JPS5921554B2 - developing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a developing device used in an image forming apparatus such as an electrophotographic copying machine.

In an image forming apparatus such as an electrophotographic copying machine, the background of the obtained image changes and becomes dirty depending on the operating conditions of the machine, the condition and type of the document on which the image is to be formed, etc.

For example, in an electrophotographic copying method using a photoconductor having a photoconductive layer, the residual potential during continuous use of the photoconductor, that is, the surface potential on the photoconductor after static electricity removal (static charge removal level), and the photosensitive area corresponding to the background part of the document Surface potential on the body (background potential)
increases as the photoreceptor is used.

This is because the photoreceptor is affected by fatigue due to exposure and charging, wear, deterioration of the exposure light source, dirt on the exposure mirror, and temperature of the developer. It is well known that in a normal electrophotographic copying machine, the residual potential changes by about 100 to 230 volts with respect to a charging potential of about 800 to 1,000 volts (for example, Japanese Patent Application No.
10) Patent application 1977-67714). Therefore, in order to prevent the background staining of the sheet by considering the variation range of the residual potential, the residual potential is detected and a bias potential proportional to the detected potential is applied to the developing electrode as shown in Figure 1a. A method has been proposed in which the toner is applied to only the portion of the member to be developed whose surface potential is equal to or higher than this bias potential for development.

Most commonly used manuscripts have an information area of 3
(~20% positive original) In this case, it is easy to detect the background potential, and even if a potential containing the information part (average potential) is detected, this average potential will be slightly smaller than the background potential. Since the bias potential is only high, no problem occurs even if the bias potential is applied as shown in FIG. 1a.

However, for example, in the case of a negative original where the background is black and the information area is white, the black area is 80 to 90, contrary to a positive original.
7%, therefore, if a developing bias potential is applied in accordance with this background potential, almost no toner will adhere to the electrostatic latent image. In this case, it is extremely difficult to detect the potential of the white portion, and even if the average potential is detected, it is no longer possible to solve the problem. SUMMARY OF THE INVENTION It is an object of the present invention to provide a developing device which eliminates the above-mentioned drawbacks and can perform development without losing information regardless of whether the background of the document is white or black. An embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 2, in an electrophotographic copying machine, a photoreceptor 11 on which a negatively charged electrostatic latent image has been formed according to the image of an original is guided by rollers 12 and 13 and passed through a developer along a guide path. After being developed, it is sent out by rollers 15 and 16.

The guide path is formed by a guide plate 17 and a developing electrode 18,
A detection electrode 19 is provided on the entrance side. The developer 14 is supplied from the developer tank 20 onto the phantom guide plate 18 by a pump. The detection electrode 19 is larger than the plurality of spot detection electrodes 191 to 193 and the average value detection electrode 194 as shown in FIG. The surface potential of a portion is detected through the toner, and the average value detection electrode 194 is made relatively large to detect the surface potential of a large portion of the photoreceptor 11 through the toner. The detection electrodes 191 to 194 detect residual potential from the original 11 within the image forming range, and therefore detect not only the operating status of the copying machine but also the background of the original. Since the average value detection electrode 194 detects the surface potential of a relatively large area on the photoconductor 11, it detects the average value of a relatively large area of the image, which is not included in the information part of the original. There are very few cases where the background cannot be detected. Since the spot detection electrodes 191 to 193 detect the surface potential of a relatively small portion on the photoconductor 11, it is often included in the information part of the document, but they accurately detect the residual potential of the photoconductor 11. be able to. Therefore, by taking the minimum output of each of these detection electrodes 191 to 194, the residual potential of the photoreceptor 11 can be detected more accurately. Each of the detection electrodes 191 to 194 is connected to the gates of N-channel field effect transistors 25 to 28 via input overvoltage protection resistors 21 to 24, respectively. The field effect transistors 25 to 28 have sources commonly connected to a negative DC power supply (-Vccl) via a resistor 29 and the base of a PNP transistor 30, and drains connected to the collector of the PNP transistor 31 and the transistor 30. Constant voltage diodes 32 to 35 are connected in common to the connection point with the emitter, and between the drain and the source, respectively. The emitter of transistor 30 is connected to a negative DC power supply (-Vcc2), and the collector of transistor 31 is grounded via resistor 36. The base of the transistor 31 is connected to the connection point of the resistors 3r and 38, and the resistors 37 and 38 are connected to a negative DC power supply (-C
c2) and the ground point. Therefore, the field effect transistors 25 to 28 are connected to each detection electrode 191.
In response to the detection potential of ~194, the current is shunted from between the base and emitter of the transistor 30 to each of the detection electrodes 191 to 19.
The smallest of the four outputs is the field effect transistor 25.
-28 are selected. The transistor 31 and the resistors 36 to 38 constitute a constant current circuit, and the transistor 3
A constant current is supplied to the emitter of field effect transistors 25 to 28, and the constant current circuit and the transistor 30 divide the output Vcc2 of the DC power supply into the selected minimum outputs of the field effect transistors 25-28. Further, the connection point between the emitter of the transistor 30 and the collector of the transistor 31 is connected to the shield wire of the cables 39 to 42 through a resistor 43 and a capacitor 44 in parallel, and is connected to the voltage regulator diodes 46 to 48 through a resistor 45. It is connected to one end and the developing electrode 17. Cables 39-42 connect each detection electrode 191-194 to the gate side of field effect transistors 25-28. The other ends of the constant voltage diodes 46 to 48 are connected to the switch 4
The movable contact piece of the switch 49 is connected to each fixed terminal of the switch 49 and grounded. The constant voltage diodes 46 to 48 have different constant voltage values, and together with the resistor 45 and the switch 49, constitute a limiter 50. Therefore, the minimum output selected by the field effect transistors 25 to 28 is applied as a bias potential to the developing electrode 17 through the limiter 50. As a result, the bias potential of the developing electrode 17 is proportional to the residual potential detection voltage as shown in FIG.
The limiter 50 limits the voltage to a certain voltage or less, and the limit voltage is changed by the switch 49. Photoreceptor 1
In No. 1, toner adheres to areas where the surface potential is higher than the bias potential of the developing electrode 17 and is visualized, but toner is attracted to the developing electrode 17 side and does not adhere to areas where the surface potential is lower than the bias potential. This prevents surface stains. Therefore, when the background of the original is white, the bias potential of the developing electrode 17 is proportional to the residual potential of the photoreceptor 11 corresponding to the background of the original, and the electrostatic charge on the photoreceptor 11 is reduced without loss of information. The latent image is developed. Further, when the background of the document is a black portion, the bias potential of the developing electrode 17 is limited to a potential lower than the residual potential of the photoreceptor 11, and therefore the information portion is also developed. In this case, if the limit level of the bias potential is set according to the whitest part of the information part of the document, development can be performed without losing information. should be selected in the best way. Further, if it is better to ground the sheet-like photosensitive paper, the rollers 13 and 16 may be grounded, and in this case, the roller pairs 12, 13 and 15 may be grounded.
, 16 must be shorter than the photosensitive paper, but of course any known grounding means may be used. In the above embodiment, the voltage of the DC power supply -Vcc
2 can be varied to arbitrarily set the restriction level.

Further, the present invention can be applied to a developing device using a photosensitive drum instead of a photosensitive paper, and can also be applied to a dry type developing device. The present invention can also be applied to a case where a sleeve is used as a developing electrode in a magnetic brush developing device and a bias potential is applied thereto, and the present invention can be implemented with various modifications within the scope of the gist. As described above, according to the present invention, the bias potential applied to the developing electrode is limited to a predetermined level or less according to the surface potential of the member to be developed, so information is not lost even if the background of the document is black or white. It becomes possible to carry out development.

[Brief explanation of the drawing]

Figures 1A and b are detection voltage vs. bias voltage characteristic diagrams for explaining the present invention, Figure 2 is a configuration explanatory diagram of an embodiment of the present invention, and Figure 3 is a diagram of the arrangement of detection electrodes in the same embodiment. It is. DESCRIPTION OF SYMBOLS 11... Photoreceptor, 14... Developer, 17... Developing electrode, 19... Detection electrode, 50... Restrictor.

Claims (1)

[Claims] 1. In a developing device that detects a surface potential within an image forming range from a member on which an electrostatic latent image is formed and applies a bias potential to a developing electrode according to the surface potential to develop the electrostatic latent image, the bias potential is What is claimed is: 1. A developing device comprising means for limiting the amount of water to below a predetermined level.