| 摘要 |
In a telephone system wherein a polarity reversible dc voltage is provided by a central office, a telephone outpulsing circuit utilizing field-effect transistors (21-24) performs the additional function of a polarity guard. Arranged in a rectifying configuration, the field-effect transistors connect the telephone line (10, 11) from a central office to a telephone set requiring a single polarity. The low ON resistance of the field-effect transistors consumes relatively little power from the central office, thus allowing a higher voltage to the telephone set than is available with a conventional polarity guard. The circuit also performs a switchhook flash function and senses the voltage on the telephone line such that if a predetermined level is exceeded, the field-effect transistors are turned OFF and this higher voltage is prevented from reaching the telephone set.
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