US 5,329,584 AGranted

Interface circuit to quickly establish a loop current

AssigneeFujitsu Limited

InventorTakashi Nagato

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Abstract

According to the invention, an interface circuit for quickly establishing a DC loop current in response to off-hook detection is provided by a circuit which uses a transistor, where its collector electrode is connected to the first one of the telephone lines; an emitter resistor connected between an emitter electrode of the transistor and the second one of the telephone lines; a bias resistor having a first terminal connected to the collector electrode; a Zener diode connected between a second terminal of the bias resistor and the second line; a first capacitor connected in parallel to the constant-voltage means; switching means connected between the second terminal of the bias resistor and a base electrode of the transistor for the switching means to be conductive in response to an off-hook signal; and charging means, typically formed of a series connection of a capacitor, a resistor and a Zener diode, connected in parallel to the bias resistor so as to allow a current to flow through the series connection to charge the first capacitor only during a transient of applying a Dc voltage from the telephone lines onto the transistor.

Claims8

Dependent claims

An interface circuit, for receiving a pair of telephone lines having a first line and a second line in a telephone switching system, and for providing a DC current to actuate a ring-trip circuit upon an Off-hook state, comprising:

a transistor for allowing a DC current supplied from the telephone lines to flow therethrough, said transistor having a collector electrode connected to said first line;
an emitter resistor connected between an emitter electrode of said transistor and said second line;
a bias resistor having a first terminal connected to said collector electrode;
constant-voltage means connected between a second terminal of said bias resistor and said second line;
a first capacitor connected in parallel to said constant-voltage means;
conductive means connected between said second terminal of said bias resistor and a base electrode of said transistor, for conducting a voltage from said constant voltage means to said base electrode;
and
a charging circuit, connected in parallel to said base resistor, said charging circuit having a current path having a lower impedance to a voltage transient on said collector electrode than an impedance of said bias resistor, said current path also having a higher impedance to a steady state voltage on said collector electrode than said impedance of said bias resistor.

5 more elements

Dependencies are inferred from the claim text.