Precautions for All Switches
Contact Protection Circuits
Using a contact protection circuit to increase the contact durability,
prevent noise, and suppress the generation of carbide or nitric acid.
Be sure to apply the contact protection circuit correctly, otherwise
adverse results may occur.
The following tables shows typical examples of contact protection
circuits. If the 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
when it
reacts with moisture. Consequently, the internal metal parts may
corrode and the Switch may fail. Be sure to select the best contact
protection circuit from the following table.
Typical Examples of Contact Protection Circuits
Do not use the following types of contact protection circuit.
Using Switches for Microloads
Contact failure may occur if a Switch for a general load is used to
switch a microload circuit. Use Switches in the ranges shown in the
diagram
right
. However, even when using microload models within
the operating range shown here, if inrush current occurs when the
contact is opened or closed, it may increase contact wear and so
decrease durability. Therefore, insert a contact protection circuit
where necessary. The minimum applicable load is the N-level
reference value. This value indicates the malfunction reference level
for the reliability level of 60% (
O
60
) (JIS C5003). The equation,
O
60
=
0.5×10
6
/operations indicates that the estimated malfunction rate is
less than 1/2,000,000 operations with a reliability level of 60%.
Circuit example
Applicable
current
Features and remarks
Element selection
AC
DC
CR
*
(Yes)
Yes
* Load impedance must be much smaller than
the CR circuit impedance when using the
Switch for an AC voltage.
Use the following as guides for C and R
values:
C: 1 to 0.5
P
F per 1 A of contact current (A)
R: 0.5 to 1
:
per 1 V of contact voltage (V)
These values depend on various factors,
including the load
characteristics
. Confirm
optimum values experimentally.
Capacitor C suppresses the discharge when
the contacts are opened, while the resistor
R limits the current applied when the
contacts are closed the next time.
Generally, use a capacitor with a low
dielectric strength of 200 to 300 V. For
applications in an AC circuit, use an AC
capacitor (with no polarity).
Yes
Yes
The operating time of the contacts will be
increased 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
No
Yes
The energy stored in the coil reaches the coil as
current via the diode connected in parallel, and
is dissipated as Joule heat by the resistance of
the inductive load. This type of circuit increases
the release time more than the CR type.
Use a diode having a reverse breakdown
voltage of more than 10 times the circuit
voltage, and a forward current rating greater
than the load current.
Diode +
Zener diode
No
Yes
This circuit effectively shortens the reset time in
applications where the release time of a diode
circuit is too slow.
Use a Zener diode with a low breakdown
voltage.
Varistor Yes
Yes
This circuit prevents a high voltage from being
applied across the contacts by using the
constant-voltage characteristic of a varistor.
This circuit also somewhat increases the reset
time. Connecting the varistor across the load is
effective when the supply voltage is 24 to 48 V,
and across the contacts when the supply
voltage is 100 to 200 V.
---
Power
supply
Inductive
load
C
R
Power
supply
C
R
Inductive
load
Power
supply
Inductive
load
Power
supply
Inductive
load
Power
supply
Inductive
load
Although it is thought that switching a DC
inductive load is more difficult than a resistive
load, an appropriate contact protection circuit
can achieve almost the same characteristics.
Power
supply
C
Load
This circuit arrangement is very
effective for diminishing arcing at
the contacts when breaking the
circuit. However, since electrical
energy is stored in C (capacitor)
when the contacts are open, the
current from C flows into the
contacts when they close. This
may lead to contact welding.
Power
supply
C
Load
This circuit arrangement is very
useful for diminishing arcing at the
contacts when breaking the circuit.
However, since the charging current
to C flows into the contacts when
they are closed, contact welding
may occur.
V
oltage (
V
DC)
30
24
12
5
0
1
10
100
1,000
Current (mA)
0.1
1 mA
Operating
range for
standard
models
Operating range for
micro-load models
26 mA
0.16 mA
800 mW
5 mW
100 mA 160 mA
Unusable
range
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C-7
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