xxv
Conformance to EC Directives
6
must be connected to the load side or other appropriate countermeasures
must be provided external to the PC.
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 EN61000-6-4 for more details.
Countermeasures are not required if the frequency of load switching for the
whole system including the PC is less than 5 times per minute.
Countermeasures are required if the frequency of load switching for the whole
system including the PC is 5 times or more per minute.
Countermeasure Examples
When switching an inductive load, connect a 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
circuit 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
µ
F 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 testing,
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.
No
Yes
The diode connected in parallel with
the load changes energy accumulated
by the coil into a current, which then
flows into the coil so that the current
will be converted into Joule heat by the
resistance of the inductive load.
This time lag, between the moment the
circuit is opened and the moment the
load is reset, caused by this method is
longer than that caused by the CR
method.
The reversed dielectric strength value
of the diode must be at least 10 times
as large as the circuit voltage value.
The forward current of the diode must
be the same as or larger than the load
current.
The reversed dielectric strength value
of the diode may be two to three times
larger than the supply voltage if the
surge protector is applied to electronic
circuits with low circuit voltages.
Yes
Yes
The varistor method prevents the impo-
sition of high voltage between the con-
tacts by using the constant voltage
characteristic of the varistor. There is
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 varistor in parallel with the
load. If the supply voltage is 100 to 200
V, insert the varistor between the con-
tacts.
---
CR method
Power
supply
Inductiv
e
load
Diode method
Power
supply
Inductiv
e
load
Varistor method
Power
supply
Inductiv
e
load
