Protection circuit

#The invention relates to a protective circuit arrangement for protecting circuit elements connected to a line against the damaging effects of transient abnormal high voltage surges on the line caused by, for example, lightning or fluctuations in adjacent power transmission lines. The circuit arrangement of the present invention is particularly, though not exclusively, suitable for protecting line associated elements of a telephone subset connected to an exchange line via a solid state line switch, the line switch providing a hook-switch function and loop-current interrupt functions.

#Protection circuits for telephone subsets of the aforementioned kind are known and comprises a voltage limiting circuit and a current limiting circuit. The voltage limiting circuit comprises a voltage dependent nonlinear resistor connected across the subset's line terminals to substantially absorb transient high voltage surges on the line, thus protecting circuit elements in the subset that are associated with the line terminal end of the subset's circuit. In addition to the voltage limiting circuit, a current limiting circuit is also provided. The current limiting circuit comprises an arrangement of a resistor in series with and on the line side of the conductive path of the line switch, and a diode arrangement connected between the line end of the resistor and the line switch's control element. The value of the resistor is chosen so that when a current threshold is exceeded, the voltage drop across the resistor exceeds the forward drop across the diode arrangement thus limiting the current through the conductive path of the line switch.

#A disadvantage of the known protection circuit is that a proportion of the current produced by the transient voltage surge passes through the line switch and therefore the line switch's power rating must be sufficient to accommodate this transient current and the power rating chosen accordingly. This is reflected in the cost of the transistors used in the line switch.

#In order that the invention may be carried into effect, embodiments thereof will now be described in relation to the drawings, in which:

#FIG. 1 shows part of the line circuit of a telephone subset incorporating a known protection circuit;

#FIG. 2 shows part of the line circuit of a telephone subset incorporating the protection circuit of the present invention; and

#Referring to FIG. 1, the circuit comprises line terminals L1 and L2 for connection to an exchange line (not shown). Across terminals L1 and L2 is connected a voltage protection varistor VC1. A line switch in the form of a complementary pair of transistors comprising an NPN transistors TR1 and a PNP transistor TR2 is serially connected between the line terminals and the subset's transmission circuit TX. The collector/emitter junction of a line switch control transistor TR3 is connected between the base element of transistor TR2 and terminal L2. A resistor R2 is connected between the base of transistor TR2 and the collector element of transistor TR3 to provide a small but sufficient base current for transistors TR2. The base element of transistor TR3 is coupled to an output of a micro-processor (not shown) in the transmission circuit. Signals at this output control the functions of the line switch in a known manner. A resistor R1 is serially connected between terminal L1 and the collector/emitter junction of transistor TR1, and a serial arrangement of two diodes D1 and D2 are connected between terminal L1 and the base element of transistor TR2. Resistor R1 and diodes D1 and D2 form the current limiting section of the protection circuit. The operation of this known current protector circuit has been described above.

#Referring to FIG. 2, except for the protection circuit, the circuit is identical to that of FIG. 1 and will not be described. The protection circuit shown in FIG. 2 comprises resistor R1 serially connected between line terminals L1 and the collector/emitter junction of transistor TR1; diodes D1 and D2 serially connected between terminal L1 and the base element of transistor TR2; a varistor VC1 connected between the junction of resistor R1 and the emitter of transistor TR2, and terminal L2; and a further diode D3 connected across the emitter and base of transistor TR2. The resistance value of resistor R1 is chosen so that the voltage drop across it at normal operating line current (.ltoreq.150 ma) is less than the forward bias voltage of diode D1. Typically, the resistance of resistor R1 is approximately 3 ohma.

#In operation, with normal line current flowing through resistor R1 and the saturated line switch, the forward bias voltage across the base/emitter junction of transistor TR2 is 0.6 volts and the voltage drop across resistor R1 is less than 0.6 volts. Therefore the voltage drop across diodes D1 and D2 is the sum of the forward bias voltage across the base/emitter junction of transistor TR2 and the voltage drop across resistor R1, which when normal line current is flowing is less than the sum of the forward bias voltages of diodes D1 and D2 (1.2 volts).