Telephone subscriber's line circuit - has earth key detection with increased resistance if key is held too long

摘要

A subscriber's line (b, a) is connected via the hybrid transformer (U) to a resistive transmission ridge consisting of a capacitor (C) shunted by a resistor (Rh) and two resistors (Rb, Ra) connected to ground and battery (-UB) respectively. Operation of the subscriber's earth key is detected by an operational amplifier (V) with its input across the capacitor and its output compared with a threshold (UtHl) by a second op-amp (K). If teh subscriber holds this key down too long, a contact (po) opens to put a high resistance (Ri) in series with the battery supply to the line. For test, the contact can be opened with the earth key not operated. Output from the detector then indicates faulty line insulation. If line insulation appears O.K., a lower threshold (Uth2) can then be used to check that the contact (po) is operating correctly.