Publication 1762-UM001D-EN-P - March 2004
2-6 Installing Your Controller
Power Supply Inrush
During power-up, the MicroLogix 1200 power supply allows a brief
inrush current to charge internal capacitors. Many power lines and
control 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 1200 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.
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