Longitudinally balanced, dual voltage line-feed circuit
First Claim
1. A power-conserving, current-limiting line-feed circuit for a telephone subscriber loop, comprising:
- current controlling transistor means (Q101, Q102) for connecting one of the conductors (Ring) of said telephone loop with a source of dc voltage to supply a current to said loop determined by a source of reference potential;
a resistor (R101) in circuit with said current controlling transistor means and a source of dc voltage;
first means (OAA, Q103, OAC, Q105) for driving said current controlling transistor means to limit the current in said loop to a first value in accordance with the dc voltage developed across said resistor by said current supplied to said one of said conductors when said loop is off hook;
second means (C3, R122, R110, OAD, R111, OAC) for driving said current controlling transistor means inversely in accordance with the presence of any ac voltage developed between said one of said conductors and said source of dc voltage; and
third means (VR) for driving said current controlling means to limit the current in said loop to a second value when said loop is on hook.
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Accused Products
Abstract
A power-conserving, loop start signalling, dual-voltage line-feed circuit that maintains ac longitudinal balance on the loop conductors and which limits average and peak loop current not only when the loop is in the off-hook transmission state but also during part-time on-hook transmission. The current-limiting circuitry employs feedback so that a virtual ground is presented toward the line feed transformer at voice frequencies to reduce battery noise and potential crosstalk that might otherwise couple into the circuit because of non-zero battery impedance. The circuit provides for injecting a voltage-controlled feed current into the current regulator to maintain sufficient drive current so the current regulator can continue to maintain the virtual ground when the loop is on hook. The circuit employs a battery switch and switch hook state detector which controls the connection of a higher voltage battery to the loop when the loop is on-hook and a lower voltage battery when the loop is off-hook to conserve power. To reduce the likelihood of oscillation between on and off-hook indications, the switchhook status detector responds to a loop resistance threshold that is lower when the high voltage battery is being used than when the low voltage battery is being used.
86 Citations
24 Claims
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1. A power-conserving, current-limiting line-feed circuit for a telephone subscriber loop, comprising:
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current controlling transistor means (Q101, Q102) for connecting one of the conductors (Ring) of said telephone loop with a source of dc voltage to supply a current to said loop determined by a source of reference potential; a resistor (R101) in circuit with said current controlling transistor means and a source of dc voltage; first means (OAA, Q103, OAC, Q105) for driving said current controlling transistor means to limit the current in said loop to a first value in accordance with the dc voltage developed across said resistor by said current supplied to said one of said conductors when said loop is off hook; second means (C3, R122, R110, OAD, R111, OAC) for driving said current controlling transistor means inversely in accordance with the presence of any ac voltage developed between said one of said conductors and said source of dc voltage; and third means (VR) for driving said current controlling means to limit the current in said loop to a second value when said loop is on hook.
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2. A power-conserving, current-limiting line-feed circuit for a telephone subscriber loop comprising:
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a source of dc voltage (HVBAT, LVBAT); a source of switched reference potential exhibiting one reference potential when said loop is off-hook and a different reference potential when said loop is on-hook; current controlling transistor means (Q101, Q102) for connecting one of the conductors (Ring) of said telephone loop with said source of dc voltage to supply a current determined by said source of reference potential; a resistor (R101) in circuit between said current controlling transistor means and said source of dc voltage; first means (OAA, Q103, OAC, Q105) for driving said current controlling transistor means in accordance with the dc voltage developed across said resistor by said current supplied to said one of said conductors, said first means driving said current controlling transistor means to limit said current supplied to said one of said conductors to one value when said loop is off-hook and to another value when said loop is on-hook; and second means (C3, R122, R110, OAD, R111, OAC) for driving said current controlling transistor means inversely in accordance with the presence of any ac voltage developed between said one of said conductors and said source of dc voltage. - View Dependent Claims (3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. A power-conserving, current-limiting line-feed circuit for a telephone subscriber loop, comprising:
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current controlling transistor means (Darlingtons) for connecting one of the conductors (Ring) of said telephone loop to a source of dc voltage; a resistor (R101) in circuit between said current controlling transistor means and said source of dc voltage; a first source of reference potential operative when said loop is on hook and a second source of reference potential when said loop is off hook; and means (OAA, OAC, Q105) for driving said current controlling transistor means in accordance with the difference between said first source or second source of reference potential and the potential across said resistor to limit current through said loop. - View Dependent Claims (14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24)
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Specification