JP5588927B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus using an electrophotographic system, and more particularly to an image forming apparatus that uses a two-component developer composed of a carrier and a toner and develops an electrostatic latent image by holding only the toner on a developing roller. About.

  In recent image forming apparatus development systems, attention has been paid to a touch-down development system that has advantages of both a one-component development system that uses only toner as a developer and a two-component development system that uses toner and a carrier. In the touchdown development method, a magnetic brush is formed on the surface of the magnetic roller with a two-component developer containing toner and a carrier, and only the toner is moved from the magnetic brush to the surface of the developing roller to form a thin toner layer. In this method, toner is ejected onto the surface of the photosensitive drum on which the electrostatic latent image is formed, and developed as a toner image. In the touch-down development method, a two-component development method is adopted for the toner charging area to extend the life, and a one-component development method is adopted for the development area to improve the image quality.

  As a method of generating a high voltage to be applied to the developing roller and the magnetic roller in such an image forming apparatus, a method of superimposing a DC voltage on an AC voltage as described in Patent Document 1 is known.

  FIG. 6 shows a conventional power supply circuit for generating a bias voltage supplied to the developing roller and the magnetic roller. Regarding the subscripts of the reference numerals assigned to the respective elements, K indicates black, M indicates magenta, C indicates cyan, and Y indicates yellow, which indicates that the component corresponds to the developing device of each color. Since the configurations of the power supply circuits 90K, 90M, 90C, and 90Y are all the same, the following description will be comprehensively described without adding any subscripts.

  An AC power supply 91 is connected to the primary side of the transformer 92. On the secondary side of the transformer 92, a superimposed voltage of the boosted AC voltage and the DC voltage output from the developing roller DC power source 93 is applied to the developing roller 96, and the AC voltage and the superimposed voltage output from the magnetic roller DC power source 94 are applied. A voltage is applied to the magnetic roller 97.

JP 2010-85592 A

  In the conventional power supply circuit shown in FIG. 6, the power supply device 90 is provided on a one-to-one basis for each color developing device. Accordingly, since a plurality of power supply devices 90 are required and a plurality of transformers 92 are also required, the number of parts of the power supply device 90 is increased, resulting in a cost increase.

  SUMMARY An advantage of some aspects of the invention is to provide an image forming apparatus in which the number of parts is reduced and the cost is reduced.

The image forming apparatus according to the first aspect of the present invention is provided with a developer carrying member that conveys a two-component developer composed of toner and a carrier, and is disposed to face the developer carrying member, and is supplied from the developer carrying member. A toner carrier that carries the toner, and an image carrier that carries a toner image that is visualized by flying an electrostatic latent image on the surface of the toner carrier to the electrostatic latent image. A plurality of image forming units having different colors of toner images formed on the image carrier, and connected in common to the plurality of image forming units, and an AC voltage of an AC power source input from the primary side, the transformer means for outputting a voltage transformer from the secondary side to the respective image forming means, which is connected to each of said developer carrying member of each image forming unit, the first DC voltage the first DC voltage output hands for outputting If the connected to each of the toner carrying member of each image forming unit, and a second DC voltage output means for outputting a second DC voltage, has, the developer of the respective image forming means for the carrier, a first bias voltage connected to said first DC voltage output means first DC voltage and the transformer means output is generated by superimposing an AC voltage outputted respectively applied The second DC voltage output from the second DC voltage output means connected to the toner carrier of each image forming means is superimposed on the AC voltage output from the transformer means. Voltage applying means for applying the second bias voltage, and control means for controlling the DC voltage output by each of the DC voltage output means , the image forming means for a period during which the toner image is not formed. Forming means In order to cause the voltage applying unit to generate a bias voltage for forcibly moving the toner from the toner carrier having the toner carrier to the developer carrier disposed opposite to the toner carrier, the control unit includes the development unit. The first DC voltage output from the first DC voltage output means connected to the agent carrier and the second DC voltage output from the second DC voltage output means connected to the toner carrier are used for the development. The difference between the first bias voltage applied to the developer carrier and the second bias voltage applied to the toner carrier causes a leak to begin to occur between the developer carrier and the toner carrier. The potential difference between the developer carrier and the toner carrier is set between the developer carrier and the toner carrier which are controlled by the image forming unit. , The image A constant voltage circuit is connected for limiting the voltage to a predetermined reference voltage that is greater than a potential difference between the developer carrying member and the toner carrying member when the forming unit forms the toner image and lower than the leak voltage. It is .

  Conventionally, the transforming means is provided on a one-to-one basis for each color image forming means. However, by making the transforming means common to a plurality of image forming apparatuses, the number of parts of the voltage applying means can be reduced. Therefore, it is possible to reduce the size and cost of the apparatus.

  If the voltage transforming means is common to the plurality of image forming means, it is necessary to perform secondary output of the voltage transforming means while any one of the image forming means of each color forms a toner image. Further, in order to extend the life of the image carrier, it is common to turn off the charging of the image carrier except during the formation of the toner image. If a bias voltage is applied to the toner carrier during this time, the toner The toner moves from the carrier to the image carrier, leading to wasteful consumption of toner.

  Therefore, for any image forming unit in which the image carrier is off during the period during which any of the image forming units is forming a toner image, development is performed from the toner carrier instead of the development bias voltage used during printing. A bias voltage for forcibly moving the toner is applied to the agent carrier. As a result, even if the charging of the image carrier is off, the toner does not move from the toner carrier to the image carrier, so that useless toner is not consumed.

  According to this configuration, when a bias voltage for forcibly moving the toner from the toner carrier to the developer carrier is applied, the output value of the AC power supply must be changed between the toner carrier and the developer carrier. The potential difference becomes large and leakage occurs. However, if there is at least one image forming means for forming a toner image, the voltage transforming means is shared by each color, so that an AC voltage output is also output to the image forming means in which the charging of the image carrier is off. It cannot be changed.

  Therefore, by connecting a constant voltage element between the toner carrier and the developer carrier, a predetermined voltage or more is not applied between the toner carrier and the developer carrier, and leakage can be prevented.

According to a second aspect of the invention, an image forming apparatus according to claim 1, the color toner image in which the plurality of image forming means is formed on an image bearing member having respective, non-black color different It is.

  In general, the frequency of monochrome printing is overwhelmingly higher than that of color printing, and the operation rate of the image forming unit that forms a black image among the plurality of image forming units is the highest. That is, since the supply time of the bias voltage to the toner carrier and developer carrier included in the image forming apparatus for forming a black image becomes longer, troubles such as damage to circuit components are likely to occur. Accordingly, the image forming means for forming an image other than black has a common transformer, and the image forming means for forming a black image is provided with a dedicated transformer, thereby facilitating maintenance.

  According to the present invention, the number of parts of the voltage applying means can be reduced, and the apparatus can be reduced in size and cost.

1 is a cross-sectional view showing an internal structure of an image forming apparatus according to an embodiment of the present invention. 1 is a block diagram illustrating an electrical configuration of an image forming apparatus. FIG. 4 is a diagram for explaining a process in which toner is supplied from a developing device to a photosensitive drum. FIG. 3 is a circuit diagram of a power supply circuit in this embodiment. 6 is a timing chart showing the charging timing of the photosensitive drum and the application timing of the bias voltage (DC component) applied to the developing roller and the magnetic roller. The circuit diagram of the conventional power supply circuit.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing an outline of the internal structure of an image forming apparatus 1 according to an embodiment of the present invention. The image forming apparatus 1 is a tandem color printer. In this embodiment, a printer is described as an example. However, the present invention can be applied not only to a printer, but also to a facsimile machine, a copier, a multifunction machine having these functions, and the like, and is a tandem type image forming apparatus. That's fine.

  The image forming apparatus 1 includes a sheet storage unit 110, an image forming unit 130, and a fixing unit 160. The sheet storage unit 110 is disposed at the lowermost part of the image forming apparatus 1 and includes a sheet tray 111 that can store a bundle of sheets P. The paper tray 111 is inserted into the image forming apparatus 1 and attached. When replenishing the paper P, the paper tray 111 is pulled out from the image forming apparatus 1. In the bundle of sheets P stored in the sheet tray 111, the uppermost sheet P is fed out toward the sheet conveyance path 115 by driving the pickup roller 113. The paper P is transported to the image forming unit 130 through the paper transport path 115.

  The image forming unit 130 forms a toner image on the conveyed paper P. The image forming unit 130 includes a magenta unit 133M, a cyan unit 133C, a yellow unit 133Y, and a black unit 133Bk arranged in tandem according to the order of transferring the toner images to the transfer belt 131. These units have the same configuration, and a magenta unit 133M will be described as an example.

  The magenta unit 133M includes a photosensitive drum 135. Around the photosensitive drum 135, a charger 137, an exposure device 139, a developing device 141, and a cleaner 143 are arranged. The charger 137 uniformly charges the peripheral surface of the photosensitive drum 135. The exposure device 139 generates light corresponding to magenta data in the image data transmitted from a personal computer or the like, and irradiates the circumferential surface of the uniformly charged photosensitive drum 135. As a result, an electrostatic latent image corresponding to the magenta data is formed on the peripheral surface of the photosensitive drum 135. In this state, by supplying magenta toner from the developing device 141 to the peripheral surface of the photosensitive drum 135, a toner image corresponding to magenta data is formed on the peripheral surface.

  The transfer belt 131 can move in the D direction (clockwise) while being sandwiched between the photosensitive drum 135 and the primary transfer roller 145. The toner image corresponding to the magenta data is transferred from the photosensitive drum 135 to the transfer belt 131. The magenta toner remaining on the peripheral surface of the photosensitive drum 135 is removed by the cleaner 143. The above is the description of the magenta unit 133M.

  A toner image corresponding to magenta data is transferred to the transfer belt 131, and a toner image corresponding to cyan data, a toner image corresponding to yellow data, and a toner image corresponding to black data are superimposed and transferred onto the toner image. The As a result, a color toner image is formed on the transfer belt 131. The color toner image is transferred to the paper P conveyed from the paper storage unit 110 by the secondary transfer roller 149.

  The sheet P on which the color toner image is transferred is sent to the fixing unit 160. The fixing unit 160 includes a heating roller 161 and a pressure roller 163. The paper P on which the color toner image is transferred is sandwiched between these rollers. As a result, heat and pressure are applied to the color toner image, and the color toner image is fixed to the paper P. The paper P is discharged to the paper discharge unit 169.

  FIG. 2 is a block diagram illustrating an electrical configuration of the image forming apparatus 1. The image forming apparatus 1 has a configuration in which a sheet storage unit 110, an image forming unit 130, a fixing unit 160, a control unit 200, an operation display unit 300, and a power supply circuit 6 are connected to each other via a bus. The sheet storage unit 110, the image forming unit 130, and the fixing unit 160 have been described with reference to FIG.

  The control unit 200 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), an image memory, and the like, and performs control necessary for operating the image forming apparatus 1. This is executed for the hardware that constitutes the above. The ROM stores software necessary for controlling the operation of the image forming apparatus 1. The RAM is used for temporary storage of data generated during execution of software, storage of application software, and the like. The image memory temporarily stores image data (such as image data transmitted from a personal computer).

  The operation display unit 300 is provided with operation keys and a display screen. The display screen displays various operations and details of operations. The power supply circuit 6 generates a bias voltage obtained by superimposing a DC voltage and an AC voltage, and applies the bias voltage to a roller in the developing device 141.

  Next, the configuration and operation of the developing device 141 will be described. FIG. 3 is a diagram illustrating a process in which the toner 53 in the two-component developer 51 is supplied from the developing device 141 to the photosensitive drum 135. Reference numeral 55 denotes a carrier constituting the two-component developer 51.

  The developing device 141 includes a developing roller 11, a magnetic roller 13, and a stirring screw (not shown). The agitating screw is disposed in an agitating chamber (not shown) that accommodates the two-component developer 51, and agitates the two-component developer 51 by rotating. As a result, the toner 53 and the carrier 55 are frictionally charged, and the toner 53 and the carrier 55 are electrostatically coupled to each other and are pumped up by the magnetic roller 13.

  The magnetic roller 13 sucks and conveys the two-component developer 51 in which the toner 53 and the carrier 55 are electrostatically coupled while rotating in the rotation direction R2 as the shaft 33 rotates. The blade 35 regulates the thickness of the two-component developer 51 carried on the magnetic roller 13.

  At the facing portion 37, the toner 53 in the two-component developer 51 is supplied to the developing roller 11 that rotates in the rotation direction R1. Specifically, in a state where the carrier 55 is attracted to the magnetic roller 13, only the toner 53 is attracted to the developing roller 11 by the magnetic force and attracts to the developing roller 11.

  The developing roller 11 is disposed opposite to the magnetic roller 13, sucks the toner 53 at the facing portion 37, and conveys the toner 53 by rotating in the rotation direction R <b> 1 while carrying the toner 53. Then, the toner 53 is supplied to the photosensitive drum 135 at the facing portion 47.

  Specifically, the photosensitive drum 135 rotates in a rotation direction R3 opposite to the rotation directions R1 and R2. The conveyed toner 53 is attracted by the electrostatic latent image formed on the peripheral surface of the photosensitive drum 135 and moves to the photosensitive drum 135. As a result, the electrostatic latent image on the peripheral surface of the photosensitive drum 135 is developed, and a toner image is formed on the peripheral surface. A transfer belt 131 in contact with the photosensitive drum 135 is disposed on the photosensitive drum 135. The transfer belt 131 moves in the D direction, and the toner image is transferred to the transfer belt 131.

  As described above, the toner 53 in the two-component developer 51 conveyed by the magnetic roller 13 moves to the developing roller 11 at the facing portion 37. Further, the toner 53 conveyed by the developing roller 11 moves to the photosensitive drum 135 at the facing portion 47. Thus, in order to cause the toner 53 to fly and move at the facing portion 47 and the facing portion 37, the power supply circuit 6 biases the developing roller 11 and the magnetic roller 13 with a DC voltage and an AC voltage superimposed on each other. Is applied.

  FIG. 4 is a circuit diagram of the power supply circuit 6. Regarding the subscripts of the reference numerals attached to the respective elements, K indicates black, M indicates magenta, C indicates cyan, and Y indicates yellow, which indicates that the component corresponds to the developing device 141 of each color.

  First, the black power supply circuit 600 that supplies the power supply voltage to the developing device 141 of the black unit 133Bk will be described. The black power supply circuit 600 has a configuration similar to that of the conventional power supply circuit 90K, and an AC power supply 61 is connected to the primary side of the transformer 62. On the secondary side of the transformer 62, the boosted AC voltage and the developing roller are used. The superimposed voltage of the DC voltage output from the DC power supply 63 is applied to the developing roller 11K, and the superimposed voltage of the AC voltage and the DC voltage output from the DC power supply 64 for the magnetic roller is applied to the magnetic roller 13K. The resistor R connected to the preceding stage of the developing roller 11K and the magnetic roller 13K is a resistor for limiting the current flowing to each roller.

  Next, the color power supply circuit 610 that supplies a power supply voltage to the developing devices 141 of the magenta unit 133M, the cyan unit 133C, and the yellow unit 133Y will be described. The color power supply circuit 610 includes an AC power supply 68 and a transformer 69 that are common to the developing devices 141 of the respective colors. An AC power source 68 is connected to the primary side of the transformer 69.

  A superposed voltage of an AC voltage output from the transformer 69 and a DC voltage output from the DC power supply 65M for the developing roller is applied to the developing roller 11M, and an AC voltage output from the transformer 69 and the magnetic roller are applied to the magnetic roller 13M. The superimposed voltage of the DC voltage output from the DC power source 66M for the application is applied.

  The developing roller 11C is applied with the superimposed voltage of the AC voltage output from the transformer 69 and the DC voltage output from the DC power supply 65C for the developing roller, and the magnetic roller 13C is applied with the AC voltage output from the transformer 69 and the DC for the magnetic roller. A superimposed voltage of the DC voltage output from the power supply 66C is applied.

  A superposed voltage of an AC voltage output from the transformer 69 and a DC voltage output from the DC power supply 65Y for the developing roller is applied to the developing roller 11Y, and an AC voltage output from the transformer 69 and the DC for the magnetic roller are applied to the magnetic roller 13Y. A superimposed voltage of the DC voltage output from the power supply 66Y is applied.

  A series circuit of a resistor R and a capacitor C is connected to the preceding stage of the developing roller 11M and the magnetic roller 13M. The resistor R is for limiting the current flowing to each roller, and the capacitor C is for superimposing an AC voltage and a DC voltage. The same applies to the resistor R and the capacitor C connected to the preceding stage of the developing roller 11C and the magnetic roller 13C, the developing roller 11Y, and the magnetic roller 13Y.

  Further, a constant voltage element ZD is connected between the developing roller 11M and the magnetic roller 13M. The constant voltage element ZD is, for example, a Zener diode whose Zener voltage is a predetermined reference voltage, and may be any constant voltage circuit that can limit the potential difference between the developing roller 11M and the magnetic roller 13M to a reference voltage or less. The same applies to the constant voltage element ZD connected between the developing roller 11C and the magnetic roller 13C, and the developing roller 11Y and the magnetic roller 13Y. The reference voltage is set by obtaining a voltage value slightly smaller than a potential difference at which a leak current starts to flow between the developing roller 11S and the magnetic roller 11M, for example, through experiments.

  Thus, by arranging one transformer 69 in common with the color developing device 141, the number of components of the power supply circuit 6 can be reduced, and the size and cost of the device can be reduced.

  In the present embodiment, the common transformer 69 is arranged for the color developing devices 141 other than black, but a common transformer may be arranged for all colors. However, in general, the frequency of monochrome printing is overwhelmingly higher than that of color printing, and the operating rate of the black unit 133Bk is the highest. That is, since the supply time for supplying the bias voltage to the developing roller 11K and the magnetic roller 13K becomes longer, troubles such as component damage are likely to occur. Therefore, in order to facilitate maintenance, it is desirable to provide a dedicated power supply circuit 600 for the black unit 133Bk.

  In the tandem type image forming apparatus, each color developing device 141 performs color printing by superimposing and transferring the toner image of the own color onto the toner image of the other color transferred to the transfer belt 131, and the developing device for each color. The timing at which the toner image 141 is formed on the photosensitive drum 135 is different. When the timing at which the toner image is formed by the developing device 141 is different, the application timing of the bias voltage that the power supply circuit 6 supplies to the developing device 141 of each color also differs. Conventionally, as shown in FIG. 6, the power supply circuit 90 is provided for each color developing device, and therefore, the bias voltage on / off control may be performed for each power supply circuit 90.

  However, in the image forming apparatus 1 of the present invention, the transformer 69 is common to the color developing device 141. Based on this point, control at the time of image formation of the power supply device 6 in the present embodiment will be described.

  The black power supply circuit 610 supplies a developing bias voltage to the developing roller 11B and the magnetic roller 13B at a timing necessary for forming a black toner image on the photosensitive drum 135, as in the past. The supply timing of the developing bias voltage is controlled by the control unit 200.

  In the color power supply circuit 610, since the transformer 69 is provided in common for each color, the bias voltage is not applied to the color developing device 141 only at the timing of forming the toner image as in the prior art, but the color toner image is not generated. During the formation, a bias voltage is applied to all the color developing devices 141.

  On the other hand, in order to keep the life of the photosensitive drum 135 long, the photosensitive drum 135 is charged only at the timing of forming a toner image, and the charging is turned off during other periods. However, if a developing bias voltage is applied to the developing roller 11 when the photosensitive drum 135 is not charged, the toner moves to the peripheral surface of the photosensitive drum 135 due to a potential difference, and wasteful toner is consumed. turn into.

  Accordingly, the color power supply circuit 610 is configured so that when the charging of the photosensitive drum 135 is off, the magnetic roller 610 is connected to the magnetic roller so that the toner does not move from the developing roller 11 to the photosensitive drum 135 when the charging of the photosensitive drum 135 is off. A bias voltage for moving the toner to 13 is applied.

  As a bias voltage at which the toner moves from the developing roller 11 to the magnetic roller 13, a bias voltage at the time of refresh operation (hereinafter referred to as “refresh voltage”) is employed. The refresh operation means that the magnetic roller 13 peels off the toner thin layer formed on the developing roller 11 to form a new toner thin layer. In the touch-down development method, the toner thin layer is formed on the developing roller 11 with a magnetic brush and a bias voltage. The toner is fixed to the developing roller 11 by applying an electric field in one direction, or the developing roller 11 is exposed to light. A development ghost (history phenomenon) phenomenon in which an afterimage after the toner flies to the body drum 135 appears in the second round may occur. The refresh operation is for solving this problem, and is generally performed at a timing between sheets, that is, between sheets.

  When the color power supply circuit 610 applies a bias voltage to the developing device 141 in which the charging of the photosensitive drum 135 is off, the developing bias used during printing is used to prevent the toner from moving to the photosensitive drum 135. Apply a refresh voltage instead of a voltage. By doing so, useless toner is not consumed even if a voltage is applied to the developing device 141 while the photosensitive drum 135 is charged off.

  Switching between the development bias voltage and the refresh voltage output from the color power supply circuit 610 is performed by the control unit 200 changing the output values of the development roller DC power supply 65 and the magnetic roller DC power supply 66. Conventionally, during the refresh operation, the output value of the AC power supply is lowered to reduce the potential difference between the developing roller and the magnetic roller, thereby preventing leakage. However, in the power supply circuit 6 in the present invention, since the transformer 69 is common to the developing devices 141 of the respective colors, the AC power supply 68 needs to maintain the output during development. That is, in this state, the potential difference between the developing roller 11 and the magnetic roller 13 at the time of refresh operation becomes large, and a leak occurs. The occurrence of leak causes image deterioration, component damage, and the like.

  Therefore, by connecting the constant voltage element ZD between the developing roller 11 and the magnetic roller 13, a predetermined voltage or higher is not applied between the developing roller 11 and the magnetic roller 13, and leakage can be prevented. When a Zener diode is used as the constant voltage element ZD, the Zener voltage is set to a voltage that is larger than the potential difference between the developing roller 11 and the magnetic roller 13 during printing and lower than the leakage voltage at which leakage starts to occur. And the magnetic roller 13 can be prevented from having a potential difference higher than the leakage voltage.

  FIG. 5 is a timing chart showing the charging timing of the photosensitive drum 135 and the application timing of the bias voltage (DC component) applied to the developing roller 11 and the magnetic roller 13. When charging of the magenta photosensitive drum 135 for toner image formation is started (time t1), the color power supply circuit 610 applies a developing bias voltage to the developing roller 11M and the magnetic roller 13M, and the developing rollers 11C and 11Y. Then, a refresh voltage is applied to the magnetic rollers 13C and 13Y.

  Specifically, at time t1, the AC power supply 68 starts outputting an AC voltage, and the DC power supply 65M for the developing roller and the DC power supply 66M for the magnetic roller use the DC voltage for the developing bias voltage set by the control unit 200. Output. The developing roller DC power supplies 65C and 65Y and the magnetic roller DC power supplies 66C and 66Y output a DC voltage for refresh voltage under the control of the control unit 200.

  When charging of the magenta photosensitive drum 135 is completed (time t2), the color power supply circuit 610 switches the voltage applied to the developing roller 11M and the magnetic roller 13M from the developing bias voltage to the refresh voltage. Specifically, at time t2, the controller 200 switches the voltage output from the developing roller DC power supply 65M and the magnetic roller DC power supply 66M from the developing bias voltage DC voltage to the refresh voltage DC voltage.

  Subsequently, when charging of the cyan photosensitive drum 135 is started (time t3), the color power supply circuit 610 applies a developing bias voltage to the developing roller 11C and the magnetic roller 13C, and the developing rollers 11M and 11Y and the magnetic roller 13M. And a refresh voltage is applied to 13Y.

  When the charging of the cyan photosensitive drum 135 is completed (time t4), the color power supply circuit 610 switches the voltage applied to the developing roller 11C and the magnetic roller 13C from the developing bias voltage to the refresh voltage.

  Subsequently, when charging of the yellow photosensitive drum 135 starts (time t5), the color power supply circuit 610 applies a developing bias voltage to the developing roller 11Y and the magnetic roller 13Y, and the developing rollers 11C and 11M and the magnetic roller 13C. And a refresh voltage is applied to 13M.

  When the charging of the yellow photosensitive drum 135 is completed (time t6), the color toner image formation is completed, and the color power supply circuit 610 supplies the developing rollers 11M, 11C, and 11Y to the magnetic rollers 13M, 13C, and 13Y. Turn off voltage application. That is, the AC power supply 68, the developing roller DC power supply 65, and the magnetic roller DC power supply 66 stop voltage output. As described above, from the secondary side of the transformer 69, the AC voltage is applied after the charging of the magenta photosensitive drum 135 is started (time t1) until the charging of the yellow photosensitive drum 135 is completed (time t6). Is output continuously.

  Next, when charging of the black photosensitive drum 135 is started (time t7), the AC power supply 61 starts to output an AC voltage, and the developing roller DC power supply 63 and the magnetic roller DC power supply 64 are used for developing bias voltage. DC voltage is output. Thereby, a developing bias voltage is applied to the developing roller 11B and the magnetic roller 11B. When charging is completed (time t8), the black power supply circuit 610 turns off the voltage application to the developing roller 11B and the magnetic roller 13B.

  As described above, by arranging the common transformer 69 for the color developing device 141, the number of parts of the power supply circuit 6 can be reduced, and the size and cost of the device can be reduced. .

  When the transformer 69 is used in common with the color developing device 141, the AC power source 68 needs to maintain a constant output while the color developing device 141 forms a toner image. Further, in order to extend the life of the photosensitive drum 135, charging is turned off except during toner formation. During this time, when a bias voltage is applied to the developing device 141, the developing roller 11 applies the photosensitive drum 135 to the photosensitive drum 135. This will cause toner movement. Therefore, a refresh voltage is applied to the developing device 141 in which the charging of the photosensitive drum 135 is off, not the developing bias voltage used during printing. As a result, the toner does not move from the developing roller 11 to the photosensitive drum 135 while the photosensitive drum 135 is charged off, so that useless toner is not consumed.

  When the output values of the DC power supply 65 for the developing roller and the DC power supply 66 for the magnetic roller are refresh voltage values, the potential difference between the developing roller 11 and the magnetic roller 13 increases unless the output value of the AC power supply 68 is changed. Leakage will occur. Therefore, by connecting the constant voltage element ZD between the developing roller 11 and the magnetic roller 13, a predetermined voltage or more is not applied between the developing roller 11 and the magnetic roller 13, and leakage can be prevented.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 6 Power supply circuit (voltage application means)
11 Development roller (toner carrier)
13 Magnetic roller (developer carrier)
130 Image forming unit (image forming means)
61, 68 AC power supply (AC power supply)
62, 69 Transformer
63, 65M, 65C, 65Y DC power supply for developing roller (DC voltage output means)
64, 66M, 66C, 66Y DC power supply for magnetic roller (DC voltage output means)
110 Paper storage unit 135 Photosensitive drum (image carrier)
141 Developing Device 200 Control Unit (Control Unit)
300 Operation display unit 600 Black power supply circuit (voltage application means)
610 Color power circuit (voltage application means)
ZD constant voltage element

Claims (2)

A developer carrier for conveying a two-component developer comprising toner and carrier;
A toner carrier disposed opposite to the developer carrier and carrying toner supplied from the developer carrier;
An image carrier that carries an electrostatic latent image formed on the surface, and carries a toner image that is visualized by the toner of the toner carrier flying to the electrostatic latent image;
A plurality of image forming means having different colors of toner images formed on the image carrier,
Transformer means connected in common to the plurality of image forming means, inputting an AC voltage of an AC power source from the primary side, and outputting a voltage transformed from the secondary side to each of the image forming means,
Which is connected to each of said developer carrying member of each image forming unit, a first DC voltage output means for outputting a first DC voltage,
A second DC voltage output means connected to each of the toner carriers of each of the image forming means and outputting a second DC voltage;
Have the for the developer carrying member of each image forming unit, a first DC voltage and AC voltage said transformer means has output that outputs the first DC voltage output means connected to each A second DC voltage output from the second DC voltage output means connected to the toner carrier of each image forming means by applying a first bias voltage generated by superimposing ; Voltage application means for applying a second bias voltage generated by superimposing the AC voltage output by the transformer means ;
Control means for controlling the DC voltage output by each DC voltage output means;
Equipped with a,
Bias for forcibly moving toner from the toner carrier of the image forming unit to the developer carrier disposed opposite to the toner carrier during a period when the image forming unit does not form the toner image In order to cause the voltage application means to generate a voltage, the control means outputs a first DC voltage output from a first DC voltage output means connected to the developer carrier and a first DC voltage connected to the toner carrier. The difference between the first bias voltage applied to the developer carrier and the second bias voltage applied to the toner carrier is the second DC voltage output by the second DC voltage output means. Control so that the leakage voltage between the toner carrier and the toner carrier starts to be higher than the leakage voltage,
A potential difference between the developer carrier and the toner carrier between the pair of the developer carrier and the toner carrier that each of the image forming units has, indicates that the image forming unit has the toner image. An image forming apparatus to which a constant voltage circuit that limits a voltage below a predetermined reference voltage that is larger than a potential difference between the developer carrying member and the toner carrying member and lower than the leakage voltage is formed. A black image forming means for forming a black toner image;
A black voltage applying means for transforming an alternating voltage of the alternating current power source and supplying a bias voltage generated by superimposing the transformed voltage and the direct current voltage to the black image forming means;
The image forming apparatus according to claim 1, further comprising: the plurality of image forming units corresponding to a plurality of colors other than black.

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