FLOATING DC CURRENT SOURCE FOR A SUBSCRIBER LOOP CIRCUIT

摘要

A FLOATING DC CURRENT SOURCE FOR A SUBSCRIBER LOOP CIRCUIT by CONSTANTINE VICTOR SKIDANENKO and GREGOR DONALD McGIBBON Under an off-hook condition a pulse width control circuit is periodically turned on by narrow pulses from a clock source. This initiates current flow from a primary dc source via the single primary winding of a transformer, and through power transistors connected in a Darlington configuration. A ramp voltage is developed across an emitter resistor of one of said power transistors and this voltage is feedback to said pulse width control circuit. When the ramp voltage reaches a predetermined value, the pulse width control circuit turns off the power transistors and the energy stored in the transformer is transferred by the collapsing magnetic field into alternating current in the secondary winding. This alternating current is rectified and filtered to provide loop current to the telephone subscriber loop. The voltage reflected from the secondary to the primary is stored in a peak detector circuit which operates to control the length of time during which the ramp voltage builds to its predetermined amplitude. The magnitude of the reflected voltage is a function of the loop resistance. Thus, a controlled loop curent is provided which is dependent upon the loop resistance. When the loop resistance reaches a predetermined value, a breakdown diode in the output of the peak detector allows current to flow in an "idle power shutdown circuit" which interrupts the clock pulse input to the pulse width control circuit. This permits a reduction in power during the on-hook state.