9
3. Protection of contact of switch
・ Contact protection circuit is used to extend the service life of contact. It reduces generation of noise at
the time of switching on and off. It also minimizes carbide and nitric acid gas generated by arc. Use this
circuit correctly, otherwise it will cause reverse effect, making the matter worse.
・ In case of using contact protection circuit, pay attention the operating time is possibility of delaying a
little. The following are typical examples of contact protection circuit.
a) Typical example of protective circuit
Example of circuit
Power supply
Special remarks for application and selection
DC
AC
Varistor
method
○
○
This method is intended not to apply voltage beyond
limit of varistor to inbetween contacts. It delays a little
time to return inductive load such as relay.
Select the most conformed rated model in terms of
power supply voltage and load capacity in selected
article by varistor maker.
C･R
method
○
○
It delays a little time to activate or return inductive
load such as relay.
As for CR value, to contact current and contact voltage,
the estimate value is C : 0.5 μ F / A, R : 1.0 Ω / V.
However, it differs according to the characteristics
of load, therefore, be sure to confirm to select by
experiment.
Diode
method
○
×
This method is intended to consume counter
electromotive force of inductive load by diode
and avoid to apply high voltage to inbetween the
contacts.
This method makes the return time of inductive load
slower.Choose the diode rated current is more than
load current and counter withstand voltage more than
10 times of power supply voltage.
Diode ＋
Zener diode
method
○
×
Diode method proves to be effective when the return
time takes too long for inductive load such as relay.
When you choose zener voltage of zener diode,
choose base on power supply voltage. Be careful with
limitations having this method. Because load is too
big, larger capacity zener diode is needed for reverse
surge power.
P.S. : Power supply, I.L. : Inductive load
b) Example of wrong protection circuit
Condenser method
This is very effective to remove arc at the time of
off-contact, but this is likely to run charged current
to condenser leading to contact sticking and short
service life at the time on-contact.
This is very effective to remove arc at the time of off-
contact, but this is likely to run short circuit current
accumulated in the condenser leading to contact
sticking on contacting.
This is likely to short service life at the time on-
contact.
P.S. : Power supply, I.L. : Inductive load
If the load is larger than the specifi ed contact capacity, use a secondary relay. In general, the load voltage and
the corrent is too less to the life of contact is longer. However, this does not expect for a low electirical load.
Manostar switches cannot directly shut off the circuit of 250 V AC or more.
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