Brochure

22
Technical Information – Relays
The following table lists examples of contact protection circuits.
Avoid use of a surge suppressor in the manner shown below.
Although it is considered that switching a DC inductive load is more difficult than a resistive load, an appropriate contact protection circuit
can achieve almost the same characteristics.
Circuit example Applicability Features and remarks Element selection
AC DC
CR Fair Good
Good Good
Diode Poor Good
Diode + Poor Good
Zener
Diode
Varistor Good Good
Load impedance must be much
smaller than the RC circuit when the
relay operates on an AC voltage.
Optimum C and R values are:
C: 1 to 0.5 µF for 1–A switching
current R: 0.5 to 1Ω for 1–V
switching voltage
These values do not always agree
with the optimum values due to the
nature of the load and the
dispersion in the relay
characteristics. Confirm optimum
values experimentally. Capacitor C
suppresses discharge when the
contacts are opened, while resistor
R limits the current applied when
the contacts are closed the next
time. Generally, employ a capacitor
C whose dielectric strength is 200
to 300 V. If the circuit is powered by
an AC power source, employ an
AC capacitor (non–polarized).
The release time of the contacts will
be delayed when a relay solenoid is
used as a load. This circuit is
effective if connected across the
load when the supply voltage is 24
to 48 V. When the supply voltage is
100 to 240 V, connect the circuit
across the contacts.
The energy stored in a coil
(inductive load) reaches the coil as
current via the diode connected in
parallel with the coil, and is
dissipated as Joule (measurable)
heat by the resistance of the
inductive load. This type of circuit
delays the release time more than
the RC type.
This circuit effectively shortens
release time in applications where
the release time of a diode
protection circuit proves to be too
slow.
By utilizing the constant-voltage
characteristic of a varistor, this
circuit prevents high voltages from
being applied across the contacts.
This circuit also somewhat delays
the release time. This circuit, if
connected across the load, is
effective when the supply voltage is
24 to 48 V. If the supply voltage is
100 to 240 V, connect the circuit
across the contacts.
–
The zener diode breakdown voltage
should be about the same as the
supply voltage.
Employ 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 to
three times that of the supply
voltage can be used in an electronic
circuit where the circuit voltage is
not particularly high.
Power
source
Inductive
load
Inductive
load
Power
source
Inductive
load
Power
source
Inductive
load
Power
source
Inductive
load
Power
source
This circuit arrangement is very effective for diminishing sparking
(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. Therefore, metal deposition is likely to occur
between mating contacts.
This circuit arrangement is very useful for diminishing sparking
(arcing) at the contacts when breaking the circuit. However, since
the charging current to C flows into the contacts when they are
closed, metal deposition is likely to occur between the mating
contacts.
Power
supply
Load
Power
supply
Load
Omron A5 Catalogue 2007 1-282 11/9/06 10:16 am Page 22