Image forming apparatus having a voltage controlled contact charger

#The present invention relates to a charging apparatus and an image forming apparatus provided with a charging member to which a voltage is applied for charging (or discharging) a member to be charged, such as a photosensitive member.

#In recent years, contact type charging apparatuses with special characteristics such as no ozone generation or low power consumption have been attracting public attention and some of them have been put to practical use. In these apparatuses, a conductive charging member is placed in contact with the member to be charged, such as the photosensitive member, and then a voltage is applied to this charging member to trigger an electric discharge to the member to be charged so that the surface of the member to be charged is charged to a predetermined potential.

#It is also possible to charge the member to be charged without placing the charging member in contact with the member to be charged. In other words, the charging member may be positioned to hold from the member to be charged, a minute air gap across which the electric discharge occurs from the charging member to the member to be charged, as a necessary amount of charge bias is applied to this charging member. This method can charge the member to be charged in the same manner as the charging member is placed in contact with the member to be charged.

#FIG. 1 is a schematic section of a preferred embodiment of the image forming apparatus (laser beam printer) according to the present invention, depicting the general structure.

#FIG. 2 is a graph showing the relation between the voltage VDC applied to the charge roller and the current I.sub.d flowing to the photosensitive drum.

#FIG. 3 is a schematic section of the second embodiment of the image forming apparatus in which the charging member is in the form of a charging blade.

#FIG. 1 is a schematic section of a preferred embodiment of the image forming apparatus according to the present invention. This image forming apparatus is a laser beam printer employing a transfer type electrophotographic process.

#A reference numeral 1 designates a photosensitive drum as an image bearing member (member to be charged). This photosensitive drum 1 is a cylinder having a diameter of 30 mm and is rotatively driven about the central axis which runs in the direction perpendicular to the surface of this page, in the clockwise direction X indicated by an arrow at a predetermined process speed (peripheral velocity). In the case of this embodiment, it is rotated at 23 mm/sec. The photosensitive drum 1 comprises an organic photoconductive layer 1a which is 25 mm thick, and a grounded conductive substrate 1b which supports the organic photoconductive layer.

#A reference numeral 2 designates a charge roller as a charging member placed in contact with the photosensitive drum 1. This charge roller 2 is rotated by the rotation of the photosensitive drum 1 and as a predetermined amount of charge bias voltage is applied to it from a voltage supplying member 3 (HVT, power source), the peripheral surface of the rotary photosensitive drum 1 is uniformly charged to a predetermined polarity and potential (in this embodiment, to the negative polarity).