A-53
Appendices
NJ-series CPU Unit Hardware User’s Manual (W500)
A-2 Specifications of Basic I/O Units
A
A-2-3 Precautions on Contact Output Units
Precautions for Correct UsePrecautions for Correct Use
Do not connect a capacitor as an arc killer in parallel with an inductive load
as shown in the diagram on the right. This arc killer is very effective for pre-
venting spark discharge at the moment when the circuit is opened. How-
ever, at the moment 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
CR
method
Yes Yes If the load is a relay or solenoid, there
is a delay in the resetting time.
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 should be
approx. 1 to 0.5 F per contact current of 1 A and
resistance of the resistor should be approx. 0.5 to
1 per contact voltage of 1 V. These values,
however, vary with the load and the characteris-
tics of the relay. Decide these values from experi-
ments, 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.
Diode
method
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
delay in the resetting time 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 elec-
tronic circuits with low circuit voltages.
Varistor
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
delay in the resetting time.
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.
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Power
supply
Inductive load
C
R
Power
supply
Inductive load
Power
supply
Inductive load
C
Power
supply
Inductive load
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