Rockwell Automation 1770 Industrial Automation Wiring and Grounding Guidelines - Manual (Page 12)

Industrial Automation Wiring and Grounding Guidelines 12
Publication 1770-4.1 – February 1998
Common Power Source for I/O
Unless each I/O of a module or block is individually isolated,
multiple I/O within the block or module share a common terminal
for one side of the power source. All I/O sharing a common
terminal must share a common power source (i.e., from the same
pole of a disconnect or from the same transformer tap).
If a module or block has multiple commons, each common and its
I/O may be isolated from the other commons. In that case, each
common and its I/O can have a separate power source. If each I/O is
individually isolated, each I/O can have a separate power source. If
a module or block has individually isolated I/O or multiple isolated
commons and multiple power sources are used, be certain that the
difference in potential between any two power sources does not
exceed the specified maximum continuous voltage that can be
applied between the channels.
Under-Voltage Shutdown
Each power supply with under-voltage shut-down protection
generates a shut-down signal on the backplane when the ac line
voltage drops below its lower voltage limit. The power supply
removes the shut-down signal when the line voltage comes back
above the lower voltage limit. This shut-down is to guard against
invalid data being stored in memory.
Because a capacitive-input power supply converting ac to dc draws
power only from the peak of the ac voltage wave-form, the external
transformer load (in VA) of each power supply is 2.5 times its real
power dissipation (in Watts). If the transformer is too small, the
peaks of the sine wave are clipped. Even if the voltage is still above
the lower voltage limit, the power supply senses the clipped wave as
low voltage and sends the shut-down signal.
Sizing the Transformer
To determine the required rating of the transformer add the
external-transformer load of the power supply and all other power
requirements (input circuits, output circuits). The power
requirements must take into consideration the surge currents of
devices controlled by the processor. Choose a transformer with the
closest standard transformer rating above the calculated
requirements.