JP2003295633A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming device reliably starting a circuit for generating a forward bias by superimposing a reverse bias on the forward bias, impressing it on a transfer roller, and cancelling a leakage current from a photoreceptor. <P>SOLUTION: When a TRCVON signal is turned 'ON' at a T4 timing, a reverse bias circuit part outputs the reverse bias. When the leakage current from a photosensitive drum is cancelled by the reverse bias which is transited into the maximum value at a T5 timing, a TRCCON signal is turned 'ON', so that a forward bias circuit part is started. When the transition of forward bias potential is completed at a T6 timing, the potential of the transfer roller is made to be stable. When the TRCVON signal is turned 'OFF' at a T7 timing, reverse bias potential is transited into 0 V during T7 and T8 timings. Then the forward bias with a higher potential transition speed is synchronized with the reverse bias, and the potential is transited. The potential of the transfer roller to be a synthetic output is kept to be fixed potential. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a printer, a copying machine, a facsimile, or a combination thereof, and more specifically, it applies toner on a photoconductor to a transfer roller for transferring it to a recording medium. The present invention relates to an image forming apparatus capable of eliminating a leakage current from a photoconductor by applying a reverse bias to a forward bias and applying it to a transfer roller in order to reliably activate a circuit for generating the forward bias.

[0002]

2. Description of the Related Art In a conventional image forming apparatus such as a laser printer or a copying machine, a photoconductor having a charge generation layer and a charge transport layer laminated on a base material layer is charged by corona discharge. An electrostatic latent image is formed on the body by exposure to light from a laser or LED, and a developer such as toner is visualized with a developing roller. The image is transferred onto a recording medium such as paper with a transfer roller. Then, the image is formed by heating and fixing with a fixing device. In addition, cleaning of the photoconductor and reuse of toner are performed by a so-called cleanerless system in which residual toner remaining on the photoconductor that has not been transferred is temporarily collected by a cleaning roller and returned to the developing roller during non-transfer. ing.

In the image forming apparatus using the cleanerless system, the toner on the photoconductor is transferred onto the recording medium passing between the photoconductor and the transfer roller at the time of transfer. A bias is applied to the transfer roller, and at the time of cleaning, a reverse bias having the same polarity as the toner is applied to the transfer roller in order to return the toner adhering to the transfer roller onto the photoconductor and clean the cleaning roller. .

[0004]

However, in the conventional image forming apparatus, when the leakage current from the photoconductor contacting the transfer roller flows into the control circuit of the transfer roller and the control circuit is the self-excited oscillation circuit. However, due to the influence of this leakage current, a situation in which the circuit for generating the forward bias cannot be started occurs, and there is a problem that a separately excited oscillation circuit must be used in order to reliably start it. .

The present invention has been made in order to solve the above-mentioned problems. By applying a reverse bias to a forward bias and applying it to the transfer roller, the leakage current from the photosensitive member is prevented from flowing into the control circuit. Thus, it is an object of the present invention to provide an image forming apparatus capable of reliably activating a circuit for generating a forward bias.

[0006]

In order to solve the above problems, an image forming apparatus according to a first aspect of the present invention is directed to an image carrier on which an electrostatic latent image is formed, and an electrostatic latent image on the image carrier. A transfer roller for transferring the developer image, which has been developed with a developer, onto a recording medium; and a constant current type forward bias applying means for applying a forward bias having a reverse polarity to the developer to the transfer roller. , Reverse bias applying means for applying a reverse bias having the same polarity as the developer to the transfer roller, and applying the reverse bias when the forward bias applying means applies a forward bias to the transfer roller. The means is characterized in that a reverse bias is applied to the transfer roller so as to be superposed on the forward bias.

In the image forming apparatus having this structure, when the forward bias applying means applies the forward bias to the transfer roller,
The reverse bias applying means can apply the reverse bias to the transfer roller by superposing it on the forward bias.

Further, in the image forming apparatus according to a second aspect of the present invention, in addition to the configuration of the first aspect of the invention, the forward bias applying means includes a self-excited oscillation circuit.

In the image forming apparatus of this structure, in addition to the operation of the invention according to claim 1, since the forward bias applying means is provided with the self-excited oscillation circuit, it can be started without using an oscillator, and the apparatus structure is Can be simplified.

An image forming apparatus according to a third aspect of the invention has a configuration in which the reverse bias applying means is of a constant voltage system in addition to the configuration of the first or second aspect of the invention. ing.

In the image forming apparatus having this structure, in addition to the operation of the invention according to claim 1 or 2, since the reverse bias applying means is a constant voltage method, a voltage applying method different from the forward bias applying means can be adopted. it can.

Further, in the image forming apparatus according to a fourth aspect of the present invention, in addition to the configuration of the third aspect of the invention, a reverse bias potential applied to the transfer roller by the constant voltage type reverse bias applying means is applied. The absolute value is smaller than the absolute value of the potential of the forward bias applied to the transfer roller by the constant current type forward bias applying means.

In the image forming apparatus having this structure, in addition to the operation of the invention according to the third aspect, the absolute value of the potential of the reverse bias applied to the transfer roller by the constant voltage type reverse bias applying means is determined by the constant current method. It can be made smaller than the absolute value of the forward bias potential applied to the transfer roller by the bias applying means.

Further, in the image forming apparatus according to a fifth aspect of the invention, in addition to the configuration of the third aspect of the invention, in the case where the forward bias applying means applies a forward bias to the transfer roller, A configuration in which the reverse bias is applied to the transfer roller by the reverse bias applying means for a period until the potential of the forward bias applied by the forward bias applying means to the transfer roller reaches a predetermined potential. Has become.

In the image forming apparatus of this structure, in addition to the operation of the invention according to claim 3 or 4, the reverse bias applying means is
The reverse bias can be applied to the transfer roller for a period until the potential of the forward bias applied to the transfer roller by the forward bias applying means reaches a predetermined potential.

An image forming apparatus according to a sixth aspect of the present invention is the image forming apparatus according to any one of the first to fifth aspects, further comprising the forward bias applying means from the image carrier through the transfer roller. And a reverse bias applying means for applying a reverse bias to the transfer roller based on the current value detected by the detecting means. .

In the image forming apparatus having this structure, in addition to the operation of the invention according to any one of claims 1 to 5, the reverse bias applying means applies the reverse bias to the transfer roller based on the current value detected by the detecting means. can do.

The image forming apparatus of the invention according to claim 7 is necessary for the operation of the forward bias applying means based on the current value detected by the detecting means, in addition to the configuration of the invention according to claim 6. The reverse bias applying means includes a calculating means for calculating a voltage value to be applied to the transfer roller so that a predetermined current can be obtained.

In the image forming apparatus having this structure, in addition to the operation of the invention according to the sixth aspect, the reverse bias applying means applies the current to the transfer roller so that a predetermined current necessary for the operation of the forward bias applying means can be obtained. The calculation means can calculate the voltage value for the calculation based on the current value detected by the detection means.

An image forming apparatus according to an eighth aspect of the present invention is the image forming apparatus according to any one of the first to seventh aspects, further comprising a reverse bias potential applied to the transfer roller by the reverse bias applying means. When the potential detecting means determines that the potential of the reverse bias applied by the reverse bias applying means to the transfer roller has reached a predetermined potential, the forward bias applying means determines the forward bias applying means. The configuration is characterized in that a forward bias is applied to the transfer roller.

In the image forming apparatus having this structure, in addition to the operation of the invention according to any one of claims 1 to 7, when the reverse bias potential applied to the transfer roller by the reverse bias applying means reaches a predetermined potential, The forward bias applying means can apply the forward bias to the transfer roller when the detecting means determines.

Further, in the image forming apparatus according to a ninth aspect of the present invention, in addition to the configuration according to the first aspect of the present invention, the reverse bias applying means always applies a predetermined potential to the transfer roller. Is applied.

In the image forming apparatus having this structure, in addition to the operation of the invention according to any one of claims 1 to 8, the reverse bias applying means can always apply a predetermined potential to the transfer roller.

Further, in the image forming apparatus according to a tenth aspect of the present invention, in addition to the configuration of the invention according to any one of the first to ninth aspects, the forward bias applying means applies a forward bias to the transfer roller. If the reverse bias applying means applies a reverse bias to the transfer roller, the developer attached to the transfer roller is transferred to the image by utilizing the potential difference between the image carrier and the transfer roller. The structure is characterized in that it is transferred to a carrier.

In the image forming apparatus having this structure, in addition to the operation of the invention according to any one of claims 1 to 9, when the forward bias applying means does not apply the forward bias to the transfer roller, the reverse bias applying means operates. By applying a reverse bias to the transfer roller, the developer attached to the transfer roller can be transferred to the image carrier by utilizing the potential difference between the image carrier and the transfer roller.

Further, in the image forming apparatus according to the invention of claim 11, in addition to the structure of the invention of claim 10, the reverse operation is performed when the developer attached to the transfer roller is transferred to the image carrier. The reverse bias potential applied to the transfer roller by the bias applying means is the reverse bias applied by the reverse bias applying means while superposing the forward bias when the forward bias applying means applies the forward bias to the transfer roller. The structure is characterized in that it is larger than the electric potential.

According to the image forming apparatus having this structure,
In addition to the operation of the invention according to the first aspect, the reverse bias applying means applies the reverse bias potential applied to the transfer roller to the transfer roller when the developer attached to the transfer roller is transferred to the image carrier. When a bias is applied, it can be made higher than the reverse bias potential applied by the reverse bias applying means by superimposing it on the forward bias.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of an image forming apparatus embodying the present invention will be described below with reference to the drawings. First, the overall configuration of the laser printer 1 will be described with reference to FIG. FIG. 1 is a central sectional view of a laser printer 1 according to this embodiment. As shown in FIG. 1, the laser printer 1 has a feeder section 4 for feeding a sheet 3 as a recording medium into a main body case 2 in a side sectional view.
Also, an image forming unit 5 for forming a predetermined image on the fed paper 3 is provided. In the laser printer 1, the left-hand direction in the drawing is the front surface.

The paper discharge tray 46 is formed on the rear end side of the upper part of the main body case 2 so as to hold the printed papers 3 in a stack at the rear side of the upper surface of the main body case 2.
In addition, there is an open space on the front side of the upper surface of the main body case 2 for inserting the process cartridge 17, and the space is vertically moved around the support shaft 54a provided on the front end side of the paper discharge tray 46. The rotating top cover 54 is configured to cover a cartridge storage portion 57, which will be described later, which is a space for inserting the process cartridge 17.
The position of the top cover 54 when opened is indicated by a two-dot chain line in the figure.

At the rear portion (on the right-hand side in the figure) of the main body case 2, the paper 3 discharged from the fixing device 18 of the image forming unit 5 provided at the lower rear end side of the main body case is moved to the upper rear end side. A paper discharge path 44 is provided so as to draw a half arc in the vertical direction along the back surface of the main body case 2 so as to be guided to the provided paper discharge tray 46, and the paper 3 is conveyed onto the paper discharge path 44. A paper discharge roller 45 for performing the above is provided. In the laser printer 1, since the paper discharge path 44 is formed in a semi-arc shape in this manner, the paper 3 printed on the upper surface is discharged toward the lower surface on the paper discharge tray 46, that is, so-called face down. It is possible to perform paper discharge of the type, and when printing multiple sheets,
Since the papers 3 are stacked with the printing surface facing downward in the paper discharge order, the printed papers 3 can be arranged in the print order.

The feeder unit 4 is provided in the bottom of the main body case 2, a paper feed roller 8, a paper feed tray 6 that is detachably mounted, and a paper feed tray 6, and holds the papers 3 in a stack. The paper pressing plate 7 that presses the paper 3 against the paper feeding roller 8
Is provided above one end of the paper feed tray 6 and is pressed toward the paper feed roller 8 to sandwich the paper 3 with the paper feed roller 8 at the time of paper feeding and to convey the paper 3 in a double feed manner. Separation pad 9 for preventing, conveyance roller 11 which is provided at two places on the downstream side in the conveyance direction of sheet 3 with respect to sheet feed roller 8 and conveys sheet 3, and sheet 3 is passed through each of conveyance rollers 11 And the paper dust removing roller 10 that removes the paper dust by making contact with the transport roller 11 and transports the paper 3 in cooperation with the transport roller 11, and is provided on the downstream side of the transport roller 11 in the transport direction of the paper 3 and A registration roller 12 is provided for adjusting the timing of feeding the sheet 3 at that time.

The paper pressing plate 7 is capable of stacking the papers 3 in a stack, and a support shaft 7a provided at the end farther from the paper feed roller 8 is supported on the bottom surface of the paper feed tray 6. As a result, the nearer end of the support shaft 7a can be moved up and down with the center of rotation as the center of rotation, and a spring (not shown) is urged from the back side toward the paper feed roller 8. ing. Therefore, the sheet pressing plate 7 swings downward against the biasing force of the spring with the support shaft 7a as a fulcrum as the stacking amount of the sheets 3 increases. Paper feed roller 8
The separation pad 9 is disposed so as to face each other, and the separation pad 9 is pressed toward the paper feed roller 8 by the spring 13 disposed on the back side of the separation pad 9.

The feeder 4 is provided on the front surface of the main body case 2 (on the left-hand side in the figure) and is opened / closed in the front-rear direction (left-right direction in the figure) about the support shaft 14a as a fulcrum. A manual feed tray 14 including a tray portion 14b capable of stacking 3 and a cover portion 14c configured to slide with respect to the tray portion 14b so as to become a part of the main body case 2 when the tray portion 14b is closed. A manual feed roller 15 for feeding the sheets 3 stacked on the tray portion 14b of the manual feed tray 14 and a separation pad 25 for preventing double feeding of the sheets 3 are provided.

Manual feed roller 15 and separation pad 25
Are arranged so as to face each other, and the separation pad 25 is pressed toward the manual feed roller 15 by a spring (not shown) arranged on the back side of the separation pad 25. When printing, the paper 3 stacked on the manual feed tray 14
It is sent by the frictional force of the rotating manual feed roller 15 and is prevented from being double-fed by the separation pad 25 so that it is conveyed to the registration rollers 12 one by one.

Next, the configuration of the image forming section 5 will be described with reference to FIG. FIG. 2 is a cross-sectional view of the image forming unit 5 viewed from the side. As shown in FIG. 2, the image forming unit 5
Is composed of a scanner unit 16, a process cartridge 17, a fixing device 18, a duct 100, etc. so as to form an image on the sheet 3 conveyed by the feeder section 4.

The scanner unit 16 is arranged in the upper part of the main body case 2 below the paper discharge tray 46, and has a laser emitting portion (not shown) for emitting laser light, which is driven to rotate and emits from the laser emitting portion. The polygon mirror 19 for scanning the scanned laser beam in the main scanning direction, the fθ lens 20 for keeping the scanning speed constant, the reflection mirror 21 for reflecting the scanned laser beam, and the laser beam reflected by the reflection mirror 21 for the photosensitive drum. It is composed of a relay lens 22 and the like for adjusting the focal position for focusing on 27. Based on predetermined image data, the scanner unit 16 passes the laser light emitted from the laser emitting section through a polygon mirror 19, an fθ lens 20, a reflection mirror 21, and a relay lens 22 in this order, as indicated by a chain double-dashed line A. Alternatively, the surface of the photosensitive drum 27 of the process cartridge 17 is exposed and scanned by being reflected.

The process cartridge 17 includes a photosensitive drum 27, a scorotron charger 29, a developing roller 31,
Supply roller 33, toner box 34, transfer roller 3
0, a cleaning roller 51, a secondary roller 52, and the like.

The photosensitive drum 27 is disposed on the side of the developing roller 31 so that its rotation axis is parallel to the rotation axis of the developing roller 31, and in the state of being in contact with the developing roller 31, the direction of the arrow (counterclockwise direction in the figure). ) Is rotatably arranged. The photoconductor drum 27 includes a charge generation layer in which an organic photoconductor such as an azo pigment or a phthalocyanine pigment is dispersed as a charge generation material in a binder resin on a conductive base material, or a resin such as polycarbonate in a hydrazone type or aryl type. It is a drum in which a charge transport layer in which an amine-based compound is mixed is laminated. When the photoconductor drum 27 is irradiated with a laser beam or the like, light is generated in the charge generation layer by light absorption, and the charge is transported to the surface of the photoconductor drum 27 by the charge transport layer. By eliminating the charged surface potential, a potential difference can be provided between the potential of the irradiated part and the potential of the non-irradiated part. An electrostatic latent image is formed on the photosensitive drum 27 by exposing and scanning the laser light based on the image data. The photosensitive drum 27 is
The “image carrier” in the present invention.

Scorotron type charger 2 as charging means
The reference numerals 9 are provided above the photoconductor drum 27 at predetermined intervals so as not to contact the photoconductor drum 27. The scorotron type charger 29 is a scorotron type charger for positive charging that generates corona discharge from a discharge wire such as tungsten, and is configured to uniformly charge the surface of the photosensitive drum 27 to a positive polarity. Has been done.
In addition, this scorotron charger 29 is used in the charging circuit unit 2
02 (see FIG. 3). An opening 171 is provided so that products such as ozone generated during charging can be discharged to the outside of the process cartridge 17. The opening 171 is opened on the upper surface of the casing of the process cartridge 17 at the portion where the scorotron charger 29 is provided, and is in communication with the outside air.

Further, the developing roller 31 is the photosensitive drum 2
7 is disposed downstream of the position where the scorotron charger 29 is arranged in the rotation direction of 7 (counterclockwise in the figure), and is rotatable in the arrow direction (clockwise in the figure). The developing roller 31 has a metal roller shaft covered with a roller made of a conductive rubber material.
A developing bias is applied from (see FIG. 3).

Next, the supply roller 33 is the developing roller 31.
At the side position of the photosensitive drum 2 with the developing roller 31 interposed therebetween.
7 is rotatably arranged at a position opposite to 7, and abuts against the developing roller 31 in a compressed state. The supply roller 33 has a metal roller shaft covered with a roller made of a conductive foam material, and frictionally charges toner supplied to the developing roller 31.

The toner box 34 is provided at a side position of the supply roller 33, and the inside thereof is filled with the toner supplied to the developing roller 31 via the supply roller 33. In the present embodiment, a positively chargeable non-magnetic one-component toner is used as the developer, and this toner includes a polymerizable monomer, for example, a styrene-based monomer such as styrene, an acrylic acid, an alkyl ( It is a polymerized toner obtained by copolymerizing an acrylic monomer such as C1 to C4 acrylate and alkyl (C1 to C4) methacrylate by a known polymerization method such as suspension polymerization. To such a polymerized toner, a coloring agent such as carbon black, a wax, and the like are mixed, and an external additive such as silica is added to improve fluidity. The particle size is about 6 to 10 μm.

The toner in the toner box 34 is stored in the rotating shaft 35 provided at the center of the toner box 34.
The agitator 36 supported by is rotated in the arrow direction (counterclockwise direction in the figure) to stir. Further, a window 38 for detecting the remaining amount of toner is provided on the side wall of the toner box 34, and is cleaned by a cleaner 39 supported by the rotating shaft 35.

A transfer roller 30 is arranged downstream of the developing roller 31 in the rotational direction of the photosensitive drum 27 and below the photosensitive drum 27, and rotates in the arrow direction (clockwise direction in the figure). Supported as possible. The transfer roller 30 has a metal roller shaft covered with a roller made of an ion conductive rubber material, and is configured such that a forward bias is applied from the high voltage generation circuit 200 during transfer.

Next, the cleaning roller 51 is arranged at a position lateral to the photosensitive drum 27. This arrangement position is a downstream position of the transfer roller 30 in the rotation direction of the photosensitive drum 27 and an upstream position of the scorotron charger 29. A secondary roller 52 is provided at a position on the opposite side of the photosensitive drum 27 with the cleaning roller 51 interposed therebetween so as to come into contact with the cleaning roller 51.
A scraping member 53 is in contact with the next roller 52.

In this laser printer 1, the photosensitive drum 27 is cleaned by the cleanerless system. After the toner is transferred from the photosensitive drum 27 to the sheet 3 by the transfer roller 30, residual toner or paper dust remaining on the surface of the photosensitive drum 27 applies a cleaning bias from the cleaning circuit unit 201 (see FIG. 3). It is electrically sucked by the cleaned cleaning roller 51. The cleaning roller 51 is the secondary roller 52.
Only the paper dust is electrically sucked by the secondary roller 52.
The paper dust sucked in is scraped off by the scraping member 53.

An exposure window 69 is provided above the photoconductor drum 27 so that the laser beam from the scanner unit 16 is directly applied to the photoconductor drum 27.
The exposure window 69 is located on the upper surface of the casing of the process cartridge 17 closer to the toner box 34 than the opening 171 of the scorotron charger 29, so that the photosensitive drum 27 can communicate with the outside of the process cartridge 17. It is open to.

The fixing device 18 is disposed on the downstream side of the process cartridge 17, and is provided on the heating roller 41, the pressing roller 42 that presses the heating roller 41, and the downstream side of the heating roller 41 and the pressing roller 42. A pair of transport rollers 43 is provided. The heating roller 41 is
A halogen roller for heating is provided inside a cylindrical roller made of metal, and the toner transferred onto the sheet 3 in the process cartridge 17 is transferred between the heating roller 41 and the pressing roller 42 by the sheet 3 between the rollers. The paper 3 is heated and fixed while passing, and then the paper 3 is carried to the paper discharge path 44 by the carry roller 43.

The duct 100, which is sucked by a fan (not shown) and discharges the atmosphere to the outside of the main body case, is extended in the width direction of the process cartridge 17 by the length in the width direction (direction orthogonal to the insertion direction). The exhaust passage has a tubular shape and has a V-shape when viewed from the side. The inside is
A duct 100a that is divided into two chambers by a partition wall that divides the width direction into two parts and that mainly exhausts products such as ozone generated from the scorotron charger 29.
A duct 100b mainly for discharging the hot atmosphere generated from the fixing device 18 is configured.

Further, when the process cartridge 17 is mounted in the main body case 2, the scorotron type charger 29 of the upper surface of the casing of the process cartridge 17 is installed.
Near the opening 171 provided near the shutter 1
03, the lower surface of the duct 100a, the partition member 104 made of an elastic member such as rubber, and the left and right side plates of the cartridge storage unit 57 (not shown), the exhaust chamber 101 is configured. Then, the ozone generated from the scorotron type charger 29 is filled in the exhaust chamber 101, and the scorotron type charger 29 of the lower surface of the duct 100a is filled so that the ozone atmosphere is sucked and discharged to the duct 100a.
An opening 105 is formed in a portion facing to.

The partition member 104 has a length corresponding to the length of the duct 100 in the width direction (insertion direction) of the lower surface of the duct 100a, which is in contact with the tip of the insertion direction when the process cartridge 17 is mounted. It is provided so as to extend in a direction orthogonal to the direction). Further, it also plays a role as a shock absorbing material for shock when the process cartridge 17 is inserted. The shutter 103 is a long plate-like member extending in the width direction of the process cartridge 17, and a support shaft 103a provided at one edge in the short side direction thereof is supported by a lower wall surface of the duct 101a. . The supporting portion is located on the downstream side in the insertion direction of the process cartridge 17, and supports the shutter 103 so that the free end side of the shutter 103 can move in the vertical direction.

The opening 1 is also formed on the lower surface of the duct 100b.
06 is provided and mounted in the process cartridge 17
The atmosphere of the exhaust chamber 102, which is constituted by the wall surface of the front end portion in the insertion direction of the, the lower surface of the duct 100b, the fixing device 18, the static elimination plate 107, etc., is exhausted. still,
The neutralizing plate 107 is provided between the process cartridge 17 and the fixing device 18 on the conveyance path of the paper 3 so as to neutralize the paper 3 charged by passing through the process cartridge 17 during printing. , Has a shape in which a plurality of grooves are arranged in a line in the conveyance direction of the paper 3, and functions as a paper guide.

Next, with reference to FIGS. 3 and 4, the electrical configuration around the photosensitive drum 27 of the laser printer 1 of the present embodiment will be described. FIG. 3 is a block diagram showing an electrical configuration around the photosensitive drum 27 of the first embodiment. FIG. 4 shows the photosensitive drum 27 of the second embodiment.
It is a block diagram which shows the surrounding electric constitution.

As shown in FIG. 3, in the laser printer 1 of the first embodiment, the grounded photosensitive drum 27 is used.
Cleaning roller 51, secondary roller 52, scorotron charger 29, developing roller 31 arranged around
And the transfer roller 30 are respectively connected to the high voltage generation circuit 20.
0 is connected to the cleaning circuit unit 201, the charging circuit unit 202, the developing circuit unit 203, and the transfer circuit unit 300, and a control bias is applied to each of them. The transfer circuit unit 300 is controlled by the CPU 400 to apply a forward bias having a polarity opposite to that of the positively charged toner and a reverse bias having the same polarity as the toner to the transfer roller 30. Incidentally, the CPU 400 is the "calculating means" in the present invention.
Is.

The transfer circuit section 300 transfers the toner electrostatically adsorbed on the photosensitive drum 27 onto the sheet 3 during transfer, so that the forward bias circuit section 301 generates a forward bias to be applied to the transfer roller 30. In order to return the toner adhering on the transfer roller 30 to the photosensitive drum 27 at the time of cleaning, the reverse bias circuit section 302 generates a reverse bias applied to the transfer roller 30.

The forward bias circuit section 301 is a so-called constant current circuit and outputs a constant current which is not influenced by impedance to the transfer roller 30. The forward bias circuit unit 301 controls the control signal (TRCCO) from the CPU 400.
Forward bias ON / OFF for switching whether to apply a forward bias to the transfer roller 30 based on the N signal)
The circuit portion 314, the forward bias control circuit portion 313 having an IC for controlling the output of the forward bias to be applied, and the output from the IC of the forward bias control circuit portion 313 are amplified to operate the forward bias boosting portion 311. Forward bias booster drive circuit unit 31 for controlling the voltage load for
2 and a forward bias boosting unit 311 for generating a forward bias to be applied to the transfer roller 30, and a forward bias of about −8000 V is applied to the transfer roller 30 during transfer. In addition, this forward bias circuit unit 3
01 uses a self-excited oscillation method as a circuit start-up method and is provided with a known circuit for self-excited oscillation. The forward bias circuit section 301 is the "forward bias applying means" in the present invention.

The reverse bias circuit section 302 is a so-called constant voltage circuit, and applies a constant voltage which is not affected by impedance to the transfer roller 30. Reverse bias circuit unit 30
Reference numeral 2 denotes a reverse bias ON / OFF circuit unit 325 that switches whether to apply a reverse bias to the transfer roller 30 based on a control signal (TRCVON signal) from the CPU 400, and a control signal (TRCVCH from the CPU 400.
Based on the ON signal), an output voltage variable circuit section 324 for switching the reverse bias between the two potentials applied to the transfer roller 30, and a reverse bias control circuit section having an IC for controlling the output of the reverse bias to be applied. 323, a reverse bias booster drive circuit 322 that drives the reverse bias booster 321 by amplifying the output from the IC of the reverse bias control circuit 323, and a reverse bias that is applied to the transfer roller 30. Reverse bias booster 321
And a reverse bias of about 1700 V is applied to the transfer roller during cleaning. Also, the reverse bias booster 321 is a forward bias booster 311.
Connected to the transfer roller 30 via a resistor R
1, R2 are connected and grounded via the resistor R1, and at the same time, the contact between the resistors R1 and R2 is connected to the forward bias control circuit unit 313. Further, the resistor R1 is configured as a forward bias output current detection unit 331 that detects whether or not a forward bias is output. The reverse bias circuit unit 302 is the "reverse bias applying means" in the present invention.

Next, as shown in FIG. 4, in the reverse bias circuit section 302 of the high voltage generation circuit 200 of the laser printer 1 of the second embodiment, the forward bias output current detection in the first embodiment is detected. The unit 331 is provided as the forward bias output current inflow current detection unit 332. Further, the forward bias control circuit unit 313 is connected to the inflow current detection unit 401 of the CPU 400 via an operational amplifier 350 that buffers signals outside the high voltage generation circuit 200. Other configurations are the same as those in the first embodiment. The forward bias output current inflow current detection unit 332
Is the "detection means" in the present invention.

Next, the printing operation of the laser printer 1 will be described with reference to FIGS. Paper tray 6
The uppermost sheet 3 stacked on the sheet pressing plate 7 is pressed toward the sheet feeding roller 8 from the back side of the sheet pressing plate 7 by a spring (not shown). When printing is started, the paper 3 is sent by the frictional force between the paper 3 and the rotating paper feed roller 8, and is first sandwiched between the paper feed roller 8 and the separation pad 9. At this time, a plurality of sheets 3 may be double-fed due to the mutual frictional force. Therefore, a separation pad 9 is provided in order to prevent a plurality of sheets from being conveyed while being double-fed, and the front end surface of the double-fed sheets 3 in the conveyance direction causes friction with the separation pad 9. Due to the resistance due to the force, the double-fed sheets 3 are separated into single sheets. The paper 3 separated into single sheets is removed of paper dust adhering to the surface thereof when passing through the paper dust removing roller 10, and is sent to the registration roller 12 by the facing conveyance roller 11.

On the other hand, in the scanner unit 16, the laser light generated by the laser emitting section (not shown) is emitted to the polygon mirror 19 based on the laser drive signal generated by the engine controller (not shown). . The polygon mirror 19 receives the incident laser light in the main scanning direction (paper 3
Scanning in the direction orthogonal to the transport direction of the fθ lens 20.
Emit to. The fθ lens 20 converts the laser light scanned by the polygon mirror 19 at a constant angular velocity into a constant velocity scan. Then, the traveling direction of the laser light is changed by the reflection mirror 21, is converged by the relay lens 22, and is imaged on the surface of the photosensitive drum 27.

The surface potential of the photosensitive drum 27 is, for example, about 10 by the scorotron type charger 29.
It is charged to 00V. Next, the photosensitive drum 27 rotating in the direction of the arrow (counterclockwise in the figure) is irradiated with laser light. The laser light is emitted on the main scanning line of the paper 3 so that the portion to be developed is irradiated and the portion not to be developed is not irradiated, and the portion irradiated with the laser light (bright portion).
Has its surface potential lowered to, for example, about 100V. The rotation of the photoconductor drum 27 also irradiates the laser beam in the sub-scanning direction (conveyance direction of the paper 3), and the part (dark part) and the bright part which are not irradiated with the laser light are on the surface of the photoconductor drum 27. An electrically invisible image, that is, an electrostatic latent image is formed on the surface.

The toner in the toner box 34 is
By the rotation of the supply roller 33, the toner is supplied to the developing roller 31. At this time, the supply roller 33 and the developing roller 31 are positively triboelectrically charged, and further adjusted to be a thin layer having a constant thickness and carried on the developing roller 31. The developing roller 31 has a developing bias of, for example, about 5
A positive voltage of 50V is applied. Due to the rotation of the developing roller 31, when the toner carried on the developing roller 31 and positively charged comes into contact with the photosensitive drum 27 so as to face it, the static toner formed on the surface of the photosensitive drum 27 is formed. Transfer to a latent image. That is, the potential (+ 550V) of the developing roller 31 is lower than the potential (+ 1000V) of the dark portion and higher than the potential (+ 100V) of the light portion, so that the toner is selectively transferred to the light portion having the low potential. In this way, a visible image with toner is formed on the surface of the photosensitive drum 27, and development is performed.

The registration roller 12 registers the sheet 3 and sends out the sheet 3 at the timing when the leading edge of the visible image formed on the surface of the rotating photosensitive drum 27 and the leading edge of the sheet 3 coincide with each other. Then, when the sheet 3 passes between the photosensitive drum 27 and the transfer roller 30, the potential of the bright portion (+
The toner electrostatically attracted onto the surface of the photosensitive drum 27 tries to be transferred to the transfer roller 30 to which a transfer bias lower than 100 V), for example, about −8000 V is applied. However, the toner is blocked by the paper 3 and cannot be transferred to the transfer roller 30. As a result, the toner is transferred onto the paper 3. That is, the visible image formed on the surface of the photosensitive drum 27 is transferred onto the paper 3.

Then, the sheet 3 on which the toner is transferred is conveyed to the fixing device 18. Static elimination plate 10 grounded at that time
After passing over the sheet 7, the charge removal plate 107 removes the toner and the residual charge of the sheet 3. Then, the fixing device 18 applies about 200 to the paper 3 on which the toner is applied by the heating roller 41.
A certain amount of heat and pressure from the pressing roller 42 are applied to fuse the toner onto the sheet 3 to form a permanent image. The heating roller 41 and the pressing roller 42 are respectively grounded via a diode, and the surface potential of the pressing roller 42 is lower than the surface potential of the heating roller 41. Therefore, the positively charged toner that is placed on the heating roller 41 side of the sheet 3 is transferred to the pressing roller 4 via the sheet 3.
2 is electrically attracted to the heating roller 41 during fixing.
Disturbance of the image due to the toner being attracted to is prevented.

The sheet 3 to which the toner is heated and fixed is conveyed on the sheet discharge path 44 by the sheet discharge roller 45, and is discharged to the sheet discharge tray 46 with the printing surface facing downward. Similarly, the paper 3 to be printed next is stacked on the paper discharge tray 46 with the printing surface facing down on the paper 3 previously discharged. In this way, the user can obtain the sheets 3 arranged in the printing order.

Next, the operation of the high voltage generation circuit 200 of the laser printer 1 according to the first embodiment will be described with reference to FIG. As shown in FIG. 3, the high voltage generation circuit 20
0 applies high voltage to the scorotron charger 29 arranged around the photosensitive drum 27, the cleaning roller 51, the secondary roller 52, the developing roller 31, and the transfer roller 30 to form an image. Is going. Transfer circuit unit 300 for generating high voltage applied to transfer roller 30
Is operated based on a signal output from the COPU 400, and generates a different voltage for each state in image formation of the laser printer 1.

The transfer circuit section 300 generates a forward bias having a polarity opposite to that of the toner at the time of printing and at the time of cleaning.
A reverse bias having the same polarity as the toner is generated. The forward bias is generated from the forward bias circuit unit 301 which is a constant current circuit. When the CPU 400 outputs the TRCCON signal to the forward bias ON / OFF circuit unit 314, the forward bias O
The N / OFF circuit unit 314 turns on the transistor switch so that a current flows through each circuit. Then, the forward bias control circuit unit 313 operates to control the forward bias booster drive circuit unit 312, and the forward bias booster 31
1 causes forward bias to occur. Then, the generated forward bias is applied to the transfer roller 30 to attract the toner during printing.

On the other hand, the reverse bias is generated from the reverse bias circuit section 302 which is a constant voltage circuit. When the CPU 400 outputs the TRCVON signal to the reverse bias ON / OFF circuit unit 325, the reverse bias ON / OFF circuit unit 325
Turns the transistor switch "ON" so that a current flows through each circuit. At this time, if the CPU 400 outputs the TRCVCHON signal to the output voltage variable circuit unit 324, a reverse bias having a lower potential than that in the case where the TRCVCHON signal is not output is generated. Then, the reverse bias control circuit unit 323 operates to control the reverse bias boosting unit drive circuit unit 322, and the reverse bias boosting unit 32.
1 causes reverse bias to occur. Then, the generated reverse bias is applied to the transfer roller 30 so that the toner is ejected at the time of cleaning. In addition, TRCVCH
A low potential reverse bias is generated by the ON signal because it is applied to the transfer roller 30 when the forward bias circuit unit 301 is activated.

The laser printer 1 of the first embodiment
Then, when the electrostatic latent image formed on the surface of the photoconductor drum 27 transfers the toner image developed by the toner onto the sheet 3, a forward bias of about −8000 V is applied to the transfer roller 30. However, the photosensitive drum 27 is about 55 by the scorotron charger 29 and the developing roller 31.
It is charged to 0 to 1000V. Due to the influence of this charging potential,
Forward bias circuit unit 30 via the transfer roller 30 in contact
A so-called leakage current flows into 1.

On the other hand, based on the signal transmitted from the CPU 400, the forward bias ON / OFF circuit section 314 activates the forward bias circuit section 301 to generate the forward bias. However, when a leakage current has already flowed in the circuit and a current flows through the resistor R1 of the forward bias output current detection unit 331, the self-excited oscillation type forward bias circuit unit 301 is started, and it is started. Current does not flow in the circuit part for. That is, the forward bias circuit unit 301 cannot be activated.

Therefore, in the laser printer 1 of the first embodiment, the reverse bias circuit unit 302 applies the reverse bias of a low potential to the transfer roller 30 prior to the activation of the forward bias circuit unit 301. Then, the transfer roller 30
Since the potential of (1) becomes higher than the surface potential of the photosensitive drum 27, it is possible to prevent leakage current from flowing into the forward bias circuit section 301 via the transfer roller 30. Further, the self-excited oscillation transmission type forward bias circuit unit 301, once activated, does not need to flow a current in the circuit for activation, so that it is not necessary to apply a reverse bias.
The reverse bias circuit unit 302 stops applying the reverse bias.

Timings of reverse bias application under different conditions will be described below with reference to the timing charts of FIGS. Figure 5 shows TRCV
6 is a timing chart when the CHON signal is output and a reverse bias having a potential lower than the reverse bias potential at the time of cleaning is output as a starting reverse bias for starting the forward bias circuit unit 301. FIG. 6 is a timing chart in the case where the reverse bias for starting is always output when the forward bias is output. FIG. 7 is a timing chart when the transition time of the reverse bias potential is longer than the transition time of the forward bias potential. FIG. 8 is a timing chart when the transition time of the reverse bias potential is shorter than the transition time of the forward bias potential.

In the timing charts of FIGS. 5 to 8, the relationship between the reverse bias potential and the time (transition time) required to reach the maximum potential is shown at timings T0 to T3, respectively. As an example, the T0 to T4 timings in FIG. 5 will be described. As shown in FIG. 5, when the TRCVON signal output from the CPU 400 is turned “ON” at the T0 timing, the reverse bias ON / OFF circuit unit 325 causes the transistor switch of the reverse bias circuit unit 302 to generate a reverse bias. Becomes "ON",
Current flows in the circuit. At this time, the output voltage variable circuit unit 3
In No. 24, since the TRCVCHON signal is OFF, the reverse bias potential is set to the potential at the time of cleaning. Therefore, the potential of the reverse bias whose output is started at the T0 timing transitions to the potential at the cleaning at the T1 timing, and the potential of the transfer roller 30 is also the same. Then, when the TRCVON signal becomes “OFF” at the T3 timing, the reverse bias potential and the transfer roller 30 potential gradually decrease, and transition to 0 V at the T3 timing after the same period as the T0 to T1 timings.

Then, after the timing T4, processing for outputting the forward bias is performed. First, when the TRCVON signal is turned "ON" at the timing T4, the TRCVCHON signal is also turned "ON" at that timing, so that the maximum value of the reverse bias potential is lower than the potential during cleaning. Transition speed at this time, that is, T
The rate of increase in potential between timings 4 and T5 is the same as that between timings T0 and T1, and the potential of the reverse bias is the maximum potential at a value lower than the potential between timings T1 and T2 at timing T5.

At this T5 timing, the CPU 40
0 determines that the potential of the reverse bias for start-up also becomes the maximum value, and outputs the TRCCON signal. That is, the fact that the potential of the reverse bias reaches the maximum value can be regarded as the output of the reverse bias having a potential sufficient to eliminate the leakage current from the photosensitive drum 27. This T
The period from timing 4 to T5, that is, the transition period, is set in advance as a predetermined period. During this period, the potential of the transfer roller 30 increases, and at the timing T5, the potential is lower than that at the time of transfer and higher than the surface potential of the photosensitive drum 27, for example, the developing roller 31 causes the surface potential of the photosensitive drum 27 to be about 550V. So
The potential of the reverse bias for starting is about 600V.

Then, at T6 timing, the transition of the forward bias potential is completed. At this time, the electric potential of the transfer roller 30 becomes the electric potential at the time of transfer. Then, the CPU 400
At the T7 timing, it is judged that a sufficient period has elapsed for completing the transition of the forward bias potential, and TRC is determined.
Turn off the VON signal. Then, the potential of the reverse bias is transited to 0V during the timing of T7 to T8, but the forward bias which is a constant current is shorter than the time reverse bias which is required for the transition of the voltage. This means that when the reverse bias is changed from 0V to about 550V, T4 ~.
Although it takes a period of T5 timing, the forward bias is transited from, for example, 0 V to about −8000 V during the timing of T5 to T6, which is almost the same as the period, so it can be said that the transition time is shorter than that of the reverse bias. Then, since the forward bias potential transits in synchronization with the transition of the reverse bias potential, the potential of the transfer roller 30, which is a combined output thereof, is a constant potential even if the superposed and output reverse bias disappears. Can be maintained.

Next, as shown in FIG. 6, when the reverse bias for starting is always output during the output of the forward bias, T0 to
The transition of each signal at the T6 timing and the output potential of each bias is the same as in the case of FIG. And
After the T6 timing, the reverse bias is not turned “OFF”. Therefore, the state at the T6 timing is maintained.

Next, with reference to FIG. 7, a case where the transition time of the reverse bias potential is longer than the transition time of the forward bias potential will be described. The T0 to T3 timings are the same as in the case of FIG. Then, at the T4 timing, unlike the case of FIG. 5, the TRCVCHON signal remains “OFF” and the TRCVON signal becomes “ON”. In this case, the reverse bias potential is transitioned to about 1700V. Since the transition speed is the same as that in the case of FIG. 5, the transition time of the potential of the reverse bias shown at the timings T4 to T5 is further required. Then, at the timing of T5, the potential of the transfer roller 30 is about 170.
It becomes 0V.

Further, since the transition speed of the forward bias potential transiting to the timings T5 to T6 is faster than the transition speed of the reverse bias potential as in the case of FIG. 5, the TRCVON signal becomes "OFF" at the timing T7. , T7 to T8
The forward bias potential is also transitioned in synchronization with the reverse bias transition speed transitioning to 0V during the timing. The potential of the transfer roller 30, which is the combined output, is changed to the potential at the time of transfer in accordance with the transition of the potential of the forward bias at the timing of T5 to T6, and then is a constant potential during the transition of both the forward and reverse transfer biases and after the transition. Is maintained.

Next, the case where the transition time of the reverse bias potential is shorter than the transition time of the forward bias potential will be described with reference to FIG. Regarding the timings T0 to T3, the operation is the same as in the case of FIG. 5, but the transition time of the reverse bias potential is shortened. In this case, the forward bias potential transition is from T4 to T5, while the reverse bias potential transition is from T4 to T5.
It takes T6 timing. Since the transition speed of the reverse bias potential is faster than the transition speed of the forward bias potential, T4
TRCC simultaneously with TRCVON signal at timing
Even if the forward bias circuit unit 301 is activated at the same time that the ON signal is turned “ON”, that is, the reverse bias starts to be applied to the transfer roller 30, it can be activated.

Therefore, the reverse bias and the forward bias are simultaneously applied to the transfer roller 30 at the timings T4 to T5, the potential of the transfer roller 30 increases, and the increased potential of the transfer roller 30 increases at the timings T5 to T6. It decreases with the transition of the bias potential. And
After the potential of the forward bias is stabilized at the timing of T6, T
When the TRCVON signal becomes “OFF” at the 7th timing, the reverse bias becomes 0 at the T7 to T8 timings.
Transitioned to V. At that time, the forward bias is also changed in synchronization, but since it is slower than the transition speed of the reverse bias, the potential of the transfer roller 30 to which the combined voltage of the forward and reverse transfer bias is applied is not constant. That is, the difference between the reverse bias potential that decreases in a certain period of time and the forward bias potential that increases in the same period of time is added to the potential of the transfer roller 30, so that the potential of the transfer roller 30 further decreases. And
At the time when the transition of the reverse bias potential is completed at the timing of T8, the forward bias potential is still in transition, so T8
The electric potential of the transfer roller 30 increases from timing T9 to timing T9. Then, from timing T7 to T9, from timing T7 to
The same potential as the decrease in the reverse bias during the T8 timing is applied to the transfer roller 30 as the increase in the forward bias, and when the transition ends at the T9 timing, the potential of the transfer roller 30 returns to the potential at the T7 timing. Stabilize.

Next, the operation of the high voltage generation circuit 200 of the laser printer 1 according to the second embodiment will be described with reference to FIGS. FIG. 9 shows that the high voltage generation circuit 200 of the laser printer 1 according to the second embodiment has the photosensitive drum 2
7 is a flowchart of a process for determining the potential of the bias applied to 7. Hereinafter, each step of the flowchart is abbreviated as "S".

As shown in FIG. 4, in the laser printer 1 of the second embodiment, the leakage current flowing from the photosensitive drum 27 into the transfer circuit section 300 of the high voltage generating circuit 200 is
The forward bias output current can be detected by the inflow current detection unit 332, and its output can be transmitted to the inflow current detection unit 401 of the CPU 400 via the operational amplifier 350. Then, the CPU 400 determines the potential of the bias applied to the transfer roller 30 from the current value detected by the inflow current detection unit 401 according to the processing procedure of the flowchart shown in FIG. The program of this flowchart is stored in a predetermined storage area of a ROM (not shown). Hereinafter, the current value detected by the inflow current detection unit 401 is “I _i ”, the potential of the photosensitive drum 27 is “V _i ”, the reverse bias output voltage applied to the transfer roller 30 and its potential are “V ₀ ”. .

As shown in FIG. 9, the detection value obtained by detecting the leakage current from the photosensitive drum 27 by the forward bias output current inflow current detection unit 332 of the transfer circuit unit 300 is the operational amplifier 3
It is input to the inflow current detection unit 401 of the CPU 400 via 50. Then, the CPU 400 determines from this detected value that
“Inflow current I _i detection” is performed (S1). Next, CPU4
00 performs “photoconductor potential V _i estimation” from the detected I _i (S2). That is, the following formula is calculated. V _i = (1+ (R2 / R1)) × R1 · I _i + α (1) The resistors R1 and R2 are fixed resistors, and their resistance values are determined in advance. 27
The surface potential of is estimated. Note that α is an arbitrary value.

Next, the CPU 400 performs "setting of the reverse bias output voltage V ₀ " (S3). That is, V ₀ applied to the transfer roller 30 as the reverse bias output voltage is V
V ₀ having a value larger than the surface potential V _i of the photosensitive drum 27 calculated in S2 is determined so as to satisfy ₀ > V _i .

Then, based on V ₀ , "reverse bias output" is performed (S4). The CPU 400 sets the voltage applied to the transfer roller 30 as the reverse bias to V ₀ ,
A control signal is output to the output voltage variable circuit unit 324. still,
In the first embodiment, the two output voltages fixed by the control signal transmitted to the output voltage variable circuit unit 324 are switched, but in the second embodiment, for example, the output voltage variable circuit unit 324. An NPN transistor or the like is provided for the CPU 400, and the CPU 400 controls the base current thereof to control the magnitude of the current flowing between the collector and the emitter, and the reverse bias potential to be output can be controlled by this current value.

Further, the CPU 400 changes the reverse bias from 0 (V) to V ₀ (V) based on the transition speed of the reverse bias potential stored in advance in a predetermined storage area of a ROM (not shown), for example. The time to perform is calculated, and after this time has elapsed, "forward bias output" is performed (S5). At this time, the potential of the reverse bias is V ₀ , and the leakage current from the photosensitive drum 27 does not flow in, so the forward bias circuit unit 301 can be activated.

As described above, in the laser printer 1 of the first embodiment, the transfer is performed by the leakage current from the photosensitive drum 27 flowing into the transfer circuit section 300 through the transfer roller 30. In order to prevent the self-oscillation type forward bias circuit unit 301 that generates the forward bias of FIG.
The reverse bias for performing the cleaning operation of the transfer roller 30 is applied to the transfer roller 30 at a timing before the start of the above, so that the leakage current is canceled and the forward bias circuit unit 301 is removed. Allow it to start. Further, in the laser printer 1 according to the second embodiment, the leakage current flowing from the photoconductor drum 27 is detected,
The potential of the photosensitive drum 27 is estimated based on the detection result. Then, the potential of the reverse bias to be applied is determined based on the estimation result and applied to the transfer roller 30. Therefore, it is not necessary to apply an excessive potential to the transfer roller 30, and the timing of starting the forward bias circuit unit 301 It can be hastened.

Needless to say, the present invention can be modified in various ways. For example, in the first embodiment, in the timing chart, the forward bias is output after the application of the reverse bias and after the passage of the predetermined transition period.
In the case where the CPU 400 determines that the potential detection means for detecting the potential of 0 is provided, the detection result is output to the CPU 400, and the reverse bias having the potential sufficient to eliminate the leakage current from the photosensitive drum 27 is output. The TRCCON signal may be turned "ON". In addition, in the second embodiment,
Although the potential of the photosensitive drum 27 is estimated by calculation, for example, a table or the like in which a current value and an estimated potential are associated with each other is stored in a predetermined storage area of a ROM (not shown), and the photosensitive drum is referred to by referring to the table. The potential of 27 may be estimated.

[0090]

As described above, in the image forming apparatus according to the first aspect of the present invention, when the forward bias applying means applies the forward bias to the transfer roller, the reverse bias applying means applies the forward bias to the transfer roller. , And a reverse bias can be applied. Therefore, the leak current flowing from the image carrier to the forward bias applying means via the transfer roller can be eliminated by the reverse bias.

Further, in the image forming apparatus according to the second aspect of the invention, in addition to the effect of the first aspect of the invention, the forward bias applying means includes the self-excited oscillation circuit, so that the oscillator can be used without using the oscillator. It can be started and the device configuration can be simplified. Therefore, the production cost can be reduced and the production process can be simplified.

Further, in the image forming apparatus according to the third aspect of the invention, in addition to the effect of the first or second aspect of the invention, since the reverse bias applying means is a constant voltage system, a voltage different from the forward bias applying means. It can be an application method. Therefore, the control for superimposing and outputting the constant current applied by the forward bias applying means and the constant voltage applied by the reverse bias applying means to the transfer roller can be easily performed.

Further, in the image forming apparatus of the fourth aspect of the invention, in addition to the effect of the third aspect of the invention, the absolute value of the potential of the reverse bias applied to the transfer roller by the constant voltage type reverse bias applying means is set. The absolute value of the potential of the forward bias applied to the transfer roller by the constant current type forward bias applying means can be made smaller. Therefore, the reverse bias applying means can output a reverse bias having a necessary potential.

According to the image forming apparatus of the fifth aspect of the invention, in addition to the effect of the third or fourth aspect of the invention, the reverse bias applying means includes the forward bias potential applied to the transfer roller by the forward bias applying means. The reverse bias can be applied to the transfer roller until the voltage reaches a predetermined potential. Therefore, the forward bias applying means can be reliably activated.

Further, in the image forming apparatus of the invention according to claim 6, in addition to the effect of the invention according to any one of claims 1 to 5, the reverse bias applying means transfers the image on the basis of the current value detected by the detecting means. A reverse bias can be applied to the roller. Therefore, the reverse bias applying means can apply a reverse bias having a necessary potential to the transfer roller.

Further, in the image forming apparatus of the invention according to claim 7, in addition to the effect of the invention according to claim 6, reverse bias applying means is provided so that a predetermined current necessary for the operation of the forward bias applying means can be obtained. The voltage value to be applied to the transfer roller by the calculation unit can be calculated based on the current value detected by the detection unit. Therefore, the reverse bias applying means can apply a reverse bias having a necessary potential to the transfer roller.

Further, in the image forming apparatus of the invention according to claim 8, in addition to the effect of the invention according to any one of claims 1 to 7, the reverse bias potential applied to the transfer roller by the reverse bias applying means is predetermined. When the potential detecting unit determines that the potential has been reached, the forward bias applying unit can apply the forward bias to the transfer roller. Therefore, the forward bias applying means can be reliably activated.

Further, in the image forming apparatus of the invention according to claim 9, in addition to the effect of the invention according to any one of claims 1 to 8, the reverse bias applying means always applies a predetermined potential to the transfer roller. be able to. Therefore, the leak current flowing from the image carrier to the forward bias applying means via the transfer roller can be eliminated by the reverse bias.

Further, in the image forming apparatus of the invention according to claim 10, in addition to the effect of the invention according to any one of claims 1 to 9, when the forward bias applying means does not apply the forward bias to the transfer roller, By applying a reverse bias to the transfer roller by the reverse bias applying means, the developer attached to the transfer roller can be transferred to the image carrier by utilizing the potential difference between the image carrier and the transfer roller. Therefore, by applying a reverse bias to the transfer roller by the reverse bias applying means,
The transfer roller can be cleaned.

According to the image forming apparatus of the eleventh aspect of the invention, in addition to the effect of the tenth aspect of the invention, the reverse bias applying means is used when the developer attached to the transfer roller is transferred to the image carrier. The potential of the reverse bias applied to the transfer roller can be made larger than the potential of the reverse bias applied by the reverse bias applying means while being superimposed on the forward bias when the forward bias applying means applies the forward bias to the transfer roller. . Therefore, it is possible to output a reverse bias having a smaller potential than the reverse bias for cleaning.

[Brief description of drawings]

FIG. 1 is a central cross-sectional view of a laser printer 1 according to this embodiment.

FIG. 2 is a cross-sectional view of the image forming unit 5 viewed from the side.

FIG. 3 is a diagram illustrating a photosensitive drum 27 according to the first embodiment.
It is a block diagram which shows the surrounding electric constitution.

FIG. 4 is a diagram illustrating a photosensitive drum 27 according to a second embodiment.
It is a block diagram which shows the surrounding electric constitution.

FIG. 5 shows that the TRCVCHON signal is output,
9 is a timing chart in the case where a bias having a potential lower than the reverse bias potential at the time of cleaning is output as a starting reverse bias for starting the forward bias circuit unit 301.

FIG. 6 is a timing chart in the case where a reverse bias for starting is always output when a forward bias is output.

FIG. 7 is a timing chart when the transition time of the reverse bias potential is longer than the transition time of the forward bias potential.

FIG. 8 is a timing chart when the transition time of the reverse bias potential is shorter than the transition time of the forward bias potential.

FIG. 9 is a laser printer 1 according to a second embodiment.
6 is a flowchart of a process for determining the potential of the bias applied to the photosensitive drum 27 by the high voltage generation circuit 200 of FIG.

[Explanation of symbols]

1 laser printer 27 Photosensitive drum 30 transfer roller 301 Forward bias circuit section 302 Reverse bias circuit section 332 Forward bias output current Inflow current detector 400 CPU

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H200 FA17 GA13 GA16 GA23 GA34                       GA46 GA59 GB22 GB37 HA03                       HA12 HB03 HB12 HB22 JA02                       JA28 JA29 JA30 JB12 NA02                       NA09 NA15 NA16 PA03 PA04                       PA20 PB05 PB15

Claims (11)

[Claims] 1. An image bearing member on which an electrostatic latent image is formed, a transfer roller for transferring a developer image, which is the electrostatic latent image on the image bearing member developed by a developer, to a recording medium, and the developing unit. Constant current type forward bias applying means for applying a forward bias having a polarity opposite to that of the developer to the transfer roller; and reverse bias applying means for applying a reverse bias having a bias having the same polarity as the developer to the transfer roller. An image, characterized in that, when the forward bias applying means applies a forward bias to the transfer roller, the reverse bias applying means applies a reverse bias to the transfer roller by superimposing the forward bias on the transfer roller. Forming equipment. 2. The image forming apparatus according to claim 1, wherein the forward bias applying unit includes a self-excited oscillation circuit. 3. The image forming apparatus according to claim 1, wherein the reverse bias applying unit is a constant voltage type. 4. The absolute value of the reverse bias potential applied to the transfer roller by the constant voltage type reverse bias applying means is the forward bias potential applied to the transfer roller by the constant current type forward bias applying means. The image forming apparatus according to claim 3, wherein the image forming apparatus is smaller than the absolute value of. 5. When the forward bias applying unit applies a forward bias to the transfer roller, the forward bias applying unit applies a forward bias to the transfer roller until a potential reaches a predetermined potential. The image forming apparatus according to claim 3, wherein the reverse bias is applied to the transfer roller by a reverse bias applying unit. 6. A detection unit for detecting a current value flowing from the image carrier to the forward bias application unit via the transfer roller, and applying the reverse bias based on the current value detected by the detection unit. The image forming apparatus according to claim 1, wherein the means applies a reverse bias to the transfer roller. 7. The reverse bias applying means applies the current to the transfer roller so that a predetermined current necessary for the operation of the forward bias applying means is obtained based on the current value detected by the detecting means. The image forming apparatus according to claim 6, further comprising a calculation unit that calculates a voltage value. 8. The reverse bias applying means includes potential detecting means for detecting a reverse bias potential applied to the transfer roller, and the reverse bias potential applied to the transfer roller by the reverse bias applying means is a predetermined potential. 8. The image forming apparatus according to claim 1, wherein the forward bias applying unit applies a forward bias to the transfer roller when the potential detecting unit determines that the voltage has reached a predetermined value. 9. The image forming apparatus according to claim 1, wherein the reverse bias applying means always applies a predetermined potential to the transfer roller. 10. When the forward bias applying means does not apply a forward bias to the transfer roller, the reverse bias applying means applies a reverse bias to the transfer roller, whereby the image carrier and the transfer member are transferred. 10. The image forming apparatus according to claim 1, wherein the developer attached to the transfer roller is transferred to the image carrier by using a potential difference between the image carrier and the roller. 11. The reverse bias potential applied to the transfer roller by the reverse bias applying means when the developer attached to the transfer roller is transferred to the image carrier is:
11. The image according to claim 10, wherein when the forward bias applying unit applies a forward bias to the transfer roller, the reverse bias applying unit has a potential higher than a reverse bias potential applied in superimposition with the forward bias. Forming equipment.