| 摘要 |
The magnitude of current supplied to a subscriber telephone loop during both loop start and ground start modes of operation is controllably limited by employing a battery feed circuit including an adjustable line build-out impedance. Resistors are controllably switched in or out of the battery feed circuit leads as a function of the potential developed across the line build-out impedance in a predetermined one of the battery feed circuit leads. Since the line build-out impedance being adjusted in value is within the detection loop, there is hysteresis in the switch points for switching the resistors in and out of the battery feed circuit leads. This hysteresis minimizes possible oscillation at the switch points and, therefore, minimizes development of unwanted signals on the subscriber loop. The use of an adjustable line build-out impedance further allows use of small resistance value, low wattage resistors as line build-out impedance elements. This improves overall efficiency and allows use of longer subscriber loops.
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