Subscriber telephone set incorporating overvoltage protection

Claim 1

In a subscriber telephone set comprising line inputs, hook contacts, and an electronic transmission and dialing circuit associated with a push button dial and incorporating a first overvoltage protection means connected in said set to the line inputs and also a second protection means provided up-stream of the hook contacts, intended to limit the power applied to said electronic transmission and dialing circuit associated with a push-button dial, the improvement wherein said second protection device is in essence formed by a dipole electronic circuit arranged in series with one of the inputs of the transmission and dialing circuit, by means of a dialing interrupter, this dipole circuit having first and second terminals and comprising a first transistor whose main current path is connected to one of the terminals of the dipole circuit via a current measuring resistor, and to the other terminal of the dipole circuit via a diode which forms together with a second transistor whose polarity is the reverse of the polarity of the first transistor a current mirror supplying the control electrode of the first transistor, while a third transistor is connected between the one terminal of the dipole circuit and the control electrode of the first transistor, said third transistor becoming conductive when the current flowing through the current measuring resistor reaches a predetermined value.