SURGE VOLTAGE PROTECTION CIRCUIT

A circuit for surge voltage protection of therefor a telephone station has first and second input terminals for receiving a DC supply voltage via a two-wire subscriber line. Both terminals are shunted via a varistor. In parallel with the varistor is an element for protecting the circuit against polarity reversals. This element has two outputs, one of them carries the positive potential of the DC supply voltage and both of them are connected with respective terminals of the actual station circuit. A V-MOS or bipolar tranistor is arranged between the one output and the associated station circuit terminal. This transistor belongs to a current limiting circuit and serves also as a dialswitch-pulsing contact. The transistor circuit contains two further transistors and a number of dimensioning resistors. The positive output of the protection element is connected to the keyboard-sided input of the transistor circuit via a series network formed by a reverse Zener-diode and a resistor. The breakdown voltage of the this network is below the voltage at which the varistor becomes operative.