Current-controlled image transfer

Claim 1

An image transfer system comprising: image bearing means having an electrically conductive base and an imaging surface for bearing thereon a toner image of first polarity; current detecting means connected between said electrically conductive base of said image bearing means and a reference potential; corona unit for applying corona ions of second polarity opposite to said first polarity to the back side of a transfer medium which is brought into contact with said imaging surface of said image bearing means for transferring said toner image from said image bearing means to said transfer medium; a power supply connected to said corona unit for supplying a power thereto for emitting said corona ions; and control means for controlling a power level to be applied to said corona unit from said power supply responsive to information supplied from said current detecting means, said control means measuring a first current level I 1 during a non-transfer period, in which no transfer sheet is present below said corona unit, and a second current level I 2 during a transfer period, in which said transfer sheet is present below said corona unit, and, then, after comparing a difference x between I 1 and I 2 with a first predetermined value x 1 stored in said control means, controlling said power supply so as to make x-x 1 to be substantially equal to zero.