G9SB-2002-A Datasheet, PDF (14/25 Pages) Omron Electronics LLC – Ultra Slim Safety Relay Unit

C-A-4 Contact Protection Circuits
Using a contact protection circuit is effective in increasing contact
durability and minimizing the production of carbides and nitric acid.
The following table shows typical examples of contact protection
circuits. Use them as guidelines for circuit design.
Typical Examples of Contact Protection Circuits
1. Depending on factors such as the nature of the load and the
Relay characteristics, the effects may not occur at all or adverse
effects may result. Therefore be sure to check operation under
the actual load conditions.
2. When a contact protection circuit is used, it may cause the
release time (breaking time) to be increased. Therefore be sure to
check operation under the actual load conditions.
Circuit example
Applicable
current
AC
DC
CR
*See Yes
(See
remarks.
remarks.)
Power ^{C R}
supply
(Yes)
Inductive
load
Yes Yes
(See
remarks.)
Power
supply
C
Inductive
load
R
Diode
Diode + Zener
diode
Power
supply
Power
supply
No
Inductive
load
Yes
No
Inductive
load
Yes
Features and remarks
Element selection
*Load impedance must be much smaller than Use the following as guides for C and R values:
the CR circuit impedance when using the Relay _{C: 0.5 to 1 μF per 1 A of contact current (A)}
for an AC voltage.
R: 0.5 to 1 Ω per 1 V of contact voltage (V)
When the contacts are open, current flows to the
inductive load via CR.
These values depend on various factors,
including the load characteristics and variations
in characteristics. Confirm optimum values
experimentally.
The release time of the contacts will be
increased if the load is a Relay or solenoid.
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 dielectric
strength of 200 to 300 V. For applications in an
AC circuit, use an AC capacitor (with no polarity).
If there is any question about the ability to cut off
arcing of the contacts in applications with high
DC voltages, it may be more effective to connect
the capacitor and resistor across the contacts,
rather than across the load. Perform testing with
the actual equipment to determine this.
The electromagnetic energy stored in the
inductive load reaches the inductive load 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. A diode having a reverse breakdown
voltage two or three times that of the supply
voltage can be used in an electronic circuit
where the circuit voltage is not particularly high.
This circuit effectively shortens the release time The breakdown voltage of the Zener diode
in applications where the release time of a diode should be about the same as the supply voltage.
circuit is too slow.
Varistor
Power
supply
Yes Yes This circuit prevents a high voltage from being The cutoff voltage Vc must satisfy the following
Inductive
load
applied across the contacts by using the
constant-voltage characteristic of a varistor. This
circuit also somewhat increases the release
conditions. For AC, it must be multiplied by
Vc > (Supply voltage × 1.5)
2.
time. Connecting the varistor across the load is If Vc is set too high, its effectiveness will be
effective when the supply voltage is 24 to 48 V, reduced because it will fail to cut off high
and across the contacts when the supply voltage voltages.
is 100 to 240 V.
Do not use the following types of contact protection circuit.
This circuit arrangement is very effective for diminishing
arcing at the contacts when breaking the circuit. However,
C
since electrical energy is stored in C (capacitor) when the
Power
Load contacts are open, the current from C flows into the
supply
contacts when they close. This may lead to contact
welding.
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
Power C Load ^{when they are closed, contact welding may occur.}
supply
Note: 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.
C-A-5 Countermeasures for Surge from External
Circuits
Install contact protection circuits, such as surge absorbers, at
locations where there is a possibility of surges exceeding the Relay
withstand voltage due to factors such as lightning. If a voltage
exceeding the Relay withstand voltage value is applied, it will cause
line and insulation deterioration between coils and contacts and
between contacts of the same polarity.
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C-6