Abstract: The invention relates to a parallel oscillatory circuit frequency converter comprising: a rectifier (1); an inverse rectifier (4) having thyristors disposed in a bridge circuit; a parallel oscillatory circuit (5) having an inductance (L) and a series connection of two capacitors (C, C.sub.K) whereby one capacitor (C) is connected to the load points of the bridge circuit; and a gate-controlled turn-off time control device (6) for the alternate firing (switching) of the thyristors a, b forming the particular diagonally adjacent bridge branches with a cycle frequency adapted to maintain a minimum turn-off time, a safety firing circuit (24, 25) controlled by a detector circuit (11-23) to detect incorrect switching delivering firing pulses without delay to all the thyristors at least until all the thyristors are current-carrying.

Abstract: The invention relates to a parallel resonant-circuit converter having at least one intermediate-circuit inductance. In order to guarantee that the inverter will start even with different loads, the voltage produced when discharging a separately charged capacitor across the load is monitored. At some time after the maximum voltage across the load is reached, another separately charged capacitor is discharged via a thyristor of the inverter located in the biasing circuit of the intermediate-circuit inductance. Because of the blocked thyristor and of the capacitor charged to the voltage maximum, the biasing current is forced to flow via diagonal branches and including the load. In this manner, reliable starting of the inverter is possible even with loads having a different natural frequency.

Abstract: An inverter circuit with a thyristor bridge circuit, a load circuit developed as a parallel oscillating circuit connected to the bridge circuit, an impulse generating circuit for supplying ignition pulses to the bridge circuit and an arcing time regulating circuit for controlling the impulse generating circuit in accordance with the arcing time. A down-and-up integrating circuit is triggered by an ignition impulse to integrate down until a discriminator circuit produces a signal indicating zero crossover of the load current, thereafter to integrate up until a second discriminator produces a second signal indicating zero crossover of the load voltage, at which time the integrator output indicates arcing time, and is applied to the time regulating circuit to control the impulse generating circuit.