AC Power Monitor

Posted on Feb 6, 2014

This AC Power Monitor continuously watches the AC power line voltage for both under-voltage and missing cycles. When it detects a total of 5 or 6 consecutive missing half-cycles (50mS, 50/60HZ), it drops a relay and starts a timer. A power contactor is slaved to the relay contacts. Its purpose is to protect power-loss sensitive industrial equipmen

AC Power Monitor
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t from brief power glitches by preventing immediate restart. After the specified time delay (2 to 10S), the equipment may automatically restart. Although this may seem like a simple task, note that the condition sensed is the absence of a periodic signal and that the timer may or may not have external power available. electroschematics. com is not really into industrial controls, but this may be the beginning ”this is where I spent most of my life. This function could also be performed by a PLC (programmable logic controller) with a UPS (unterruptable power source), but such would be expensive and consume additional panel space. Key to this functioning properly is its power-loss ride-through capability. Note that this circuit continues to time out even after the power is interrupted. To run the circuitry for the entire time period, energy is stored in a large filter capacitor (10, 000uf). That way if the power resumes during the timeout period, the timer remains alive and functioning so that the sensitive equipment may not restart. At the end of the timeout period, the bleeder circuit kicks in to dump the remaining charge. A 4000 series CMOS latch consisting of U3B & U3C is set by the positive transition of U1-3. Should the power resume while the capacitor is bleeding down, the latch is reset via the signal at the collector of Q3 so that the bleeder driver Darlington (Q1 & Q2) turns off. When all of the charge is dissipated...

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