Mitsubishi Electric MELSEC iQ-R Series User Manual page 102

■Measures against back EMF
Provide a contact protection circuit for an extended contact life, noise prevention at contact close, and reduction of the
carbides and nitric acids formed by an arc discharge.
An incorrect circuit involves a high risk of contact welding.
With the contact protection circuit, the recovery time may be delayed.
The following table shows typical examples of the contact protection circuit.
Circuit example
Capacitor +
resistance method
(CR method)
Diode method
Diode + zener diode
method
Varistor method
*1 On AC power supply, the impedance of the CR needs to be sufficiently higher than that of the load (for preventing errors due to the
leakage current of the CR).
• Avoid using contact protection circuits like the following. Although highly effective in reducing the arc at
current cutoff, a charge current flows into the capacitor when the contact turns on or off, which leads to the
risk of contact welding. A DC inductive load, generally considered to be more difficult to open and close than
a resistive load, can achieve the same performance of a resistive load in an appropriate configuration of the
protection circuit.
• Install the protection circuit near the load or contact (module). A long distance between them may inhibit the
effect of the protection circuit. As a guide, install it at a distance of no more than 50cm.
6 INSTALLATION AND WIRING
100
6.1 Before Using the I/O Modules
Element selection criteria
Estimate the constants of a capacitor and
*1
resistance with the following as a guide.
Some differences, however, may arise from
a variation in the nature and characteristics
Inductive of the load.
load
• Capacitor: 0.5 to 1 (F) for a load current
of 1A
• Resistance: 0.5 to 1 () for a power
supply voltage of 1V
Use a capacitor whose withstand voltage is
equal to or higher than the rated voltage. In
an AC circuit, use a capacitor with no
Inductive
polarity.
load
Use a diode that satisfies the following
conditions:
• A reverse breakdown voltage is more
Inductive
than ten times as high as the circuit
load
voltage.
• A forward current is more than twice as
high as the load current.
Use a zener diode whose zener voltage is
higher than the power supply voltage.
Inductive
load
Select a varistor whose cut-off voltage (Vc)
satisfies the following conditions:
• Vc > power supply voltage  1.5 (V)
• Vc > power supply voltage  1.5 (V)  2
Inductive
(on AC power supply)
load
Note that selecting an element of a too high
Vc leads to a weaker effect.
Inductive
load
Remarks
When a relay or solenoid is used as the
load, the recovery time is delayed.
A capacitor has the effect of reducing a
discharge at contact OFF, while a
resistance has the effect of limiting a
current at contact ON.
The recovery time is delayed than the CR
method.
This method is suitable for the case where
the diode method results in a substantial
delay in the recovery time.
The recovery time is a little delayed.
Inductive
load