High voltage capacitor charge control by low voltage cycle counting

We describe a low voltage open loop control for high voltage capacitor charging circuits. The control is achieved by counting the number of halfcycles from the line supply that are allowed to excite the HV transformer primary. This number of half cycles determines the amount of charge stored in the capacitor, therby setting its voltage to the desired value. This open loop system avoids the use of HV sampling or switching. An implementation is presented for a RC charging circuit, together with graphs and formulas for design purposes. LC resonant charging is also discussed.