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Conformance to EC Directives 6
6-4 Relay Output Noise Reduction Methods
The CS/CJ-series PLCs conforms to the Common Emission Standards
(EN50081-2) of the EMC Directives. However, noise generated by relay out-
put switching may not satisfy these Standards. In such a case, a noise filter
must be connected to the load side or other appropriate countermeasures
must be provided external to the PLC.
Countermeasures taken to satisfy the standards vary depending on the
devices on the load side, wiring, configuration of machines, etc. Following are
examples of countermeasures for reducing the generated noise.
Countermeasures
(Refer to EN50081-2 for more details.)
Countermeasures are not required if the frequency of load switching for the
whole system with the PLC included is less than 5 times per minute.
Countermeasures are required if the frequency of load switching for the whole
system with the PLC included is more than 5 times per minute.
Countermeasure Examples
When switching an inductive load, connect an surge protector, diodes, etc., in
parallel with the load or contact as shown below.
Circuit Current Characteristic Required element
AC DC
Yes Yes If the load is a relay or solenoid, there is
a time lag between the moment the cir-
cuit is opened and the moment the load
is reset.
If the supply voltage is 24 or 48 V, insert
the surge protector in parallel with the
load. If the supply voltage is 100 to
200 V, insert the surge protector
between the contacts.
The capacitance of the capacitor must
be 1 to 0.5 µF per contact current of
1 A and resistance of the resistor must
be 0.5 to 1 Ω per contact voltage of 1 V.
These values, however, vary with the
load and the characteristics of the
relay. Decide these values from experi-
ments, and take into consideration that
the capacitance suppresses spark dis-
charge when the contacts are sepa-
rated and the resistance limits the
current that flows into the load when
the circuit is closed again.
The dielectric strength of the capacitor
must be 200 to 300 V. If the circuit is an
AC circuit, use a capacitor with no
polarity.
CR method
Power
supply
Inductive
load
C
R
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