d and c g so as to produce a rising voltage indicated at 36 of the secondary output voltage v s of fig2 f at t 1 when v 1 becomes zero the capacitances c d and c g discharge so that the voltage across them falls as indicated at waveform portion 38 of v s corona discharge takes place during the last portion of the rising voltage waveform portion 36 e g above a line 40 note that because of the low slope of the waveform section 36 the corona discharge occurs for a longer time than would be the case if the waveform sections 36 of v s had the same slope as waveform section 38 i e a greater slope the operation during a negative half cycle of v i when thyristor 14 conducts is similar to the operation just described for a positive half cycle of v i for example at t 1 i s goes to zero and thyristor 12 turns off between t 1 and t 2 both thyristor 12 and 14 are turned off and v 1 is at zero volts at t 2 trigger pulse 31 of v g2 triggers thyristor 14 into conduction causing v 1.