Current limiter for telephone signalling

Details Current limiter for telephone signalling Current limiter for telephone signalling

H - Electricity

04

M

379/12

H04M 3/02 (2006.01) H03K 17/64 (2006.01) H04M 19/08 (2006.01)

Patent

CA 1206639

CURRENT LIMITER FOR TELEPHONE OFFICE SIGNALLING by JOHN M. HUFT ABSTRACT OF DISCLOSURE A ring-ground circuit in a telephone system comprises a control transistor that selectively receives base drive from a microprocessor and that has its emitter connected through a low voltage LED and through the series combination of the base-emitter junction of a switching transistor and its emitter resistor to ground. The LED has a sharp transition in its conduction characteristics when the voltage across it reaches a pre- determined value which is a reference voltage. Conduction of the control transistor provides base drive to the switching transistor for causing collector current in the latter to initiate a ground-start condition in a relay coil that is connected to a central office battery voltage. When current in the switching transistor increases to a prescribed value, that is set by the LED reference voltage and the emitter resistor, conduction of the switching transistor is automatically controlled for limiting the maximum value of its collector current. - 8 -

448952

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