Fixing device and image forming apparatus

Abstract

To provide a technique that enables a reduction in the amount of a negative charge flowing from a pressure rotating body through a rectifier to the ground of a fixing device, thereby reducing noise generated at a grounding place.SOLUTION: A fixing device 8 comprises: a heating unit 81 that has an endless belt 81A that rotates around a heater 90 that is formed on a substrate and applied with AC voltage; a pressure roller 82 that has a conductive shaft 82A, and a roller part 82B that covers an outer peripheral surface of the shaft 82A; a diode 85 that has a cathode connected to the shaft of the pressure roller, and has an anode connected to the belt of the heating unit; and a ground circuit 87 that has one end T1 connected to the anode of the diode 85 and the other end T2 connected to a fixing ground unit 80, wherein a capacitor C and a first resistance R1 are connected in series between one end T1 and the other end T2, and a second resistance R2 is connected in parallel with the capacitor C and the first resistance R1.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present application relates to a fixing device for fixing a developer image formed on a sheet to the sheet, and an image forming apparatus including the fixing device.

Background Art

[0002] In electrophotographic image formation, a fixing device that transfers a developer image onto a sheet and fixes the developer image transferred onto the sheet is generally known.

[0003] Patent Document 1 describes an image forming apparatus in which the anode side of a rectifying element is connected to a rotating body on the heating side and the cathode side of the rectifying element is grounded in order to create a difference between the surface potential of the rotating body on the heating side and the surface potential of the roller that is a pressure roller, so that a negatively charged toner image transferred onto a sheet does not adhere to the rotating body on the heating side.

[0004] Patent Document 2 describes a fixing device in which two diodes are connected to a heating roller and a pressure roller respectively, and a positive charge is accumulated on the heating roller and a negative charge is accumulated on the pressure roller by the diodes so that a positively charged developer transferred onto a sheet does not adhere to the heating roller. In this fixing device, the positive charge and the negative charge accumulated on the heating roller and the pressure roller respectively are accumulated until they exceed the reverse breakdown voltage of the diode, and when they exceed the reverse breakdown voltage of the diode, they flow all at once towards the ground.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in the image forming apparatus described in Patent Document 1 and the fixing device described in Patent Document 2, since the direction of the rectifying element is arranged in the direction of storing negative charges, negative charges are accumulated on the pressure rotating body side, and when the accumulated negative charges exceed the reverse voltage withstand of the rectifying element, they flow into a grounding point such as the ground of the fixing device all at once, so noise has occurred at the grounding point.

[0007] An object of the present application is to provide a technique capable of reducing noise generated at a grounding point compared to the prior art by reducing the amount of negative charges flowing from the pressure rotating body through the rectifying element toward the ground.

Means for Solving the Problems

[0008] To achieve the above object, the fixing device of the present application is a fixing device that fixes a developer image formed on a sheet to the sheet, and includes a heater having a substrate and a resistive heating element to which an alternating voltage is applied and formed on the substrate, and an endless belt that rotates around the heater. A heating unit, a pressure rotating body that forms a nip portion by sandwiching the belt between the heater, the pressure rotating body having a conductive shaft extending in the rotation axis direction of the pressure rotating body and an elastic layer covering the outer peripheral surface of the shaft; A rectifying element having a cathode connected to the shaft of the pressure rotating body and an anode connected to the belt of the heating unit, and a grounding circuit having one end connected to the anode of the rectifying element and the other end connected to the ground, wherein a capacitor element and a first resistor element are connected in series between the one end and the other end, and a second resistor element is connected in parallel to the capacitor element and the first resistor element. It is characterized by having a grounding circuit.

[0009] In the fixing device of the present application, since the direction of the rectifying element is the direction of allowing current to flow rather than the direction of storing negative charges, the negative charges generated by the sheet conveyed to the fixing device flow from the rectifying element toward the ground circuit. The negative charges generated by the friction between the sheet conveyed to the fixing device and the elastic layer (e.g., made of rubber) of the pressure rotating body temporarily increase. The temporarily increased negative charges due to friction can pass through the capacitive element. The amount of the negative charges that have passed through the capacitive element is reduced by passing through the first resistance element connected in series to the capacitive element, so that the noise generated at the grounding location such as the ground of the fixing device can be reduced.

[0010] Also, in the fixing device of the present application, the negative charges that constantly flow toward the pressure rotating body via the sheet do not accumulate in the capacitive element but flow toward the ground through the second resistance element. Since the amount of the negative charges is reduced by passing through the second resistance element, the noise generated at the grounding location such as the ground of the fixing device can be reduced.

[0011] Further, the heating unit includes a holder that holds the heater with the substrate surface of the substrate facing the inner peripheral surface of the belt and guides the belt, and a conductive stay that holds the holder inside the belt. The stay is electrically connected to the anode of the rectifying element and connected to one end of the ground circuit. Thus, since the stay is connected to the ground circuit, the electric charges flowing from the stay can be made to flow into the ground circuit.

[0012] Further, the image forming apparatus of the present application is an image forming apparatus including the fixing device of the present application, and includes a photosensitive drum, a charger for charging the photosensitive drum, a developing device for supplying a developer to an electrostatic latent image formed on the charged photosensitive drum to form a developer image, a transfer member disposed opposite to the photosensitive drum for transferring the developer image onto a sheet passing through a transfer nip portion between the photosensitive drum, a high-voltage power supply board for applying a charging voltage to the charger and a transfer voltage to the transfer member respectively, and a control unit. The control unit uses the high-voltage power supply board to apply a charging voltage of a first polarity, which is opposite in polarity to the charge generated by the sheet being conveyed and the nip portion of the fixing device, to the charger, supplies a positively charged developer charged to the first polarity from the developing device to the photosensitive drum to form a developer image, and when executing an image forming process of transferring the formed developer image onto the sheet, uses the high-voltage power supply board to apply a transfer voltage of a second polarity, which is the same in polarity as the charge generated by the sheet being conveyed and the nip portion of the fixing device, to the transfer member.

[0013] As described above, since the image forming apparatus of the present application includes the fixing device of the present application, the image forming apparatus of the present application can also obtain the same effects as the fixing device of the present application.

[0014] Also, the length from the transfer nip portion between the photosensitive drum and the transfer member to the nip portion of the fixing device is shorter than the length of the sheet being conveyed in the sheet conveyance direction, the polarity of the surface potential of the pressure rotating body generated by the rectifying element is the same as the second polarity of the transfer voltage, and the high-voltage power supply board applies a transfer voltage greater than the voltage of the second polarity generated on the pressure rotating body by the rectifying element to the transfer member.

[0015] When the distance between the nip portion between the photosensitive drum and the transfer member and the nip portion of the fixing device is shorter than the length of the sheet being conveyed in the sheet conveyance direction, when the high-voltage power supply board applies a transfer voltage of the second polarity, a charge of the second polarity constantly flows toward the pressure rotating body via the sheet. This is because the surface potential of the pressure rotating body by the rectifying element is about -(minus) several hundred V, and the transfer voltage of the second polarity is about -(minus) several thousand V.

[0016] In the image forming apparatus of the present application, the charges of the second polarity flowing through the pressurized rotating body pass through the rectifying element and flow toward the ground via the second resistance element. In this way, the constantly flowing negative charges can be made to flow through the second resistance element and toward the ground without being accumulated by the capacitive element, so that noise can be prevented from occurring.

[0017] Further, the control unit detects a transfer current value flowing between the transfer member and the photosensitive drum using the high-voltage power supply board, and controls the transfer voltage output by the high-voltage power supply board so that the detected transfer current value becomes the target current value. This makes it possible to control the transfer current value flowing between the transfer member and the photosensitive drum to the target current value.

[0018] The fixing device further includes a temperature sensor that detects the temperature of the heater, and the image forming apparatus further includes a switching element that switches an AC voltage input from an external commercial power supply via a power cable, and a low-voltage power supply board that supplies the AC voltage switched by the switching element to a resistance heating element of a heating unit of the fixing device. The low-voltage power supply board is connected to an external ground via a power cable, and the other end of the grounding circuit is connected to the external ground via the low-voltage power supply board.

[0019] When the low-voltage power supply board is configured to be connected to an external ground via a power cable, it is affected by noise generated at the grounding location of the fixing device. Therefore, with the configuration of the rectifying element and the grounding circuit as in the configuration of the present application, noise generated at grounding locations such as the ground of the fixing device can be reduced.

[0020] Further, the resistance value of the first resistance element is smaller than the resistance value of the second resistance element, and the resistance value of the first resistance element is a value in the range of 1 kΩ to 1000 kΩ. This makes it possible to sufficiently reduce the amount of negative charges flowing through the capacitive element by the first resistance element in the range of 1 kΩ to 1000 kΩ, and then flow the negative charges to a grounding location such as the ground of the fixing device.

[0021] Also, the resistance value of the first resistance element is smaller than the resistance value of the second resistance element, and the resistance value of the second resistance element is in the range of 10 MΩ to 100 MΩ. As a result, the negative charges flowing constantly can flow through the second resistance element to a grounding point such as the ground of the fixing device without being blocked by the capacitance element. As a result, it is possible to reduce the generation of noise at a grounding point such as the ground of the fixing device.

[0022] Also, the anode of the rectifying element is electrically connected to the belt of the heating unit via a static eliminator brush. Since it is electrically connected to the belt via the static eliminator brush, when the belt rotates, the static eliminator brush can contact the belt appropriately, so that the belt can be appropriately connected to the grounding circuit.

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0024] Hereinafter, embodiments of the present application will be described in detail with reference to the drawings.

[0025] FIG. 1 is a cross-sectional view showing the schematic configuration of a monochrome laser printer 1 according to an embodiment of the present application. The monochrome laser printer 1 is an example of an image forming apparatus. Hereinafter, the monochrome laser printer 1 will be simply referred to as the printer 1. In the following description, as shown in FIG. 1, the description will be based on the direction seen from the user who uses the printer 1. That is, in FIG. 1, the right side is referred to as "front", the left side as "rear", the upper side as "upper", the lower side as "lower", the front side as "left", and the back side as "right".

[0026] As shown in FIG. 1, the printer 1 includes a main body housing 2, a sheet supply unit 3, a process unit PR, and a fixing device 8.

[0027] The sheet supply unit 3 is a mechanism for supplying the sheet S to the process unit PR, and is provided in the lower part of the main body housing 2. The sheet supply unit 3 includes a supply tray 31 for accommodating the sheet S, a sheet pressing plate 32, and a sheet supply mechanism 33. The sheet supply mechanism 33 includes a pickup roller 33A, a separation roller 33B, a first conveyance roller 33C, and a registration roller 33D. In the sheet supply unit 3, the sheet S in the supply tray 31 is brought close to the pickup roller 33A by the sheet pressing plate 32 and sent to the separation roller 33B by the pickup roller 33A. The sheet S is separated into single sheets by the separation roller 33B and conveyed by the first conveyance roller 33C. After aligning the positions of the leading ends of the sheet S, the registration roller 33D conveys the sheet S toward the process unit PR. Here, the direction in which the sheet S is conveyed is referred to as the conveyance direction, and the direction orthogonal to the conveyance direction within the plane of the sheet S is referred to as the width direction. Hereinafter, the width direction of the sheet S will be simply referred to as the "width direction".

[0028] The process unit PR has a function of forming a toner image on the sheet S supplied from the sheet supply unit 3. The process unit PR is a toner image forming unit. The process unit PR includes an exposure device 4 and a process cartridge 5.

[0029] The exposure device 4 is disposed at the upper part inside the main body housing 2 and includes a laser light source (not shown), a polygon mirror (shown with symbols omitted), a lens, a reflecting mirror, etc. In the exposure device 4, laser light based on image data is emitted from the laser light source as shown by the dashed-dotted line and scanned on the surface of the photosensitive drum 61, thereby exposing the surface of the photosensitive drum 61.

[0030] The process cartridge 5 is disposed below the exposure device 4 and is detachable from the main body housing 2 through an opening formed when the front cover 21 provided on the main body housing 2 is opened. The process cartridge 5 includes a drum unit 6 and a developing unit 7.

[0031] The drum unit 6 includes a photosensitive drum 61, a charger 62, and a transfer roller 63. The developing unit 7 is detachable from the drum unit 6 and includes a developing roller 71, a supply roller 72, a layer thickness regulating blade 73, a toner storage portion 74 for storing toner which is dry toner, and an agitator 75.

[0032] In the process cartridge 5, after the surface of the photosensitive drum 61 is uniformly positively charged by the charger 62, it is exposed by the laser light from the exposure device 4, thereby forming an electrostatic latent image based on image data on the photosensitive drum 61. Also, the toner in the toner storage portion 74 is supplied to the developing roller 71 through the supply roller 72 while being agitated by the agitator 75. The toner enters between the developing roller 71 and the layer thickness regulating blade 73 as the developing roller 71 rotates and is carried on the developing roller 71 as a thin layer of a certain thickness.

[0033] The toner carried on the developing roller 71 is supplied from the developing roller 71 to the electrostatic latent image formed on the photosensitive drum 61. As a result, the electrostatic latent image is visualized, and a toner image is formed on the photosensitive drum 61. Then, the sheet S supplied from the sheet supply unit 3 is conveyed through the transfer nip TN between the photosensitive drum 61 and the transfer roller 63, so that the toner image formed on the photosensitive drum 61 is transferred onto the sheet S.

[0034] The fixing device 8 fixes the toner image on the sheet S. The fixing device 8 includes a heating unit 81 and a pressure roller 82. One of the heating unit 81 and the pressure roller 82 is biased against the other by a biasing mechanism (not shown). In the fixing device 8, the sheet S onto which the toner image has been transferred is conveyed between the heating unit 81 and the pressure roller 82, so that the toner image is thermally fixed on the sheet S. The sheet S on which the toner image has been thermally fixed is discharged onto the discharge tray 22 by the second conveying roller 23 and the discharge roller 24. The detailed configuration of the fixing device 8 will be described later with reference to FIGS. 3 and 4.

[0035] Next, the electrical configuration of the printer 1 will be described with reference to FIG. 2. As shown in FIG. 2, the printer 1 further includes a control unit 101, a ROM 102, a RAM 103, a non-volatile memory 104, a pre-registration sensor SE1, a post-registration sensor SE2, and a discharge sensor SE3. The control unit 101, the ROM 102, the RAM 103, and the non-volatile memory 104 are mounted on the main board 100. Further, a main board ground portion 107 is provided on the main board 100. The main board ground portion 107 is a member that serves as the reference potential for the circuit operation of the mounted components such as the control unit 101.

[0036] The control unit 101 is constituted by, for example, an ASIC and performs overall control of each part of the printer 1. The control unit 101 is electrically connected to the ROM 102, the RAM 103, the non-volatile memory 104, the main motor 108, the pre-registration sensor SE1, the post-registration sensor SE2, the fixing device 8, the low-voltage power supply board 110, and the high-voltage power supply board 112.

[0037] The ROM 102 stores various control programs for controlling the printer 1, various settings, etc.

[0038] The RAM 103 is used as a work area from which various control programs are read and a storage area for temporarily storing image data included in a print job. The control unit 101 controls each part of the printer 1 while storing the processing results in the RAM 103 or the non-volatile memory 104 according to the control programs read from the ROM 102 and the signals output from various sensors.

[0039] The main body housing 2 is provided with an inlet 200. The inlet 200 is for inputting an AC voltage (for example, AC 100V) supplied by an external commercial power supply. A power cable 210 having a power plug P is connected to the inlet 200. The power plug P consists of three terminals including an earth terminal, and the power cable 210 consists of three cores: a live-side cable PCL, a neutral-side cable PCN, and an earth cable EC. Then, the live-side cable PCL and the neutral-side cable PCN are connected to the connector 110A of the low-voltage power supply board 110 via the inlet 200. Also, the earth cable EC is connected to the ground portion 110C of the low-voltage power supply board 110 via the inlet 200. The ground portion 110C of the low-voltage power supply board 110 is a member that serves as a reference potential for the circuit operation of mounted components such as the zero-crossing circuit 120 (see FIG. 5) described later.

[0040] By connecting the earth cable EC to the ground portion 110C of the low-voltage power supply board 110, the ground portion 110C of the low-voltage power supply board 110 is grounded. Then, the main board ground portion 107 is connected to the ground portion 110C of the low-voltage power supply board 110 by a sheet metal member (not shown) and becomes a member that serves as a reference potential for the circuit operation of mounted components such as the control unit 101.

[0041] As another example, the low-voltage power supply board 110 is covered with a box-shaped metal enclosure, and the ground portion 110C of the low-voltage power supply board 110 is connected to the enclosure. Then, by connecting the ground cable EC to the enclosure, the enclosure may be configured to be grounded.

[0042] The live-side terminal T11 of the connector 110A is connected to the live-side terminal T21 of the connector 110B via the relay 110E, and the neutral-side terminal T12 of the connector 110A is connected to the neutral-side terminal T22 of the connector 110B via the switching element 110F. Then, the live-side terminal T21 of the connector 110B is connected to one power supply terminal PT1 of the heater 90, and the neutral-side terminal T22 of the connector 110B is connected to the other power supply terminal PT2 of the heater 90.

[0043] The control unit 101 outputs a control signal toward the low-voltage power supply board 110. The relay 110E is for turning on / off the AC100V input. The control unit 101 performs on / off control of the relay 110E by the control signal. The switching element 110F is an element including, for example, a triac or the like. The control unit 101 controls the on / off of the switching element 110F by the control signal. The control unit 101 performs phase control and frequency control of AC100V by the control signal to adjust the amount of electric power supplied to the heater 90. The specific circuit configuration including the relay 110E and the switching element 110F will be described later with reference to FIG. 5.

[0044] Also, an AC-DC conversion circuit 110D is provided on the low-voltage power supply board 110. The AC100V input to the connector 110B of the low-voltage power supply board 110 is also input to the AC-DC conversion circuit 110D (not shown), and the AC-DC conversion circuit 110D converts the AC100V into a DC voltage, for example, DC24V. The low-voltage power supply board 110 outputs the converted DC24V to the main board 100 and the high-voltage power supply board 112.

[0045] The high-voltage power supply board 112 boosts the DC 24V from the low-voltage power supply board 110 to generate a high voltage. The high-voltage power supply board 112 is provided with a charging voltage application circuit 112A, a developing voltage application circuit 112B, and a transfer voltage application circuit 112C. The charging voltage application circuit 112A applies a positive-polarity charging voltage to the charger 62. The charging voltage is, for example, from 5 kV to 8 kV. The developing voltage application circuit 112B applies a positive-polarity developing voltage to the developing roller 71. The developing voltage is, for example, from 300 V to 500 V.

[0046] The transfer voltage application circuit 112C applies a transfer voltage to the transfer roller 63. The control unit 101 controls the transfer voltage using the transfer voltage application circuit 112C so that the current flowing through the transfer roller 63 becomes a predetermined current value. At this time, the transfer voltage is a negative-polarity voltage and has a value of about minus (-) several thousand volts.

[0047] The high-voltage power supply board 112 is further provided with a ground portion 112D that serves as a reference potential for the circuit operation of the mounted components such as the charging voltage application circuit 112A, the developing voltage application circuit 112B, and the transfer voltage application circuit 112C. The ground portion 112D of the high-voltage power supply board 112 is connected to the ground portion 110C of the low-voltage power supply board 110 and a sheet metal member (not shown). Therefore, the ground portion 112D of the high-voltage power supply board 112 is a member that serves as a reference potential for the circuit operation of the mounted components such as the charging voltage application circuit 112A.

[0048] The main motor 108 transmits a driving force to the roller group 35, the pressure roller 82, and the photosensitive drum 61. The roller group 35 includes a pickup roller 33A, a first conveyance roller 33C, a registration roller 33D, a developing roller 71, a second conveyance roller 23, a discharge roller 24, and the like. When the control unit 101 drives the main motor 108, a driving force is transmitted to the roller group 35, the pressure roller 82, and the photosensitive drum 61. Then, the roller group 35, the pressure roller 82, and the photosensitive drum 61 rotate in the direction of conveying the sheet S in the conveying direction.

[0049] The pre-registration sensor SE1 is a sensor that is arranged upstream of the registration roller 33D in the conveyance path 40 and detects the passage of the sheet S. As the pre-registration sensor SE1, a sensor having an actuator that swings when the sheet S abuts, an optical sensor, or the like can be used. The pre-registration sensor SE1 outputs an on signal when the sheet S is passing and an off signal when the sheet S is not passing. The detection signal from the pre-registration sensor SE1 is output to the control unit 101.

[0050] The post-registration sensor SE2 is a sensor that is arranged upstream of the fixing device 8 in the conveyance path 40, specifically, between the registration roller 33D and the transfer roller 63, and detects the passage of the sheet S. The post-registration sensor SE2 has the same configuration as the pre-registration sensor SE1. The detection signal from the post-registration sensor SE2 is output to the control unit 101.

[0051] The discharge sensor SE3 is arranged between the nip portion NP and the second conveyance roller 23 in the conveyance path 40 and detects the passage of the sheet S. The discharge sensor SE3 has the same configuration as the pre-registration sensor SE1. The detection signal from the discharge sensor SE3 is output to the control unit 101.

[0052] FIG. 3 shows a schematic configuration of the fixing device 8. As shown in FIG. 3(a), the fixing device 8 includes a heating unit 81, a pressure roller 82 as an example of a pressure rotating body, a static elimination brush 83, a diode 85 which is a rectifying element, and a grounding circuit 87. One of the heating unit 81 and the pressure roller 82 is biased with respect to the other by a biasing mechanism (not shown).

[0053] The heating unit 81 includes a heater 90, a belt 81A, a holder 81B, and a stay 81C. The heater 90 is a flat heater that heats the sheet S via the belt 81A and is supported by the holder 81B. The structure of the heater 90 will be described later based on FIG. 4.

[0054] The holder 81B is made of, for example, resin, and contacts the inner peripheral surface 81A1 of the belt 81A to guide the belt 81A. The stay 81C is a member that supports the holder 81B, and is formed by bending a plate material having higher rigidity than the holder 81B, such as a steel plate, into a U-shaped cross section.

[0055] The belt 81A is an endless belt having heat resistance and flexibility, and is made of metal or resin. The heater 90, the holder 81B, and the stay 81C are arranged inside the belt 81A, and the belt 81A is configured to rotate around the heater 90 and the holder 81B. The inner peripheral surface 81A1 of the belt 81A contacts the heater 90, and the outer peripheral surface 81A2 of the belt 81A contacts the pressure roller 82 or the sheet S.

[0056] As shown in FIG. 3(b), the pressure roller 82 has a columnar shaft 82A and a cylindrical roller portion 82B. The shaft 82A is made of, for example, metal. The roller portion 82B, which is an example of an elastic layer, is an elastic layer that covers a part of the shaft 82A and is made of, for example, rubber. The pressure roller 82 forms a nip portion NP (see FIG. 3(a)) for heating and pressurizing the sheet S by sandwiching the belt 81A with the heater 90. ·Pressurizing.

[0057] The cathode of the diode 85 is connected to the shaft 82A of the pressure roller 82 via the first sheet metal member PL1. The anode of the diode 85 is connected to one terminal T1 of the ground circuit 87 via the second sheet metal member PL2.

[0058] The static elimination brush 83 is a member for eliminating static electricity from the belt 81A, and is composed of a brush portion 83A made of a plurality of conductive fibers. And, as shown in Fig. 3(b), the brush portion 83A contacts the surface of the longitudinal end of the belt 81A. One end 83B, which is on the opposite side of the brush portion 83A of the static elimination brush 83, is connected to the second sheet metal member PL2 via the third sheet metal member PL3. The end 83B of the static elimination brush 83 is connected to one terminal T1 of the grounding circuit 87 via the third sheet metal member PL3 and the second sheet metal member PL2. Note that the first to third sheet metal members PL1 to PL3 are not limited to sheet metal members and may be conductive wires.

[0059] The grounding circuit 87 is a circuit for grounding (earthing) the belt 81A, which is the heating unit 81, and the pressure roller 82. The belt 81A is connected to the grounding circuit 87 via the static elimination brush 83, so that the surface potential of the belt 81A becomes approximately 0V.

[0060] The grounding circuit 87 is a circuit for grounding (earthing) the belt 81A, which is the heating unit 81, and the pressure roller 82. The grounding circuit 87 has two terminals T1 and T2. One terminal T1 is connected to the anode of the diode 85 via the second sheet metal member PL2 as described above, and the other terminal T2 is connected to the fixing ground portion 80.

[0061] The fixing ground portion 80 is electrically connected to the ground portion 110C of the low-voltage power supply board 110 via a sheet metal member (not shown). As a result, the fixing ground portion 80 is grounded (earthed), and the other terminal T2 of the grounding circuit 87 is grounded (earthed).

[0062] The grounding circuit 87 is configured by connecting a capacitor C and a first resistor R1 in series between one terminal T1 and the other terminal T2, and connecting the second resistor R2 in parallel therewith. The resistance value of the first resistor R1 is, for example, in the range of 1 kΩ to 1000 kΩ, and the resistance value of the second resistor R2 is, for example, in the range of 10 MΩ to 100 MΩ. Depending on the size of the first resistor R1 or the second resistor element, the voltage generated between the terminal T1 and the fixing ground portion 80 becomes a value close to 0V.

[0063] The belt 81A is connected to the grounding circuit 87 via the static eliminator brush 83, so that the surface potential of the belt 81A becomes approximately 0V. The pressure roller 82 is connected to the grounding circuit 87 via the diode 85, and the cathode of the diode 85 is connected to the pressure roller 82. Depending on the connection direction of the diode 85 and the value of the reverse breakdown voltage of the diode 85, the surface potential of the pressure roller 82 is offset from the potential of the fixing ground portion 80 by the reverse breakdown voltage of the diode 85, becoming approximately several hundred V negative (minus). The function of the grounding circuit 87 will be described later with reference to FIGS. 6 and 7.

[0064] FIG. 4 shows a schematic configuration of the heater 90. FIG. 4(a) is a plan view of the heater 90, and FIG. 4(b) is a cross-sectional view taken along the A-A arrow in FIG. 3(b). As shown in FIG. 4(a), the heater 90 includes a substrate 91, a resistance heating element 92 composed of a first resistance heating element 92A and a second resistance heating element 92B, conductive wires 93A to 93C, and power supply terminals PT1 and PT2.

[0065] The substrate 91 is made of an elongated rectangular plate of ceramic with aluminum oxide as the material. On the surface 91A of the substrate 91, a linear first resistance heating element 92A and a second resistance heating element 92B are formed in parallel, for example, by printing copper paste. The first resistance heating element 92A and the second resistance heating element 92B each extend in the longitudinal direction of the substrate 91, which is parallel to the axial direction of the roller portion 82B, and are formed apart from each other in the short direction orthogonal to the longitudinal direction. The length of the first resistance heating element 92A and the second resistance heating element 92B in the longitudinal direction is longer than the length in the width direction of the sheet S.

[0066] One end 92A1 of the first heating resistor 92A is connected to the power supply terminal PT1 via the conducting wire 93A, and one end 92B1 of the second heating resistor 92B is connected to the power supply terminal PT2 via the conducting wire 93B. And, the other end 92A2 of the first heating resistor 92A and the other end 92B2 of the second heating resistor 92B are connected via a U-shaped conducting wire 93C. The power supply terminals PT1 and PT2 are for inputting AC 100V. When AC 100V is input from the power supply terminals PT1 and PT2, the first heating resistor 92A and the second heating resistor 92B generate heat. Note that the first heating resistor 92A and the second heating resistor 92B are sealed by the glass material 94.

[0067] Two thermistors TH1 and TH2 are provided in contact with the back surface 91B of the substrate 91. One thermistor TH1 is provided in contact with approximately the center in the longitudinal direction of the back surface 91B of the substrate 91, and the other thermistor TH2 is provided in contact with the vicinity of one end in the longitudinal direction of the back surface 91B of the substrate 91 (see Fig. 3(b)). The thermistors TH1 and TH2 detect the temperature of the heater 90 and output it as a temperature signal to the control unit 101. The control unit 101 controls the temperature of the heater 90 to reach the target temperature based on the temperature signals from the thermistors TH1 and TH2. Note that the number of the thermistors TH1 and TH2 is not limited to two, and may be one or three or more.

[0068] The heater 90 is supported by the holder 81B such that its surface 91A faces the inner peripheral surface 81A1 of the belt 81A (see Fig. 3(a)).

[0069] Fig. 5 shows the relationship among the heater 90, the low-voltage power supply board 110, and the main board 100. As shown in Fig. 5, the low-voltage power supply board 110 has, in addition to the relay 110E and the switching element 110F described above with reference to Fig. 2, a fuse FS, a zero-cross circuit 120, etc.

[0070] Between the live-side terminal T11 and the neutral-side terminal T12 of the connector 110A of the low-voltage power supply board 110, a fuse FS, a relay 110E, a heater 90, and a switching element 110F are connected. Specifically, the live-side terminal T11 of the connector 110A and one power supply terminal PT1 of the heater 90 are connected via a first connection line L11 and the live-side terminal T21 of the connector 110B. And, a fuse FS and a relay 110E are connected on the first connection line L11. Also, the other power supply terminal PT2 of the heater 90 is connected to the switching element 110F via the neutral-side terminal T22 of the connector 110B and a second connection line L12. And, the switching element 110F is connected to the neutral-side terminal T12 of the connector 110A via a third connection line L13.

[0071] The fuse FS blows when an overcurrent flows through the heater 90, preventing further overcurrent from flowing through the heater 90. The relay 110E, as described above, turns on / off the input of AC100V, which is an AC voltage from the connector 110A, based on a control signal.

[0072] The switching element 110F performs a switching operation on the AC voltage applied to the heater 90 based on a control signal from the control unit 101. For example, a triac can be cited as the switching element 110F. A triac is a semiconductor element and is a switching element that switches an AC voltage showing a voltage waveform including a positive polarity and a negative polarity.

[0073] The zero-cross circuit 120 is composed of a diode bridge circuit 121 and a signal output circuit 122. The zero-cross circuit 120 is a circuit for detecting the zero-cross point of an AC voltage.

[0074] The diode bridge circuit 121 has four diodes, namely the first to fourth diodes D1 to D4, and is configured by connecting the cathode of the first diode D1 to the anode of the second diode D2 to form a first series-connected diode, and connecting the cathode of the third diode D3 to the anode of the fourth diode D4 to form a second series-connected diode, and connecting them in parallel. And the connection point J1 between the cathode of the first diode D1 and the anode of the second diode D2 is connected to the first connection line L11 via the fourth connection line L14, and the connection point J2 between the cathode of the third diode D3 and the fourth diode D4 is connected to the third connection line L13 via the fifth connection line L15.

[0075] And the input side of the signal output circuit 122 is connected between the connection point J3 between the anode of the second diode D2 and the anode of the fourth diode D4 and the connection point J4 between the cathode of the first diode D1 and the cathode of the third diode D3 via the sixth connection line L16.

[0076] The input voltage Vin is an alternating voltage applied between the live side terminal T11 and the neutral side terminal T12, that is, AC100V. When the input voltage Vin is applied to the live side terminal T11 and the neutral side terminal T12, the zero-crossing circuit 120, particularly the signal output circuit 122, outputs an output signal voltage to the terminal Vout of the control unit 101. The output signal voltage input to this terminal Vout is the zero-crossing signal output by the zero-crossing circuit 120. The control unit 101 detects the zero-crossing point of the alternating voltage based on the zero-crossing signal input to the terminal Vout and outputs a control signal to the switching element 110F.

[0077] FIG. 6 is a diagram for explaining the functions of the diode 85 and the grounding circuit 87 included in the fixing device 8, and shows a state where the leading end of the sheet S is conveyed to the nip portion NP of the fixing device 8 with a toner image formed on the sheet S.

[0078] Note that FIG. 6(a) shows the connection state of the diode 85 in the present embodiment, and FIG. 6(b) shows the connection state of the diode 85' in the fixing device as a comparative example. The diode 85' shown in FIG. 6(b) has its anode connected to the shaft 82A and its cathode connected to the ground circuit 87. That is, the diode 85' is connected in the reverse direction to the diode 85 shown in FIG. 6(a).

[0079] Since the surface of the photoreceptor drum 61 is positively charged in the range of 5 kV to 8 kV, a positive developing voltage in the range of 300 V to 500 V is also applied to the developing roller 71, and a negative transfer voltage with a digit in the minus (-) kV range is applied to the transfer roller 63, as shown in FIG. 6, a toner image by positively charged toner is formed on the sheet S. In both cases of FIG. 6(a) which is the present embodiment and FIG. 6(b) which is the comparative example, since the belt 81A is directly connected to the ground circuit 87, the surface potential of the belt 81A becomes approximately 0 V.

[0080] In the case of FIG. 6(a) which is the present embodiment, the diode 85 is arranged in such a direction that positive charges do not flow from the pressure roller 82 toward the fixing ground portion 80, and negative charges flow from the pressure roller 82 toward the fixing ground portion 80. The surface potential of the pressure roller 82 becomes minus (-) several hundred V by the ground circuit 87 and the diode 85.

[0081] In the case of FIG. 6(b) which is the comparative example, the diode 85' is arranged in such a direction that positive charges flow from the pressure roller 82 toward the fixing ground portion 80, and negative charges do not flow from the pressure roller 82 toward the fixing ground portion 80. Since the pressure roller 82 is configured to accumulate negative charges in the direction of the diode 85', the surface potential of the pressure roller 82 becomes minus (-) several thousand V by the ground circuit 87 and the diode 85'.

[0082] In both cases of FIGS. 6(a) and 6(b), since the surface potential of the belt 81A is approximately 0V and the surface potential of the pressure roller 82 is a negative potential, the positively charged toner on the sheet S does not adhere to the belt 81A of the heating unit 81. Also, in terms of the positively charged toner not adhering to the belt 81A, when comparing FIGS. 6(a) and 6(b), it is more difficult for the toner to adhere in the comparative example of FIG. 6(b) than in the present embodiment of FIG. 6(a).

[0083] When the leading edge of the sheet S is conveyed to the nip portion NP of the fixing device 8, a negative charge is temporarily generated by the friction between the leading edge of the sheet S and the roller portion 82B of the pressure roller 82. The negative charge temporarily increased by the friction flows to the fixing ground portion 80 via the diode 85 and the ground circuit 87 as shown in FIG. 6(a). The negative charge temporarily increased by the friction finally flows to the ground of the external commercial power supply via the fixing ground portion 80, the inlet 200, and the earth cable EC.

[0084] The diode 85' in FIG. 6(b) which is a comparative example is connected to the pressure roller 82 in the opposite direction to the direction of the diode 85 in FIG. 6(a). In the case of FIG. 6(b), the negative charge temporarily increased by the friction does not flow in the direction of the ground circuit 87 via the diode 85', but is once accumulated in the pressure roller 82.

[0085] If the negative charge continues to accumulate in the pressure roller 82, the amount of the accumulated negative charge may exceed the reverse breakdown voltage of the diode 85'. In that case, the negative charge accumulated in the pressure roller 82 will flow into the fixing ground portion 80 all at once via the diode 85' and the ground circuit 87. Then, the accumulated negative charge will also flow into the ground of the external commercial power supply all at once via the fixing ground portion 80, the inlet 200, and the earth cable EC. Therefore, in the case of FIG. 6(b) which is a comparative example, when the negative charge accumulated in the pressure roller 82 flows into the fixing ground portion 80 all at once exceeding the diode 85', noise will be generated in the fixing ground portion 80, the inlet 200, and the earth cable EC.

[0086] In the case of FIG. 6(a) which is this embodiment, the temporarily increased negative charge generated by the friction between the tip of the sheet S and the roller portion 82B of the pressure roller 82 flows to the fixing ground portion 80 via the diode 85 and the ground circuit 87. Therefore, unlike FIG. 6(b) which is a comparative example, the negative charge does not flow into the fixing ground portion 80 all at once.

[0087] In the description of FIG. 3 mentioned above, it was explained that the ground circuit 87 is configured by connecting a capacitor C and a first resistor R1 in series and connecting them in parallel with a second resistor R2. In the ground circuit 87 of FIG. 3, the resistance value of the first resistor R1 is, for example, a value in the range from 1 kΩ to 1000 kΩ, and the resistance value of the second resistor R2 is, for example, a value in the range from 10 MΩ to 100 MΩ. The resistance value of the second resistor R2 is larger than the resistance value of the first resistor R1 and has a different order of magnitude. Since the resistance value of the second resistor R2 is much larger than the resistance value of the first resistor R1 in terms of a different order of magnitude, the negative charge generated by the friction between the tip of the sheet S and the roller portion 82B of the pressure roller 82 does not flow through the second resistor R2 but flows to the side where the capacitor C and the first resistor R1 are connected in series. The temporarily increased negative charge due to friction can pass through the capacitor C of the ground circuit 87 and, after passing through the capacitor C, will pass through the first resistor R1 in the range from 1 kΩ to 1000 kΩ. The amount of the temporarily increased negative charge due to friction becomes sufficiently small by passing through the first resistor R1, making it difficult for noise to occur in the fixing ground portion 80 and the like. Therefore, as described in FIG. 6(a), in the configuration of this embodiment, it is possible to make it difficult for noise to occur at the grounding locations such as the fixing ground portion 80.

[0088] FIG. 7(a) is a diagram showing a case where a sheet S being conveyed exists between the transfer nip TN between the photoreceptor drum 61 and the transfer roller 63 and the nip portion NP of the fixing device 8. FIG. 7(b) is a comparative example, and the configuration of the ground circuit 87' is different from the configuration of the ground circuit 87 in FIG. 7(a).

[0089] Further, FIG. 7(a) shows a state where the rear end side of the conveyed sheet S is nipped by the photoreceptor drum 61 and the transfer roller 63, and the front end side of the conveyed sheet S is nipped by the belt 81A and the pressure roller 82 simultaneously. As described above, since the surface potential of the pressure roller 82 by the diode 85 is several hundred V negative (minus), and the transfer voltage is about several thousand V negative (minus), when the transfer voltage application circuit 112C applies a negative-polarity transfer voltage to the transfer roller 63, a steady negative charge flows from the transfer roller 63 through the sheet S toward the pressure roller 82.

[0090] At this time, the negative charge that has flowed to the pressure roller 82 passes through the diode 85 and flows to the fixing ground portion 80 via the ground circuit 87. The negative charge that steadily flows from the pressure roller 82 to the ground circuit 87 through the sheet S flows to the fixing ground portion 80 via the second resistor R2 without passing through the capacitor C.

[0091] FIG. 7(b) illustrated as a comparative example shows a ground circuit 87' in which the capacitor C and the first resistor R1 excluding the second resistor R2 from the ground circuit 87 are connected in series. In the case of the ground circuit 87' in FIG. 7(b), the capacitor C allows the temporarily increased negative charge generated by friction to pass through, but the steadily flowing negative charge will be blocked. The negative charge is blocked by the capacitor C. If the negative charge continues to be blocked, the negative charge will flow to a location not assumed by the printer 1, for example, a location where the potential of the printer 1 is low, and noise will occur at the unassumed location.

[0092] The grounding circuit 87 shown in FIGS. 3(a) and 7(a) forms a route through which a negative charge flows from the pressure roller 82 via the second resistor R2 due to the presence of the second resistor R2. Therefore, the grounding circuit 87 in FIGS. 3(a) and 7(a) cannot block the negative charge constantly flowing from the pressure roller 82 with the capacitor C. The grounding circuit 87 in FIGS. 3(a) and 7(a) can flow the negative charge constantly flowing from the pressure roller 82 to the fixing ground portion 80 via the second resistor R2. As a result, it is possible to make it difficult for noise to occur at the grounding location such as the fixing ground portion 80.

[0093] In particular, since the resistance value of the second resistor R2 is, for example, in the range of 10 MΩ to 100 MΩ, the amount of the negative charge constantly flowing is made sufficiently small by the second resistor R2 and flows to the fixing ground portion 80.

[0094] FIG. 8 shows a fixing device 8' having a connection mode partially different from that of the fixing device 8 in FIG. 3. The fixing device 8' in FIG. 8 is different from the fixing device 8 in FIG. 3 in that the stay 81C is grounded. Specifically, the stay 81C and the third sheet metal member PL3 are connected via the fourth sheet metal member PL4. Thereby, the stay 81C is grounded by the connection of the fourth sheet metal member PL4, the third sheet metal member PL3, the grounding circuit 87, and the fixing ground portion 80. Since the stay 81C is made of metal as described above, it is preferably grounded.

[0095] As described above, the fixing device 8 of the present embodiment is a fixing device 8 that fixes the toner image formed on the sheet S to the sheet S, and includes a heater 90 having a substrate 91 and a resistance heating element 92 to which an alternating voltage is applied and formed on the substrate 91, and an endless belt that rotates around the heater 90. A pressure roller 82 that forms a nip portion NP by sandwiching a belt 81A between a heating unit 81 having a roller 81A and a heater 90, the pressure roller 82 having a conductive shaft 82A extending in the rotational axis direction of the pressure roller 82 and a roller portion 82B covering the outer peripheral surface of the shaft 82A; a diode 85 having a cathode connected to the shaft 82A of the pressure roller 82 and an anode connected to the belt 81A of the heating unit 81; and a grounding circuit 87 having one end T1 connected to the anode of the diode 85 and the other end T2 connected to the ground, wherein a capacitor C and a first resistor R1 are connected in series between the one end T1 and the other end T2, and a second resistor R2 is connected in parallel to the capacitor C and the first resistor R1.

[0096] Thus, in the fixing device 8 of the present embodiment, since the direction of the diode 85 is not the direction of storing negative charges but the direction of flowing them, the negative charges generated by the sheet S conveyed to the fixing device 8 flow from the diode 85 toward the grounding circuit 87. The negative charges generated by the friction between the leading end of the sheet S conveyed to the fixing device 8 and the roller portion 82B (e.g., made of rubber) of the pressure roller 82 temporarily increase. The temporarily increased negative charges due to friction can pass through the capacitor C. Since the amount of the negative charges that have passed through the capacitor C is reduced by passing through the first resistor R1 connected in series to the capacitor C, noise generated at a grounding location such as the fixing ground portion 80 of the fixing device 8 can be reduced.

[0097] Also, in the fixing device 8 of the present embodiment, the negative charges that constantly flow toward the pressure roller 82 via the sheet S do not accumulate in the capacitor C but flow through the second resistor R2 toward the fixing ground portion 80. Since the amount of the negative charges is reduced by passing through the second resistor R2 as the negative charges constantly flow, noise generated at a grounding location such as the fixing ground portion 80 of the fixing device 8 can be reduced.

[0098] Further, the heating unit 81 includes a holder 81B that holds the heater 90 while opposing the substrate surface of the substrate 91 to the inner peripheral surface of the belt 81A and guides the belt 81A, and a conductive stay 81C that holds the holder 81B inside the belt 81A. The stay 81C is electrically connected to the anode of the diode 85 and connected to one end T1 of the ground circuit 87. As a result, the stay 81C is also grounded. By electrically connecting the stay 81C to the ground circuit 87, the electric charge flowing from the stay 81C can be made to flow through the ground circuit 87.

[0099] As described above, in the printer 1 including the fixing device 8, a photosensitive drum 61, a charger 62 that charges the photosensitive drum 61, a developing roller 71 that supplies toner to an electrostatic latent image formed on the charged photosensitive drum 61 to form a toner image, a transfer roller 63 that is disposed opposite to the photosensitive drum 61 and transfers the toner image to a sheet S passing through a transfer nip TN between the photosensitive drum 61 and the sheet S, a high-voltage power supply board 112 that applies a charging voltage to the charger 62 and a transfer voltage to the transfer roller 63, and a control unit 101. The control unit 101 uses the high-voltage power supply board 112 to apply a positive charging voltage having a polarity opposite to the charge polarity generated by the conveyed sheet S and the nip portion NP of the fixing device 8 to the charger 62, supplies positively charged forward-charged toner from the developing roller 71 to the photosensitive drum 61 to form a toner image, and when executing an image forming process of transferring the formed toner image to the sheet S, uses the high-voltage power supply board 112 to apply a negative transfer voltage having the same polarity as the charge polarity generated by the conveyed sheet and the nip portion NP of the fixing device 8 to the transfer roller 63.

[0100] Thus, since the printer 1 of the present embodiment includes the fixing device 8 of the present embodiment, the printer 1 of the present embodiment can also obtain the same effects as the fixing device 8 of the present embodiment.

[0101] Further, the length from the transfer nip TN between the photoreceptor drum 61 and the transfer roller 63 to the nip portion NP of the fixing device 8 is shorter than the length of the conveyed sheet S in the sheet conveyance direction, the polarity of the surface potential of the pressure roller 82 generated by the diode 85 is the same as the negative polarity of the transfer voltage, and the high-voltage power supply board 112 applies a transfer voltage larger than the negative-polarity voltage generated on the pressure roller 82 by the diode 85 to the transfer roller 63.

[0102] When the length between the transfer nip TN between the photoreceptor drum 61 and the transfer roller 63 and the nip portion NP of the fixing device 8 is shorter than the length of the conveyed sheet S in the sheet conveyance direction, when the high-voltage power supply board 112 applies a negative-polarity transfer voltage, negative charges constantly flow toward the pressure roller 82 via the sheet S. This is because the surface potential of the pressure roller 82 by the diode 85 is about -(minus) several hundred V, and the negative-polarity transfer voltage is about -(minus) several thousand V.

[0103] In the above configuration, the negative charges constantly flowing from the transfer roller 63 toward the pressure roller 82 via the sheet S pass through the diode 85 and flow toward a grounded location such as the fixing ground portion 80 via the second resistor R2. Thus, the negative charges constantly flowing from the transfer roller 63 toward the pressure roller can flow through the second resistor R2 and toward a grounded location such as the fixing ground portion 80 without being accumulated by the capacitor C. In the above configuration, noise can be prevented from occurring at a grounded location such as the fixing ground portion 80.

[0104] Further, the control unit 101 detects a transfer current value flowing between the transfer roller 63 and the photoreceptor drum 61 using the high-voltage power supply board 112, and controls the transfer voltage output by the high-voltage power supply board 112 so that the detected transfer current value becomes the target current value. Thereby, the transfer current value flowing between the transfer roller 63 and the photoreceptor drum 61 can be controlled to the target current value.

[0105] Further, the fixing device 8 further includes thermistors TH1 and TH2 that detect the temperature of the heater 90, and the printer 1 further includes a switching element 110F that switches an AC voltage input from an external commercial power supply via a power cable 210, and a low-voltage power supply board 110 that supplies the AC voltage switched by the switching element 110F to a resistive heating element of the heating unit 81 of the fixing device 8. The low-voltage power supply board 110 is connected to an external ground via the power cable 210, and the other end T2 of the ground circuit 87 is connected to the external ground via the low-voltage power supply board 110, which is characterized by this.

[0106] When the low-voltage power supply board 110 is configured to be connected to an external ground via the power cable 210, it is affected by the noise generated in the fixing ground portion 80. Therefore, with the configuration of the diode 85 and the ground circuit 87 as in the configuration of this embodiment, the noise generated at the grounding location such as the fixing ground portion 80 can be reduced.

[0107] The resistance value of the first resistor R1 is smaller than the resistance value of the second resistor R2, and the resistance value of the first resistor R1 is in the range of 1 kΩ to 1000 kΩ, which is characterized by this. The amount of negative charge flowing through the capacitor C becomes sufficiently small due to the first resistor R1 in the range of 1 kΩ to 1000 kΩ and then can flow to the fixing ground portion 80.

[0108] The resistance value of the first resistance element is smaller than the resistance value of the second resistance element, and the resistance value of the second resistor R2 is in the range of 10 MΩ to 100 MΩ, which is characterized by this. The negative charge that constantly flows from the transfer roller 63 toward the pressure roller 82 via the sheet S can flow to the fixing ground portion 80 via the second resistor R2 without being blocked by the capacitor C. As a result, it is possible to make it difficult for noise to occur in the fixing ground portion 80 and the like.

[0109] Further, the anode of the diode 85 is electrically connected to the belt 81A of the heating unit 81 via the static eliminator brush 83, which is characterized in that. Since it is electrically connected to the belt 81A via the static eliminator brush 83, when the belt 81A rotates, the static eliminator brush 83 can appropriately contact the belt 81A, so that the belt 81A can be appropriately connected to the ground circuit 87.

[0110] Note that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit thereof.

[0111] (1) In the above embodiment, the scorotron type charger is described as an example of the charger 62, but the charger 62 may be a charging roller.

[0112] (2) In the above embodiment, the monochrome laser printer 1 is described as an example of the image forming apparatus, but the present invention is not limited thereto, and a color laser printer may be used.

Description of Reference Numerals

[0113] 1... Monochrome laser printer (image forming apparatus), 61... Photoconductor drum, 62... Charger, 63... Transfer roller (transfer member), 71... Developing roller (developer), 80... Fixing ground portion, 81... Heating unit, 81A... Belt, 81B... Holder, 81C... Stay, 82... Pressing roller (pressing rotating body), 82A... Shaft, 82B... Roller portion (elastic layer), 83... Static eliminator brush, 85... Diode (rectifying element), 87... Ground circuit, 90... Heater, 91... Substrate, 92A... First resistance heating element, 92B... Second resistance heating element, 101... Control unit, 107... Main substrate ground portion, 110... Low voltage power supply substrate, 110C... Ground portion, 110F... Switching element, 112... High voltage power supply substrate, 112D... Ground portion, 210... Power cable, C... Capacitor (capacitance element), NP... Nip portion, PR... Process portion, R1... First resistance (first resistance element), R2... Second resistance (second resistance element), S... Sheet, TN... Transfer nip (transfer nip portion).

Claims

1. A fixing device for fixing a developer image formed on a sheet to the sheet, comprising: a heater having a substrate and a resistance heating element formed on the substrate to which an alternating voltage is applied; and a heating unit having an endless belt that rotates around the heater; a pressure rotating body that forms a nip portion by sandwiching the belt between the pressure rotating body and the heater, the pressure rotating body having a conductive shaft extending in the rotation axis direction of the pressure rotating body and an elastic layer covering the outer peripheral surface of the shaft; a rectifying element having a cathode connected to the shaft of the pressure rotating body and an anode connected to the belt of the heating unit; a grounding circuit having one end connected to the anode of the rectifying element and the other end connected to ground, wherein a capacitance element and a first resistance element are connected in series between the one end and the other end, and a second resistance element is connected in parallel to the capacitance element and the first resistance element; A fixing device characterized by comprising the above.

2. The heating unit includes: a holder that holds the heater with the substrate surface of the substrate facing the inner peripheral surface of the belt and guides the belt; a conductive stay that holds the holder inside the belt; and has The stay is electrically connected to the anode of the rectifying element and connected to the one end of the grounding circuit. The fixing device according to claim 1, characterized by the above.

3. An image forming apparatus including the fixing device according to claim 1 or 2, comprising: a photosensitive drum; a charger that charges the photosensitive drum; a developing device that supplies a developer to an electrostatic latent image formed on the charged photosensitive drum to form a developer image; a transfer member that is disposed opposite to the photosensitive drum and transfers the developer image to a sheet passing through a transfer nip portion between the photosensitive drum and the transfer member; a high-voltage power supply substrate that applies a charging voltage to the charger and a transfer voltage to the transfer member, respectively; a control unit; and includes The control unit is When executing an image forming process of applying a charging voltage of a first polarity having a polarity opposite to that of the charges generated by the conveyed sheet and the nip portion of the fixing device to the charger using the high-voltage power supply substrate, supplying a positively charged developer charged with the first polarity from the developing device to the photosensitive drum to form a developer image, and transferring the formed developer image to the sheet. Using the high-voltage power supply board, a transfer voltage of a second polarity that is the same as the polarity of the charge generated by the conveyed sheet and the nip portion of the fixing device is applied to the transfer member. An image forming apparatus characterized by the above.

4. The length from the transfer nip portion between the photoreceptor drum and the transfer member to the nip portion of the fixing device is shorter than the length of the conveyed sheet in the sheet conveyance direction. The polarity of the surface potential of the pressure rotating body generated by the rectifying element is the same as the second polarity of the transfer voltage. The high-voltage power supply board applies a transfer voltage greater than the voltage of the second polarity generated by the rectifying element to the pressure rotating body to the transfer member. The image forming apparatus according to claim 3, characterized by the above.

5. The control unit detects a transfer current value flowing between the transfer member and the photoreceptor drum using the high-voltage power supply board, and controls the transfer voltage output by the high-voltage power supply board so that the detected transfer current value becomes a target current value. The image forming apparatus according to claim 4, characterized by the above.

6. The fixing device further has a temperature sensor for detecting the temperature of the heater. The image forming apparatus further has a switching element that switches an AC voltage input from an external commercial power supply via a power cable, and a low-voltage power supply board that supplies the AC voltage switched by the switching element to a resistance heating element of the heating unit of the fixing device. The low-voltage power supply board is connected to an external ground via the power cable. The other end of the grounding circuit is connected to the external ground via the low-voltage power supply board. The image forming apparatus according to claim 5, characterized by the above.

7. The resistance value of the first resistance element is smaller than the resistance value of the second resistance element. The resistance value of the first resistance element is a value in the range of 1 kΩ to 1000 kΩ. The fixing device according to claim 1, characterized by the above.

8. The resistance value of the first resistance element is smaller than the resistance value of the second resistance element. The resistance value of the second resistance element is a value in the range of 10 MΩ to 100 MΩ. The fixing device according to claim 1, characterized by the above.

9. The anode of the rectifying element is electrically connected to the belt of the heating unit via a static eliminator brush. The fixing device according to claim 1, characterized by the above.

Images