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Zero DC Offset Transformer for Off Hook Sensing in Telephone Network Applications

IP.com Disclosure Number: IPCOM000085949D
Original Publication Date: 1976-Jun-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 31K

Publishing Venue

IBM

Related People

Bajorek, CH: AUTHOR [+3]

Abstract

A circuit senses the removal of a telephone receiver from its rest position on the hook, and includes a Hall sensor 10 in the gap 11 of an isolation transformer 12 driving a DC operational amplifier 13. Amplifier 13 operates an "off-hook" signal relay 14 and produces a bucking current via transistor 16 in the secondary winding 15 of transformer 12, which relieves DC saturation current from transformer 12.

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Zero DC Offset Transformer for Off Hook Sensing in Telephone Network Applications

A circuit senses the removal of a telephone receiver from its rest position on the hook, and includes a Hall sensor 10 in the gap 11 of an isolation transformer 12 driving a DC operational amplifier 13. Amplifier 13 operates an "off-hook" signal relay 14 and produces a bucking current via transistor 16 in the secondary winding 15 of transformer 12, which relieves DC saturation current from transformer 12.

This "off-hook" sensing system is adapted for a telephone exchange office which needs to sense when a telephone connected to street line 17 is lifted off a hook or cradle, in order to stop the ringing signals from the telephone exchange 18 which are too loud to be acceptable to the person listening at the telephone receiver.

When the telephone is lifted from the hook, a DC current of a large 50 ma value flows into the primary 19 of the transformer which tends to saturate the core 20 (shown dotted as an E), through the windings about the center of the E and a magnetic bar 21 bridging the legs of the E. The purpose of this circuit is to provide (in an off-hook sensor) a system which requires only a small core which, nevertheless, is not saturated by the DC current and also serves as an AC-DC current "transformer".

This is achieved by using Hall effect flux sensor 10 in the gap 11 between the control leg 22 of core 20 and bar 21, in order to detect the total flux in the core caused by both AC and DC fields. Amplifier 13 responds only to DC signals to produce a DC output which produces an equal DC output signal in secondary 15, which bucks the flux of the field from the primary DC current, thereby permitting the AC voice signals to pass through transformer 12.

Transformer 12 protects...