JPS6231345B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a developing bias device for an electrophotographic copying machine.

Generally, the surface of the photoreceptor drum used in electrophotographic copying machines has the property of being a good insulator and retaining electrical charges in a dark place.On the other hand, when exposed to light, the electrical resistance decreases and the surface becomes electrically charged. It consists of a photoconductive layer that has the property of dissipating surface charges.

When exposure is performed on a photoreceptor drum with the surface of the photoconductive layer uniformly charged, charges remain in the black areas (image areas) that are not exposed to light and are irradiated with light, depending on the image. The charge in the white area (non-image area) is dissipated. That is, an electrostatic latent image is formed.

When fine particles called toner, which are charged with a polarity opposite to that of the photoreceptor, approach or come into contact with this electrostatic latent image area, the image area attracts a large amount of the particles, while the non-image area does not.

This process of applying toner to the electrostatic latent image on the photoreceptor drum is called development, and after this development step, the toner-adhered image is transferred onto paper or the like. However, the charge on the non-image area of the photoreceptor drum cannot be completely removed by exposure, and a small amount of charge remains, and toner adheres in proportion to the strength of the electric field, which is then transferred to paper etc. The skin becomes dirty.

In order to prevent this background staining, an electrode plate called a developing electrode is placed near the photoreceptor drum so as to face the surface of the photoreceptor drum. By applying a large bias potential, the toner that tends to adhere to the non-image area is attracted to the developing electrode without affecting the toner in the image area, and a clear transferred image without background stains can be obtained. I can do it.

This causes some of the toner to adhere to the developing electrode, which has an adverse effect on developing the next electrostatic latent image. The toner is released from the electrode plate by applying a bias of opposite polarity to the
Efforts are being made to clean the electrode plates. This operation is called developing electrode cleaning, and the circuit that effectively controls the developing electrode is a bias circuit.

FIG. 1 is a block diagram showing an example of a conventional developing bias device. In the figure, 1 is a photosensitive drum, 2 is a detection electrode that detects the potential (residual potential) of a non-image area of a latent image, and 3 is a developing electrode. , 4 is an impedance conversion circuit that converts the detected value of the detection electrode 2 into a residual potential, 5
6 is an operational amplifier circuit, SW1 is a switch that closes at the timing of storing the residual potential, and SW2 is a switch for forward bias and reverse bias. Each switch for selecting the application timing is shown. Moreover, FIG. 3 is a detailed perspective view of the developing electrode.

In FIG. 1, the potential of the non-image area of the photosensitive drum on which the electrostatic latent image is formed, that is, the residual potential, is stored in the lowest value storage circuit 5, and this value continues to be supplied to the operational amplifier circuit 6. In this operational amplifier circuit 6, a voltage of an appropriate value is added to the input voltage, and the period during which the image area of the photoreceptor drum passes within the developing electrode,
Continue to apply to the developing electrode.

Next, the moment the trailing edge of the image comes off the developing electrode,
By switching SW2 to the storage battery 7 side, cleaning of the developing electrode 3 is performed.

In the conventional apparatus shown in FIG. 1, development can be performed by automatically applying a slightly larger positive bias depending on the residual potential of the photoreceptor drum, but on the other hand, applying a reverse bias can A DC power supply 7 is used that supplies a relatively large voltage assuming the worst conditions induced in the photoreceptor drum.

Therefore, automatic bias control is performed on the side with the same polarity as the photoreceptor drum, and fixed bias control is performed on the side with the opposite polarity than the photoreceptor drum. Therefore, an excessive reverse bias is applied to the toner, which has the disadvantage that the toner, which should be used repeatedly over a long period of time, deteriorates in a short period of time.

The present invention has been made in order to eliminate the above-mentioned drawbacks, and aims to provide a developing bias device for an electrophotographic copying machine that can suppress toner deterioration caused by cleaning the developing electrode and extend its life. purpose.

In order to achieve this object, the present invention includes a developing electrode disposed opposite to the photoconductor with a predetermined gap, and a device for detecting the residual potential of the photoconductor to prevent toner from adhering to the background of the electrostatic latent image. a first bias circuit that outputs a positive bias voltage (this does not mean only a positive voltage with respect to the ground potential, but a voltage with the same polarity as the polarity with which the photoreceptor is charged); A second bias circuit outputs a reverse bias voltage that has a polarity opposite to this positive bias voltage and changes in accordance with the magnitude of this positive bias voltage, and an image portion of the electrostatic latent image faces the developing electrode. The switching means applies the voltage of the first bias circuit to the developing electrode when the developing electrodes are facing each other, and applies the output voltage of the second bias circuit to the developing electrodes when they are not facing each other.

In this invention, when the image area of the electrostatic latent image faces the developing electrode, a positive bias voltage corresponding to the residual potential is applied to the developing electrode to prevent toner from adhering to the background, and the electrostatic latent image is When the image area of the image is not facing the developing electrode, a reverse bias voltage is applied to the developing electrode that has the opposite polarity to this positive bias voltage and that changes in accordance with the magnitude of this positive bias voltage. Unlike conventional devices that apply excessive reverse bias for cleaning, only a reverse bias voltage that changes in accordance with the positive bias voltage is applied, reducing the deterioration of toner that is repeatedly used. It can be kept low and at the same time extend its life.

Hereinafter, one embodiment of the present invention will be described with reference to the accompanying drawings.

FIG. 2 is a block diagram showing the configuration of the developing bias applying device of the present invention, in which a second operational amplifier circuit 6a is added in place of the storage battery 7 of the conventional device shown in FIG. That is.

This second operational amplifier circuit 6a differs from the aforementioned operational amplifier circuit 6 only in output polarity, and adds an appropriate value of voltage to the output of the lowest value storage circuit to apply a reverse bias corresponding to a positive bias value. A value is applied to the development electrode.

Originally, the reverse bias value corresponds to the bias energy applied to the development electrode, so the product of the reverse bias value and its application time is equal in absolute value to the product of the positive bias value and its application time. If you do this, you will be able to clean it.

In this case, when comparing increasing the bias value and shortening the application time with decreasing the bias value and increasing the application time, various experiments have revealed that the former causes greater deterioration. There is.

On the other hand, copying machines are required to increase the copying speed as much as possible, and in order to meet this demand, it is advantageous to increase the reverse bias value in that the bias application time can be shortened. However, I had to be prepared for the toner to deteriorate more quickly.

In this embodiment, it is assumed that the application of a reverse bias to the same extent as the application of a forward bias is allowed, and if the forward bias changes, the reverse bias is also changed accordingly, thereby increasing the copying speed. Based on this assumption, toner deterioration can be kept to a minimum.

FIG. 4 is a circuit diagram showing another embodiment of the developing bias device according to the present invention, in which the developing electrode is in a floating state without a special positive bias applying circuit.

In FIG. 4, the developing electrode 3 is arranged with an appropriate gap from the photoreceptor drum as in the previous embodiment, but due to the electric charge on the photoreceptor drum, the surface potential of the drum is much higher than that of the drum. A low voltage, higher than the residual potential, is induced, and development is carried out by this voltage.

In this case, the voltage induced in the developing electrode is detected by the induced voltage detection circuit 8 having a high impedance, and is applied to the limiter circuit 9 via the switch SW1, which is closed only during detection. This limiter circuit 9 has an amplifier section, a Zener diode _ZD , and a relay RA.When the input voltage exceeds a certain level, the relay RA operates, and the switching contact ra of this relay is activated.
It operates. In addition, this switching contact ra
The common terminal of is connected to the developing electrode 3 via switch SW2.
The switching terminal on the normally closed side is connected to the negative side of the storage battery 10 whose output is slightly higher than the residual potential, and the switching terminal on the normally open side is connected to the negative side of the storage battery 11 whose output is about 10% higher than that of the storage battery 10. ing. next,
SW2 is a switch that opens when the image area is within the developing electrode and closes when it is outside the electrode.
The timing is controlled by a cam etc. not shown,
Further, SW1 is opened when the detection range is exceeded, but capacitor _C1 in the limiter circuit gives relay RA the same effect as if SW1 were closed.

Here, just before the leading edge of the image comes into the developing electrode,
SW1 is closed, SW2 is open, the potential induced in the electrode is detected, and the electrode is in a floating state (essentially, it is grounded with high impedance).
When the rear edge of the image comes off the electrode, the
SW2 is closed and different reverse biases are applied depending on the detection potential.

In this case, if SW1 is closed at a point where the leading edge of the image has passed about 20 to 100 mm beyond the width of the developing electrode, a value close to the average value of the photoreceptor surface potential can be detected.

As is clear from the above description, according to the present invention, it is possible to extend the life of the toner by applying reverse bias energy corresponding to the positive bias energy during development when cleaning the developing electrode.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the structure of a developing bias device of a conventional electrophotographic copying machine, FIG. 2 is a block diagram showing the structure of a developing bias device according to an embodiment of the present invention, and FIG. 3 is a block diagram showing the structure of a developing bias device of an embodiment of the present invention. FIG. 4 is a detailed perspective view of the developing electrode and a circuit diagram showing the configuration of another embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Photosensitive drum, 2... Developing electrode, 3... Detecting electrode, 4... Impedance conversion circuit, 5... Minimum value storage circuit, 6, 6a... Operational amplifier circuit, 7, 10, 1
1...Storage battery, 8...Induced voltage detection circuit, 9...Limiter circuit.

Claims (1)

[Claims] 1: a developing electrode disposed opposite to the photoreceptor with a predetermined gap; and a first developing electrode that detects the residual potential of the photoreceptor and outputs a positive bias voltage for preventing toner from adhering to the background of the electrostatic latent image. a bias circuit; a second bias circuit that outputs a reverse bias voltage that has a polarity opposite to that of the positive bias voltage and changes in accordance with the magnitude of the positive bias voltage; and an image portion of the electrostatic latent image. Applying the output voltage of the first bias circuit to the developing electrode when the developing electrode is facing the developing electrode, and applying the output voltage of the first bias circuit to the developing electrode when the image portion of the electrostatic latent image is not facing the developing electrode. A developing bias device for an electrophotographic copying machine, comprising: switching means for applying an output voltage of 1 to the developing electrode.