Mitsubishi Electric NZ2MFB2-16A User Manual page 50

Load type Waveform
Capacitive load Capacitive load
i
i: Inrush current
io: Rated current
t: 0.008 to 0.33 seconds (0.5 to 2 cycle)
*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.
Circuit example
Capacitor +
resistance method
(CR method)
Diode method
Diode + zener diode
method
Varistor method
6 INSTALLATION AND WIRING
48
6.1 Before Using the I/O Modules
*2
io
t
*1
Inductive
load
Inductive
load
Inductive
load
Inductive
load
Inductive
load
Inrush current Waveform
i/rated current
io

Approx. 20 to 40
times
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 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.
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.
Use a zener diode whose zener voltage is
equivalent to or higher than the power
supply voltage.
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.
Inrush current
i/rated current
io

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.