Abstract: A twelve-pulse converter has an a.c.-input and a d.c.-output which is connected with an armature via two thyristors of a change-over circuit. An extinguishing circuit comprises a resistor, a capacitor, an inductor and a thyristor which is also connected with the armature via related diodes. A current transformer in the armature circuit causes ignition of the thyristor in the extinguishing circuit at the occurrence of excess current. The extinguishing capacitor is continuously connected with a charging circuit and is connected parallel to the thyristor of the extinguishing circuit and with the thyristors connected to the d.c.-negative pole. The d.c.-negative pole connected thyristors are cut-off when the thyristor of the extinguishing circuit is ignited and the energy appearing in the armature circuit is dissipated by the resistor in the extinguishing circuit. The zero-crossing of the current is detected by means of two voltage converters which are connected with the d.c.-negative pole and the d.c.