Balanced constant current sensing circuit inherently immune to longitudinal currents

• US 4,581,691 A
• Filed: 04/23/1984
• Issued: 04/08/1986
• Est. Priority Date: 04/23/1984
• Status: Expired due to Term

• Current Assignee: Bell Telephone Laboratories, Inc. (Nokia Corporation)
• Original Assignee: AT&T, Inc.
• Inventors: Hock, Richard H.
• Primary Examiner(s): Salce, Patrick R.
• Assistant Examiner(s): REBSCH, DONALD
• Application Number:
  US06/603,072
• Time in Patent Office: 715 Days
• Field of Search: 179/18 FA, 179/27 G, 179/170 D, 363/19, 363/21, 363/22, 363/97
• US Class Current: 363/21.12
• CPC Class Codes: H04M 19/008 Using DC/DC converters DC/DC converters per se H02M3/28

Abstract

A current converter circuit that supplies send and return currents to a balanced transmission line includes current sensing circuitry that responds to a current unbalance or a differential current, independently of longitudinal currents in the balanced line. A reference voltage and two inputs to a comparator monitor the voltage drop across a voltage divider connecting the send and return leads of the transmission line. The voltage divider has a grounded center tap so that longitudinal currents flow to ground and provide equal and opposite polarity voltages across the divider, which cancel each other. Unbalanced currents provide a corresponding differential voltage input to the comparator, whose control voltage output is utilized to initiate corrective action in a current control circuit.

Claims

1. A current converter coupled to energize a transmission line and including:

current control circuitry for limiting differential current in a transmission line having send and return leads comprising;

an amplifier circuit including first and second inputs,

a reference ground, a first resistance coupling the send lead to the reference ground,

a second resistance coupling the return lead to the reference ground,

a reference voltage source connected to the first resistance and to the first input of the amplifier circuit, the reference voltage source connecting a terminal of the first resistance other than that connected to the reference ground to the first input of the amplifier circuit, and

the second input of the amplifier circuit being directly connected to a terminal of the second resistance other than that connected to the reference ground.

4. A current converter as defined in claim 1 wherein the first and second resistance are of equal value.

2. A current converter coupled to energize a pair of conductors and including:

circuitry for detecting differential current flow while rejecting longitudinal current in the pair of conductors comprising;

a first resistor for generating a first voltage in response to current flow in a first line of the pair of conductors,

a second resistor for generating a second voltage in response to current flow in a second line of the pair of conductors,

the first and second resistor each having a first and second terminal,

means for combining the first and second resistors so that differential current flow causes the first and second voltages to be additive and longitudinal current flow causes the first and second voltage to cancel each other,

the means for combining comprising a grounded reference node connected to a first terminal of each of the first and second resistors,

means for accepting a reference voltage,

means for amplifying the first voltage as summed with the reference voltage with the second voltage,

the means for amplifying comprising an operational amplifier having an inverting and non-inverting input,

the means for accepting the reference voltage coupling a second terminal of the first resistor to the non-inverting input, and

the second terminal of the second resistor being connected to the inverting input.

3. A current converter coupled to energize a balanced transmission line including:

current control circuitry for monitoring a current balance between send and return leads of the transmission line comprising;

first and second current sensing means for generating voltages responsive to current levels and each coupling a ground of the balanced transmission line to the send and return leads respectively, so that voltages due to longitudinal currents nullify each other and voltages due to differential currents are additive,

the first and second current sensing means comprising first and second resistors connected to the ground of the balanced transmission line and having their non-grounded ends connected to the send and return lead respectively and the first and second resistors being of equal magnitude,

a reference voltage source,

a comparator circuit,

the reference voltage source being connected between the first resistor and the comparator circuit,

the comparator circuit having a first terminal connected to the first current sensing means and to the reference voltage and a second terminal connected to the second current sensing means, and

converter control means responsive to the comparator circuit.