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4-11-3 Programmable Controller Power Interruptions
A sequential circuit is built into the PC to handle power interruptions. This circuit
prevents malfunctions due to momentary power loss or voltage drops. A timing
diagram for the operation of this circuit is shown below.
The PC ignores all momentary power failures if the interruption lasts no longer
than 10 ms. If the interruption lasts between 10 and 25 ms, the interruption may
or may not be detected. If the supply voltage drops below 85% of the rated volt-
age for longer that 25 ms (less for the DC Power Supply), the PC will stop operat-
ing and the external outputs will be automatically turned OFF.
Operation is resumed automatically when the voltage is restored to more than
85% of the rated value. The diagram below shows the timing of PC operation
and stopping during a power interruption. The time it takes to detect the power
failure is shorter when the power supply is DC.
Power supply
Power
interrupted
Power
restored
Power failure detection signal
+5 V
CPU operating voltage
Power supply reset
Output during operation
Time lapse until
detection
1 s
System Design and Safety Considerations Section 4-11
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