JP2011069872A - Charging device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a charging device capable of suppressing potential variation of a grid electrode caused by a change in Zener voltage due to a temperature change of a Zener diode, and to provide an image forming apparatus. <P>SOLUTION: The charging device 21 includes: a constant-current power source device 211 configured to supply a high voltage causing corona discharge to a corona electrode 212 while maintaining the constant current; the Zener diode 214 connected to the grid electrode 213 with a reverse polarity so that the potential of the grid electrode 213 may be maintained to be a predetermined target grid voltage, and having the Zener voltage equal to or less than the target grid voltage; and a variable resistor 216 which is parallel-connected to the Zener diode 214 and has a resistance value by which a voltage drop occurring when supplying the high-voltage from the constant-current power source device 211 to the corona electrode 213 becomes equal to the Zener voltage of the Zener diode 214. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a scorotron charging device having a corona electrode and a grid electrode for charging a surface of an electrostatic latent image carrier that carries an electrostatic latent image provided in an image forming apparatus to a predetermined voltage, and more particularly to the grid electrode. The present invention relates to a technique for maintaining the potential at a preset target grid voltage.

An electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or a multifunction machine is a charging device that charges a surface of a photosensitive drum (electrostatic latent image carrier) carrying an electrostatic latent image to a predetermined voltage. It has. Specifically, as the charging device, a discharge wire (corona electrode) that generates corona discharge with the photosensitive drum, and a grid wire that converges the charging potential of the photosensitive drum due to the corona discharge of the discharge wire to a constant value. There is a scorotron charging device including a (grid electrode) (see, for example, Patent Document 1).
In this scorotron charging device, it is important to maintain the grid wire potential at a preset target grid voltage in order to keep the charging potential of the photosensitive drum constant.
For example, Patent Document 1 (see FIG. 1) discloses a configuration in which a potential of a grid wire can be adjusted to a target grid voltage by a variable resistance element connected to the grid wire. In Patent Document 1, a Zener diode (varistor) set higher than a target grid voltage is connected in parallel to the variable resistance element for the purpose of protecting the potential of the grid wire from becoming too high. A configuration is disclosed.

Further, as shown in Patent Document 1 (see FIG. 10), a configuration in which the potential of the grid wire is maintained at the target grid voltage by a zener diode (varistor) connected to the grid wire with a reverse polarity has been conventionally known. It has been.
However, in such a configuration, an error may occur between the potential of the grid wire adjusted by the Zener voltage and the target grid voltage due to individual differences in the Zener voltage of the Zener diode. Therefore, it is conceivable to configure so that the potential of the grid wire is corrected to the target grid voltage error by connecting a resistor in series with the Zener diode. Specifically, a Zener diode whose Zener voltage is slightly lower than the target grid voltage and a resistor having a resistance value capable of correcting an actual error between the Zener voltage and the target grid voltage in the Zener diode are connected in series. That's fine.

Japanese Patent Laid-Open No. 11-24371

However, the Zener diode has a characteristic that the Zener voltage varies greatly with temperature. For this reason, in the configuration in which the potential of the grid wire is maintained at the target grid voltage by the Zener diode connected to the grid wire with the reverse polarity as described above, the potential of the grid wire is changed with the change of the Zener voltage of the Zener diode due to the temperature change. There was a problem that it fluctuated greatly and the target grid voltage could not be maintained.
Accordingly, the present invention has been made in view of the above circumstances, and its object is to maintain the potential of the grid electrode at a preset target grid voltage by a Zener diode connected to the grid electrode with a reverse polarity. It is an object of the present invention to provide a charging device and an image forming apparatus capable of suppressing fluctuations in the potential of a grid electrode caused by a change in Zener voltage due to a change in temperature of the Zener diode.

In order to achieve the above object, the present invention is applied to a scorotron charging device having a corona electrode and a grid electrode for charging a surface of an electrostatic latent image carrier carrying an electrostatic latent image to a predetermined voltage. A constant current power supply for supplying a high voltage that causes corona discharge to the corona electrode while maintaining a constant current, and the grid electrode for maintaining the potential of the grid electrode at a preset target grid voltage. A zener diode having a zener voltage less than the target grid voltage and connected in parallel to the electrode, and a voltage when a high voltage is supplied from the constant current power supply device to the corona electrode And a first resistance element having a resistance value whose drop is the same as the Zener voltage of the Zener diode. Configured as a collector.
According to the present invention, even if the Zener voltage of the Zener diode largely fluctuates due to a temperature change, the first resistance element connected in parallel to the Zener diode has a small fluctuation in resistance value due to the temperature change. Variations in the potential of the grid electrode can be suppressed by the first resistance element. In addition, if a change occurs in the current value supplied from the constant current power supply device to the first resistance element, the voltage applied to the first resistance element greatly fluctuates. Since the Zener voltage of the Zener diode connected in parallel to the first resistance element has a small fluctuation due to the current value, the Zener diode can suppress the fluctuation of the potential of the grid electrode. Therefore, even if a change in temperature or a change in supply current occurs, the potential of the grid electrode can be maintained at the target grid voltage, and desired image forming performance can be stably obtained in an image forming apparatus including the charging device. be able to.

Incidentally, the Zener diode has some variation in Zener voltage due to individual differences. Therefore, the Zener diode has a Zener voltage smaller than the target grid voltage, is connected in series with the Zener diode, and a voltage drop when a high voltage is supplied from the constant current power supply device to the corona electrode. It is desirable to further include a second resistance element having a resistance value at which the sum of the zener voltage of the zener diode and the zener voltage of the zener diode becomes the target grid voltage. Thereby, the individual difference of the Zener voltage of the Zener diode can be corrected by the second resistance element, and the potential of the grid electrode can be adjusted to the target grid voltage with high accuracy.
Moreover, it is desirable that the first resistance element and the second resistance element are variable resistance elements. Accordingly, the resistance values of the first resistance element and the second resistance element can be appropriately changed even after the charging device is manufactured.
Furthermore, when at least the first resistance element is a variable resistance element, a voltage detection means for detecting the voltage across the Zener diode and a high voltage from the constant current power supply device to the corona electrode are supplied. It is desirable to further comprise resistance value setting means for gradually setting the resistance value of the first resistance element until the voltage detected by the voltage detection means does not vary. Thereby, it is possible to easily set the resistance value of the first resistance element in each of the charging devices.
The present invention can also be understood as an invention of an image forming apparatus including the charging device.

  According to the present invention, in a configuration in which the potential of the grid electrode is maintained at a preset target grid voltage by a Zener diode connected to the grid electrode with a reverse polarity, the Zener voltage is caused by a change in Zener voltage due to a temperature change of the Zener diode. It is possible to provide a charging device and an image forming apparatus that can suppress fluctuations in the potential of the grid electrode.

1 is a diagram illustrating a main part of an image forming apparatus according to an embodiment of the present invention. 1 is a diagram showing a schematic configuration of a charging device according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings so that the present invention can be understood. The following embodiment is an example embodying the present invention, and does not limit the technical scope of the present invention.
The image forming apparatus X according to the embodiment of the present invention is an electrophotographic image forming apparatus such as a color copier, a printer, a facsimile machine, and a complex machine thereof. Of course, a monochrome image forming apparatus may be used.
As shown in FIG. 1, the image forming apparatus X is a so-called tandem-type image forming apparatus, and includes a plurality of image forming units 1 to 4, an intermediate transfer belt 5, a belt support roller 7, a secondary transfer roller 9, and the like. It has.

The image forming units 1 to 4 sequentially transfer toner images corresponding to the respective colors of black (Bk), yellow (Y), cyan (C), and magenta (M) onto the moving intermediate transfer belt 5 in order. Thus, the electrophotographic image forming unit forms a color image on the intermediate transfer belt 5. In the example shown in FIG. 1, in order from the downstream side in the moving direction of the intermediate transfer belt 5, a black image forming unit 1, a yellow image forming unit 2, a cyan image forming unit 3, and a magenta image forming unit. 4 are arranged in a line.
The image forming units 1 to 4 are provided corresponding to the respective colors, and include photosensitive drums 11 to 14 (an example of an electrostatic latent image carrier) that carry an electrostatic latent image, and the photosensitive drums 11 to 14. Charging devices 21 to 24 for charging the surface, exposure devices 31 to 34 for exposing the surfaces of the charged photosensitive drums 11 to 14 to write electrostatic latent images, and electrostatic latent images on the photosensitive drums 11 to 14 Developing devices 41 to 44 that develop the image with toner and primary transfer devices 51 to 54 that transfer the toner image developed on the photosensitive drums 11 to 14 to the intermediate transfer belt 5 are provided. Although not shown in FIG. 1, the image forming units 1 to 4 include a cleaning device that removes a residual toner image on the photosensitive drums 11 to 14.

The surface of each of the photosensitive drums 11 to 14 is formed of conductive amorphous silicon, stainless steel, or the like, and is rotationally driven by four stepping motors (not shown) connected to each of them.
The intermediate transfer belt 5 is an endless belt made of a material such as rubber or urethane, and is supported by a plurality of belt support rollers 7. The surface of the intermediate transfer belt 5 comes into contact with the surfaces of the photosensitive drums 11 to 14 when the support roller 7 connected to a predetermined motor among the support rollers 7 is rotationally driven by the motor. Travel (move) while. Thus, when passing between the photosensitive drums 11 to 14 and the primary transfer devices 51 to 54, the toner images are sequentially transferred from the photosensitive drums 11 to 14 to the intermediate transfer belt 5 in a superimposed manner. The In place of the intermediate transfer belt 5, an intermediate transfer roller may be used as an intermediate transfer member.
The secondary transfer roller 9 transfers the toner image transferred to the intermediate transfer belt 5 onto a recording sheet. In the image forming apparatus X, the toner image transferred onto the recording paper by the secondary transfer roller 9 is melted and fixed by a fixing device (not shown).
As described above, the image forming apparatus X transfers the color toner images to the intermediate transfer belt by sequentially superimposing and transferring the toner images of the respective colors onto the intermediate transfer belt 5 by the plurality of image forming units 1 to 4. Further, the color toner image is transferred from the intermediate transfer belt 5 to the recording paper by the secondary transfer device to form a color image on the recording paper.

The image forming apparatus X according to the embodiment of the present invention is characterized by the configuration of the charging devices 21 to 24. This point will be described in detail below.
First, a schematic configuration of the charging devices 21 to 24 provided in the image forming apparatus X will be described with reference to FIG. Since each of the charging devices 21 to 24 has the same configuration, only the charging device 21 will be described here.
As shown in FIG. 2, the charging device 21 is a scorotron charging device having a corona electrode 212 and a grid electrode 213 for charging the surface of the photosensitive drum 11 to a predetermined voltage. The corona electrode 212 and the grid electrode 213 are disposed opposite to the surface of the photosensitive drum 11.
The charging device 21 includes a constant current power supply device 211 that supplies a high voltage (for example, about 5 kv) for causing corona discharge to the corona electrode 212 while maintaining a constant current (for example, about 100 μA). ing. Accordingly, the corona electrode 212 generates a corona discharge with the surface of the photosensitive drum 11. At this time, the grid electrode 213 converges the charging potential of the photosensitive drum 11 due to corona discharge of the corona electrode 212 to a constant value.

The charging device 21 includes a Zener diode 214 connected to the grid electrode 213 with a reverse polarity in order to maintain the potential of the grid electrode 213 at a preset target grid voltage. Here, the Zener diode 214 has an error (for example, about ± 20 V) in the Zener voltage due to individual differences.
Therefore, there is a possibility that the potential of the grid electrode 213 cannot be maintained at the target grid voltage. Therefore, the charging device 21 is configured to prevent the influence of individual differences of the Zener diode 214.

Specifically, the Zener diode 214 has a Zener voltage smaller than the target grid voltage. For example, when the target grid voltage is 650V and the Zener voltage error due to individual difference is ± 20V, the Zener diode 214 having the Zener voltage of about 630V is used. In this case, considering an error due to a fixed difference, the Zener voltage of the Zener diode 214 is 610V to 650V.
The zener diode 214 is connected in series with a variable resistor 215 (an example of a second resistor element). The variable resistor 215 has a resistance value at which the sum of the voltage drop when the high voltage is supplied from the constant current power supply 211 to the corona electrode 212 and the Zener voltage of the Zener diode 214 becomes the target grid voltage. It is what you have. For example, when the target grid voltage is 650 V and the actual Zener voltage of the Zener diode 214 is 620 V, the resistance value of the variable resistor 215 is set so that a voltage drop of 30 V that is the difference occurs. That's fine. If the variable resistor 215 is used, the resistance value can be easily changed after the charging device 21 is manufactured, but a fixed resistor having the same resistance value is provided instead of the variable resistor 215. Are also considered as other embodiments.
As described above, in the charging device 21, the potential of the grid electrode 213 is increased to the target grid voltage by the variable resistor 215 even when an error occurs in the Zener voltage due to the individual difference of the Zener diode 214. It can be adjusted with accuracy. In the configuration without the variable resistor 215, the Zener diode 214 having the same Zener voltage as the target grid voltage may be used. That is, in the present invention, the Zener diode 214 may be equal to or lower than the target grid voltage.

By the way, the Zener diode 214 generally has a characteristic that a Zener voltage fluctuation due to a change in temperature is small, but a Zener voltage fluctuation due to a temperature change is large.
Therefore, as described above, even if the potential of the grid electrode 213 is adjusted to the target grid voltage by the variable resistor 215, the potential of the grid electrode 213 is changed to the target voltage due to the fluctuation of the Zener voltage due to temperature change. The problem is that the grid voltage cannot be maintained.
Therefore, in the charging device 21, a variable resistor 216 (an example of a first resistance element) in which a change in resistance value due to a temperature change is small and a change in applied voltage is small is connected in parallel to the Zener diode 214.
Here, the variable resistor 216 has a resistance value equal to the Zener voltage of the Zener diode 214 when the high voltage is supplied from the constant current power supply device 211 to the corona electrode 212. It is set in advance. For example, when the constant current power supply device 211 applies a high voltage to the corona electrode 212, the current flowing through the grid electrode 213 is 0.1 mA, and the actual Zener voltage of the Zener diode 214 is 620V. The resistance value of the variable resistor 216 is 6.2 MΩ. If the variable resistor 216 is used, the resistance value can be easily changed after the charging device 21 is manufactured, but a fixed resistor having a similar resistance value is provided in place of the variable resistor 216. Are also considered as other embodiments.

According to such a configuration, even if the Zener voltage of the Zener diode 214 greatly fluctuates due to a temperature change, the variable resistor 216 connected in parallel to the Zener diode 214 has a small fluctuation in resistance value due to the temperature change. , The variable resistor 216 can suppress fluctuations in the potential of the grid electrode 213.
On the other hand, since the voltage drop of the variable resistor 216 greatly fluctuates due to fluctuation of the supplied current value, the variable resistor 216 is applied to the variable resistor 216 when the current value supplied to the grid electrode 213 fluctuates. The voltage value to be fluctuated greatly. However, in the charging device 21, the Zener diode 214 that is less affected by fluctuations in the current value is connected in parallel to the variable resistor 216, so that the Zener diode 214 suppresses fluctuations in the potential of the grid electrode 213. be able to.
Therefore, in the charging device 21 according to the embodiment of the present invention, the potential of the grid electrode 213 can be maintained at the target grid voltage even when a temperature change or a supply current change occurs, and the image forming apparatus. In X, a desired image forming performance can be stably obtained.

By the way, in the said embodiment, the resistance value of the said variable resistance 215,216 was demonstrated as what was preset to the appropriate value. Here, a method for setting the resistance values of the variable resistors 215 and 216 in the image forming apparatus X will be described. The variable resistor 215 and the variable resistor 216 are variable by a control circuit (not shown) provided in the image forming apparatus X.
First, in the charging device 21, the voltage across the Zener diode 214 is determined by the resistance value of the variable resistor 216 until the voltage exceeds the Zener voltage of the Zener diode 214.
Therefore, a voltage detection circuit (an example of voltage detection means) for detecting the voltage across the Zener diode 214 is provided, and a current is supplied from the constant current power supply 211 to the corona electrode 212 by the control circuit (not shown). While supplying a constant high voltage, the resistance value of the variable resistor 216 may be set gradually smaller until the voltage detected by the voltage detection circuit (not shown) does not fluctuate. Here, the control circuit (not shown) when executing such processing corresponds to a resistance value setting means.
As a result, the resistance value of the variable resistor 216 is such that the voltage applied to the variable resistor 216 when the constant current power supply device 211 supplies a high voltage of a constant current to the corona electrode 212 is the zener diode 214. It is set to a value that matches the Zener voltage.

The detection voltage when the detection voltage by the voltage detection circuit (not shown) does not change is the actual Zener voltage of the Zener diode 214.
Therefore, the control circuit (not shown) calculates the difference between the target grid voltage and the detection voltage when the detection voltage by the voltage detection circuit (not shown) no longer fluctuates, and the calculated difference value The resistance value of the variable resistor 215 is set so that the voltage drop occurs at the variable resistor 215.
In the image forming apparatus X, the control circuit (not shown) may periodically update the resistance values of the variable resistors 215 and 216 by periodically executing the series of setting processes.

X: Image forming devices 1 to 4: Image forming unit 5: Intermediate transfer belt 7: Belt support roller 9: Secondary transfer rollers 11 to 14: Photoconductor drums 21 to 24: Charging devices 31 to 34: Exposure devices 41 to 44 : Development devices 51 to 54: Primary transfer device 211 ... Constant current power supply device 212 ... Corona electrode 213 ... Grid electrode 214 ... Zener diode 215 ... Variable resistance (an example of a second resistance element)
216... Variable resistance (an example of a first resistance element)

Claims (5)

A scorotron charging device having a corona electrode and a grid electrode for charging a surface of an electrostatic latent image carrier carrying an electrostatic latent image to a predetermined voltage,
A constant current power supply for supplying a high voltage that causes a corona discharge to the corona electrode while maintaining a constant current;
A Zener diode connected to the grid electrode in reverse polarity to maintain the potential of the grid electrode at a preset target grid voltage, and having a Zener voltage equal to or lower than the target grid voltage;
A first resistance element connected in parallel to the Zener diode and having a resistance value at which a voltage drop when a high voltage is supplied from the constant current power supply to the corona electrode is the same as the Zener voltage of the Zener diode; ,
A charging device comprising: The zener diode has a zener voltage smaller than the target grid voltage;
The resistance is such that the sum of the voltage drop and the Zener voltage of the Zener diode when the high voltage is supplied from the constant current power supply device to the corona electrode is the target grid voltage. The charging device according to claim 1, further comprising a second resistance element.   The charging device according to claim 1, wherein the first resistance element and / or the second resistance element is a variable resistance element. At least the first resistance element is a variable resistance element;
Voltage detection means for detecting a voltage across the Zener diode; and the first resistance element until the voltage detected by the voltage detection means does not vary while supplying a high voltage from the constant current power supply to the corona electrode. The charging device according to claim 3, further comprising resistance value setting means for gradually decreasing the resistance value of the resistance value.   An image forming apparatus comprising the charging device according to claim 1.

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