JPS6255141B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a method of applying a bias to a developing electrode, which is a magnetic brush carrier in an electrophotographic copying device, in which a non-exposed area (a charged but not image exposed area) is applied. The feature is that the bias is switched and grounded via a varistor (voltage-dependent resistance element) to induce a self-bias. As one measure to prevent toner loss in non-exposed areas, a method has been conventionally used in which the bias applied to the developing electrode is switched and a high voltage is applied in order to prevent toner from adhering to the non-exposed areas of the photoreceptor. The high voltage is required to be at least higher than the potential of the image area in order to obtain the above effect, and usually by applying 800 to 1000 V, toner adhesion can be prevented. However, as a bias power supply, in addition to the bias potential of the image exposure area (image area), a high potential bias is applied to the non-exposed area, or a separate power supply is provided.
The former case requires a complicated device and circuit for adjusting the bias potential to an appropriate level, while the latter case has the drawback of significantly increasing power supply costs.

The present invention improves the above-mentioned drawbacks of the prior art, and is superior in that it has a simple configuration that sufficiently prevents toner loss in non-exposed areas without applying a high voltage bias or using an external power source. The aim is to obtain the desired effect.

In the present invention, when a magnetic brush developing machine containing developer is operated, the developer attached to the surface of the rotating developing electrode while being attracted to the magnet located inside the rotating developing electrode is moved around the magnet along with the rotating developing electrode. We focused on the phenomenon that the electromotive force induced by rotating the photoconductor, or the potential induced by frictional charging between the magnetic brush and the rotating developing electrode, is generated at the rotating developing electrode as a self-bias potential. When the area other than the exposed area corresponding to The developing electrode is grounded via a varistor so that the sum of the potential and the potential of the developing electrode is approximately constant.
By selecting the varistor, the developing electrode potential for the non-exposed area can be maintained at a substantially constant value that is appropriately selected in accordance with the initial charging potential of the photoreceptor.

If the resistance of the developer is R _I , the resistance of the varistor is R _B , the potential induced only by the rotation of the developing electrode is V _O , and the surface potential of the photosensitive drum is V _DRUM , the developing electrode roller grounded through the varistor is The potential V _R (ie, self-bias potential) can be considered as V _R =R _B /R _I +R _B V _DRUM +V _O. The resistance value of the varistor changes in accordance with a change in the potential at both ends, that is, a change in the developing electrode potential due to a change in drum potential; when V _DRUM is high, R _B is low, and when V _DRUM is low, R _B is high. Therefore, the potential across the varistor, that is, the potential of the developing electrode V _R is:
If V _O is constant, it changes as V _DRUM changes. Here, to give an example based on the above formula, if a varistor is selected such that R _B /R _I +R _B is 0.5 when V _DRUM is 800 V, the potential V induced in the developing electrode when V _O is 400 V _R will be 800V.

Conventionally, the potential induced in the developing electrode by the charge on the surface of the photoreceptor was considered to be the self-bias potential of the developing electrode. However, according to the present invention, the self-bias of the magnetic brush developing electrode is not limited to the potential induced in the developing electrode according to the potential on the surface of the photoreceptor;
Induced electromotive force due to relative movement between the magnet in the developing electrode and the developer on the developing electrode as the developing electrode rotates;
It has also been found that the induced potential due to frictional charging between the magnetic brush and the developing electrode is added to the rotating developing electrode. In fact, for example, in magnetic brush development using a two-component magnetic developer, if the development electrode is simply floated in the non-exposed area to prevent toner loss, carrier may adhere to the photoreceptor, and the cause is It has become clear that this is because the self-bias applied to the developing roll electrode becomes much higher than the potential of the non-exposed portion of the photoreceptor than necessary, and that this is due to the above-mentioned two-dimensionality of the self-bias.

The present invention provides a self-bias potential of a developing electrode for a non-exposed area that can control the potential of the developing electrode induced by the photoreceptor potential to a desirable approximately constant potential by grounding the developing electrode via a varistor. This is the application method.

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

In FIG. 1, the present device mainly comprises a bias power source 1 for a developing electrode, a varistor 2, and a relay 3 for bias switching.

In this configuration, when the image area of the photoreceptor 5 faces the magnetic brush developing roller, the developing roller 4 is connected to the power supply side 1 through the contact a.
When the non-exposed part of the photoreceptor faces the developing roller, the relay 3 is operated by a signal from a detection device (not shown) that detects the potential on the photoreceptor, and the developing roller is grounded via the varistor by contact b. do. Other signals that activate the relay are
It may be based on a detection signal of the document size, or it may be based on a program signal for operating each device of the copying machine.

The developing roller of this example is forcibly applied with 200V (background potential of the photoconductor + 100V) at the exposure section.
Varistor in non-exposed area (Matsushita Electric ERZ-08D
3K102). The image developed with this developing roller had no fog, and no toner was attached to the photoreceptor in the non-exposed areas, preventing toner loss.

FIG. 2 is a characteristic diagram (actually measured values) showing the relationship between the surface potential of the photoreceptor 5 and the induced potential of the developing roller via the varistor. As shown in the figure, the self-bias potential due to the varistor changes resistance depending on the applied voltage. Due to this characteristic, regardless of the magnitude of the photoreceptor surface potential shown on the horizontal axis, the self-bias potential by the varistor shown on the vertical axis is maintained at approximately 700 to 800 V, so development of non-exposed areas can be sufficiently suppressed. . At the same time, according to the present invention, it is possible to prevent development in which the self-bias potential becomes too high than necessary and carrier particles adhere to the surface of the photoreceptor.

As mentioned above, the method of the present invention connects the varistor in parallel to a normal bias power supply simply via a relay, and when the non-exposed part of the photoreceptor faces the developing electrode,
By grounding the developing electrode via a varistor, a self-bias is induced in the developing electrode, so it is possible to control the developing electrode to a nearly constant high potential when passing through the non-exposed area, and it is possible to control the developing electrode to a nearly constant high potential while applying a conventional high potential bias. Alternatively, the developing electrode can be controlled to a high potential much more easily than the method of separately adding a high voltage power source.

The method of the present invention can be applied not only to a two-component magnetic brush developing device but also to, for example, a one-component magnetic brush developing device.

[Brief explanation of the drawing]

FIG. 1 shows a bias application system in an electrophotographic copying machine according to an embodiment of the present invention, and FIG. 2 is a characteristic diagram showing the relationship between the surface potential of the photoreceptor and the induced potential of the developing roller via the varistor. be. Symbols in the figure: 1...bias power supply, 2...varistor, 3...relay, 4...magnetic brush developing roller, 5...photoreceptor, R...relay contact.

Claims (1)

[Claims] 1. In a magnetic brush development method characterized by inducing a self-bias by grounding the developing electrode through a varistor when the non-exposed part of the photoreceptor faces a rotating developing electrode that is a magnetic brush carrier. Development electrode bias application method.