Abstract: An automatic voltage control system controls the firing angle of a solid state switch (20), in turn the voltage applied to the primary winding (12) of a step-up transformer (10), in turn the voltage applied to the grid (38) and in turn the electric field intensity between grid elements (38) in an electrostatic separation process. Two voltage control signals are generated in the control system. The first voltage control signal is a protective control signal generated to maximize the voltage applied to the primary winding under normal operating conditions and to reduce the voltage to zero for a short time period following abnormal operating conditions then to reapply voltage to the primary winding (12) again. The second voltage control signal is a periodic signal of adjustable rate and frequency that functions as a variable upper limit to the magnitude of the first control signal.