Mitsubishi MELSEC-Q Series User Manual page 24

1 GENERAL SPECIFICATIONS AND PRECAUTIONS FOR USE OF I/O
MODULE
Circuit example
Capacitor +
Resistor method
(CR method)
Diode method
Diode + Zener
diode method
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(c) Measures against back EMF
When inductive load such as a magnetic contactor and solenoid is shut off, high
back EMF is generated between the contacts and arc discharge occurs.
Especially when the power factor is small, the life shortens due to arc discharge.
Therefore, take measures against arc discharge.
The following is four methods to protect a relay contact against back EMF.
• Capacitor + Resistor method (CR method)
• Diode method
• Diode + Zener diode method
• Varistor method
*1
Capacitor
Inductive
load
Resistor
Capacitor
Inductive
load
Resistor
Inductive
Diode
load
Diode
Inductive
load
Zener Diode
Method for selecting elements
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.
• Capacitor 0.5 to 1 ( F)
against contact current of 1A
• Resistor 0.5 to 1 ( ) against
contact voltage of 1V
Use a capacitor whose
withstanding voltage is 200 to
300V.
In AC circuit, use a capacitor
having no polarity.
Use a diode whose reverse
breakdown voltage is 10 times
as large as the circuit voltage
or more and whose forward
current is equal to or more than
the load current.
Use zener voltage for the
zener diode equal to or more
than the power supply voltage.
MELSEC-Q
Remarks
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.
*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)
The recovery time is later than
the CR method.
The diode method is effective
when the recovery time is too
late.
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