Abstract: A power supply system for transfer rollers associated with electrostatographic marking engines. The power supply system includes a power source having a fixed and constant output parameter, such as current or voltage. Additional resistance is connected to the output of the power source to provide the most desirable bias level on the transfer roller under actual operting and material conditions. With a constant current source, a resistance is connected in parallel with the load impedance of the roller system. With a constant voltage source, the resistance is connected in series between the voltage source and the roller system.

Abstract: A control mechanism for a toner monitor used in an electrographic reproduction apparatus having interchangeable developer stations, respectively containing developer material of different colors, and in which the toner monitor measures reflectivity of the developer material and produces a signal corresponding to the measured reflectivity in order to enable regulation of toner concentration. The mechanism comprises a sensor, associated with a developer station located in the reproduction apparatus, for differentiating between at least two colors of toner and producing signals corresponding respectively to such colors. In response to the corresponding to a particular color, a signal is produced representative of the concentration of toner of that color. The signals respectively representative of toner concentration are substantially equal for like toner concentrations.