Mitsubishi Electric NZ2MFB1-32D, User Manual

Page: 50 / 126

48
6 INSTALLATION AND WIRING
6.1 Before Using the I/O Modules
*1 A typical discharge lamp circuit is configured with a combination of discharge tubes, transformers, choke coils, capacitors and others.
Because of this, be especially careful of the case of a high power factor and a low power supply impedance, where the inrush current
flowing into the output module can be 20 to 40 times as high as the rated current.
*2 When the wiring is long, be careful with the cable capacity as well.
■
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.
Capacitive load Capacitive load
*2
i: Inrush current
io: Rated current
t: 0.008 to 0.33 seconds (0.5 to 2 cycle)
Approx. 20 to 40
times
 
Circuit example Element selection criteria Remarks
Cap
resistance method
(CR method) 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 characteristics
of the 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
polarity.
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.
Diode method Use a diode that satisfies the following
conditions:
• A reverse breakdown voltage is ten or
more times as high as the circuit voltage.
• A forward current is two or more times as
high as the load current.
The recovery time is delayed than the CR
method.
Diode + zener diode
method Use a zener diode whose zener voltage is
equivalent to or higher than the power
supply voltage. This method is suitable for the case where
the diode method results in a substantial
delay in the recovery time.
Varistor method Select a varistor whose cut-off voltage (Vc)
satisfies the following condition:
• Vc > Power supply voltage

1.5 (V)
• Vc > Power supply voltage

1.5 (V)
 
2
(on AC power supply)
Note that selecting an element of an
excessively high Vc leads to a weaker
effect.
The recovery time is a little delayed.
Load type Waveform Inrush current
i/rated current
io Waveform Inrush current
i/rated current
io
io
t
i
*1
Inductive
load
Inductive
load
Inductive
load
Inductive
load
Inductive
load