It has been known for a long time to use devices for protecting telephone exchanges against overvoltages caused by such as lightning on subscriber lines. Many different kinds of protective devices have been proposed for this purpose.
It is known to use two anti series connected field effect transistors as switches, inter alia in circuits for connecting a line circuit in a telephone station to a subscriber line. To protect such switches against excessive loss effects due to voltage transients which can occur on the subscriber line, it is known to supplement the circuit with electronic shut off means for the switches.
Such a shut off means described in EP No. 0133789 includes a drive voltage means and electronic switches intended for supplying the anti series connected field effect transistors with a common drive voltage between their source electrodes and gate electrodes such that current can flow in both directions between the line and the protected circuit as well as being intended to supply a common drive voltage between the source electrodes and gate electrodes such that the current cannot flow in any direction between the conductor and circuit via both the anti series connected field effect transistors. The means described in EP No. 0133789 also includes voltage sensing means connected to the conductor and the protected circuit so that it is sensitive to the voltage drops across the transistors. The voltage sensing means is adapted, when the voltage over either of the transistors in a conductive state exceeds a given magnitude, to actuate the switches so that the drive voltage will be such that one of the field transistors is shut off. MOS field effect transistors are used as switches and voltage sensing means in circuits described in EP No. 0133789.
In each of U.S. Pat. No. 3,369,129 and U.S. Pat. No. 3,656,025 it is taught that for limiting the current in a circuit, e.g. for medicinal purposes, two series connected field effect transistors of the JFET type are connected into the circuit, and by coming into a saturated conductive state these transistors can limit the current in the circuit.