TELEPHONE INTERFACE CIRCUIT

• US 20100119054A1
• Filed: 11/11/2008
• Published: 05/13/2010
• Est. Priority Date: 11/11/2008
• Status: Active Grant

• Current Assignee: Uniden Corporation (Cornwall Capital Management LP)
• Original Assignee: Uniden Corporation (Cornwall Capital Management LP)
• Inventors: Fujiwara, Yoshinobu
• Granted Patent: US 8,411,848 B2
• Time in Patent Office: Days
• US Class Current: 379/395.10
• CPC Class Codes: H04M 1/745 Protection devices or circuits for voltages surges on the line

Abstract

A telephone interface circuit comprises: a first transistor which switch-controls a connection between a speech circuit and a pair of subscriber lines; a second transistor which controls an on/off state of the first transistor; a positive feedback circuit which connects a collector terminal of the first transistor to a base terminal of the second transistor; and an internal power source which supplies current for driving the second transistor. A circuit constant is set such that the first transistor is to operate in a saturated region when a voltage in a range of voltage for normal use is applied in between the pair of the subscriber lines, and such that the first transistor is to operate in an unsaturated region when an overvoltage exceeding the range of voltage for normal use is applied in between the pair of the subscriber lines.

Claims

1. A telephone interface circuit, comprising:

a first transistor which switch-controls a connection between a speech circuit and a pair of subscriber lines;

a second transistor which controls an on/off state of the first transistor;

a positive feedback circuit which connects a collector terminal of the first transistor to a base terminal of the second transistor; and

an internal power source which supplies current for driving the second transistor,

a circuit constant being set such that the first transistor is to operate in a saturated region when a voltage in a range of voltage for normal use is applied in between the pair of the subscriber lines and such that the first transistor is to operate in an unsaturated region when an overvoltage exceeding the range of voltage for normal use is applied in between the pair of the subscriber lines.

2. The telephone interface circuit according to claim 1, wherein

the circuit constant is to determine a base current of the first transistor.

3. The telephone interface circuit according to claim 1, wherein

the first transistor and the second transistor are to oscillate while alternating between an on state and an off state when the overvoltage is applied in between the pair of the subscriber lines.

4. The telephone interface circuit according to claim 1, wherein

the internal power source is a source of power that can be obtained by converting an alternate-current power supplied by a commercial power source into a direct-current power.

5. The telephone interface circuit according to claim 1, wherein

the first transistor is an NPN transistor whereas the second transistor is a PNP transistor.

6. The telephone interface circuit according to claim 5, further comprising:

a microcomputer which drive-controls the second transistor, wherein

the microcomputer is to conduct negative logic-based control on the second transistor.