Mitsubishi M80 series Connection And Set Up Manual page 142

M800S/M80 Series Connection and Setup Manual
4 General Specifications
(2) 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.
Capacitor +
resistance
method (CR
method)
Diode
method
Diode + zener
diode method
Varistor
method
(Note 1) On AC power supply, the impedance of the CR needs to be sufficiently higher than that of the
POINT
Circuit example
(Note 1)
Capacitor
Resistor
Capacitor
Resistor
Diode
Diode
Zener Diode
Varistor
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.
Capacitor
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.
Element selection criteria
Estimate the constants of a capacitor
and resistance with the following as a
guide. Some differences, however, may
arise from a variation in the nature and
Inductive
characteristics of the load.
load
・Capacitor: 0.5 to 1 (μF) for a load
current of 1A
・Resistance: 0.5 to 1 (Ω) for a power
supply voltage of 1V
Use a capacitor whose withstand
Inductive
voltage is higher than the rated voltage.
load
In an AC circuit, use a capacitor with no
polarity.
Use a diode that satisfies the following
conditions:
・A reverse breakdown voltage is more
than ten times as high as the circuit
Inductive
load
voltage.
・ A forward current is more than twice as
high as the load current.
Use a zener diode whose zener voltage
Inductive
is higher than the power supply voltage.
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)× √
Inductive
2 (on AC power supply)
load
Note that selecting an element of a too
high Vc leads to a weaker effect.
Inductive
load
105
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
Capacitor
load
IB-1501269-F