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C-3
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Technical Guide for Pushbutton Switches
3. Contact Protective Circuit
Apply a contact protective circuit to extend the contact life, prevent
noise, and suppress the generation of carbide or nitric acid. Be sure
to apply the contact protective circuit correctly, otherwise an adverse
effect may occur. The following provides typical examples of contact
protective circuits. If the Limit Switch is used in an excessively humid
location for switching a load that easily generates arcs, such as an
inductive load, the arcs may generate NOx, which will change into
HNO
3
if it reacts with moisture. Consequently, the internal metal parts
may corrode and the Limit Switch may fail. Be sure to select the ideal
contact preventive circuit from the following.
Typical Examples of Contact Protective Circuits
Do not apply contact protective circuits as shown below.
4. Switching
x
Do not use the Switch for loads that exceed the rated switching capacity or other
contact ratings. Doing so may result in contact weld, contact separation failures,
or insulation failures. Furthermore, the Switch may be broken or damaged.
x
Do not touch the charged switch terminals while power is supplied,
otherwise an electric shock may be received.
x
The life of the Switch varies greatly with switching conditions.
Before using the Switch, be sure to test the Switch under actual
conditions. Make sure that the number of switching operations is
within the permissible range. If a deteriorated Switch is used
continuously, insulation failures, contact weld, contact failures,
switch damage, or switch burnout may result.
x
Do not apply excessive or incorrect voltages to the Switch or
incorrectly wire the terminals. Otherwise, the Switch may not
function properly and have an adverse effect on external circuitry.
Furthermore, the Switch itself may become damaged or burnt.
x
Do not use the Switch in locations where flammable or explosive
gases are present. Otherwise switching arcs or heat radiation may
cause a fire or explosion.
x
Do not drop or disassemble the Switch, otherwise it may not be
capable of full performance. Furthermore, it may be broken or
burnt.
Circuit example
Applicable
current
Feature and details
Element selection
AC
DC
CR circuit
*
Yes
*
When AC is switched, the load impedance
must be lower than the CR impedance.
C: 1 to 0.5
P
F
u
switching current (A)
R: 0.5 to 1
: u
switching voltage (V)
The values may change according to the
characteristics of the load. The capacitor
suppresses the spark discharge of current
when the contacts are open. The resistor
limits the inrush current when the contacts
are closed again. Consider the roles of the
capacitor and resistor and determine ideal
capacitance and resistance values through
testing. Basically, use a capacitor with a
dielectric strength between 200 and 300 V.
When AC is switched, make sure that the
capacitor has no polarity.
Yes
Yes
The operating time will be greater if the load
is a relay or solenoid. Connecting the CR
circuit in parallel to the load is effective when
the power supply voltage is 24 or 48 V and in
parallel to the contacts when the power
supply voltage is 100 to 200 V.
Diode method
No
Yes
Energy stored in the coil is changed into
current by the diode connected in parallel to
the load. Then the current flowing to the coil
is consumed and Joule heat is generated by
the resistance of the inductive load. The
reset time delay with this method is longer
than that in the CR method.
The diode must withstand a peak inverse
voltage 10 times higher than the circuit
voltage and a forward current as high or
higher than the load current.
Diode and
Zener diode
method
No
Yes
This method will be effective if the reset time
delay caused by the diode method is too
long.
Use a Zener diode with a Zener voltage that
is approximately 1.2
u
power supply voltage
as, depending on the environment, the load
may not operate.
Varistor
method
Yes
Yes
This method makes use of constant-voltage
characteristic of the varistor so that no
high-voltage is imposed on the contacts.
This method causes a reset time delay.
Connecting a varistor in parallel to the load is
effective when the supply voltage is 24 to 48
V and in parallel to the contacts when the
supply voltage is 100 to 200 V.
---
This circuit effectively
suppresses arcs when the
contacts are OFF. The
capacitor will be charged,
however, when the contacts
are OFF. Consequently,
when the contacts are ON
again, short-circuited current
from the capacitance may
cause contact weld.
This circuit effectively sup-
presses arcs when the
contacts are OFF. When the
contacts are ON again,
however, charge current will
flow to the capacitor, which
may result in contact weld.
C
Power
supply
Inductive
load
R
C
Inductive
load
Power
supply
R
Inductive
load
Power
supply
Inductive
load
Power
supply
Inductive
load
Power
supply
Switching a DC inductive load is usually
more difficult than switching a resistive
load. By using an appropriate contact
protective circuit, however, switching a DC
inductive load will be as easy as switching a
resistive load.
Load
Power
supply
C
Load
Power
supply
C
