Importance

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Abstract

A telephone apparatus powered directly from the telephone lines without external power includes a current converter in which current from a telephone line is limited by a transistor, current-sense resistor, and optical coupler acting together as a constant current device. This limited current is then converted from direct current (d.c.) to alternating current (a.c.) at the primary of a current transformer by power switching transistors driven by a flip-flop which is in turn driven by a free-running constant frequency oscillator. The alternations are symmetrical and their period or timing controlled so the current transformer does not saturate, thus maintaining maximum efficiency of conversion. The transformer outputs, after being rectified and filtered, are isolated from the primary, and provide d.c. current to maintian the charge on a battery across one output and to power telephone circuitry within a telephone device connected to the d.c. outputs. An optical coupler acting as an isolated current switch turns the constant current device off and on to facilitate pulse dialing. The current converter boosts the low telephone line current to a higher current or voltage for use by telephone devices such as pay phones, key phones, modems, or other telephone line connected devices.

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First Claim