Electrophotographic charging system and method

A system for applying charge to a photoconductive surface wherein an electrode is in proximity to the surface and spaced between the surface and a shield comprising:

• a) means for applying voltage to said electrode such that current flowing in said electrode is the sum of surface charging current and shield current;
• b) signal developing means operatively connected to said shield for utilizing said shield current for obtaining an electrical signal proportional to said surface charging current;
• and
• c) control means operatively connected to said signal developing means and connected in controlling relation to said voltage applying means for controlling the application of voltage to said electrode;
• d) so that said surface charging current is maintained constant as the distance between said surface and said electrode and shield varies.

A system for applying charge to a photoconductive surface including electrode means in proximity to the surface and shield means spaced from said electrode means such that said electrode means is between the surface and said shield means:

• a) high voltage supply means having an output, a return input and a control input such that variation in a control signal applied to said control input causes variation in the voltage of said output;
• b) means for coupling said output of said supply means to said electrode means;
• c) means defining a current summing node connected to said return input;
• d) means for connecting said shield means to said summing node;
• and
• e) control means having an output connected to said control input of said supply means, a first input adapted to receive a control signal and a second input connected to said summing node so that shield current flows from said shield means to said summing node and the sum of said charging current and shield current flows from said summing node and so that as charging current for said surface varies as represented by variations in voltage at said summing node said control means applies a signal to said control input of said supply means to control the output of said supply means and thereby said charging as determined by the nature of said control means.

A method for applying charge to a photoconductive surface wherein an electrode is in proximity to the surface and spaced between the surface and a shield comprising the steps of:

• a) applying a voltage to said electrode such that current flowing in said electrode is the sum of surface charging current and shield current;
• b) utilizing said shield current to obtain an electrical signal proportional to said surface charging current;
• and
• c) utilizing said signal to control the application of voltage to said electrode;
• d) so that said surface charging current is maintained constant as the distance between said surface and said electrode and shield varies.