Improvements in and relating to thyristor circuits

1,032,191. Semi-conductor switching circuits. LICENTIA PATENT - VERWALTUNGS - G.m.b.H. Feb.11, 1963, No. 5490/63. Heading H3T. [Also in Division H2] A circuit for switching off a semi-conductor thyristor comprises a quenching capacitor and means for charging the capacitor in dependence upon the current through the thyristor when the latter is conducting and for discharging the capacitor in opposition to the current flow through the thyristor to terminate the current flow. With reference to Fig. 1 a semi-conductor thyristor 9 is arranged to short-circuit resistors 3, 4 of a rotor resistance of a motor when thyristor 9 is rendered conducting by pulses from 11. During the period when thyristor 9 is conducting quenching capacitor 14 charges over the circuit including inductive reactance 10 connected as an auto-transformer in series with the parallel R.C. circuit 13 and with the diode 15, the peak voltage attained by the capacitor 14 being determined by reactance 10 and the current flow through thyristor 9. When a second thyristor 12 in series with capacitor 14 is fired the quenching capacitor 14 is discharged through it to the thyristor 9 in the opposite direction to the current flow in the latter to quench it. The thyristor 12 is quenched thereafter by the oscillatory return flow of current. In a second embodiment, Fig. 2 (not shown), the circuit is used to short-circuit the load resistance in the stator circuit of a motor in order to control the speed of rotation of the latter.