Telephonic communications – Subscriber line or transmission line interface – Protective circuit
Patent
1996-05-08
1997-09-16
Zele, Krista M.
Telephonic communications
Subscriber line or transmission line interface
Protective circuit
379373, 379399, 361 56, 361119, H04M 174, H02H 904
Patent
active
056688660
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to apparatus for protecting a subscriber line interface circuit against overvoltages appearing on its two input wires and exceeding the voltage of the supply voltage source of the subscriber line interface circuit.
BACKGROUND ART
It is known to protect subscriber line interface circuits against overvoltages, primarily against overvoltages in the form of lightening-induced overvoltage pulses caused, with the aid of a so-called primary protector which may, for instance, consist in a spark gap which functions to reduce the overvoltages on the subscriber line to a voltage of about one kilovolt. A so-called secondary overvoltage protector is fitted to the subscriber line, to deal with these residual pulses of about one kilovolt. The secondary overvoltage protector may have the form of a known circuit marketed by General Semiconductor under the trademark TRANSZORB, or a circuit marketed by RCA under the trademark SURGECTOR.
Current standards require the secondary overvoltage protector to be earthed to the apparatus stand with a separate wire. However, this can create problems that are manifested in the form of residual pulses in the order of magnitude of some hundred volts across a subscriber line interface circuit connected to the subscriber line. Modern line interface boards, in which the line interface circuit is a monolithic integrated circuit, are very often broken-down by lightening, due to residual pulses from the secondary overvoltage protector appearing across the two input wires of the circuit board.
U.S. Pat. No. 4,456,940 teaches an overvoltage protector which includes, among other things, two diodes whose anodes are each connected to a respective input terminal of a subscriber line interface circuit and whose cathodes are connected to earth, and further includes two thyristors whose cathodes are each connected to a respective input terminal on the subscriber line interface circuit and whose anodes are connected to earth. The thyristor gates are connected to the supply voltage of the subscriber line interface circuit. This protective circuit protects the subscriber line interface circuit against overvoltages. However, the thyristors are relatively space-consuming and a reduction in their number would be a significant advantage. It is sometimes necessary to match the thyristors, so that they will ignite or fire simultaneously, i.e. for the same voltage across the cathodes with equal voltage across the gate. However, the document does not describe several cascade-connected overvoltage protectors.
SUMMARY OF THE INVENTION
The object of the invention is to eliminate the effect on a subscriber line interface circuit of residual pulses that emanate from secondary overvoltage protectors.
This object is achieved with apparatus according to the invention and having the characteristic features set forth in the following claims.
One advantage afforded by the present invention is that the number of thyristors required in the tertiary overvoltage protector has been reduced in comparison with the number required in the known protectors, therewith reducing the space required by the overvoltage protector. Another advantage is that matching between two or more thyristors to ensure that the thyristors will be activated at one and the same voltage level is no longer necessary.
BRIEF DESCRIPTION OF THE DRAWING
The invention will now be described in more detail with reference to the accompanying drawing, in which FIG. 1 is a block schematic which illustrates a primary overvoltage protector circuit, a secondary overvoltage protector circuit and a tertiary overvoltage protector circuit connected to a subscriber line interface circuit; and FIG. 2 illustrates an embodiment of a tertiary overvoltage protector circuit according to the invention for a subscriber line interface circuit.
BEST MODE OF CARRYING OUT THE INVENTION
FIG. 1 is a block schematic illustrating an inventive total overvoltage protector for a subscriber line interface circuit 1. The subscriber line in
REFERENCES:
patent: 4254442 (1981-03-01), Dijkmans et al.
patent: 4377832 (1983-03-01), Toney et al.
patent: 4456940 (1984-06-01), Hammerberg et al.
patent: 4661979 (1987-04-01), Jakab
patent: 4908854 (1990-03-01), Lofmark
patent: 5003588 (1991-03-01), Wingerath
Saint-Surin Jacques M.
Telefonaktiebolaget LM Ericsson
Zele Krista M.
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