22 Rockwell Automation Publication 1766-UM001O-EN-P - September 2021
Chapter 2 Install Your Controller
transformers can supply inrush current for a brief time. If the power source
cannot supply this inrush current, the source voltage may sag momentarily.
The only effect of limited inrush current and voltage sag on the MicroLogix
1400 is that the power supply capacitors charge more slowly. However, the
effect of a voltage sag on other equipment should be considered. For example,
a deep voltage sag may reset a computer connected to the same power source.
The following 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 will 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 10 milliseconds and 3 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 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 as described above is generally longer than the
turn-on and turn-off times of the inputs. Because of this, the input state
change from On to Off that occurs when power is removed may be recorded by
the processor before the power supply shuts down the system. Understanding
this concept is important. The user program should be written to take this
effect into account.
Other Types of Line Conditions
Occasionally the power source to the system can be temporarily interrupted. It
is also possible that the voltage level may drop substantially below the normal
line voltage range for a period of time. Both of these conditions are considered
to be a loss of power for the system.
Prevent Excessive Heat For most applications, normal convective cooling keeps the controller within
the specified operating range. Ensure that the specified temperature range is
maintained. Proper spacing of components within an enclosure is usually
sufficient for heat dissipation.
In some applications, a substantial amount of heat is produced by other
equipment inside or outside the enclosure. In this case, place blower fans
inside the enclosure to assist in air circulation and to reduce hot spots near the
controller.
Additional cooling provisions might be necessary when high ambient
temperatures are encountered.
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