Telephonic communications – Subscriber line or transmission line interface – Protective circuit
Reexamination Certificate
1999-07-27
2003-05-13
Isen, Forester W. (Department: 2644)
Telephonic communications
Subscriber line or transmission line interface
Protective circuit
C379S399010, C379S405000, C379S413000, C379S027050, C379S093050, C379S351000, C361S111000, C361S118000
Reexamination Certificate
active
06563926
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to automatically resetting a surge protection circuit, and specifically to systems and methods for automatically resetting a surge protection circuit within a telephone line interface circuit after receiving a transient energy surge, such as lightning or an AC voltage spike, by disabling and then re-enabling the surge protection circuit based on discharge current.
BACKGROUND OF THE INVENTION
Telephone line interface circuits are electronic circuits that service the conventional two-wire (the tip and ring wires) telephone subscriber lines. Active telephone line interface circuits, in which the tip and ring wires of a telephone subscriber are driven via active line circuits, are well known in the art. Exemplary active telephone line interface circuits are described in U.S. Pat. No. 5,274,702 issued to Rosch et al. on Dec. 28, 1993 entitled “Wideband Telephone Line Interface Circuit” and in U.S. Pat. No. 5,323,461 issued to Rosenbaum et al. on Jun. 21, 1994 entitled “Telephone Line Interface Circuit With Voltage Switching.”
As described in the above-referenced patents, a line drive circuit in a telephone subscriber line interface circuit typically includes two unity-gain DC amplifiers whose outputs are coupled to the tip and ring wires, respectively, of the telephone subscriber line via a switching circuit and a sensing circuit. The telephone line interface circuit also includes a controlled voltage generator (more generally referred to as a power source) and a control circuit. The sensing circuit serves to monitor AC and DC conditions on the line and includes a sensing transformer and series resistors, which are generally referred to as feed resistors. The switching circuit serves, in association with software control via the control circuit, to interconnect the line, the line drive circuit, the controlled voltage generator, and a battery in various configurations to provide for different operating states of the line interface circuit. These states include configurations in which relay contacts connect the outputs of the line drive circuit or the output of the controlled voltage generator selectively to the tip and ring wires of the line, and in which relay contacts and an electronic switch selectively connect the output of the controller voltage generator or the battery as a supply voltage for the line drive circuit.
It is also well known to protect the line drive circuit in an active telephone line interface circuit from high transient voltages and currents (more generally referred to as transient energy surges) due to lightning, AC surges, and inductive coupling. To this end, it is known to couple a front-end protection circuit, such as a diode bridge, to points between the outputs of the line drive circuit and the feed resistors. As a front-end protection circuit, the diode bride clamps positive transient voltages to ground and clamps negative transient voltages to a breakdown voltage of a transient voltage suppressor (such as a thyristor), which is connected between a negative supply voltage and ground. In order to provide sufficient power dissipation for negative transient voltages or currents, a common solution uses several transient voltage suppression diodes (frequently referred to by the trade name TRANZOR™ or TRANSIL™ from SGS-Thomson Microelectronics) in combination. However, such a solution has the disadvantages of incurring a relatively high and undesirable cost. Additionally, such a solution requires a relatively large amount of physical space.
It would be desirable to be able to replace the transient voltage suppression diodes by a single crow-bar type protection device, frequently referred to by the trade name TRISIL™ or SIDAC™. However, such a device has the disadvantage that, if used in an active telephone line interface circuit, the current supplied by the line drive circuit can be sufficient to maintain or hold the protection device operative at and after the end of a negative transient voltage or surge. This commonly results in a locked up state of the telephone line interface card. In other words, the surge protection circuit within the telephone line interface circuit protects the card from damage in an inexpensive and space saving manner, but may possibly leave the card in an inoperable or locked up state.
The applicant discovered and patented an improved telephone line interface circuit aimed at avoiding these disadvantages. In particularly, U.S. Pat. No. 5,539,820 (hereinafter the 820 patent) issued on Jul. 23, 1996 to the applicant and is entitled “Protection of Active Telephone Line Interface Circuits” describes surge protection circuitry used for protecting the telephone line interface circuit from transient energy surges and is hereby incorporated by reference. As can be seen in the 820 patent, the surge protection circuitry uses a current limiter portion to protect a power supply in the interface and a front-end protection portion to protect against positive and negative transient voltages and currents.
In more detail, the front-end protection portion is described as a clamping diode, which is connected to a return on the telephone line interface circuit's power source. In this configuration, the clamping diode channels positive surge currents to the return when the potential rises to a diode drop above the return. The transient voltage suppression portion of the front-end protection circuitry is described as one or more specially designed diodes, such as a series combination of a thyristor and a clamping zener device. These diodes are designed to clamp excess voltages and to dissipate high transient power. When a negative voltage caused by a surge exceeds a predetermined threshold, the thyristor will breakover and create a path to the return for the negative surge current through the zenering device. In both instances (positive and negative surges), the surge current is diverted via the surge protection circuit to the return instead of to the power source where damage may occur.
Unfortunately, the applicant has found that in some situations, it is possible that the surge protection circuitry may become latched in an intermediate state. In particular, the applicant has found that the current supplied by the line drive circuit in intermittent situations may still be sufficient to maintain the operation of the surge protection circuit after the end of a negative transient voltage or surge without resetting back to a nominal state. In those rare but frustrating situations, continued use of the line interface circuit is not possible. While the surge protection circuit successfully protects the telephone line interface circuit, the protection circuit remains operative and undesirably blocks normal operation of the telephone line interface circuit.
In light of the foregoing, a need exists for an improved technique and system that prevents lockup of the surge protection circuit within the telephone line interface circuit. A need also exists for an improved technique and system to quickly and automatically reset the surge protection circuit within the telephone line interface circuit.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a method and apparatus for resetting a surge protection circuit within a telephone line interface that substantially obviates one or more of the limitations and disadvantages of the described prior arrangements.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve these and other objects and advantages, and in accordance with the purpose of the invention as embodied and broadly described herein, a telephone line interface's surge reset circuit consistent with the present invention includes a sensor and a feedb
Gorecki John C.
Isen Forester W.
Singh Ramnandan
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