Mitsubishi Electric M800VS Series Connection And Set Up Manual page 157

M800VS/M80V Series Connection and Setup Manual
4 General Specifications
4.9 Remote I/O Unit
 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 + re-
sistance method
(CR method)
Diode method
Diode + zener di-
ode method
Varistor method
Note
(*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 con-
tact 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.
POINT
 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.
IB-1501613-A
Estimate the constants of a capacitor and resis-
(*1)
tance with the following as a guide. Some differ-
ences, however, may arise from a variation in
the nature and characteristics of the load.
Capacitor
Inductive
 Capacitor: 0.5 to 1 (μF) for a load current of 1 A
load
Resistor  Resistance: 0.5 to 1 (Ω) for a power supply
voltage of 1 V
Use a capacitor with voltage resistance of the
rated voltage or higher. In an AC circuit, use a
capacitor with no polarity.
Capacitor
Inductive
load
Resistor
Use a diode that satisfies the following condi-
tions:
 A reverse breakdown voltage is more than ten
Inductive
Diode
times as high as the circuit voltage.
load
 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.
Diode
Inductive
load
Zener Diode
Select a varistor whose cut-off voltage (Vc) sat-
isfies the following conditions:
 Vc > power supply voltage × 1.5 (V)
 Vc > power supply voltage × 1.5 (V) × √ 2 (on
Inductive
AC power supply)
Varistor
load
Note that selecting an element of a too high Vc
leads to a weaker effect.
Inductive
Capacitor
load
126
Element selection criteria
Inductive
Capacitor
load
Remarks
When a relay or solenoid is
used as the load, the recov-
ery time is delayed.
A capacitor has the effect of
reducing a discharge at
contact OFF, while a resis-
tance has the effect of limit-
ing a current at contact ON.
The recovery time is de-
layed than the CR method.
This method is suitable for
the case where the diode
method results in a substan-
tial delay in the recovery
time.
The recovery time is a little
delayed.