Abstract: A pulse generating circuit for a plasma ignition system includes a thyristor connected between a supply terminal and earth. A primary winding (Wp) of a transformer (TR) and a first capacitor (C.sub.1) are connected across the thyristor. The supply terminal is also connected through a secondary winding (W.sub.s) and a diode (D) to an output terminal. A saturable core inductor (L) and a second capacitor (C.sub.2) are connected in series across the output terminal and earth. A plasma plug is also connected across the output terminal and earth. In operation, the first and second capacitors are charged and the thyristor is fired. A high voltage pulse is applied by the secondary winding (W.sub.s) to the plasma plug causing electric breakdown. The second capacitor (C.sub.2) then discharges through the inductor (L) and the plasma plug. Four alternative circuits are also described.