| 摘要 |
An a.c. input circuit for converting a sensed a.c. voltage into a logic level voltage includes a rectifier for converting the sensed a.c. voltage to a d.c. voltage which is supplied through a coupling impedance to the light emitter of an optical isolator. While the a.c. input voltage is below a first predetermined voltage, a switchable low impedance circuit path in parallel with the light emitter is rendered conductive to maintain a low impedance across the light emitter to prevent the light emitter from becoming conductive. A switchable high impedance circuit path in parallel with the switchable low impedance circuit path becomes conductive once the sensed a.c. input voltage reaches the predetermined voltage to drive the light emitter into hard conduction. The switchable high impedance circuit path remains conductive until the sensed a.c. input voltage drops below a second predetermined value, at which point the light emitter is turned off and the switchable low impedance path becomes conductive.
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