WO1995018502A1

WO1995018502A1 - Telephone switching circuit

Info

Publication number: WO1995018502A1
Application number: PCT/NO1993/000198
Authority: WO
Inventors: Ivar J. Haanes
Original assignee: Haanes Ivar J
Priority date: 1993-12-27
Filing date: 1993-12-27
Publication date: 1995-07-06
Also published as: SE505338C2; SE9602265D0; SE9602265L; AU2400395A

Abstract

Telephone switching circuit for use with telephone terminals in case of two or more telephone terminals being connected in parallel to a common subscriber line in a telephone system, whereby the telephone terminals (1, 2) at the common subscriber line have restricted access to the telephone system in that only one of the terminals at a time can have telephone connection within the telephone system. The telephone switching circuit (11) comprises for each terminal (1) two anti-parallel connected semiconductor elements (3, 5) of thyristor character inserted in one line branch (11A) between the terminal (1) and the subscriber line with output electrodes (3A, 5A) of the semiconductor elements lying in series in the line branch (11A). Gate electrodes (3C, 5C) of the semiconductor elements (3, 5) are connected to each other through a resistor (4).

Description

TELEPHONE SWITCHING CIRCUIT
The invention generally relates to control of the telephone connection between two or more telephone sets (terminals) being connected in parallel to one and the same subscriber line in a telephone network or system, and a terminal at another location in the telephone network.

Without particular circuit arrangements in the line branches between the individual, parallel connected terminals and the subscriber line, a telephone conversation being conducted between one of the terminals and a terminal somewhere else in the network may be picked up by the other parallel connected terminals when the microtelephone concerned is lifted. In order to avoid undesired listening or possible intervention into a running conversation, internal telephone exchanges are employed. Other alternatives may be mechanical relays or electronic circuit arrangements mounted in each individual parallel terminal line branch.

Telephone exchanges have a complicated structure and are often based on manual operation (switchboard). The number of terminals which can be serviced will be limited. The cost of buying and installing such internal exchanges and extra wiring in many instances is so high that this solution is not a real alternative to the telephone subscriber concerned.

This applies not the least in the case of private households.

Mechanical relays are more expensive and require more space and besides are less reliable in operation than electronic circuit arrangements having corresponding functions.

Electronic circuit arrangements for blocking against undesired connection are also known. The known techniques can be considered as represented by European patent application, publication No. 0.009.293, Finnish published patent application No. 77.343 and Finnish published patent application No. 79.638. These employ one circuit arrangement or electronic circuit per terminal in the parallel configuration.

The circuit according to EP 0.009.293 has a relatively complicated structure and comprises approximately seven semiconductors, one capacitor and two resistors. The circuit is connected in parallel to the telephone terminal. During a conversation the telephone current flows through three semiconductors. Installation of this circuit requires expert knowledge, because one output terminal of the circuit shall be connected to a point being accessible only inside the telephone terminal.

The circuit according to FI 77.343 consists of two thyristors connected in anti-parallel and connected in series with the telephone terminal. Across each of the thyristors, i.e. between an anode and a gate electrode thereof, there is connected a diac. When the microtelephone is lifted, it is assumed that one of the diacs is activated and this in turn activates the associated thyristor. The circuit must be regarded as very critical as to its function.

The circuit according to FI 79.638 consists of two thyristors connected in anti-parallel and connected in series with the telephone terminal. When the microtelephone is lifted and provided that the line voltage is sufficiently high, one of the thyristors is activated by means of a parallel activating circuit. The activating circuit consists of 1-3 semiconductors and two resistors. The activating voltage can be adjusted by means of resistance in the activating circuit.

Thus, more particularly and in view of the above known techniques, this invention is directed to a telephone switching circuit for use with telephone terminals in case of two or more telephone terminals being connected in parallel to a common subscriber line in a telephone system, whereby the telephone terminals at the common subscriber line have restricted access to the telephone system in that only one of said terminals at a time can have telephone connection within the telephone system, the telephone switching circuit comprising for each terminal two anti-parallel connected semiconductor elements of thyristor character inserted in one line branch between said terminal and said subscriber line with output electrodes of said semiconductor elements lying in series in said line branch.

The novel and specific features in the circuit according to the invention primarily consist therein that gate electrodes of said semiconductor elements are connected to each other through a resistor.

Compared to EP 0.009.293 the solution according to the present invention involves the advantages of being significantly more simple for obtaining the same function, there will exist a voltage drop in one semiconductor element only as compared to three voltage drops in such elements in the known circuit arrangement during a telephone conversation, and there will be a simpler installation with no requirement for intervention into the telephone terminal or apparatus.

In order for the circuit according to FI 77.343 to operate, it is a requirement that the thyristor is activated in consequence of activation of the diac. When the diac is activated, however, the voltage across the diac and the thyristor is reduced and it is not obvious that the thyristor has become conductive in the meantime. If in this known arrangement the thyristor is activated, it is maintained conductive as long as the current does not decrease below the conducting current limit. The current distribution between the diac and the thyristor therefore will be a critical factor. Moreover, the circuit arrangement described in FI 77.343 does not have any possibility of adjustment with respect to the voltage requirements of the telephone system.

Finally, in FI 77.343 there is also proposed a simplified solution consisting of a triac connected in parallel to a diac. In addition to the above drawbacks this variant of the circuit has the deficiency of not being completely independent of polarity.

Turning now to FI 79.638 it is seen that the circuit arrangement described therein is somewhat similar to the telephone switching circuit according to the present invention. The latter circuit, however, does not require any separate activating circuit. Because of its simple structure the present telephone switching circuit is more reliable, cheaper to produce and in subscriber use, these being substantial advantages for private households.

In summary, the main advantage of the circuit according to the invention primarily is its simple design as it consists of three elements only, i.e. of two semiconductors and a resistor connecting the semiconductor gate electrodes to each other.

In the following description the invention will be explained more closely with reference to the drawings, in which:
Fig. 1 is a block schematic illustration of two subscriber
telephone terminals being connected in parallel to
a common subscriber line,
Fig. 2 in a generalized schematic form shows one switching
circuit according to the invention, two such cir
cuits being incorporated in the arrangement in Fig.

1, and
Fig. 3 shows the circuit of Fig. 2 somewhat more in
detail.

Fig. 1 shows an example of an arrangement with a subscriber line consisting of two wires or branches AL1 and AL2, to which two subscriber terminals 1 and 2 are connected in parallel. With respect to telephone terminal 1 the usual microtelephone or handset operated hook switch 1A is indicated, which as conventional when closed serves to establish a DC current path from the common subscriber line through line branches llA and l1B connecting terminal 1 to the subscriber line AL1-AL2. In Fig. 1 the particular telephone switching circuit according to the invention is shown at 11 (for terminal 1) and at 12 (for terminal 2).

As shown in Fig. 2 the circuit 11 according to the invention may be considered to have the general structure of a bridge circuit, comprising two semiconductor elements 3 and 5 with a resistor 4 interconnecting control or gate electrodes of the two semiconductor elements.

Turning now to a practical embodiment based on the general principle illustrated in Fig. 2, there is shown in
Fig. 3 two thyristor type elements which constitute the semiconductor elements 3 and 5. These semiconductor elements 3 and 5 are connected in an anti-parallel configuration, with input electrode SB of element 5 as well as output electrode 3A of element 3 connected to the line branch llA. At the opposite side of circuit 11 output electrode 5A of element 5 and input electrode 3B of element 3 are interconnected.

Moreover, as mentioned above, resistor 4 is connected between gate electrodes 3C and 5C respectively, of the two semicon ductor elements. Accordingly the output electrodes of the semiconductor elements constitute the output terminals of switching circuit 11 as a two-pole inserted in line branch llA leading to telephone terminal 1.

Telephone switching circuit 12 being connected to terminal 2 has the same configuration as the circuit described with reference to Fig. 3. The voltage treshold for activating the semiconductor elements 3 and 5 is adjusted to the requirements of the telephone system concerned, by means of the resistor 4 between gate electrodes 3C and 5C. This resistor circuit can be supplemented by additional resistors or resistor arrangements 6 and 7 in parallel to each of the semiconductor elements 3 and 5 respectively.

By arranging the two semiconductors or thyristors 3 and 5 in anti-parallel and using bi-directive thyristors of the triac type this switching circuit is made independent of the polarity of the subscriber line current.

For simplicity of illustration in Fig. 1 only two telephone terminals 1 and 2 are shown connected in parallel to one and the same subscriber line AL1-AL2. It is obvious, however, that the number of parallel telephone terminals sharing a common subscriber line, can be higher than two.

The operation of the switching circuit in the embodiment described above with reference to the drawings, is as follows:
When the subscriber line ALl-AL2 is not in use, the switching circuit, for example circuit 11, does not conduct current. During signalling (calling) a pulsating high voltage is applied to all switching circuits, such as circuits 11 and 12 in Fig. 1, and one of the semiconductor elements in each of the circuits becomes conductive and nonconductive respectively, according to the signal amplitude variation, whereby the call signal is received by all telephone terminals 1 and 2.

When the handset is lifted from one of the terminals, for example terminal 1, a voltage will be applied across switching circuit 11 and one of the semiconductor elements 3 or 5 therein will be activated, i.e. become conductive. The polarity of the supply voltage in subscriber line AL1-AL2 will decide which one of semiconductor elements 3 and 5 will be activated. Switching circuit 11 having entered into operation in this manner, shows that a telephone connection has been established and a DC current flows to the telephone terminal 1. The circuit 11 will remain conducting until the current flowing drops below the current threshold of the semiconductor when the telephone connection is broken.

Because of the resistance in the subscriber line the voltage across the points of connection of line branches llA and llB to the subscriber line will be decreased when the connection is established. If the handset of any one of the other terminals is lifted during a running conversation, the switching circuit, for example 12, of the terminal concerned, for example 2, is not activated, because the voltage across the switching circuit is now lower than the activating threshold voltage of the semiconductors. Accordingly, access to the remaining terminals will be blocked until the running conversation is terminated.

The resistor 4 and possibly resistors 6 and 7 if present, determine the gate voltage applied and makes it possible to adjust the lowest activating voltage threshold of the switching circuit, according to the conditions or requirements in the telephone system concerned.

Thus, when the handset of one terminal is lifted, this terminal is connected to the telephone system or network at the same time as connection to the remaining terminals is blocked. The same occurs upon establishing an outgoing call when the subscriber line is unoccupied.

A telephone connection having been established can be transferred from one terminal, for example terminal 1, to another terminal, for example terminal 2, by putting down the handset at terminal 1 and simultaneously lifting the handset at terminal 2. In the case of outgoing calls the handset at terminal 2 must be lifted before the handset is put down at terminal 1.

As described above the present telephone switching circuit automatically maintains a desired telephone connection between any one of several telephone terminals being connected in parallel to a common subscriber line, and the telephone system, whereas at the same time any possibility of connection between the remaining terminals in parallel and the telephone system is blocked.

Claims

1. Telephone switching circuit for use with telephone terminals in case of two or more telephone terminals being connected in parallel to a common subscriber line (AL1, AL2) in a telephone system, whereby the telephone terminals (1,2) at the common subscriber line have restricted access to the telephone system in that only one of said terminals at a time can have telephone connection within the telephone system,

the telephone switching circuit (11) comprising for each terminal (1) two anti-parallel connected semiconductor elements (3,5) of thyristor character inserted in one line branch (llA) between said terminal (1) and said subscriber line (AL1) with output electrodes (3A,5A) of said semiconductor elements lying in series in said line branch (ill), char a c t e r i z e d in that gate electrodes (3C,5C) of said semiconductor elements (3,5) are connected to each other through a resistor (4).

2. Telephone switching circuit according to claim 1, c h a r a c t e r i z e d b y an additional resistor (6,7) connected between the input electrode (3B,5B) and the gate electrode (3C,5C) of each semiconductor element (3,5).