JPS633307B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a bias device for a variable magnification copying machine.

A bias device for a variable magnification copying machine includes an auto-bias system that detects the surface potential of a photoreceptor with a sensor and changes the bias voltage to the developing section based on the output. Conventionally, in this auto-bias system, the sensor is configured to a fixed size that matches the smallest electrostatic latent image formed on the photoreceptor in order to work not only when magnifying the image at the same magnification but also when changing the magnification. However, when the sensor is made to have a fixed size as described above, even if the same image is copied at the same magnification and at variable magnification, the bias voltage may be different, resulting in copies with different image quality. This is because the image corresponding to the sensor differs between when the magnification is constant and when the magnification is changed. In other words, since the size of the sensor is set to the image width on the photoconductor when changing the magnification (reduction), the sensor adjusts the average value of the surface potential of the photoconductor and the image width in response to the full width of the photoconductor when changing the magnification. However, at the same magnification, it only corresponds to the photoreceptor and a part of the image width, so the average value of the surface potential of that part is detected. Here, there is no problem if the image is uniform in the width direction, but if it is not, there will be a difference in the detection potential of the sensor when copying the same image at the same magnification and when the magnification is changed. Therefore, even if the same original is used, different copies will be made when the original size is changed and when the original size is changed. The same thing can be said when the average value or minimum value of the surface potential of only a part of the image width on the photoreceptor is detected during zooming.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a bias device for a variable magnification copying machine that improves the above-mentioned drawbacks and makes it possible to obtain copies of the same image quality from the same original regardless of the difference in copy magnification.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a variable magnification copying machine according to this embodiment.
This variable-magnification copying machine can perform copying at a plurality of types of copying magnifications, and can arbitrarily set the copying magnification using an operation section. The photosensitive drum 1 is rotationally driven by a driving section, and after being uniformly charged by a charging device 2, a document image is exposed by an exposure device to form an electrostatic latent image.
This electrostatic latent image is developed by a wet type developing device using a developing plate 3, excess developer is removed by a squeeze roller 4, and transferred by a transfer device 6 onto a transfer paper 5 from a paper feed device. The image on the transfer paper 5 is fixed by a fixing device and becomes a copy, and the photosensitive drum 1 is cleaned by a cleaning device 7 and reused.

The developing device connects the photosensitive drum 1 to the liquid discharge port 8.
Development is carried out by sending a developer between the developing plate 3 and the developing plate 3. The developing plate 3 also serves as a developing electrode, and a bias voltage is applied to the developing plate 3. Further, a sensor 9 is arranged behind the liquid discharge port 8, and this sensor 9 detects the surface potential of the photoreceptor drum 1.

As shown in FIG. 2, the sensor 9 includes three sensor electrodes 10 to 12 arranged in a row in the width direction of the photoreceptor drum 1 and a protective electrode 1 arranged around the sensor electrodes 10 to 12.
3, and the sensor electrodes 10 to 12 are arranged with a small interval from the photoreceptor drum 1. Although the sensor electrodes 10 to 12 are spaced apart from each other,
The switches 14 to 16 connect to the terminal 17 in combinations depending on the copying magnification. This switch 14~
16 are closed in a combination according to the copying magnification by setting the copying magnification by the operation section, and the total length of the sensor electrode connected to the terminal 17 changes according to the copying magnification when the copying magnification is varied. Therefore, when the same original is copied at each copying magnification, the detection part that contributes to the detection operation, that is, the sensor electrode connected to the terminal 17, corresponds to the same image regardless of the difference in copying magnification. The detection potential is also the same voltage during each copy regardless of the difference in copy magnification, making it possible to obtain copies of the same image quality.

As shown in FIG. 3, the detected potential at the terminal 17 is applied to an amplifier comprising a field effect transistor Q ₁ , transistors Q ₂ and Q ₃ , diodes D ₁ and D ₂ , and resistors R ₁ to R ₆ . The output of this amplifier is applied to the protective electrode 13, integrated by the capacitor _C1 , and applied to the capacitor _C2 via the switch SW and the resistor _R7 . The switch SW is closed only when the leading edge of the electrostatic latent image on the photoreceptor drum 1 enters the developing area, and the capacitor _C2 receives the potential applied when the switch SW is closed. switch
It is memorized after the SW is released. This capacitor C ₂
The stored potential is amplified by transistor Q ₄ in an arithmetic circuit consisting of transistor Q ₄ , Zener diode ZD ₁ , and resistors R ₈ and R ₉ , and a constant voltage is applied by Zener diode ZD ₁ to become a bias voltage for development. applied to dish 3. Here, Zener diodes ZD ₂ , ZD ₃ and resistance R ₁₀
The constant voltage circuit that consists of creates a constant voltage from the power supply voltage DC and supplies it to each part.

As described above, in the bias device of the variable magnification copying machine according to the present invention, the size of the detection part of the sensor is changed by varying the copying magnification, so the image quality is the same from the same document regardless of the difference in the copying magnification. It becomes possible to obtain a copy of

[Brief explanation of the drawing]

FIG. 1 is a schematic side view showing a variable magnification copying machine according to an embodiment of the present invention, FIGS. 2a and 2b are perspective views and wiring diagrams showing a sensor of the same embodiment, and FIG. 3 is a schematic side view showing a variable magnification copying machine according to an embodiment of the invention. FIG. 10-12...sensor electrode, 13-15...switch.

Claims (1)

[Claims] 1. In a bias device for a variable magnification copying machine that detects the surface potential of a photoreceptor with a sensor and automatically changes a bias voltage to a developing section based on this potential, a plurality of sensor 0-electrodes constituting the detection section of the sensor and A variable magnification copying machine, characterized in that the plurality of sensor electrodes are connected in a combination according to the copying magnification by changing the copying magnification using an operation section, and a switch is provided to change the size of the detection section of the sensor. Bias device.