| 摘要 |
PURPOSE:To prevent transfer of toner to a drum when abnormality is generated in a main electrifying device by changing over the potential impressed to a developing means from a specified developing bias to a non developing bias when current value fed to a grid is below a specified value. CONSTITUTION:Negative ion generated in a vicinity of a wire of the main charger 12 is captured by a grid 14, fed to the ground through a Zener diode 16 and a resistor R, and the potential between the diode 16 and the resistor R is compared with a specified reference potential EREF by a comparator 18. Then, a changeover switch 24 is controlled by the output of the comparator 18, when the main charger 12 is normally activated, the bias of a developing roll 22 is made EDEU, but when the current i fed to the grid 14 is smaller than the specified value fixed by the reference potential EREF, it is changed over to ENON side which is positive potential in a reverse direction. Thus, the negatively electrified toner is retained on the developing roll 22, and transfer of needless toner to the drum 20 can be prevented. |
