Rockwell Automation Publication 2080-UM001B-EN-E - November 2011 9
About Your Controller Chapter 2
Power Considerations
The following explains power considerations for the micro controllers.
Isolation Transformers
You may want to use an isolation transformer in the AC line to the controller.
This type of transformer provides isolation from your power distribution system
to reduce the electrical noise that enters the controller and is often used as a step-
down transformer to reduce line voltage. Any transformer used with the
controller must have a sufficient power rating for its load. The power rating is
expressed in volt-amperes (VA).
Power Supply Inrush
During power-up, the Micro800 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 Micro800 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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