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Specifications of Basic I/O Units Appendix A
Arc killer circuit examples are listed in the following table.
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.
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 arc killer in parallel with the
load. If the supply voltage is 100 to
200 V, insert the arc killer 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.
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 cir-
cuit 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 arc
killer 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
contacts.
---
CR method
Power
supply
Inductive
load
Power
supply
Inductive
load
Diode method
Varistor method
Inductive
load
Power
supply
Power
supply
Inductive
load
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