Abstract: A self-excitation type switch power supply unit with overcurrent protection. For adopting a simple and reliable current negative feedback protective circuit to coordinate with a complementary bridge circuit, a positive feedback circuit without DC biasing capacitive coupling, an oscillation-starting circuit of the AC power supply, etc., so that the self-excitation type switch power supply can become a practical power supply unit with simple circuitry, small size and low cost.

Abstract: A control device for switching a thyristor in a circuit including an a.c. voltage source, which comprises firing signal circuit having an input for receiving control signals, an output for providing firing signals for a thyristor and a detector for detecting zero-crossings of an a.c. voltage provided by the a.c. voltage source. The firing signal circuit further comprises an accumulating circuit for accumulating the control signals and for providing a firing signal when both the detector detects a zero-crossing of the a.c. voltage and the value of the accumulated signals is equal to or exceeds a first predetermined value, and subtracting circuit for subtracting a second predetermined value from the value of the accumulated signals when the firing signal is provided.