Bridge lifter for telephone line

Claim 2

An electronic bridge lifter adapted for installation in transmission loops multiply connected to a common transmission line and responsive to levels of currents in said loops for isolating idle ones of said loop from said line and providing a low-loss connection to said line for active ones of said loops, CHARACTERIZED IN THAT said bridge lifter comprises: first impedance means serially connected in a respective said loop for isolating said loop from said line; semiconductor switch means for shunting said first impedance means with a low-loss path when said switch means is in a closed state, said switch means being in a normally open state; and loop current measuring means for measuring said level of current in said respective loop and activating said switch means to shunt said first impedance means when said level exceeds a preselected threshold and thereby connect said loop to said line through said low-loss path.

Claim 3

Apparatus in accordance with claim 1 wherein said first impedance means comprises a first resistor; said switch means comprises a first transistor having base, emitter, and collector electrodes with said emitter and collector electrodes connected to respective terminals of said first resistor; and said measuring means comprises a second resistor serially connected in said loop, a second transistor having base, emitter and collector electrodes with said emitter and base electrodes connected to respective terminals of said second resistor and said collector electrode connected to said base electrode of said first transistor, said second resistor having a value sufficient to hold said level of current below said threshold when said loop is open and idle and allowing said level of current to exceed said threshold when said loop is closed and active.

Claim 4

Apparatus in accordance with claim 1 wherein said first impedance means includes first and second resistors serially connected in said loop; said switch means comprises first and second transistors hAving respective base, emitter and collector electrodes with said emitter and collector electrodes of said first and second transistors connected to respective terminals of said first and second resistors, respectively; and said measuring means comprises third and fourth resistors serially connected in said loop, third and fourth transistors having base, emitter and collector electrodes with said emitter and base electrodes of said third and fourth transistors connected to respective terminals of said third and fourth resistors, respectively, and said collector electrodes of said third and fourth electrodes connected to said base electrodes of said second and first transistors, respectively, said third and fourth resistors have a total value sufficient to hold said level of said current below said threashold when said loop is open and idle and allowing said level of current to exceed said threshold when said loop is closed and active.

Claim 5

Apparatus in accordance with claim 3 including: first and second diodes connected between said collector electrodes of said first and second transistors, respectively, and said respective terminals of said first and second resistors, respectively; and third and fourth diodes connected between said collector electrodes of said third and fourth transistors, respectively, and said base electrodes of said second and first transistors, respectively.

Claim 6

Apparatus in accordance with claim 4 including: fifth and sixth resistors connected between said collector electrodes of said fourth and third transistors, respectively, and said base electrodes of said first and second transistors, respectively; and seventh and eighth resistors connected between said emitter and base electrodes of said first and second transistors, respectively.

Claim 7

Apparatus in accordance with claim 3 including: fifth, sixth, seventh and eighth transistors having base, emitter, and collector electrodes and having polarities opposite to the polarities of said first, second, third and fourth transistors, respectively; said fifth and sixth transistors having said emitter and collector electrodes connected with said respective terminals of said first resistor and second resistors, respectively, in parallel with said first and second transistors, respectively; said seventh and eighth transistors having said emitter and base electrodes connected to respective terminals of said third and fourth resistors, respectively, in parallel with said third and fourth transistors, respectively; said collector electrodes of said third and seventh transistors are connected to said base electrodes of said second and sixth transistors; and said collector electrodes of said fourth and eighth transistors are connected to said base electrodes of said first and fifth transistors, whereby said bridge lifter operates with either polarity of said current in said loop.

Claim 8

Apparatus in accordance with claim 6 including: first, second, fifth and sixth diodes connected between said collector electrodes of said first, second, fifth, and sixth transistors, respectively, and said respective terminals of said first and second resistors, respectively; and third and fourth diodes and seventh and eighth diodes connected between said collector electrodes of said third and fourth transistors and said seventh and eighth transistors, respectively, and said base electrodes of said second and sixth transistors and said first and fifth transistors, respectively.

Claim 9

Apparatus in accordance with claim 7 including: fifth and sixth resistors connected between said collector electrodes of said fourth and eighth transistors and said third and seventh transistors, respectively, and said base electrodes of said first and fifth transistors and said second and sixth transistors, respectively; and seventh and eighth resistors connected between said base and emitter electrodes of said first and fifth transistors and said second and sixth transistors, respectively.

Claim 10

Apparatus in accordance with claim 8 including: a first bipolar diode means connected between said emitter electrodes of said first and fifth transistors and said base electrodes of said third and seventh transistors; and a second bipolar diode connected between said emitter electrodes of said second and sixth transistors and said base electrodes of said fourth and eighth transistors, whereby said bridge lifter is protected from current and voltage surges in said loop.

Claim 11

Apparatus in accordance with claim 9 wherein said first and second bipolar diode means comprise back-to-back Zener diodes.

Claim 12

In a communications system comprising a transmission line, a plurality of subscriber loops bridged across said line and terminated by a telephone set, bridge lifting apparatus in said loops that differentiates between levels of current in said loops to permit the transmission of signals to respective said sets when off-hook and to substantially block said signals when respective said sets are on-hook CHARACTERIZED IN THAT said bridge lifting apparatus comprises an electronic bridge lifter including: first impedance means serially connected in a respective said loop for blocking said signals and isolating said loop from said line; semiconductor switch means for shunting said first impedance means, said switch means being in a normally open state; and loop current measuring means for measuring said levels of current in said respective loops and activating said switch means to shunt said first impedance means when said level exceeds a preselected threshold thereby to allow said signals to reach said set.

Claim 13

In a communications system comprising a transmission line, a plurality of subscriber loops bridged across said line and terminated by a telephone set, bridge lifting apparatus in said loops that differentiates between levels of current in said loops to permit the transmission of signals to respective said sets when off-hook and to substantially block said signals when respective said sets are on-hook CHARACTERIZED IN THAT said bridge lifting apparatus comprises an electronic bridge lifter including: first impedance means serially connected in a respective said loop for blocking said signals and isolating said loop from said line; semiconductor switch means for shunting said first impedance means, said switch means being in a normally open state; and loop current measuring means for measuring said levels of current in said respective loops and activating said switch means to shunt said first impedance means when said level exceeds a preselected threshold thereby to allow said signals to reach said set.