| 摘要 |
The loss of an AC supply voltage is detected by sensing the amplitude of the AC voltage relative to the reference potential about which it periodically goes positive and negative. Whenever the amplitude of the signal is below a given threshold, a sub-threshold signal is produced and its duration is sensed. If its duration is shorter than a predetermined sampling period, the sub-threshold signal is treated as a normally occurring signal and is filtered out of the system. If its duration exceeds the predetermined sampling period, it indicates that the AC voltage has remained "low" for too long a period of time, and an error signal is generated.
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