gefran I3 Series Instruction Manual page 35

Contact protection circuit
Use of contact protective devices or protection circuits can suppress the counter emf to a low level.
However, note that incorrect use will result in an adverse effect.
Typical contact protection circuits are given in the table below.
Circuit
CR
circuit
Diode
circuit
Diode
and
zener
circuit
Varistor
circuit
(G: Good, NG: No Good, C: Care)
80175G_MAN_I3-IE-IN-IX_I7_11-2018_ENG
Figure 9
Contact protection circuit: Inductive load
Voltage
Features/Others
CA CD
If the load is a timer, leakage
current flows through the CR
circuit causing faulty opera-
tion.
C* G
* If used with AC voltage, be
sure the impedance of the
load is sufficiently smaller
than that of the CR circuit
If the load is a relay or sole-
noid, the release time leng-
G C thens.
Effective if the power supply
voltage is 24 or 48V.
The diode connected in
parallel causes the energy
stored in the coil to flow to the
coil in the form of current and
dissipates it as joule heat at
the resistance component of
NG G
the inductive load.
This circuit further delays the
release time compared to the
CR circuit.
(2 to 5 times the release time
listed in the catalog)
Effective when the release
NG G time in the diode circuit is too
long.
Using the stable voltage cha-
racteristics of the varistor, this
circuit prevents excessively
G G high voltages from being ap-
plied across the contacts.
This circuit also slightly de-
lays the release time.
Devices Selection
As a guide in selecting "c" and "r",
c : 0.5 μF to 1μF for 1A contact current
r : 0.5 Ω to 1 Ω per 1V contact voltage
Values vary depending on the properties of the
load and variations in relay characteristics.
Capacitor "c" acts to suppress the discharge
the moment the contacts open.
Resistor "r" acts to limit the current when the
power is turned on the next time.
Use a capacitor "c" with a breakdown voltage
of 200 to 300V.
Use AC type capacitors (non polarized) for AC
circuits.
Use a diode with a reverse breakdown volta-
ge at least 10 times the circuit voltage and a
forward current at least as large as the load
current.
In electronic circuits where the circuit voltages
are not so high, a diode can be used with a re-
verse breakdown voltage of about 2 to 3 times
the power supply voltage.
Use a zener diode with a zener voltage about
the same as the power supply voltage.
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