Rockwell Automation 1762-Lxxxx MicroLogix 1200 Programmable Controllers - Manual (Page 20)

Publication 1762-UM001G-EN-P - March 2011
2-6 Install 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.
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.