provides overall system control and timing may take any one of several forms it is preferable that microcomputer com be constructed with a commercially available microprocessor however many alternate structures will be readily apparent to persons skilled in the art in fact in some embodiments analog circuitry may be employed for example selectable storage capacitors may be charged to potentials representing the signals on inputs in1 17 at various instants of time these stored charges may be selectively coupled to a combining network to produce a control signal microcomputer com establishes the rates and sequence in which each of the inputs in1 in7 transmits its respective signal to command means com in this embodiment this rate will be normally twice the power line frequency but subject to substantial increase under predetermined conditions it is apparent that other rates may be employed to suit the characteristics of a specific voltage controller and precipitator primary power lines 12 are also connected to a pair of inputs of shaper 84 shaper 84 an amplifier rapidly saturates to produce a square wave output synchronously with the power line frequency this output of shaper 84 is applied as one comparison input to phase comparator pc phase comparator pc has another input 60 which is nominally at 60 hz in a well known fashion a phase differential between line 60 and the output of shaper 84 produces an error signal from phase comparator pc this error signal is applied to voltage controlled oscillator vc to regulate its frequency of oscillation oscillator vc.