Contact Protective Circuit For Relay Modules - WAGO I/O-SYSTEM 750 Manual

12 • Digital Output Modules

Contact Protective Circuit for Relay Modules

2.1.2 Contact Protective Circuit for Relay Modules
Switching off inductive loads such as contactors and solenoid valves can gen-
erate 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 circuits. Furthermore, they
cause an arc, which may destroy the switching contact, and have 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:
Load Protection
Diode
Series connection
diode/zener diode
Suppressor diode
Varistor
R/C combination
Addi-
tional Off
Delay
long
Load
U
D
g051211e
medium to
short
U Load
ZD
g051212e
medium to
U Load
short
ZD
g051213e
medium to
short
VDR U Load
VDR
g051214e
R
medium to
U Load
short
RC
C
g051215e
Defined Induction Bipolar
Voltage Effective
Limitation Attenuation
yes (U ) no
D
yes (U ) no
ZD
yes (U ) yes
ZD
yes (U ) yes
VDR
no yes
Advantages/Disadvantages
Advantages:
- easy implementation
- cost-effective, reliable
- uncritical dimensioning
- low induction voltage
Disadvantages:
- attenuation only via load
resistor
- long off delay
Advantages:
- uncritical dimensioning
Disadvantages:
- attenuation only above
U
ZD
Advantages:
- cost-effective
- uncritical dimensioning
- limitation of positive)/
negative peaks
- suitable for A.C. voltage
Disadvantages:
- attenuation only above
U
ZD
Advantages:
- high power absorption
- uncritical dimensioning
- suitable for A.C. voltage
Disadvantages:
- attenuation only above
U
VDR
Advantages:
- HF attenuation via power
storage
- suitable for A.C. voltage
- level-independent attenua-
tion
Disadvantages:
- exact dimensioning re-
quired
- high inrush current
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I/O Modules