Isolated Circuit Digitally Indicates 120-/220-V Line Voltage (.PDF Download)

For applications such as motor control or power supplies, it’s often necessary to know whether the ac line is at 120 V or 220 V to adjust the operation. The circuit illustrated in the figure monitors the ac line and provides a basic output indicating whether it is at 120 V or 220 V, with the output at the transistor collector going low for 120-V and high for 220-V inputs...

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