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Compact I/O Expansion Power Supplies
The only effect of limited inrush current and voltage sag on the system is that the power supply
capacitors charge more slowly. However, the effect of a voltage sag on other equipment must be
considered. For example, a deep voltage sag can reset a computer that is connected to the same
power source. The following considerations determine whether the power source must be
required to supply high inrush current:
• Power-up sequence of devices in a system
• Amount of the power source voltage sag if the inrush current cannot be supplied
• Effect of voltage sag on other equipment in the system
If the entire system is powered up simultaneously, a brief sag in the power source voltage typically
does not affect any 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 operational during power loss is called "program scan hold-up
time after loss of power." The duration of the power supply hold-up time depends on the type
and state of the I/O, but is typically between 5 milliseconds...10 seconds. When the duration of
power loss reaches this limit, the power supply signals the processor that it can no longer provide
adequate DC power to the system. This condition is referred to as a power supply shutdown.
The processor then performs an orderly shutdown of the controller.
Input States on Power Down
The power supply hold-up time is longer than the turn-on and turn-off times of the inputs.
Because of this, the processor can record the input state change from 'On' to 'Off ' that occurs
when power is removed before the power supply shuts down the system. Comprehension of this
concept is important. Write the program to compensate for this effect.
Other Types of Line Conditions
Occasionally the power source to the system can be temporarily interrupted. It is also possible
that the voltage level can drop substantially below the normal line voltage range for a time. Both
of these conditions are considered to be a loss of power for the system.
User Power Overcurrent Condition
The power supply shuts down if there is an overcurrent condition. All outputs latch off and
remain off until you remove the overcurrent and cycle power. Reload the user program following
a power supply shutdown.
ATTENTION: To avoid unexpected operation due to 24V DC user power shutdown (1769-PA2
only), monitor the 24V DC user output with a 24V DC input channel.
16
Rockwell Automation Publication 1769-IN028C-EN-P - August 2016
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