Wiring - Hitachi MICRO-EHV+ Applications Manual

4.2

Wiring

(1) Separation of power system
Several different power sources are used with PLC, such as main power of PLC, power for I/O signal and power
for external devices. These power sources should be separated as much as possible.
If these power sources come from one power source, install transformers or noise filters to separate those power
lines as much as possible.
(2) Fail safe
1] Construct an interlock circuit outside the PLC.
When the PLC power supply is turned ON/OFF, the lag time and the difference in the startup time between the
basic unit's power and the expansion unit's power or the external power (particular DC power supply) for the
PLC I/O signals may temporarily cause the I/O not to operate normally.
For this reason, apply the power to the expansion unit before the basic unit or apply the power to the basic unit
and the expansion unit at the same time. In addition, the external power (particular DC power supply) for the
PLC I/O signals should be applied before the PLC units.
Also, it is conceivable that a fault in the external power and a failure in the PLC unit lead to abnormal actions.
To prevent such actions from causing abnormal operation the entire system, and from a point of view of creating
a fail safe mechanism, construct circuit such as an emergency stop circuit, the protect circuit, and the interlock
circuit, for the sections that lead to a mechanical breakdown and accident from abnormal actions outside the
PLC.
2] Install a lightning arrester
To prevent damage to equipment as a result of being struck by lightning, we recommend setting up a lightning
arrester for each PLC power supply circuit.
MICRO-EHV+ series PLC detects power failures from a voltage drop of the internal 3.3 V DC power supply. For
this reason, the load in the 3.3 V DC power of the unit is light, the 3.3 V DC is retained for a long time and
operations may continue for more than 100ms. Therefore, when using the AC input unit, an OFF delay timer for
coordinating with the internal 3.3 V DC is needed because the AC input signal turns off more quickly than the
internal 3.3 V DC.
Main power supply
100 V AC
to
200 V AC
Transformer
Transformer
Figure 4.3 Example of power system diagram
4 – 3
Chapter 4
NF
Power for PLC unit
NF: Noise filter
NF
Power for I/O signals
NF
Power for general equipment
Installation