Omron C200HE-CPU11 Installation Manual page 149

Specifications
Arc killer circuit examples are listed in the following table.
Circuit
CR method
Power
supply
Diode method
Power
supply
Varistor method
Power
supply
Note Do not connect a capacitor as an arc killer in parallel with an inductive load as shown in the following dia-
gram. This arc killer is very effective for preventing spark discharge at the moment when the circuit is
opened. However when the contacts are closed, the contacts may be welded due to the current charged in
the capacitor.
DC inductive loads can be more difficult to switch than resistive loads. If appropriate arc killers are used,
however, DC inductive loads will be as easy to switch as resistive loads.
Current
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 arc killer in parallel with the
load. If the supply voltage is 100 to
200 V, insert the arc killer between the
contacts.
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.
Yes Yes The varistor method prevents the
imposition of high voltage between the
contacts by using the constant voltage
characteristic of the varistor. There is
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 varistor in parallel with the
load. If the supply voltage is 100 to
200 V, insert the varistor between the
contacts.
Characteristic
Power
supply
Appendix B
Required element
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
experiments, and take into
consideration that the capacitance
suppresses spark discharge when the
contacts are separated 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.
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
arc killer is applied to electronic
circuits with low circuit voltages.
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