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AC Line Under/Over Voltage Alarm
March 29, 2009 |
| 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. |
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