VOLTAGE AND CURRENT PROTECTION ARRANGEMENT FOR A TELEPHONE SUBSCRIBER LINE INTERFACE CIRCUIT

摘要

A protection arrangement for a telephone subscriber line interface circuit is disclosed. The arrangement is particularly useful for protecting an electronic telephone set from over-voltage and over-current fault conditions. The arrangement provides a FET (34) that operates in saturation mode to connect an office battery (22) to the subscriber line under normal operation. The FET (34) also provides isolation capabilities for protecting the line circuit from an over-current condition on the subscriber line. Over-voltage protection is provided by way of an isolation relay (12) between the line circuit and the subscriber line. Both the FET and isolation relay are operated by a controller (16) that uses timers in the methods of over-voltage and over-current protection that it performs. A further capability of the arrangement is that is resets itself after the fault condition has ended. This feature is particularly useful in the case of fault conditions of short duration.