Corona Discharge Device

1. Field of the Invention

The present invention relates to a corona discharge device of the scorotron system used for electrophotographic copying machines, which requires only one connector that connects to a corona discharge wire, and which is adapted to be easily positioned with respect to the main body and is adapted to prevent poor electrical connection.

2. Description of the Prior Art

Corotron and scorotron have widely been known as corona discharge devices. In particular, the scorotron charger has been frequently employed as a discharge device for negatively charging owing to its small dispersion in discharge performance. With regard to the construction, however, a connector for grid wires is separately provided from a connector for corona discharge wire, such that the grid wires are impressed with a constant voltage from an external power supply. Since at least two or more connectors are necessary, the shape of the connector portion tends to become complex. Besides, the device requires clumsy positioning operation and imposes a problem of poor connection.

In view of such circumstances, the object of the present invention is to provide a corona discharge device which requires only one connector to facilitate the positioning operation as well as to prevent the occurrence of poor connection.

The present invention deals with a corona discharge device comprising a discharger having a wire for corona discharging, an electrically conductive shielding plate and grid wires; and an impedance element which is connected in series between the grid wires and the ground and which generates a voltage responsive to a corona current flowing into the grid wires; wherein the impedance element is connected between the grid wires and the electrically conductive shielding plate and is formed as a unitary structure in the discharger. Thus, since the impedance element is connected between the grid wires and the electrically conductive shielding plate and is formed as a unitary structure together with the discharger, the connector is needed only for the corona discharge wire, enabling the positioning operation to be facilitated and poor connection to be prevented.

Furthermore, according to the present invention, the impedance of the impedance element is made variable to compensate the aging of photosensitive materials, as well as to change the picture characteristics .

The drawings illustrate an embodiment according to the present invention; in which;

FIG. 1 is a plan view;

FIG. 2 is a side view of FIG. 1;

FIG. 3 is a plan view showing, in a partially cut-away manner, the device of the invention on an enlarged scale;

FIG. 4 is a vertically cut-away side view on an enlarged scale;

FIG. 5 is a vertically cut-away front view;

FIG. 6 is a vertically cut-away front view showing a modified embodiment; and

FIG. 7 is a diagram illustrating characteristics.

An embodiment of the invention is illustrated below with reference to the drawings. Referring, first, to FIGS. 1 to 4, a casing 3 which serves as an electrically conductive shielding plate is formed in a ]-shape by a back plate 1 and side plates 2 (refer to FIG. 5). On both sides of the casing 3 are provided electrically insulating brackets 4 and 5 across which are stretched a plurality of grid wires 6 which are located on the open side of the casing 3 and are pulled by spring 7. Further, nearly at the center of the casing 3, a corona discharge wire 8 is stretched being pulled by a spring 9 between the brackets 4 and 5 in the lengthwise direction of the casing 3. Thus, a discharger 10 of the scorotron system is constructed by the casing 3, grid wires 6 and corona discharge wire 8. A constant-voltage element 11 which is an impedance element is provided at one end of the casing 3. One end of the constant-voltage element 11 is connected to the ends on one side of the grid wires 6, and the other end thereof is connected to the side plate 2. To the other end of the casing 3 is provided, in a protruded manner, a connector 12 to which is connected one end of the corona discharge wire 8. The connector 12 is connected to a high-voltage power supply 13 accommodated in a main body which is not shown. The high-voltage power supply 13 is grounded together with the casing 3. Both ends of the back plate 1 forming the casing 3 serve as engaging portions 14 that will fit to rails formed in the main body which is not shown.

With the thus constructed device, the engaging portions 14 are fitted to rails formed in the main body, and the connector 12 is connected to the high-voltage power supply 13, such that the device is electrically connected to the main body. In this case, only one connector 12 is required, and connectors are not needed for the grid wires 6. As compared with the conventional devices which require a plurality of connectors, therefore, the device of the invention minimizes the probability of developing poor electrical contact, and further enables the positioning operation to be easily carried out. Moreover, since only one connector 12 is required, the device can be simply constructed in small size.

In this embodiment, the constant-voltage element 11 used as impedance element may have either two-way characteristics or one-way characteristics. When the constant-voltage element having one-way characteristics is employed, however, the connecting direction must be changed depending upon the characteristics to be obtained. Examples of the constant-voltage element 11 will be a varistor, Zener diode and the like. The constant-voltage element may further be constituted by a constant-current circuit.

Further, as shown in FIG. 6, the constant-voltage element 11 may be constructed by a plurality of constant-voltage elements 11a, 11b, 11c,--which will be selected by a switch 15. In this case, if the constant-voltage element 11 is selected by the switch 15 to change the Zener voltage, i.e., to change the grid voltage of grid wires 6, the surface potential of a photosensitive member 16 is controlled as illustrated in FIG. 7. Characteristics illustrated in FIG. 7 are obtained when the casing 3 is made of a zinc-plated steel plate of 0.8 mm in thickness, grid wires 6 are composed of tungsten 0.2 mm in diameter, corona discharge wire 8 is composed of a stainless steel of a diameter of 0.06 mm, the distance of the corona discharge wire 8 to the side plates 2 is selected to be 28 mm, the distance between the back plate 1 and the corona discharge wire 8 is selected to be 11 mm, the distance between the corona discharge wire 8 and the photosensitive member 16 is selected to be 10 mm, the distance between the grid wires 6 and the photosensitive member 16 is selected to be 2 mm, the total discharge current is set to be 800 .mu.A, and the moving speed of the photosensitive member 16 is 128 mm per second.

Since the surface potential of the photosensitive member 16 can be controlled as mentioned above, it is allowed to compensate the aging of the photosensitive members 16. Namely, for the photosensitive members which increase the surface potential and residual potential with the use, the grid potential should be decreased and for the photosensitive members which decrease the surface potential and residual potential with the use, the grid potential should be increased. In this case, there is no need of intensifying the exposure.

Moreover, when it is desired to obtain pictures with high contrast, the grid potential should be increased to increase the surface potential of the photosensitive member. If the operation is effected in a reverse manner, pictures with soft tone can be obtained. Thus, the device of the invention makes it possible to change the picture characteristics.

The above operation can also be attained by connecting a variable resistor or the like.