VOLTAGE CONTROLLED TRANSIENT PROTECTION UNIT

Abstract: An arrangement for protection of circuits in a telephone exchange against overvoltages like for example lightening transients or so called longitudinal disturbances appearing on the lines connected to the exchange. The arrangement comprises at least two non-linear electronic circuit elements (T2, T3) which selectively shunt said overvoltages to earth. The junction between the conduction states, conducting-non-conducting, for at least one of said circuit elements (T2, T3) is controllable.

Description: / VOLTAGE CONTROLLED TRANSIENT PROTECTION UNIT TECHNICAL FIELD Th,e present invention relates to an arrangement for overvoltage pro¬ tection of circuits in a telephone exchange. Such overvoltages , for example, can be lightning transients on lines connected to the ex¬ change or so called longitudinal disturbances on subscriber lines caused by short circuit to earth. BACKGROUND ART In the journal Electronics of October 25, 1979, page 42 an arrange¬ ment for protection of circuits connected to the telephone network is described. The arrangement is " intended to protect against for example lightning transients on subscriber Lines and consists mainly of four so called clamp diodes, which connect the two wires of the subscriber line to the two poles of the power supply source. By this arrangement the maximal voltage between the a-wires and the b-wires is limited to the pole voltage of the supply source. The disadvantage with this design of the protection circuit is that the heavy transient currents which can be conducted to the negative pole of the supply source give rise to a voltage-drop in the power supply among other things due to its inherent resistance. Such voltage-drops can destroy other equip¬ ment in its turn. To handle this disadvantage it has been suggested, compare for example the Swedish patent specification 7^14316-2, to conduct the negative transient currents to earth by means of Zener diodes instead of towards the negative pole of the battery. The disadvantage with this solution is that the transient protection is not active to voltages in the in¬ terval between the varying battery voltage and the Zener voltage. A further problem with known arrangements resides in the cooperation of the protection unit with the ringing relay of the subscriber line. Most often connection of the ringing signal to the line is made by means of a relay. The ringing signal generator generates an alterna ¬ ting voltage for example in the interval from -200 V t + 150 V while the working interval for the line circuit connected to the line is OMPI between 0 V and the battery voltage, for example -56 V. The transient protection for the line circuit, thus intended to protect against vol- tages lower than -56 V and higher than 0 V, must be located between the ringing relay and the line circuit so that the protection circuit does not short circuit the ringing voltage. The disadvantage with such an arrangement is that the sometimes heavy transient currents at, for example, lightning disturbances, pass through the relay contact, which consequently have to be dimensioned for this load. DISCLOSURE OF THE INVENTION The technical problem to be solved by the present invention resides in the simultaneous elimination with one ' " arrangement of the above mentioned disadvantages according to prior art. The invention presents a solution characterized as appears from the appended claims. The advantages with the solution according to the invention are obviou as the protection circuit is effective to all realistic values of the disturbance voltage at the same time as the rirrg ' irfg relay only need to be dimensioned with reference to its actual function as connecting element for ringing voltage to the subscriber. BRIEF DESCRIPTION OF DRAWING. The invention will be described below with reference to the accompany- ing drawing on which Figure 1 illustrates an arrangement according to prior art Figure 2 illustrates an arrangement according to prior art Figure 3 illustrates an arrangement according to prior art Figure 4 illustrates an arrangement according to the invention Figure 5 illustrates more in detail an arrangement according to the invention. OM? BEST MODE FOR CARRYING OUT THE INVENTION Figure 1, according to the above, illustrates an arrangement according to prior art. The two wires a, b of a subscriber line are connected to a so called Subscriber Line Interface Circuit or line circuit 1. To protect the circuit 1 against positive transients coming on the wire, 5 a current limiting resistor R1 has been connected in series with the line and a diode D2 has been connected to earth. By means of this arrangement the voltage at the connection of the wire a to the cir¬ cuit 1 can maximally be equal to zero. A similar arrangement protects the circuit 1 against positive transient coming on the wire b. Nega- 10 tive transients are short circuited to the negative pole of the supply voltage source having the voltage -U1. A protection circuit designed in this way has the above mentioned disadvantages. Figure 2 illustrates another protection circuit according to prior art Two Zener diodes DZ1 and DZ2 connect the transient currents, positive 15 and negative, through two current limiting resistors R1 and R2 to earth. In Figure 3 the arrangement according to Figure 2 is shown together with a conventionally connected ringing signal generator 2. A ringing relay 3, controlled from the exchange, can, for signalling to the 20 subscriber, connect the a-wire and b-wire of the subscriber Line to the two poles of the generator. In Figure 4 a circuit diagram for an arrangement according to the in¬ vention is illustrated. Positive as well as negative transient currents on both speech wires are, as will be shown, connected to *- 25 earth. In this way the problem with a voltage-drop in the power supply to the line circuits 1 is avoided. The problem with the solution according to Figure 2 is avoided by making the ignition voltage for the units, which protect against negative transients, follow the supply voltage of the line circuit 1. Finally the contacts of the 30 ringing relay 3 are protected against the transient currents by placing the relay between the protection circuit and the line c r¬ The arrangement is illustrated for the unactivated state in Figure 4. A positive transient on the wire a is conducted to earth through the diode D5 and the thyristor T1 and the gate junction of which is kept forward biased by means of a voltage +U3 relative earth. In a similar 5 manner a positive transient on the wire b is conducted through the diode D6 and the thyristor T1 to earth. In the same way as described above two current limiting resistors R1 and R2 are connected between the protection device and the a-wires and b-wires. A negative transient, of a magnitude greater than the supply voltage 10 U1, on the wire a is conducted to earth through the thyristor T2 and the diode D7. The gate of the thyristor is kept at a voltage -U1 equal to the supply voltage. The thyristor T3 and the diode D7 con¬ duct the negative transients on the wire b. The circuit 1 is thus in unactivated state, protected against positive transients coming from 15 the line and negative transients having a magnitude greater than the supply voltage U1. A lightning disturbance, however, can occur at the same time as ringning signal is sent to the subscriber. The ringing relay 3 has in this case been activated and connected the poLes of the ringing 20 signal generator through two current limiting resistors R3 and R4 to the line. With the dashed-dotted line in the figure it has functio¬ nally been indicated that at the same time as the ringing relay is activated a second voltage -U2 is fed to the control electrodes of the thyristors T2 and T3 and the voltage -HJ3 to the control electrode 25 of the thyristor T1 is disconnected. The Figure, however, is not to be understood in such a way that the connection and disconnection of these latter voltages have to be done by means of relay contacts on the relay 3. The connected ringing voltage, being for example +_ 200 V must of 30 course not be shunted to earth by the protection device. On the con¬ trary voltages higher than + 200 V and lower than - 200 V shall acti¬ vate the protection circuit. This is achieved in the following way. A Zener diode DZ3 with the Zener voltage 200 V is connected between the anode and the gate of the thyristor T1. Positive transients on the a-wire and the b-wire respectively of a magnitude greater than 200 V is then shunted through the diode D5 and D6 respectively and the thyristor T1 to earth. The gates of the thyristors T2 and T3 are, as above mentioned, connected to the voltage -U2, suitably equal to -200 V. By this arrangement negative transients of a magnitude grea¬ ter than 200 V, will be shunted by the thyristors T2 and T3 and the diode D7 to earth. In Figure 5 a_n embodi ent of the arrangement according to the invention is illustrated somewhat more in detail. .The protection circuit is designed essentially as above mentioned. The two transistors TR1 and TR3 have constant emitter voltage + U3 for example equal to +5V. In unactivated state base current is drawn from the transistors to the supply voltage -U1, for example -48 V, through a resistor R9. The transistor TR1 is conducting and delivers control current to the thyristor T1 according to the above. Also the transistor TR3 is con¬ ducting and the collector current passes through the resistor R8 and the chain of diodes D8, D9, D10 to the supply voltage source -U1. A voltage lower than -U1 on for example the wire a then switches on the thyristor T2, which takes control current through the resistor R5 and the transistor TR2. When ringing voltage is fed to the subscriber the line circuit 1 is disconnected in the same way as earlier described at the same time as a third contact on the ringing relay connects the voltage + U3 to the base of the transistors TR1 and TR3, which consequently are cut off. The ringing voltage, for example +, 200 V is supplied onto the line without being influenced by the protection circuit. Positive transients on the a-wires and the b-wires greater than 200 V are shunted to earth in a way as described above. Negative transients on the a-wire, for example, of a magnitude greater than 200 V opens a current path from earth through the diode D7, the Zener diode DZ4, with the Zener voltage 200 V, the resistor R5 and the gate-cεthode junction of the thyristor T2. Then the thyristor T2 switches on and shunts the trasient through the diode D7 to earth. A similar function applies to negative transients on the b-wire.

Claims: WHAT WE CLAIM IS : An , arrangement for protection of circuits in a telecommunication equipment against overvoltages appearing on lines connected to the circuits comprising at least two non-linear electronic circuit ele¬ ments, which selectively shunt said overvoltages to earth, charac¬ terized in that the conduction state for at least one of said non¬ linear electronic circuit elements is controllable. OMPI

Inventors: GHISLER WALTER, HAMMARBERG LARS ERIK, HJORTENDAL ROYNE GUNNAR
Applicants: ERICSSON TELEFON AB L M, GHISLER WALTER, HAMMARBERG LARS ERIK, HJORTENDAL ROYNE GUNNAR
Application year: 1981
Main classification: H02H9
IPC codes: H02H9, H04M3, H04Q3