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Line Current Monitoring Circuit for Call Logging

IP.com Disclosure Number: IPCOM000060373D
Original Publication Date: 1986-Mar-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 2 page(s) / 36K

Publishing Venue

IBM

Related People

Taub, DM: AUTHOR

Abstract

A line current monitoring circuit for call logging senses the current in both wires of the line differentially, using a DC-to-DC converter circuit to provide rectified voltage across resistors in both wires. A telephone line with two wires 1 and 2 connecting subscriber to selector has series resistors 3 and 4 in the wires. Across the resistors 3 and 4 are connected pairs of diodes 5, 6 and 7, 8 and capacitors 9 and 10. A DC-to-DC converter to provide rectified voltage across the resistors 3 and 4 includes a transformer 11 having a center-tapped primary winding 12. A pulsed DC voltage is applied through transistors 13 and 14 to drive the converter circuit. Two center-tapped secondary windings 15 and 16 are connected across the respective resistors 3 and 4 through the center taps and diodes 17, 18 and 19, 20.

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Line Current Monitoring Circuit for Call Logging

A line current monitoring circuit for call logging senses the current in both wires of the line differentially, using a DC-to-DC converter circuit to provide rectified voltage across resistors in both wires. A telephone line with two wires 1 and 2 connecting subscriber to selector has series resistors 3 and 4 in the wires. Across the resistors 3 and 4 are connected pairs of diodes 5, 6 and 7, 8 and capacitors 9 and 10. A DC-to-DC converter to provide rectified voltage across the resistors 3 and 4 includes a transformer 11 having a center-tapped primary winding 12. A pulsed DC voltage is applied through transistors 13 and 14 to drive the converter circuit. Two center-tapped secondary windings 15 and 16 are connected across the respective resistors 3 and 4 through the center taps and diodes 17, 18 and 19, 20. The center tap of the primary winding 12 provides an output voltage on conductor 21 which is referenced to a +5V supply through a resistor 22. The circuit is able to discriminate between the minimum off-hook line current and the maximum on-hook line current or line leakage current. The change of voltage across the resistor 22 is substantial relative to the change of line current that causes it, i.e., a good transfer resistance is obtainable. The circuit is balanced with respect to the two wires of the line and is insensitive to common-mode current, giving a true measure of the loop current.

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