Rockwell Automation 1768-PA3_PB3 CompactLogix Power Supplies Installation Instructions - Manual (Page 13)

1768 CompactLogix Power Supplies 13
Publication 1768-IN001B-EN-P - July 2006
Additional considerations determine whether the power source must be required to
supply high inrush current.
• The power-up sequence of devices in a system
• The amount of the power-source voltage sag if the inrush current cannot be
supplied
• The effect of voltage sag on other equipment in the system
If the entire system is powered-up at the same time, a brief sag in the power source
voltage typically does not affect equipment.
Loss of Power Source
The power supply is designed to withstand brief power losses without affecting the
operation of the system. The time the system is fully operational during power loss
is called the ride-through time. The duration of the ride-through time depends on
the type and state of the I/O but is typically 25 ms for the 1768-PA3 power supply
and 5 ms for the 1768-PB3 power supply. At the end of the ride-through, the power
supply’s PWR LED status indicator toggles to indicate the end of the ride-through
time.
If the power supply remains without power at the end of the ride-through time,
another period of time, the hold-up time, begins. During the hold-up time, the
power supply continues to partially power the system. The hold-up time is typically
5 ms for both power supplies.
When the duration of power loss extends beyond the hold-up time, the power
supply can no longer provide adequate dc power. At this point, the power supply
must shut down and the system performs an orderly shutdown of the controller.
During the orderly shutdown of the controller, a time period called the extended
hold-up time occurs. The extended hold-up time lasts from 8 to 12 seconds. During
this time, the power supply provides power only to the controller. The I/O,
communication, and motion modules will not receive power.
Input States on Power Down
The power supply hold-up time as described above is generally longer than the
turn-on and turn-off times of the inputs. The input-state of the controller changes
from on to off when power is removed. The controller may then record the state
before the power supply shuts down the system. This concept is important and
should be taken into account when writing the program.