Furthermore, they cause an arc, which may destroy the switching contact, and has
a significant impact on the lifetime and safety of the relay. The following
protective circuits, which are directly connected to the source of the interference
in parallel with the consumer, have proved successful in practice:
Table 16: Protective Circuits for Contacts of Relay Modules
Protective Circuits
Additional
Off-Delay
Defined
Induction
Voltage
Limitation
Bipolar
Effective
Attenuation
Advantages and
Disadvantages
Figure 15: Diode
long
yes (U
D
)
no
Advantages:
•
easy implementation
•
cost-effective and
reliable
•
uncritical
dimensioning
•
low induction voltages
Disadvantages:
•
attenuation only via
load resistor
•
long off-delay
Figure 16: Zener Diode
and Diode
medium to
short
yes (U
ZD
)
no
Advantages:
•
uncritical
dimensioning
Disadvantages:
•
attenuation only above
U
ZD
Figure 17: Suppressor
Diode
medium to
short
yes (U
ZD
)
yes
Advantages:
•
cost-effective
•
uncritical
dimensioning
•
limitation of positive
and negative peaks
•
suitable for AC
voltage
Disadvantages:
•
attenuation only above
U
ZD
Figure 18: Varistor
medium to
short
yes (U
VDR
)
yes
Advantages:
•
high power absorption
•
uncritical
dimensioning
•
suitable for AC
voltage
Disadvantages:
•
attenuation only above
U
VDR
