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AC Line Under/Over Voltage Alarm

2016-02-20 04:57  
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Power lines, which deviate much beyond normal voltages can damage expensive electronic equipment.  The circuit below sounds an alarm whenever the line voltage is higher or lower than normal.  I set the alarm limits at about -15% from standard levels.  The circuit rectifies and filters the power line signal.  I set the resistor values, so the DC voltage produced is close to 1% of the RMS value of the line.  Thus, a 120vac line would yield about 1.2v DC.  That voltage is fed to a pair of voltage comparators. The outputs of the comparators decide if the sampled voltage is above, below or acceptable.  Three LEDs indicate the voltage range.  A green LED is lit if the voltage is OK.   If the voltage is outside normal levels, the circuit sounds an alarm using a small magnet or piezoelectric beeper.  I have shown recommended resistor values for both 120vac and 240vac power lines.



AC Line Under/Over Voltage Alarm Circuit





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