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MELSEC-Q
1 GENERAL SPECIFICATIONS AND PRECAUTIONS FOR USE
(c) Measures against back EMF
Configure a contact protection circuit for extending the contact life, preventing
noise when the contact is cut off, and suppressing the generation of carbide and
nitric acid due to arc discharge.
An Incorrect contact protection circuit may cause contact welding.
Also, when using the contact protection circuit, the recovery time may be long.
The following table shows the representative examples of the contact protection
circuit.
Circuit example Method for selecting elements Remarks
Capacitor +
Resistor method
(CR method)
Refer to the following for
constants of the capacitor and
resistor. Note that the
following values may differ
depending on a nature of the
load and a variation of
characteristics of it.
• Capacitor 0.5 to 1 ( F)
against contact current of 1A
• Resistor 0.5 to 1 (Ω)against
contact voltage of 1V
Use a capacitor whose
withstand voltage is 200 to
300V. In AC circuit, use a
capacitor having no polarity.
If a load is from a relay or
solenoid, the recovery time
delays.
A capacitor suppresses
electric discharge while a
contact is off, and a resistor
restricts a flow of current
while a contact is on.
Diode method
Use a diode whose reverse
breakdown voltage is 10 times
as high as the circuit voltage
or more and whose forward
current is twice as high as the
load current or more.
The recovery time is later
than the CR method.
Diode + Zener
diode method
Use zener voltage for the
zener diode equal to or more
than the power supply
voltage.
The diode method is effective
when the recovery time is too
late.
*1: When using AC power, impedance of CR must be larger enough than that of
the load. (prevention of a malfunction due to leak current from the CR)
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