Abstract:
The present invention relates to a system for protecting against overloads in a telephone system. The telephone system includes a telephone exchange, a ringing device and an interface, coupled to receive input signals on subscriber lines. The input signals are forwarded to either the telephone exchange or the ringing device depending upon the operating mode of the telephone system. The telephone line supplies a signal indicating the operating mode. A single protection device is coupled between the input of the subscriber lines and the interface. The protection device is controlled by the operating mode signal, and includes logic for protecting both the ringing device and the telephone exchange.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the protection against electric overvoltages of telephone exchanges and, more particularly, of the interface between a telephone line and an exchange. Such an interface, currently designated in the art as a Subscriber Line Interface Circuit (SLIC), is generally comprised of a card which receives, for example, eight telephone lines. 
     Although the present invention is described hereafter within the framework of this application, it should be noted that the present invention provides a protection system that can also apply to the protection of one or several electric conductors connected to other electric or electronic circuits. 
     2. Discussion of the Related Art 
     FIG. 1 shows a conventional embodiment of an SLIC. Between conductors T and R (Tip and Ring) of a subscriber telephone line and SLIC interface  1 , is positioned a circuit  2  of inverting relays, called ringing relays. The ringing relays act to switch the line between two operating modes. A first so-called “ringing” operating mode connects conductors T and R, respectively through relays I 1  and I 2 , to a ringing generator  3  via lines  3   a  and  3   b . In a second so-called “speech” operating mode (shown in FIG.  1 ), conductors T and R are connected via relays I 1  and I 2  to circuit  1  via lines E 1  and E 2 . 
     SLIC circuit  1  manages the transistor between the two operating modes. Circuit  1  has a control output  4  issuing a two-state signal for controlling a block  5  operating relays I 1  and I 2 . The control of block  5  is generally performed via a transistor Tc, the transmitter of which is connected to the ground, the collector of which is connected to a control input of block  5  and the base of which is connected to terminal  4  via a resistor Rc. Block  5  is supplied by a positive voltage V (generally around 5 volts). Alternatively, the assembly comprised of transistor Tc and resistor Rc can be integrated into SLIC circuit  1 . 
     The usual protection of the SLIC circuit and of ringing generator  3  is provided by placing protection device  6  between the line and relay circuit  2 , and by placing protection device  7  between SLIC circuit  1  and circuit  2 . 
     Device  6  provides protection for the ringing generator  3  when relays I 1  and I 2  connect the line thereto (in ringing mode), by connecting one or the other of conductors T and R to the ground as soon as the line voltage overcomes a determined threshold, for example, a voltage of ±200 volts. The selection of the voltage threshold is determined by the operating mode of the ringing generator. In the ringing mode, an a.c. voltage called the ringing signal is applied to the line and has a peak value generally between 50 and 100 volts RMS. The function of device  6  is to suppress any voltage higher than the ringing peak voltage. 
     Protection device  7  has the function of protecting SLIC circuit  1  when the contacts of relay circuit  2  connect the line thereto (in speech mode, the position shown in FIG.  1 ). Device  7  acts to connect one or the other of connectors T or R to the ground as soon as the voltage on the line exceeds a determined threshold corresponding to the normal voltage level on the line. In the speech mode, circuit  1  is supplied by a negative voltage generally included between −30 and −70 d.c. volts (for example −48 volts) using the ground as a reference. This voltage is called “battery voltage”. The inputs, respectively E 1  and E 2 , of circuit  1  on the line side have to be protected from any positive voltage as well as from any voltage which is more negative than the battery voltage. 
     Protection devices  6  and  7  generally are so-called crowbar devices. An example implementation of a typical protection device and the current-voltage characteristic of the example protection device are respectively shown in FIGS. 2A and 2B. 
     FIG. 2A shows a conventional embodiment of a half-protection circuit of the crowbar type for one of conductors T or R. In FIG. 2A, only one half of protection device  6  has been shown since it has the same structure between each of conductors T or R and the ground. 
     For the portion shown in FIG. 2A (protection of conductor T), device  6  includes a first anode-gate thyristor Th 1  mounted between conductor T and the ground, its cathode being connected to the ground. The gate of thyristor Th 1  is connected to the cathode of an avalanche diode Z 1 , the anode of which is connected to the ground. When a positive overvoltage exceeding the avalanche voltage of diode Z 1  occurs on conductor T, diode Z 1  starts an avalanche and turns on thyristor Th 1 . Device  6  includes a second cathode-gate thyristor Th 2  mounted between conductor T and the ground, its anode being connected to the ground. The gate of thyristor Th 2  is connected to the ground via a second avalanche diode Z 2  mounted with a biasing opposite to that of avalanche diode Z 1  associated with thyristor Th 1 . Diode Z 2  starts an avalanche when a negative overload exceeding its avalanche voltage occurs on conductor T and, thereby, thyristor Th 2  turns on. 
     The avalanche voltages of diodes Z 1  and Z 2  are set to correspond to the circuit protection threshold voltages, for example, ±200 volts. 
     FIG. 2B shows the current-voltage characteristic of a protection circuit such as shown in FIG.  2 A. This drawing illustrates the shape of current It through thyristors Th 1  and Th 2  which are conducting according to the voltage Vt across them. The right portion of the curve corresponds to the action of thyristor Th 1  and diode Z 1 , the left portion of the curve corresponding to the action of thyristor Th 2  and diode Z 2 . 
     The protection device  7  between the SLIC and circuit  2  has a structure similar to that of device  6 , but with different and asymmetrical triggering thresholds, for example, 0 and −50 volts. It should be noted that in the speech mode, device  6  is not necessary since device  7  triggers first. 
     A disadvantage of a system such as that shown in FIG. 1 is that the cost of the protection is high since it requires at least two protection devices to be supplied and mounted separately. 
     Another disadvantage of such a system is that, if an overvoltage occurs as relays I 1  and I 2  are connecting the line to SLIC circuit  1 , protection device  7  will start operating. However, for the time period of the overload, a strong current flows through the contacts of relays I 1  and I 2 , thereby risking to damage them and making it impossible to use static relays. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of known protection systems by providing a novel system for protecting a subscriber interface capable of avoiding the occurrence of excess currents in the ringing relays in case of an overload. 
     More generally, the present invention provides a system for protecting circuits, connected to at least one conductor via at least one relay, and for which it is desired to have different protection thresholds. 
     To achieve these objects, the present invention provides a method for protecting against overloads between a telephone exchange and relays connected to a subscriber line. The method includes the steps of connecting, between the subscriber line and the relays, a single protection device, the operation of which is parametered according to a signal issued by the subscriber interface, for controlling the relays. 
     In other words, the present invention provides a device for protection against overloads in an interface between a telephone exchange and relays connected to a subscriber line, the protection device including, for each conductor of the line, at least one switching means for connecting the conductor to the ground in case of an overvoltage exceeding a threshold value, and at least one means for automatically setting the threshold value based on a signal for controlling the relays. 
     According to one embodiment of the present invention, the means for setting the threshold value includes a switch mounted between a voltage control terminal of the switching means and a determined potential, the switch being controlled according to a state of the signal for controlling the relays issued by the interface. 
     According to another embodiment of the present invention, the switching means includes a thyristor mounted between the conductor and the ground and the gate of which is connected to the ground via an avalanche diode, the switch configuring the switching means between a first operating mode where the threshold value corresponds to the avalanche voltage of the diode and a second operating mode where the threshold value corresponds to the determined potential. 
     According to another embodiment of the present invention, the switch includes a transistor controlled responsive to the state of the signal that controls the relays. 
     According to another embodiment of the present invention, the device includes, for each conductor, a first unit for protection against a positive overvoltage and a second unit for protection against a negative overvoltage, each unit including a switch associated with a switching means. 
     According to another embodiment of the present invention, the determined potential associated with the switch of the first unit is the ground, and the determined potential associated with the switch of the second unit corresponds to the battery potential of the interface. 
     According to another aspect of the present invention, a system for protecting against overloads between a telephone exchange and relays connected to a subscriber line includes a single protection device coupled between the line and the relays. 
     According to another aspect of the present invention, a system for protecting against overloads for two circuits connected to at least one conductor via at least one relay includes a protection device coupled between the conductor and the relay including two voltage-controlled switching means for connecting the conductor to a reference potential in case of an overvoltage exceeding one out of two threshold values, the threshold values defining an operating range associated with one of the circuits. The protection device further includes two means for automatically setting each of the threshold values according to a signal for controlling the relay. 
     According to another embodiment of the present invention, each means for setting a threshold value includes a switch connecting a control terminal of the switching means associated therewith to a determined potential, the control terminal being also connected to the ground via an avalanche diode. 
     These objects, characteristics and advantages as well as others, of the present invention, will become apparent upon review of the following detailed description of embodiments of the invention discussed in relation to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1,  2 A and  2 B, previously described, are meant to show the state of the art and the problem to solve; 
     FIG. 3 shows, in the form of block diagrams, an embodiment of a protection system according to the present invention; 
     FIG. 4 shows, partially and schematically, a first embodiment of a protection device according to the present invention adapted to the system of FIG. 3; 
     FIG. 5 shows a detailed electric diagram of a first embodiment of a portion of a protection device according to the present invention; and 
     FIG. 6 shows a detailed electric diagram of a second embodiment of a portion of a protection device according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     For clarity, the same components have been referred to by the same references in the different drawings. 
     FIG. 3 shows a first embodiment of a protection system according to the present invention. 
     A characteristic of the present invention is to provide a single protection device  10  both for an SLIC circuit  1  and for a ringing generator  3 . FIG. 3 shows the same type of diagram as in FIG. 1, but wherein the conventional protection devices  6  and  7  (FIG. 1) have been replaced with the sole protection device  10  according to the present invention. 
     As previously, a circuit  2  including coil-controlled ringing relays I 1  and I 2  or other detection and control circuits is connected, on the one hand, to the subscriber line (conductors T and R), and on the other hand to SLIC interface  1  and to ringing generator  3 . 
     A characteristic of the present invention is that protection device  10  has two operating modes according to the position of relays I 1  and I 2 . The selection of the operating mode of device  10  is performed by means of a signal for controlling circuit  2 , issued by circuit  1 , through a first output terminal  4 . 
     Device  10  is positioned between circuit  2  and the line so as to suppress the overloads from the telephone line before they reach relays I 1  and I 2 . The protection device according to the present invention thus also protects relay circuit  2 , allows for the use of static relays. 
     FIG. 4 shows, schematically, a portion of a first partial embodiment of device  10 . In FIG. 4, only one half of device  10  (for protection against overloads on one of conductors T or R) has been shown. The entire protection device  10  includes two assemblies such as that shown in FIG. 4, each respectively associated with conductors T and R. 
     According to the present invention, protection device  10  includes, for each conductor R or T, a unit  11  for positive overvoltages and a unit  12  for negative overvoltages. 
     Unit  11  includes an anode-gate thyristor Th 1  mounted between conductor T or R and the ground. The gate of thyristor Th 1  is also connected to the ground via a first avalanche diode Z 1 . 
     A characteristic of the present invention is that a first switch K 1  is mounted in parallel with diode Z 1  and is controlled by a signal  4  for controlling relay circuit  2 . 
     The function of switch K 1  is to set a determined potential for triggering the protection by short-circuiting avalanche diode Z 1  when relays I 1  and I 2  (FIG. 3) establish a contact between the line and interface  1  (speech mode) so that thyristor Th 1  is on as soon as a positive voltage appears on conductor T or R. 
     In the ringing mode, that is, when relays I 1  and I 2  establish a contact between conductors T and R and ringing generator  3 , switch K 1  is open and the thyristor Th 1  associated with avalanche diode Z 1  operates as in the conventional device shown in FIG.  2 A. 
     Unit  12  includes a cathode-gate thyristor Th 2  mounted between conductor T or R and the ground. The gate of thyristor Th 2  is also connected to the ground via an avalanche diode Z 2 . 
     A characteristic of the present invention is that unit  12  includes a second switch K 2  between the gate of thyristor Th 2  and a negative potential −Vbat, determined to correspond to the threshold of the negative voltage for protecting interface  1 , that is, battery voltage −Vbat. 
     Switch K 2  has the function of forcing the potential of the gate of thyristor Th 2  to potential −Vbat when relays I 1  and I 2  are positioned to ensure the contact between the line and SLIC interface  1  (speech mode). Potential −Vbat is obtained by connecting a terminal of switch K 2  to the battery voltage present on the subscriber card, for example, on a second output terminal  13  of SLIC interface  1 . Thus, in the speech mode, the protection acts as soon as the potential on line T or R is more negative than −Vbat. According to the present invention, switch K 2  is controlled, as switch K 1 , by signal  4 . 
     In the ringing mode, switch K 2  is off and unit  12  operates in the same way as the conventional circuit discussed in relation with FIG.  2 A. 
     Each switch K 1  or K 2  thus constitutes a means for automatically setting the triggering potential of the unit, respectively,  11  or  12 , to a desired value based on the state of the signal for controlling relays I 1  and I 2 . A first value of the triggering potential corresponds to the avalanche voltage of the diode, respectively Z 1  or Z 2 , and a second value corresponds to the determined potential, respectively 0 or −Vbat, to which the switch, respectively K 1  or K 2 , is connected. 
     Thus, the protection device  10  according to the present invention can be parametered between the two operating modes (speech and ringing). Moreover, the selection of the operating mode is performed automatically according to the present invention by means of the control signal available at the SLIC interface and meant, as in conventional circuits, for controlling relay circuit  2 . 
     An advantage of the present invention is that, a single protection device provides protection against overvoltages from the line for SLIC interface  1 , ringing generator  3  and relay circuit  2  as well. 
     FIGS. 5 and 6 respectively show two additional detailed embodiments of the protection device according to the present invention. A first embodiment shown in FIG. 5 corresponds to a practical implementation of the protection device for a system such as that shown in FIG. 3, where control transistor Tc and control resistor Rc of relay circuit  2  are external to SLIC interface  1 . A second embodiment shown in FIG. 6 may be used in a system where the SLIC circuit integrates control transistor Tc and resistor Rc and thus issues, on its output  4 , a control signal which is inverted with respect to the first embodiment. 
     In the first embodiment shown in FIG. 5, switch K 1  is comprised of an NPN bipolar transistor T 1 , the transmitter of which is connected to the ground and the collector of which is connected to the gate of thyristor Th 1 . The base of transistor T 1  is connected to the collector of an NPN transistor T 3  as well as to conductor T or R via a protection resistor Rp. Transistor T 3  receives on its base, via a resistor R 1 , the control signal issued by terminal  4  of SLIC interface  1 . The transmitter of transistor T 3  is connected to the ground as well as, via a resistor R 2 , to its base. The function of resistors R 1  and R 2  is to bias transistor T 3 . 
     In the embodiment shown in FIG. 5, it is assumed that the control signal issued by terminal  4  is in a low state (0 volts) in the speech mode and in a high state (5 volts) in the ringing mode. 
     In the ringing mode, transistor T 3  receives a base current through resistor R 1  and is thus saturated. Transistor T 1  is thus blocked since its base is at a level of zero volts. This corresponds to the off-state of switch K 1 . 
     In the speech mode, transistor T 3  is blocked and the base of transistor T 1  thus receives current when the voltage on conductor T or R is positive. Thus, as soon as the voltage on conductor T or R becomes positive, transistor T 1  saturates and thus suppresses any positive overvoltage. The protective resistor Rp of transistor T 1  is selected so that this transistor is saturated. 
     For example, in a particular embodiment, values of 1 kΩ can be chosen for resistances R 1  and R 2  and a value of 10 kΩ can be chosen for resistance R 3 . 
     The switch K 2  of unit  12  is comprised of an NPN-type bipolar transistor T 2 , the transmitter of which is connected to the gate of thyristor Th 2  and the collector of which is connected to the terminal  13  issuing voltage −Vbat. The base of transistor T 2  is connected by a resistor R 7  to the collector of a PNP bipolar transistor T 4 , the transmitter of which is grounded and the base of which is connected to terminal  4  via a resistor R 4 . The base of transistor T 2  is also connected to its transmitter via a resistor R 5  and the base of transistor T 4  is connected to the collector of transistor T 2  via a resistor R 6 . 
     In the ringing mode, the base-transmitter junction of transistor T 4  is reverse-biased since terminal  4  is at a positive potential. Transistor T 4  is thus blocked and transistor T 2  is also blocked by means of resistor R 5 . This corresponds to the off-state of switch K 2 . 
     In the speech mode, transistor T 4  is biased by means of resistors R 4  and R 6  and is thus saturated. Thus, a current flows through the base of transistor T 2  which also saturates. This corresponds to the on-state of switch K 2 . 
     The value of resistance R 5  is selected to enable the blocking of transistor T 2  in the ringing mode. Resistors R 4  and R 6  which constitute a divider bridge of the biasing voltage of transistor T 4 , are chosen so that their midpoint (base of transistor T 4 ) is at a potential lower than −0.6 volt when terminal  4  is at a zero potential and so that the base of transistor T 4  is at a potential higher than −0.6 volt when terminal  4  is at a potential corresponding to its other state (for example 5 volts). 
     For example, in a particular embodiment, resistance R 4  may have a value of 100 Ω, resistance R 5  may have a value of 1 MΩ and resistance R 6  may have a value of 4.7 kΩ. 
     Transistors T 1  (unit  11 ) and T 2  (unit  12 ) are selected to bear a voltage at least equal to the avalanche voltage of diodes Z 1  and Z 2 . This condition is not required for transistor T 3  (unit  11 ) since transistor T 3  only sees the control voltage of terminal  4 . 
     In the foregoing description, the influence of the base-transmitter voltage drops and of the resistive voltage drops over the switching of thyristors Th 1  and Th 2  in case of an over-voltage has been neglected. 
     FIG. 6 shows a second detailed embodiment of protection device  10  for the case where control transistor Tc and control resistor Rc are integrated to SLIC interface circuit  1  and where terminal  4  issues a control signal which is in a low state in the ringing mode and in a high state in the speech mode. 
     In this case, transistor T 3  of unit  11  of FIG. 5 is no longer necessary and resistor R 2  of unit  11 ′ (FIG. 6) is directly mounted on transistor T 1 . For unit  12 ′, the signal issued by terminal  4  of interface  1  must, however, be inverted. For this purpose, a resistor R 3  and an NPN bipolar transistor T 5  are interposed between terminal  4  and the base of transistor T 4 . The transmitter of transistor T 5  is connected to the ground while its collector is connected to the base of transistor T 4 . The base of transistor T 5  is connected to terminal  4  via resistor R 3 . 
     The operation of the embodiment shown in FIG. 6 can be inferred from the operation discussed in relation with FIG.  5 . 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the choice of the values of the resistances depends on the operating voltages of the protection device. Similarly, the different bipolar transistors can, in case of need, be replaced with MOS transistors. 
     In addition, the present invention also applies to any system where circuits are connected to one or several conductors via relays and for which the protection thresholds are different for each circuit. It should be noted that, in such applications, the voltage reference cannot be the ground but a positive or negative potential. Further, any unit  11  or  12  can be provided to operate symmetrically or asymmetrically between two conductors. In addition, the number of protected conductors is not necessarily two but can be any number. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.