Apparatus for electrostatic charging of paper in electrophotographic reproduction

Claim 1

A charge control circuit for electrophotographic reproducing apparatus, said charge control circuit comprising a high voltage power source, a corona discharge electrode electrically connected to said source for producing a corona discharge current, a target plate for supporting a sheet of reproducing paper, said target plate being grounded and facing said discharge electrode to expose the sheet of reproducing paper to the corona discharge current from said corona discharge electrode whereby said sheet is electrostatically charged, a detection electrode positioned adjacent said discharge electrode to detect corona discharge current from said discharge electrode, feedback means connected to said discharge electrode and to said high voltage power source to control said power source in accordance with the detected corona discharge current to maintain the value of said current constant, and means connected to said power source to interrupt the connection thereof to said discharge electrode upon elimination of discharge current between the discharge electrode and the detection electrode when a spark discharge is produced between the discharge electrode and the reproducing paper, said power source comprising a high voltage transformer with primary and secondary windings, and a high voltage rectifier connected to the secondary winding of the transformer and to the discharge electrode, said means to interrupt the connection of the power source to the discharge electrode comprising a switch means for interrupting power supply to the primary winding of the transformer, a relay controlling said switch means, and means including a timer circuit connected to said feedback means and to said relay to operate said relay and open said switch means if said feedback means detects lack of corona discharge current.