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7 I/O MODULES
MELSEC-ST
(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.
• Cap Resistor method (CR method)
• Diode method
• Diode + Zener diode method
• Varistor method
Circuit example
Method for selecting elements
Remarks
*1
Capacitor
Resistor
Inductive
load
Cap
Resistor method
(CR method)
Capacitor
Resistor
Inductive
load
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.
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)
Diode method
Diode
Inductive
load
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.
The recovery time is later than
the CR method.
Diode + Zener
diode method
Diode
Inductive
load
Zener Diode
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.
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Summary of Contents for MELESEC-ST
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Page 23: ...A 21 A 21 MEMO...
Page 114: ...10 22 10 22 10 MOUNTING AND INSTALLATION MELSEC ST MEMO...
Page 125: ...11 11 11 11 11 EMC AND LOW VOLTAGE DIRECTIVE MELSEC ST MEMO...
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