Abstract: An overvoltage and overcurrent protection for an exchange terminal circuit which has two input terminals (a, b) and a number of output terminals (T1-T). The input terminals are connected to a two-wire line and the output terminals are connected to equipment for the transmission of analog as well as digital signals over the line via the exchange terminal circuit. The exchange terminal circuit comprises an impedance matching network having a transformer at the primary as well as secondary sides of which there are networks that are symmetric in respect to earth. The network at the primary side comprises two primary winding halves (L1, L2) between which a capacitor (C) is series connected. The novel features of the invention is a varistor (V1) that is connected in parallel with the capacitor (C). When a surge pulse has charged the capacitor to the rated voltage of the varistor the varistor goes conductive thereby increasing the current through the primary winding until the core of the transformer is saturated.

Abstract: A supply circuit for a two-wire line is included in a DC-magnetized hybrid transformer in which the two primary windings (1.sub.1, 1.sub.2, 1.sub.1 ', 1.sub.2 ') are magnetically coupled to two pairs of secondary windings (1.sub.3, 1.sub.4). The primary winding is divided into two parts to form an auto transformer and the supply circuit (E.sub.o, R.sub.1b, R.sub.2b) is connected to the common connection points of the parts. The supply circuit includes two resistor networks which each consists of a PTC-resistor (R.sub.p1, R.sub.p2) in parallel with a temperature dependent resistor (R.sub.1, R.sub.2) to limit the feed current at varying line length.

Abstract: A direct current magnetized hybrid transformer in a telephone line repeater has both its primary windings magnetically coupled to two pairs of secondary windings, one pair creating the four-wire path in the repeater and the second pair being connected to a balance impedance. The primary windings are each divided into two parts with a like number of turns. The feeding voltage source included in the repeater is connected between the connection points common for the winding parts.

Abstract: A compensation circuit is provided for a piezo-electric quartz crystal in a filter circuit for reducing the influence of the secondary resonance frequencies of the crystal on the filter characteristic. The Q-value of the crystal at one or several secondary resonance frequencies is reduced by connecting to the crystal a two-terminal network the impedance of which has a real and an imaginary part so that the real part gives an ohmic contribution which at the resonance frequency of the imaginary part reduces the Q-value of the crystal.