JP6652465B2 - Photodetector and image forming apparatus - Google Patents

Description

  The present invention relates to a photodetector and an image forming apparatus, and is suitably applied to, for example, a toner density sensor that detects the density of toner transferred to a medium such as paper in an image forming apparatus.

  2. Description of the Related Art Conventionally, in an image forming apparatus, for example, an image is exposed on the surface of a photoconductor drum by a built-in exposure device, a toner is generated by attaching toner to the surface, and further transferred and fixed on a medium such as paper. Accordingly, those that form (ie, print) an image are widely used.

  Some of the image forming apparatuses have a toner density sensor that detects the density of toner attached to the surface of a photosensitive drum or the like. In such an image forming apparatus, for example, a measurement toner image including a measurement pattern is generated on the surface of the photosensitive drum, and the toner density in the measurement toner image is detected by a toner density sensor. Then, based on the detected toner density, the image forming apparatus can form an image in which the toner density is appropriately adjusted by increasing or decreasing the toner density in forming an image thereafter. .

  The toner density sensor has, for example, a light-emitting unit that emits light and a light-receiving unit that receives the light, irradiates light from the light-emitting unit toward the surface of the photosensitive drum, and reflects the light reflected at this time. The light is received by the light receiving unit, and a light receiving signal corresponding to the obtained light amount is generated. The image forming apparatus can detect the toner density by comparing the light receiving signal with a predetermined reference value.

  Among them, as a light receiving unit of the toner density sensor, for example, a current corresponding to the light amount is generated by a photodiode, and the current is converted into a voltage by a current-voltage conversion circuit using a first operational amplifier. A circuit that amplifies to a desired signal level by using an amplifier circuit has been proposed (for example, see Patent Document 1).

JP-A-2002-6683 (FIG. 6)

  However, in the toner concentration sensor having such a configuration, an operational amplifier is used for each of the current-voltage conversion circuit and the amplifier circuit, and it is necessary to appropriately arrange circuit elements such as resistors around the circuit. Problem.

  The present invention has been made in view of the above points, and has as its object to propose a light detection device and an image forming apparatus that can accurately detect light with a simple configuration.

In order to solve such a problem, a photodetector of the present invention includes a light-emitting portion that emits light and a light-receiving portion that receives light, and the light- emitting portion includes a semiconductor having a PN junction of a semiconductor based on an output voltage of a regulator. A light source emits light by supplying a current determined by a difference voltage value obtained by reducing a potential difference generated in the element and a resistance value of a resistor to which an output current from the regulator is supplied, to the light emitting diode. A light receiving element that receives light and generates a photo-induced current, a plurality of voltage dividing resistors connected in series at a connection point, and an inverting input terminal connected to the first terminal of the light receiving element A non-inverting input terminal is connected to the second terminal of the light receiving element, and an output terminal is connected to the ground through a plurality of voltage dividing resistors to amplify the photo-induced current; Connect to terminal Is, the other end is connected to the connection point, and to provide a conversion element for converting current into voltage.

  Further, in the image forming apparatus of the present invention, the above-described light detecting device and an image forming unit for forming an image by a developer are provided, and the light detecting device emits light from the light emitting unit to the image formed by the image forming unit. And the light reflected in the image is received by the light receiving unit.

In the present invention, in the light emitting unit, the current supplied to the light emitting diode is increased as the temperature rises by utilizing the fact that the semiconductor element has a negative temperature dependency in the light emitting unit, so that the light emitting amount of the light emitting diode is kept constant. can do. Further, according to the present invention, in the light receiving section, a voltage corresponding to a weak photo-induced current generated in the light receiving element can be generated by the operational amplifier, and the output voltage from the operational amplifier is divided by a plurality of voltage dividing resistors, and the connection point Then, the voltage can be amplified at an amplification factor according to each voltage dividing resistor and the resistance value of the conversion element by feeding back to the inverting input terminal via the conversion element.

  According to the present invention, it is possible to realize a light detection device and an image forming apparatus capable of detecting light with high accuracy with a simple configuration.

FIG. 2 is a schematic diagram illustrating a configuration of an image forming apparatus. FIG. 2 is a schematic diagram illustrating a configuration of an image forming unit. FIG. 3 is a schematic diagram illustrating a configuration of a toner density sensor. FIG. 2 is a circuit diagram illustrating a configuration of a light emitting circuit according to the first embodiment. FIG. 2 is a circuit diagram illustrating a configuration of a light receiving circuit according to the first embodiment. FIG. 4 is a schematic diagram illustrating changes in current and voltage with respect to temperature change. FIG. 9 is a circuit diagram illustrating a configuration of a light emitting circuit according to a second embodiment. FIG. 11 is a circuit diagram illustrating a configuration of a light emitting circuit according to a third embodiment. FIG. 14 is a circuit diagram illustrating a configuration of a light receiving circuit according to a fourth embodiment. FIG. 14 is a circuit diagram illustrating a configuration of a light receiving circuit according to a fifth embodiment. FIG. 21 is a schematic diagram illustrating various signal waveforms according to the fifth embodiment. FIG. 13 is a schematic diagram illustrating changes in current and voltage with respect to a temperature change in another embodiment. FIG. 9 is a circuit diagram illustrating a configuration of a light emitting circuit according to another embodiment. FIG. 9 is a circuit diagram illustrating a configuration of a light emitting circuit according to another embodiment. FIG. 9 is a circuit diagram illustrating a configuration of a light emitting circuit according to another embodiment. FIG. 14 is a schematic diagram illustrating various signal waveforms according to another embodiment.

  Hereinafter, embodiments for carrying out the invention (hereinafter, referred to as embodiments) will be described with reference to the drawings.

[1. First Embodiment]
[1-1. Configuration of Image Forming Apparatus]
As shown in FIG. 1, the image forming apparatus 1 according to the first embodiment is a so-called MFP (Multi Function Peripheral), which has a printer function for forming (ie, printing) an image on paper as a medium. It has a function as an image scanner for reading an image and a communication function. Therefore, by combining these functions, the image forming apparatus 1 can operate as a printer, a copying machine (copier), a facsimile machine, or the like. When functioning as a printer, the image forming apparatus 1 can print a desired color image on a sheet P having a size such as A3 size or A4 size.

  In the image forming apparatus 1, various components are arranged inside a printer housing 2 formed in a substantially box shape. In the following, the right end portion in FIG. 1 is defined as the front of the image forming apparatus 1, and the vertical direction, the left and right direction, and the front and rear direction when viewed facing the front are defined and described.

  The image forming apparatus 1 is entirely controlled by the control unit 3. The control unit 3 is connected to a host device (not shown) such as a computer device by wireless or wired. When image data representing an image to be printed is provided from the higher-level device and printing of the image data is instructed from the higher-level device, the control unit 3 executes a printing process for forming a print image on the surface of the paper P.

  A paper storage cassette 4 that stores paper P is provided at the lowermost portion in the printer housing 2. A paper feed unit 5 is provided above and in front of the paper storage cassette 4. The paper supply unit 5 includes a hopping roller 6 disposed on the front upper side of the paper storage cassette 4, a conveyance guide 7 for guiding the paper P upward along the conveyance path W, and registration rollers 8 opposed to each other across the conveyance path W. And a pinch roller 9 and the like.

  The paper feed unit 5 picks up the papers P stored in the paper storage cassette 4 while separating them one by one by appropriately rotating each roller based on the control of the control unit 3, and conveys the papers. The guide 7 is moved forward and upward along the transport path W by the guide 7, and then folded back and upward to come into contact with the registration roller 8 and the pinch roller 9. The rotation of the registration roller 8 is appropriately suppressed. By applying a frictional force to the sheet P between the registration roller 8 and the pinch roller 9, a so-called skew in which the side of the sheet P is inclined with respect to the traveling direction. After the correction, the leading and trailing edges are aligned left and right, and then sent backward.

  On the rear side of the registration roller 8 and the pinch roller 9, a conveyance path W is formed substantially along the front-rear direction, and the middle conveyance section 10 is disposed below the conveyance path W. The intermediate transport unit 10 includes a front roller 11 disposed on the front side, a rear roller 12 disposed on the rear side, and a transport belt 14 including an endless belt stretched around a lower roller 13 disposed on the lower side. It is the configuration that was done. Above the front roller 11, a suction roller 15 is provided at a position opposed to the conveyance belt 14.

  When a driving force is transmitted to a rear roller 12 from a predetermined belt drive motor (not shown), the middle conveyance unit 10 runs the conveyance belt 14 by rotating the rear roller 12 in the direction of arrow R2. Let it. Accordingly, the transport belt 14 causes the upper portion along the transport path W, that is, the portion stretched between the front roller 11 and the rear roller 12 to travel rearward. At this time, when the paper P is delivered from the paper supply unit 5, the middle transport unit 10 nips the paper P between the suction roller 15 and the front roller 11 together with the transport belt 14, and places the paper P on the upper side of the transport belt 14. In this state, the paper P is caused to travel backward as the transport belt 14 travels.

  Four image forming units 16C, 16M, 16Y and 16K are arranged in order from the rear side to the front side on the upper side of the middle conveyance unit 10 and on the opposite side of the middle conveyance unit 10 across the conveyance path W. ing. The image forming units 16C, 16M, 16Y and 16K (hereinafter collectively referred to as the image forming unit 16) correspond to each color of cyan (C), magenta (M), yellow (Y) and black (K), respectively. However, only the colors are different, and all have the same configuration.

  The image forming unit 16 includes an image forming unit 31, a toner cartridge 32, and an LED (Light Emitting Diode) head 33, as shown in FIG. The transport belt 14 is interposed. The toner cartridge 32 contains toner as a developer, is disposed above the image forming unit 31, and is mounted above the image forming unit 31. The toner cartridge 32 supplies the stored toner to the toner storage unit 34 of the image forming unit 31.

  The image forming section 31 incorporates a supply roller 35, a developing roller 36, a regulating blade 37, a photosensitive drum 38, and a charging roller 39 in addition to the toner storage section 34. The supply roller 35 is formed in a cylindrical shape with a central axis extending in the left-right direction, and an elastic layer made of a conductive urethane rubber foam or the like is formed on a peripheral side surface thereof. The developing roller 36 is formed in a cylindrical shape with a central axis extending in the left-right direction, and an elastic layer having elasticity, a surface layer having conductivity, and the like are formed on a peripheral side surface thereof.

  The regulating blade 37 is made of, for example, a stainless steel plate having a predetermined thickness. A part of the regulating blade 37 is in contact with the peripheral side surface of the developing roller 36 while being slightly elastically deformed. The photoreceptor drum 38 is formed in a cylindrical shape with the central axis extending in the left-right direction, and a thin-film charge generation layer and a charge transport layer are sequentially formed on the peripheral side surface thereof so that the photosensitive drum 38 can be charged. . The charging roller 39 is formed in a cylindrical shape with a central axis extending in the left-right direction, and a peripheral side surface thereof is covered with a conductive elastic body. The peripheral side surface is brought into contact with the peripheral side surface of the photosensitive drum 38. ing.

  Further, a static elimination light source 20 is provided at a position on the front lower side of the image forming unit 31 and at a position upstream of a contact point between the photosensitive drum 38 and the transport belt 14. The static elimination light source 20 removes the charged static electricity by irradiating the photosensitive drum 38 with predetermined light.

  The image forming unit 31 rotates the supply roller 35, the developing roller 36, and the charging roller 39 in the direction of the arrow R2 (counterclockwise in the figure) by supplying a driving force from a drum motor (not shown), The body drum 38 is rotated in the direction of arrow R1 (clockwise in the figure). Further, the image forming section 31 applies a predetermined bias voltage to each of the supply roller 35, the developing roller 36, the regulating blade 37, and the charging roller 39, thereby charging each of them.

  The supply roller 35 causes the toner in the toner storage unit 34 to adhere to the peripheral side surface by charging, and causes the toner to adhere to the peripheral side surface of the developing roller 36 by rotation. After excess toner has been removed from the peripheral surface by the regulating blade 37, the developing roller 36 contacts the peripheral surface of the photosensitive drum 38.

  On the other hand, the charging roller 39 uniformly charges the peripheral side surface of the photosensitive drum 38 by making contact with the photosensitive drum 38 in a charged state. The LED head 33 has a plurality of LED chips arranged linearly in the left-right direction. The LED head 33 emits light at predetermined time intervals in a light emission pattern based on image data supplied from the control unit 3 (FIG. 2). Thus, the photosensitive drum 38 is exposed. As a result, an electrostatic latent image is formed on the peripheral side surface of the photosensitive drum 38 near the upper end thereof.

  Subsequently, the photosensitive drum 38 rotates in the direction of the arrow R <b> 1, thereby bringing the portion where the electrostatic latent image is formed into contact with the developing roller 36. As a result, toner adheres to the peripheral side surface of the photosensitive drum 38 based on the electrostatic latent image, and the toner image based on the image data is developed.

  The transfer roller 17 is located directly below the photosensitive drum 38, and sandwiches the upper portion of the transport belt 14 between the vicinity of the upper end on the peripheral side surface and the vicinity of the lower end of the photosensitive drum 38. The transfer roller 17 is rotated in a direction indicated by an arrow R2 by a predetermined bias voltage being applied and a driving force supplied from a drum motor (not shown). Thus, when the paper P is being transported along the transport path W, the image forming unit 16 can transfer the toner image developed on the peripheral side surface of the photosensitive drum 38 to the paper P.

  In this way, each image forming unit 16 moves rearward while sequentially transferring and superimposing toner images of respective colors on the sheet P conveyed from the front along the conveyance path W.

  Incidentally, when the toner density detection processing is performed based on the control of the control unit 3, the image forming unit 16 forms a predetermined toner density detection image on the surface of the photosensitive drum 38 in a state where the paper P is not conveyed. This is transferred to the surface of the conveyor belt 14. As a result, the toner constituting the toner density detection image adheres to the surface of the transport belt 14.

  In the middle transport section 10 (FIG. 2), a toner density sensor 18 is provided diagonally below the rear roller 12 and a cleaning section 19 is provided below the lower roller 13. The toner density sensor 18 detects the toner density in the toner density detection image attached to the surface of the transport belt 14 when the toner density detection processing is performed based on the control of the control unit 3. (Details will be described later).

  The cleaning unit 19 includes a plate-shaped cleaning blade 19B pressed against the outer peripheral surface of the transport belt 14, and a waste toner container 19C disposed below the cleaning blade 19B. The cleaning unit 19 is configured to control the toner adhered to the transport belt 14 due to a transport failure of the paper P when the image forming process is performed or the toner of the toner density detection image transferred to the transport belt 14 in the toner density detection process. Is scraped off from the surface of the conveyor belt 14 to be cleaned. As a result, in the middle transport unit 10, toner adheres to the back surface of the paper P to be transported next, that is, the surface facing downward in the transport path W and on which the toner image is not transferred, so that the so-called back surface is used. The reflection can be prevented.

  A fixing unit 21 is provided near the rear end of the middle transport unit 10. The fixing unit 21 includes a heating roller 21A and a pressure roller 21B that are arranged to face each other with the conveyance path W interposed therebetween. The heating roller 21A is formed in a cylindrical shape with the central axis directed in the left-right direction, and a heater is provided inside. The pressing roller 21B is formed in the same cylindrical shape as the heating roller 21A, and presses the upper surface against the lower surface of the heating roller 21A with a predetermined pressing force.

  The fixing unit 21 heats the heating roller 21A and rotates the heating roller 21A and the pressing roller 21B in predetermined directions based on the control of the control unit 3. Thus, the fixing unit 21 applies heat and pressure to the paper P received from the middle transport unit 10, that is, the paper P on which the four color toner images are transferred in a superimposed manner, fixes the toner, and transfers the paper to the rear.

  A paper discharge unit 22 is disposed behind the fixing unit 21. The paper discharge unit 22 is configured by a combination of a guide for guiding the paper P, a plurality of transport rollers, and the like, similarly to the paper supply unit 5. The paper discharge unit 22 conveys the paper P delivered from the fixing unit 21 rearward and upward, and then folds forward by appropriately rotating each conveyance roller under the control of the control unit 3. The sheet is discharged to a discharge tray 2T formed on the upper surface.

  Further, paper sensors 25, 26, 27 and 28 for detecting the paper P are appropriately provided at a plurality of locations along the transport path W in the printer housing 2. The paper sensor 25 and the like respectively detect the presence or absence of the paper P in the transport path W, and notify the control unit 3 of the obtained detection result. In response to this, the control unit 3 appropriately controls the rotation of each transport roller, the traveling of the transport belt 14 in the middle transport unit 10, and the like.

[1-2. Configuration of Toner Density Sensor]
Next, the configuration of the toner density sensor 18 will be described. As shown in FIG. 3, the toner concentration sensor 18 is attached to an attachment member 40 fixed to the printer housing 2. The toner density sensor 18 includes a light emitting unit 42 that emits the detection light L1 toward the transport belt 14 inside the sensor housing 41, a color light receiving unit 43C that receives the detection light L1 reflected by the transport belt 14, and A monochrome light receiving unit 43M (hereinafter, also collectively referred to as a light receiving unit 43) is provided.

  The sensor housing 41 is configured such that, when viewed from behind, the light emitting section 42 is arranged such that the optical axis XE of the detection light L1 in the light emitting section 42 forms a predetermined angle θ (for example, 75 degrees) with respect to the surface of the conveyor belt 14. Is appropriately adjusted, and the mounting angle with respect to the mounting member 40 is also appropriately adjusted.

  Further, in the sensor housing 41, when viewed from the rear, the monochrome axis of the light emitting section 42 and the optical axis XM of the monochrome light receiving section 43M are symmetrical to each other with the normal 14N of the conveyor belt 14 interposed therebetween. The mounting angle and the mounting position of the light receiving unit 43M are adjusted. For this reason, in the toner density sensor 18, when the detection light L1 emitted from the light emitting section 42 travels along the optical axis XE and is reflected in various directions on the surface of the transport belt 14, the mirror-reflected component is monochrome. It can reach the light receiving section 43M.

  Further, in the sensor housing 41, a color light receiving unit 43C is arranged on the opposite side of the light emitting unit 42 from the monochrome light receiving unit 43M. Specifically, in the sensor housing 41, the mounting position of the color light receiving unit 43C and the optical axis XC of the color light receiving unit 43C are set at a predetermined angle φ (for example, 30 degrees) with respect to the optical axis XE of the light emitting unit 42. The mounting angle and the like are appropriately adjusted. For this reason, in the toner density sensor 18, when the detection light L1 emitted from the light emitting unit 42 travels along the optical axis XE and is reflected in various directions on the surface of the transport belt 14, a part of the irregularly reflected component is removed. Light can be received by reaching the color light receiving section 43C.

  Further, the light emitting section 42 incorporates a light emitting circuit 44 to which a plurality of electronic components are appropriately connected. Similarly, the color light receiving section 43C and the monochrome light receiving section 43M (that is, the light receiving section 43) have a color light receiving circuit 45C and a monochrome light receiving circuit 45M (hereinafter collectively receiving light) which are appropriately connected to a plurality of electronic components. Circuit 45). Hereinafter, each circuit configuration of the light emitting circuit 44 and the light receiving circuit 45 will be described.

[1-2-1. Configuration of Light Emitting Circuit]
The light emitting circuit 44 includes a relatively small number of electronic components such as a three-terminal regulator 51, a resistor 52, a diode 53, and a light emitting diode (ie, LED) 54, as shown in the circuit diagram of FIG.

  When a power supply voltage is applied to the input terminal 51i, the three-terminal regulator 51 generates a constant output voltage Vo between the output terminal 51o and the ground terminal 51g, and generates a constant output voltage Vo according to the load connected to the output terminal 51o. The output current Io is output from the output terminal 51o.

  The input terminal 51i of the three-terminal regulator 51 is connected to a predetermined power supply circuit (not shown), and a predetermined power supply voltage VDD is supplied from the power supply circuit. The output terminal 51o is connected to the anode terminal of the diode 53 via a resistor 52 having a resistance value of Ro [Ω]. The diode 53 has a P-type semiconductor and an N-type semiconductor made of a semiconductor material such as silicon bonded to each other, and is configured as a semiconductor having a PN junction (a so-called silicon diode).

  The cathode terminal of the diode 53 is connected to the ground terminal 51g at the connection point N1 and to the anode terminal of the light emitting diode 54. The cathode terminal of the light emitting diode 54 is connected to the ground.

  With this configuration, when the predetermined power supply voltage VDD is supplied to the input terminal 51i, the light emitting circuit 44 outputs the output current Io from the output terminal 51o, and outputs the output voltage Vo between the output terminal 51o and the ground terminal 51g. Occurs. This output voltage Vo has a predetermined value in the three-terminal regulator 51. At this time, a ground terminal current Iss flows through the ground terminal 51g.

  The output current Io output from the output terminal 51o is supplied to the diode 53 via the resistor 52. At this time, a resistance voltage Vr obtained by multiplying the resistance value Ro and the output current Io is generated as a potential difference between both ends of the resistor 52. Since the diode 53 has a PN junction, the voltage drops by the forward voltage Vf. That is, the resistance voltage Vr at the resistor 52 corresponds to a voltage value of a difference obtained by subtracting the forward voltage Vf from the output voltage Vo of the three-terminal regulator 51.

  Further, the light emitting circuit 44 supplies a forward current If corresponding to the sum of the output current Io and the ground terminal current Iss to the light emitting diode 54 from the connection point N1 into which the output current Io and the ground terminal current Iss flow. Supply. When the forward current If is supplied, the light emitting diode 54 can emit the detection light L1 (FIG. 3) having a light amount corresponding to the magnitude of the forward current If.

  By the way, in the diode 53, it is known that the forward voltage Vf decreases as the ambient temperature increases due to the nature of the PN junction. In the light emitting circuit 44, although the forward voltage Vf of the diode 53 decreases as the ambient temperature increases, the output voltage Vo of the three-terminal regulator 51 is constant, so that the resistance voltage Vr applied to the resistor 52 is relatively low. , And the output current Io increases accordingly. That is, in the light emitting circuit 44, when the ambient temperature increases, the forward current If supplied to the light emitting diode 54 can be increased by increasing the output current Io.

  Here, the relationship between the respective values in the light emitting circuit 44 will be described in more detail. First, focusing on the current flowing through the connection point N1, the following equation (1) holds among the output current Io, the ground terminal current Iss, and the forward current If.

  Io + Iss = If (1)

  When the output current Io and the ground terminal current Iss in the three-terminal regulator 51 are compared, the following equation (2) holds.

  Iss << Io (2)

  Therefore, when the equation (1) is arranged for the forward current If, it can be expressed as the following equation (3).

If ≒ Io = Vr / Ro
= (Vo−Vf) / Ro (3)

Here, as a general property of the PN junction, it is known that the forward voltage Vf of the diode 53 changes, that is, decreases at a rate of about −2 [mV] with respect to a temperature rise of 1 [° C.]. Have been. That is, the forward voltage Vf of the diode 53 has temperature dependency, and its temperature coefficient is about −2 [mV / ° C.] = − 2 × 10 ⁻³ [V / ° C.]. For this reason, from equation (3), the forward current If has such a characteristic that it increases almost linearly with an increase in temperature.

  The degree of change in the forward current If with respect to a temperature rise is defined as a temperature coefficient Tc [% / ° C.]. Further, assuming that the fluctuation width of the temperature is the fluctuation temperature ΔT [° C.] and the fluctuation width of the forward current If is the fluctuation current ΔIf [A], the temperature coefficient Tc can be expressed as the following equation (4).

Tc = (1 / If) × (ΔIf / ΔT)
= 1 / (Vo−Vf) × (−ΔVf / ΔT) (4)

  In the equation (4), (ΔVf / ΔT) represents the temperature coefficient of the forward voltage at the PN junction of the diode 53, and is about −2 [mV / ° C.] as described above. It is known that the forward voltage Vf at the PN junction of the diode 53 is generally about 0.6 [V].

  Therefore, in the light emitting circuit 44, when the output voltage Vo [V] of the three-terminal regulator 51 is determined, the temperature coefficient Tc can be calculated by substituting this value into the equation (4) together with other values. In the light emitting circuit 44, the forward current If can be calculated by substituting the value of the output voltage Vo [V] into the equation (3) together with other values.

  Specifically, in the light emitting circuit 44, for example, three different numerical examples as shown below can be employed for the output voltage Vo [V] of the three-terminal regulator 51 and the resistance value Ro [Ω] of the resistor 52.

[Numerical example 1]
When the output voltage Vo of the three-terminal regulator 51 is set to 1.2 [V] and the resistance value Ro of the resistor 52 is set to 30 [Ω], the temperature coefficient Tc is calculated by using the above equation (4). (5) can be calculated.

Tc = 1 / (Vo−Vf) × (−ΔVf / ΔT)
= 1 / (1.2−0.6) × (2 × 10 ⁻³ )
≒ 3.3 × 10 ^-2 [/ ° C]
= 0.33 [% / ° C] (5)

  In this case, the forward current If can be calculated as in the following equation (6) by using the above equation (3).

If = (Vo−Vf) / Ro
= (1.2-0.6) / 30
= 0.02 [A] = 20 [mA] (6)

[Numerical example 2]
When the output voltage Vo of the three-terminal regulator 51 is set to 0.9 [V] and the resistance value Ro of the resistor 52 is set to 15 [Ω], the temperature coefficient Tc is calculated by using the above equation (4). (7) can be calculated.

Tc = 1 / (Vo−Vf) × (−ΔVf / ΔT)
= 1 / (0.9−0.6) × (2 × 10 ⁻³ )
≒ 6.6 × 10 ^-2 [/ ° C]
= 0.66 [% / ° C] (7)

  In this case, the forward current If can be calculated as in the following equation (8) by using the above equation (3).

If = (Vo−Vf) / Ro
= (0.9-0.6) / 15
= 0.02 [A] = 20 [mA] (8)

  That is, in Numerical Example 2, the temperature coefficient Tc can be made about twice as large as in Numerical Example 1 while equalizing the forward current If.

[Numerical example 3]
When the output voltage Vo of the three-terminal regulator 51 is set to 0.8 [V] and the resistance value Ro of the resistor 52 is set to 10 [Ω], the temperature coefficient Tc is calculated by using the above equation (4). (9) can be calculated.

Tc = 1 / (Vo−Vf) × (−ΔVf / ΔT)
= 1 / (0.8−0.6) × (2 × 10 ⁻³ )
≒ 1.0 × 10 ^-2 [/ ° C]
= 1.0 [% / ° C] (9)

  In this case, the forward current If can be calculated as in the following expression (10) by using the above expression (3).

If = (Vo−Vf) / Ro
= (0.8-0.6) / 10
= 0.02 [A] = 20 [mA] (10)

  That is, in Numerical Example 3, the temperature coefficient Tc can be made about three times as large as in Numerical Example 1, while making the forward current If equal.

  As described above, in the light emitting circuit 44, the forward current If supplied to the light emitting diode 54 can be increased at a degree corresponding to the temperature coefficient Tc as the ambient temperature increases. Further, in the light emitting circuit 44, by appropriately selecting the output voltage Vo and the resistance value Ro, the value of the temperature coefficient Tc is set from a relatively wide range, and the forward current is independent of the value of the temperature coefficient Tc. The value of If can be set arbitrarily.

[1-2-2. Configuration of light receiving circuit]
As shown in FIG. 5, the light receiving circuit 45 includes a color light receiving circuit 45C and a monochrome light receiving circuit 45M. The color light receiving circuit 45C and the monochrome light receiving circuit 45M have independent circuit configurations and are equivalent to each other. Therefore, hereinafter, the color light receiving circuit 45C will be described as an example.

  The color light receiving circuit 45C is roughly composed of a current-to-voltage converter 60C in the former stage and an amplifier 70 in the latter stage. The current-voltage converter 60C mainly includes a photodiode 61 that generates a current corresponding to the intensity of the received light, and an operational amplifier 62 having a non-inverting input terminal, an inverting input terminal, and an output terminal.

  The anode terminal and the cathode terminal of the photodiode 61 are connected to the non-inverting input terminal and the inverting input terminal of the operational amplifier 62 via connection points N11 and N12, respectively. A predetermined reference voltage Vref1 is supplied to the connection point N12. For convenience of explanation, the anode terminal and the cathode terminal of the photodiode 61 are hereinafter also referred to as a first terminal and a second terminal, respectively.

  The output terminal of the operational amplifier 62 is connected to the connection points N13 and N14. A variable resistor 63 and a resistor 64 are connected in series between the connection point N13 and the connection point N11. That is, in the current-voltage converter 60C, a part of the current output from the operational amplifier 62 is supplied to the inverting input terminal via the variable resistor 63 and the resistor 64, thereby forming a negative feedback circuit. The connection point N14 is an output terminal of the current-voltage converter 60C and an input terminal of the amplifier 70C.

  With this configuration, the current-to-voltage converter 60C converts the current flowing between the two terminals of the photodiode 61 into a voltage to generate an intermediate signal of the voltage Vo1, and outputs the intermediate signal to the connection point that is the output terminal of the current-to-voltage converter 60C. The signal can be supplied from N14 to the subsequent amplification section 70C.

  The amplifying unit 70C has the same configuration as the operational amplifier 62, and is configured around an operational amplifier 71 having a non-inverting input terminal, an inverting input terminal, and an output terminal. The non-inverting input terminal of the operational amplifier 71 is connected to one end of the resistor 72 via a connection point N15. The other end of the resistor 72 is connected to a connection point N14 which is an input terminal of the amplifier 70C, and is supplied with a voltage output from the current-voltage converter 60C. That is, the voltage output from the current-voltage converter 60C is input to the non-inverting input terminal of the operational amplifier 71 via the resistor 72. Further, a predetermined reference voltage Vref2 is supplied to a non-inverting input terminal of the operational amplifier 71 via a resistor 73.

  The output terminals of the operational amplifier 71 are connected to connection points N16, N17 and N18. The connection point N17 is grounded via the resistor 74. The connection point N16 is connected to the connection point N15 via a resistor 75. That is, in the amplifier 70C, a part of the current output from the operational amplifier 71 is supplied to the inverting input terminal via the resistor 75. The connection point N18 is an output terminal of the amplifier 70C.

  As described above, the amplifying unit 70C inverts the voltage of the intermediate signal using the ratio of the resistance value R72 [Ω] of the resistor 72 and the resistance value R75 [Ω] of the resistor 75, that is, the value (R75 / R72) as the amplification factor. It is an amplification circuit. Therefore, the amplifier 70C can generate a color light receiving signal having the voltage Vout1 by amplifying the intermediate signal of the voltage Vo1 supplied from the current-voltage converter 60C. The color light receiving signal generated at this time is supplied to the control unit 3 (FIG. 1).

  Similarly, the monochrome light receiving circuit 45M (FIG. 5) generates a monochrome light receiving signal having a voltage Vout2 corresponding to the magnitude of light received by the photodiode. The monochrome light receiving signal generated at this time is supplied to the control unit 3 (FIG. 1), like the color light receiving signal.

  In response to this, the control unit 3 performs various arithmetic processing such as comparing the voltages Vout1 and Vout2 of the color light receiving signal and the monochrome light receiving signal obtained from the color light receiving circuit 45C and the monochrome light receiving circuit 45M with predetermined reference values. By doing so, it is possible to detect the density of the toner adhering to the transport belt 14.

[1-3. Effects]
In the above configuration, the toner density sensor 18 of the image forming apparatus 1 according to the first embodiment has a configuration in which the light emitting circuit 44 (FIG. 4) is incorporated in the light emitting section 42.

  Here, for comparison with the light emitting circuit 44, a conventional light emitting circuit as shown in FIG. In a conventional light emitting circuit, a fixed LED driving current is supplied to a light emitting diode by a predetermined LED driving circuit or the like regardless of the ambient temperature.

  For this reason, in the conventional light emitting circuit, when the relationship between the temperature Ta and the LED driving current is graphed, the LED driving current has a constant value regardless of the temperature Ta, as shown in FIG. Therefore, in the conventional light emitting circuit, for example, the LED drive current I25 at 25 ° C. and the LED drive current I50 at 50 ° C. have the same value.

  However, a light emitting diode has a temperature dependency, and has a property of decreasing its own light emission when its surrounding temperature rises. For this reason, when the relationship between the temperature Ta and the light emission amount in the conventional light emitting circuit is graphed, the characteristic is such that the light emission amount decreases as the temperature Ta increases, as shown in FIG. Therefore, in the conventional light emitting circuit, for example, the light emission amount P50 at 50 ° C. is smaller than the light emission amount P25 at 25 ° C.

  Here, it is assumed that a conventional light receiving circuit corresponding to the conventional light emitting circuit can generate an output voltage of a magnitude corresponding to the amount of light at a fixed ratio regardless of the temperature. In this case, when the relationship between the temperature Ta and the output voltage in the light receiving circuit is graphed, the output voltage decreases as the temperature Ta increases as shown in FIG. 6C. Therefore, in the conventional light receiving circuit corresponding to the conventional light emitting circuit, for example, the output voltage V50 at 50 ° C. is smaller than the output voltage V25 at 25 ° C.

  As described above, in the conventional light emitting circuit, when the ambient temperature of the light emitting diode changes, the light emission amount changes, and the output voltage in the light receiving circuit may change accordingly. For this reason, in a conventional toner concentration sensor using a conventional light emitting circuit and a light receiving circuit, even if the toner concentration is constant, the output voltage changes in accordance with the temperature change, and the detection accuracy of the toner concentration is low. There was a possibility that it would drop significantly.

  On the other hand, in the light emitting circuit 44 (FIG. 4) according to the present embodiment, the resistor 52 and the diode 53 are connected between the output terminal 51o of the three-terminal regulator 51 and the connection point N1, and the connection point N1 is connected to the ground. The terminal 51g is connected, and the light emitting diode 54 is further connected to the connection point N1. In this light emitting circuit 44, when the output voltage Vo of the three-terminal regulator 51 becomes constant, the forward voltage Vf of the diode 53 decreases in accordance with the rise of the ambient temperature. , The forward current If supplied to the light emitting diode 54 can be increased.

  Therefore, in the light emitting circuit 44, when the relationship between the temperature Ta and the LED drive current (that is, the forward current If) is graphed, the temperature Ta rises as shown in FIG. 6D corresponding to FIG. , The LED drive current increases. Therefore, in the light emitting circuit 44, for example, the LED driving current I50 at 50 ° C. is larger than the LED driving current I25 at 25 ° C.

  Here, with respect to the light emitting circuit 44 (FIG. 4), the temperature characteristic of the light emitting diode 54, that is, the degree of decrease in the amount of light emission with the rise in temperature is measured in advance using a predetermined measuring device (not shown) or the like. Further, a temperature coefficient TcL [% / ° C.] representing the degree is calculated.

  After that, in the light emitting circuit 44, the value obtained by inverting the sign of the temperature coefficient TcL [% / ° C.] of the light emitting diode 54 is set as the temperature coefficient Tc of the forward current If, and then the relationship of the expression (4) is satisfied. Thus, the output voltage Vo of the three-terminal regulator 51 is selected. Further, in the light emitting circuit 44, the resistance value Ro of the resistor 52 is appropriately calculated using the relationship between the output voltage Vo and the above equation (3).

  For this reason, in the light emitting circuit 44, when the light emission amount of the light emitting diode 54 is about to decrease with the rise of the temperature Ta, this is canceled out and the forward current If increases as shown in FIG. The light emission amount of the light emitting diode 54 does not change. That is, when the relationship between the temperature Ta and the light emission amount in the light emitting circuit 44 is graphed, as shown in FIG. 6E corresponding to FIG. 6B, the light emission amount is almost independent of the value of the temperature Ta. The characteristic is such that a constant value is maintained.

  In other words, in the light emitting circuit 44, the LED drive current (that is, the forward current If) has a positive temperature coefficient, so that the LED drive current increases with the rise in temperature, and the light emission amount of the light emitting diode 54 Can be compensated for.

  Accordingly, when the relationship between the temperature Ta and the output voltage (that is, the voltage Vout1 of the color light receiving signal) in the light receiving circuit 45 (FIG. 5) is graphed, as shown in FIG. 6F corresponding to FIG. In addition, the characteristic is such that the output voltage maintains a substantially constant value regardless of the value of the temperature Ta. That is, in the light receiving circuit 45, for example, the output voltage V25 at 25 ° C. and the output voltage V50 at 50 ° C. have the same value.

  Accordingly, in the toner density sensor 18 (FIG. 3), for example, when the reflectance of the detection light L1 changes according to the density of the toner attached to the surface of the transport belt 14, and the amount of light received by the light receiving circuit 45 changes, The output voltage can be changed according to only the change in the amount of received light, that is, according to the density of the toner, without being affected by the temperature change.

  Therefore, the controller 3 (FIG. 1) can accurately detect the density of the toner adhered to the surface of the transport belt 14 without being affected by the temperature, and based on this, the image forming unit 16 (FIG. 2) The density of the toner adhered to the surface of the body drum 38 can be appropriately adjusted. As a result, the image forming apparatus 1 can form (ie, print) a high-quality image on the paper P.

  By the way, Patent Literature 1 described above discloses a configuration in which a thermistor is provided in a light receiving circuit for the purpose of temperature compensation for the light emission amount of a light emitting diode. Such a thermistor is required to have a so-called positive characteristic, that is, a characteristic of increasing a resistance value with an increase in temperature. As a thermistor having this positive characteristic, one using a platinum thin film is generally known, and its characteristic is defined as a Japanese Industrial Standard (JIS) C 1604 resistance thermometer.

  However, the temperature coefficient of such a thermistor is a fixed value of about 3000 [ppm / ° C.] = 0.3 [% / ° C.]. For this reason, even if this thermistor is incorporated in an amplifier circuit or the like, its temperature coefficient becomes a fixed value and is further limited to a relatively small value of about 0.3 [% / ° C.]. This is because the resistivity of the platinum thin film is relatively small due to the material properties of the platinum thin film. For example, although a resistance of several tens [kΩ] to several hundred [kΩ] is desirable, Can only obtain a resistance value of several hundred [Ω].

  On the other hand, the temperature coefficient Tc of the light-emitting diode 54 (FIG. 4) is determined according to the type and configuration of the compound semiconductor material to be configured, and is typically from −0.3 [% / ° C.] to −1.0 [%]. / ° C] which varies within a certain range. Therefore, in the conventional light receiving circuit, depending on the type of the light emitting diode, the platinum thin film thermistor cannot sufficiently perform the temperature compensation, or the type of the light emitting diode is limited because the thermistor performs the temperature compensation. There was a problem.

  On the other hand, in the present embodiment, in the light emitting circuit 44 (FIG. 4), the light emission amount of the light emitting diode 54 can be kept constant irrespective of the temperature change (FIG. 6E). Therefore, the light receiving circuit 45 used in combination with the light emitting circuit 44 does not require the addition of a temperature compensating thermistor or the like as in Patent Document 1, and can be configured relatively simply. There are no restrictions on the types of 54.

  From another viewpoint, the three-terminal regulator 51 of the light emitting circuit 44 (FIG. 4) can easily generate a constant output voltage Vo between the output terminal 51o and the ground terminal 51g by using one circuit component. And this can be maintained. On the other hand, the forward voltage Vf of the diode 53, which is a semiconductor having a PN junction, has a temperature dependency of about −2 [mV / ° C.].

  Therefore, in the light emitting circuit 44, the diode 53 is connected in series with the resistor 52 between the output terminal 51o for generating a constant output voltage Vo and the ground terminal 51g, so that the forward voltage Vf is increased as the temperature rises. The resistance voltage Vr was increased while the resistance was reduced. Therefore, in the light emitting circuit 44, when the temperature rises, the output current Io can be increased according to the increase in the resistance voltage Vr, and the forward current If supplied to the light emitting diode 54 can also be increased.

  That is, in the light emitting circuit 44, only four circuit components such as the three-terminal regulator 51, the resistor 52, the diode 53, and the light emitting diode 54 are combined. An extraordinary operation and effect can be obtained such that the light emission amount of the light emitting element can be kept constant.

  According to the configuration described above, when the forward voltage Vf of the diode 53 decreases in the light emitting circuit 44 due to a rise in temperature, the toner density sensor 18 of the image forming apparatus 1 according to the first embodiment changes the resistance voltage of the resistor 52. Vr increases, the output current Io increases, and the forward current If supplied to the light emitting diode 54 increases. Thus, in the light emitting circuit 44, by appropriately selecting the output voltage Vo and the resistance value Ro, the negative temperature dependency of the light emitting diode 54 can be compensated for by increasing the forward current If. The light emission amount of the light emitting device 54 can be kept constant regardless of the temperature.

[2. Second Embodiment]
The image forming apparatus 101 (FIG. 1) according to the second embodiment is different from the image forming apparatus 1 according to the first embodiment in that a toner density sensor 118 is provided instead of the toner density sensor 18, but is different. Is similarly configured. The toner density sensor 118 (FIG. 3) is different from the toner density sensor 18 according to the first embodiment in that the toner density sensor 118 has a light emitting circuit 144 instead of the light emitting circuit 44, but has the same configuration in other respects. Have been.

  As shown in FIG. 7 corresponding to FIG. 4, the light emitting circuit 144 has a similar three-terminal regulator 51 and a light emitting diode 54 as compared to the light emitting circuit 44 according to the first embodiment, while replacing the diode 53. It has a transistor 153, and has an emitter resistor 152 in place of the resistor 52.

  The input terminal 51i of the three-terminal regulator 51 is connected to a predetermined power supply circuit (not shown) as in the first embodiment, and a predetermined power supply voltage VDD is supplied from this power supply circuit. Unlike the first embodiment, the ground terminal 51g is connected to the ground. The output terminal 51o is connected to the base terminal of the transistor 153. The transistor 153 is an NPN-type bipolar transistor, and forms a PN junction similar to that of the diode 53 (FIG. 4) at the base and the emitter.

  The emitter terminal of the transistor 153 is connected to the ground via the emitter resistor 152. The collector terminal of the transistor 153 is connected to the cathode terminal of the light emitting diode 54. The anode terminal of the light-emitting diode 54 is connected to the above-described power supply circuit, and is supplied with the power supply voltage VDD.

  With this configuration, when the power supply voltage VDD is applied to the input terminal 51i of the three-terminal regulator 51, the light emitting circuit 144 outputs an output current Io from the output terminal 51o and supplies the output current Io to the base terminal of the transistor 153. Hereinafter, this output current Io is also referred to as base current Ib. At this time, the three-terminal regulator 51 generates an output voltage Vo between the output terminal 51o and the ground terminal 51g. This output voltage Vo has a predetermined value in the three-terminal regulator 51, as in the first embodiment.

  On the other hand, in the light emitting diode 54, the collector current Ic supplied from the power supply voltage VDD flows from the anode terminal to the cathode terminal. Thereafter, the collector current Ic is supplied to the collector terminal of the transistor 153. Transistor 153 outputs an emitter current Ie from the emitter terminal and supplies this to emitter resistor 152. At this time, in the transistor 153, a base-emitter voltage Vbe is generated between the base terminal and the emitter terminal.

  Here, the magnitude of the emitter current Ie output from the transistor 153 is equal to the sum of the input collector current Ic and base current Ib. On the other hand, the base current Ib has an extremely small value compared to the collector current Ic and the emitter current Ie due to the characteristics of the transistor 153, and can be substantially ignored. Therefore, the collector current Ic flowing through the light emitting diode 54 can be expressed as the following equation (11).

  Ic ≒ Ie = (Vo−Vbe) / Re (11)

  Further, the base and emitter portions of the transistor 153 are semiconductors having a PN junction as described above, and have the same properties as the diode 53 (FIG. 4) in the first embodiment. For this reason, the base-emitter voltage Vbe of the transistor 153 is about 0.6 [V], and the temperature dependency is reduced similarly to the case of the forward voltage Vf in the diode 53 (FIG. 4) in the first embodiment. And its temperature coefficient is about −2 [mV / ° C.].

  That is, in the light emitting circuit 144, when the temperature rises, the base-emitter voltage Vbe of the transistor 153 decreases. Then, in the light emitting circuit 144, since the output voltage Vo of the three-terminal regulator 51 is constant, the voltage applied between both terminals of the emitter resistor 152 increases, and the emitter current Ie increases accordingly. As a result, in the light emitting circuit 144, as in the first embodiment, the collector current Ic flowing through the light emitting diode 54 increases.

  This means that the equation (11) corresponds to the above-mentioned equation (3), that is, the collector current Ic, the base-emitter voltage Vbe and the resistance Re are equal to the forward current If, the forward voltage Vf and the resistance Ro. It is clear from the fact that they correspond to each other. In the light emitting circuit 144, the temperature coefficient Tc can be calculated as in the first embodiment by replacing the forward voltage Vf with the base-emitter voltage Vbe in the above equation (4).

  Therefore, like the light emitting circuit 44 according to the first embodiment, the light emitting circuit 144 can increase the collector current Ic to be supplied to the light emitting diode 54 at a degree determined by the temperature coefficient Tc as the ambient temperature increases. .

  In the light emitting circuit 144, similarly to the light emitting circuit 44 according to the first embodiment, by appropriately selecting the output voltage Vo [V] of the three-terminal regulator 51 and the resistance value Re [Ω] of the emitter resistor 152, The value of the temperature coefficient Tc can be set from a relatively wide range, and the value of the collector current Ic can be arbitrarily set independently of the value of the temperature coefficient Tc. Thus, the light emitting circuit 144 can maintain the light emission amount of the light emitting diode 54 substantially constant regardless of a change in temperature, as shown in FIG.

  As described above, the light emitting circuit 144 according to the present embodiment can achieve the same operation and effect as the light emitting circuit 44 according to the first embodiment.

  According to the above-described configuration, the toner density sensor 118 of the image forming apparatus 101 according to the second embodiment determines that the emitter resistance 152 of the transistor 153 decreases when the base-emitter voltage Vbe of the transistor 153 decreases in the light emitting circuit 144 due to a rise in temperature. The applied voltage increases, the emitter current Ie increases, and the collector current Ic supplied to the light emitting diode 54 increases. Thus, in the light emitting circuit 144, similarly to the first embodiment, the negative temperature dependency of the light emitting diode 54 can be compensated for by increasing the collector current Ic. Regardless, it can be kept constant.

[3. Third Embodiment]
The image forming apparatus 201 (FIG. 1) according to the third embodiment is different from the image forming apparatus 1 according to the first embodiment in that a toner density sensor 218 is provided instead of the toner density sensor 18. Is similarly configured. The toner density sensor 218 (FIG. 3) is different from the toner density sensor 18 according to the first embodiment in that the toner density sensor 218 has a light emitting circuit 244 instead of the light emitting circuit 44, but has the same configuration in other respects. Have been.

  As shown in FIG. 8 corresponding to FIGS. 4 and 7, the light emitting circuit 244 is similar to the light emitting circuit 44 according to the first embodiment in having a diode 53 and a light emitting diode 54. The difference is that a shunt regulator 251 and a resistor 252 are provided instead of the three-terminal regulator 51 and the resistor 52. Further, the light-emitting circuit 244 includes a resistor 255 and a transistor 256.

  As the shunt regulator 251, for example, TLV431 manufactured by Texas Instruments can be used. In this case, the reference voltage Vref, that is, the voltage between the cathode terminal and the reference terminal is about 1.2 [V].

  The shunt regulator 251 has a cathode terminal connected to the ground, and an anode terminal connected to one end of the resistor 255 and the base terminal of the transistor 256 via the connection point N4. The other end of the resistor 255 is connected to a power supply circuit (not shown) and an anode terminal of the light emitting diode 54 via a connection point N3, and a predetermined power supply voltage VDD is supplied.

  The cathode terminal of the light emitting diode 54 is connected to the collector terminal of the transistor 256. The transistor 256 is an NPN-type bipolar transistor, and its emitter is connected to the reference terminal of the shunt regulator 251 and one end of the resistor 252 via the connection point N5. The other end of the resistor 252 is connected to the anode terminal of the diode 53. The diode 53 has a cathode terminal connected to the ground.

  With this configuration, when the power supply voltage VDD is supplied to the light emitting circuit 244, a current is supplied from the connection point N3 to the resistor 255 and the light emitting diode 54 separately. The current flowing through the resistor 255 is divided into a cathode current Ik flowing through the cathode terminal of the shunt regulator 251 and a base current Ib flowing through the base terminal of the transistor 256 at the connection point N4.

  At this time, the shunt regulator 251 controls the reference voltage Vref, which is the voltage of the reference terminal with respect to the ground terminal, to be maintained at a constant value (for example, about 1.2 [V]). The magnitude of the reference current Ir supplied to N5 is changed.

  On the other hand, the transistor 256 supplies the connection point N5 with an emitter current Ie corresponding to the sum of the collector current Ic flowing through the light emitting diode 54 and the base current Ib supplied from the connection point N4. As a result, a current having a magnitude corresponding to the sum of the emitter current Ie and the reference current Ir flows through the resistor 252 and the diode 53.

  Here, in the light emitting circuit 244, due to the nature of the transistor 256, the base current Ib has a negligibly small value compared to the emitter current Ie. Further, in the light emitting circuit 244, the reference current Ir of the shunt regulator 251 also has a negligibly small value compared to the emitter current Ie.

  Therefore, focusing on the resistor 252, the collector current Ic flowing through the light emitting diode 54 can be expressed as the following equation (12).

  Ic ≒ Ie = (Vref−Vf) / Rs (12)

  Since the diode 53 is a semiconductor having a PN junction as in the first embodiment, the diode 53 has a forward voltage Vf of about 0.6 [V] and has temperature dependency. The temperature coefficient is about -2 [mV / ° C].

  That is, in the light emitting circuit 244, when the temperature rises, the forward voltage Vf of the diode 53 decreases as in the first embodiment. Then, in the light emitting circuit 244, since the reference voltage Vref of the shunt regulator 251 is constant, the voltage applied between both terminals of the resistor 252 increases. Then, in the light emitting circuit 244, the current flowing through the resistor 252, that is, the collector current Ic flowing through the light emitting diode 54 having substantially the same magnitude as the emitter current Ie increases according to the relationship of the expression (12).

  This means that equation (12) corresponds to equation (3) described above, that is, the collector current Ic, the reference voltage Vref, and the resistance value Rs correspond to the forward current If, the output voltage Vo, and the resistance value Ro, respectively. It is clear from that. In the light emitting circuit 244, the temperature coefficient Tc can be calculated as in the first embodiment by replacing the output voltage Vo with the reference voltage Vref in the above equation (4).

  Therefore, like the light emitting circuit 44 according to the first embodiment and the light emitting circuit 144 according to the second embodiment, the light emitting circuit 244 increases the collector current Ic supplied to the light emitting diode 54 as the ambient temperature increases, by the temperature coefficient. It can be increased at a degree corresponding to Tc.

  In the light emitting circuit 244, similarly to the light emitting circuit 44 according to the first embodiment, by appropriately selecting the reference voltage Vref [V] of the shunt regulator 251 and the resistance value Rs [Ω] of the resistor 252, the temperature coefficient is increased. The value of Tc can be set from a relatively wide range, and the value of the collector current Ic can be arbitrarily set independently of the value of the temperature coefficient Tc. Thus, the light emitting circuit 244 can maintain the light emission amount of the light emitting diode 54 substantially constant regardless of the change in temperature, as shown in FIG.

  As described above, the light emitting circuit 244 according to this embodiment can provide the same operation and effects as those of the light emitting circuit 44 according to the first embodiment and the light emitting circuit 144 according to the second embodiment.

  According to the above configuration, the toner density sensor 218 of the image forming apparatus 201 according to the third embodiment causes the light emitting circuit 244 to apply the voltage to the resistor 252 when the forward voltage Vf of the diode 53 decreases due to a rise in temperature. The voltage increases, the emitter current Ie increases, and the collector current Ic flowing through the light emitting diode 54 increases. Thus, in the light emitting circuit 244, similarly to the first and second embodiments, the negative temperature dependency of the light emitting diode 54 can be compensated for by increasing the collector current Ic. Can be kept constant regardless of the temperature.

[4. Fourth Embodiment]
The image forming apparatus 301 (FIG. 1) according to the fourth embodiment is different from the image forming apparatus 1 according to the first embodiment in that a toner density sensor 318 is provided instead of the toner density sensor 18, but is different. Is similarly configured. The toner density sensor 318 (FIG. 3) has a color light receiving circuit 345C and a monochrome light receiving circuit 345M in place of the color light receiving circuit 45C and the monochrome light receiving circuit 45M as compared with the toner density sensor 18 according to the first embodiment. Although different, other configurations are the same.

  The color light receiving circuit 345C and the monochrome light receiving circuit 345M are independent of each other and have the same circuit configuration as the color light receiving circuit 45C and the monochrome light receiving circuit 45M (FIG. 5) according to the first embodiment. . Therefore, hereinafter, the color light receiving circuit 345C will be described as an example. For convenience of explanation, the color light receiving circuit 345C and the monochrome light receiving circuit 345M are collectively referred to as a light receiving circuit 345 below.

  As shown in FIG. 9 corresponding to a part of FIG. 5, the color light receiving circuit 345C differs from the color light receiving circuit 45C in the first embodiment in that it is divided into a part for performing current-voltage conversion and a part for amplifying. In addition, only one operational amplifier 62 is used.

  In the color light receiving circuit 345C, similarly to the first embodiment, the anode terminal and the cathode terminal of the photodiode 61 are connected to the non-inverting input terminal and the inverting input terminal of the operational amplifier 62 via connection points N21 and N22, respectively. I have. A predetermined reference voltage Vref1 is supplied to the connection point N22 as in the first embodiment.

  The output terminal of the operational amplifier 62 is connected to the connection point N23. The connection point N23 is connected to the ground via the voltage dividing resistors 363 and 364 connected in series at the connection point N24. The connection point N24 is connected to the connection point N21 via a resistor 365.

  With this configuration, in the color light receiving circuit 345C, the photo-induced current Is supplied from the photodiode 61 is input to the operational amplifier 62, and a color light receiving signal is output from the output terminal of the operational amplifier 62. At this time, a part of the color light receiving signal output from the operational amplifier 62 is divided by the voltage dividing resistors 363 and 364 and then supplied to the inverting input terminal via the resistor 365 to realize negative feedback.

  In the color light receiving circuit 345C, the light-induced current Is flowing from the photodiode 61 is extremely weak, but the leakage current flowing from the inverting input terminal of the operational amplifier 62 to the inside is extremely small and can be substantially ignored. . When the resistance value of the resistor 365 is Rf [Ω], the resistance value Rf is a relatively large value. Therefore, a component of a part of the photo-induced current Is flowing from the anode terminal of the photodiode 61 to the connection point N24 via the resistor 365 can be substantially ignored.

  Here, the voltage of the color light receiving signal output from the operational amplifier 62, that is, the potential at the connection point N23 (voltage between the ground) and the potential at the connection point N24 are set to Vout1 and V1 [V], respectively. And 364 are denoted by Ra [Ω] and Rb [Ω], respectively. Then, since the potential V1 becomes a value obtained by dividing the voltage Vout1 by the resistance values Ra and Rb, the potential V1 can be approximately expressed by the following equation (13).

  V1 = Vout1 × Rb / (Ra + Rb) (13)

  On the other hand, in the operational amplifier 62, the amplification factor can be generally regarded as infinity, and the potential of the non-inverting input terminal and the potential of the inverting input terminal are controlled to be substantially equal. Vref1 [V] which is equivalent to. Therefore, the potential V1 can be expressed as in the following equation (14).

  V1 = Vref1−Is × Rf (14)

  By substituting the equation (14) into the equation (13) and rearranging, the following equation (15) can be obtained.

  Vout1 = (Vref1-Is × Rf) × (1 + Ra / Rb) (15)

  Here, assuming that the reference voltage Vref1 is 0 [V] for verification, this equation (15) becomes the following equation (16).

  Vout1 = −Is × Rf × (1 + Ra / Rb) (16)

  In the equation (16), the first half term (Is × Rf) corresponds to the voltage Vo1 of the intermediate signal in the color light receiving circuit 45C (FIG. 5) according to the first embodiment. The term (1 + Ra / Rb) in the latter half corresponds to the value (R75 / R72) which is the amplification factor of the amplifier 70C in the color light receiving circuit 45C (FIG. 5).

  Incidentally, since the monochrome light receiving circuit 345M has the same configuration as the color light receiving circuit 345C, it has the same function as the color light receiving circuit 345C.

  In the above configuration, the toner density sensor 318 of the image forming apparatus 301 according to the fourth embodiment uses the color light receiving circuit 345C (FIG. 9) to convert the voltage Vout1 of the signal output from the operational amplifier 62 into the voltage dividing resistors 363 and 364. , And the voltage is returned from the connection point N14 to the inverting input terminal via the resistor 365.

  The color light receiving circuit 345C has the same configuration as the photodiode 61 and the operational amplifier 62 as compared with the current / voltage converter 60C (FIG. 5) of the color light receiving circuit 45C according to the first embodiment. The resistor 365 is connected in the same manner as the variable resistor 63 and the resistor 64 to realize negative feedback. Therefore, the color light receiving circuit 345C can convert the current flowing through the photodiode 61 into a voltage by the operational amplifier 62, similarly to the current-voltage converter 60C.

  The color light receiving circuit 345C converts the value (Is × Rf) corresponding to the voltage Vo1 of the intermediate signal in the first embodiment into an amplification factor (1 + Ra / Rb), as shown in the above equation (16). ) And output as a color light receiving signal of the voltage Vout1. Therefore, the color light receiving circuit 345C selects the resistance values Ra and Rb so that the amplification factor (1 + Ra / Rb) is the same as the amplification factor (R75 / R72) in the amplification unit 70C. The voltage can be amplified with the same amplification factor as in the above.

  That is, in the color light receiving circuit 345C, the function of the current-voltage conversion unit 60C and the function of the amplifying unit 70C in the color light receiving circuit 45C according to the first embodiment can be changed by a simple circuit configuration using one operational amplifier 62. Can also be realized. In other words, the color light receiving circuit 345C can significantly reduce the number of components and can significantly reduce the circuit size, while achieving the same function as the color light receiving circuit 45C according to the first embodiment. Therefore, the configuration can be significantly simplified.

  From another viewpoint, in the color light receiving circuit 345C, the photo-induced current Is flowing from the photodiode 61 is extremely weak, and is much smaller than the negative feedback current flowing from the connection point N24 toward the inverting input terminal. . For this reason, in the color light receiving circuit 345C, the component of the photo-induced current Is flowing into the inside from the inverting input terminal of the operational amplifier 62 is extremely small and can be ignored.

  In the color light receiving circuit 345C, the operational amplifier 62 has a premise that the amplification factor is infinite, but since the voltage of the output signal cannot exceed the power supply voltage, the voltage is suppressed to, for example, about several volts. Accordingly, the operational amplifier 62 is controlled so that the potential difference between the inverting input terminal and the non-inverting input terminal becomes substantially 0 [V] due to the nature of the operational amplifier.

  As described above, in the color light receiving circuit 345C, the light induced current Is is extremely weak, and the characteristic of the operational amplifier 62 as an operational amplifier is used in combination. Thus, both the current-voltage conversion process and the amplification process can be performed.

  Further, in the color light receiving circuit 345C, as shown in Expression (16), the voltage Vout1 of the color light receiving signal is substantially determined by the photo-induced current Is and the resistance values Rs, Ra, and Rb. Therefore, in the color light receiving circuit 345C, the voltage Vout1 of the color light receiving signal is set to a desired value only by appropriately selecting the resistance values Rs, Ra, and Rb in accordance with the magnitude of the photo-induced current Is obtained from the photodiode 61. Can easily be adapted to

  According to the above configuration, the toner density sensor 318 of the image forming apparatus 301 according to the fourth embodiment separates the voltage Vout1 of the signal output from the operational amplifier 62 by the voltage dividing resistors 363 and 364 in the color light receiving circuit 345C. And the voltage is fed back from the connection point N14 to the inverting input terminal via the resistor 365. Thus, the color light receiving circuit 345C converts the photo-induced current Is into a voltage with a simple circuit configuration using only one operational amplifier 62, and then amplifies the color induced light Is to an appropriate voltage Vout1 by the operational amplifier 62. A signal can be generated.

[5. Fifth Embodiment]
The image forming apparatus 401 (FIG. 1) according to the fifth embodiment is different from the image forming apparatus 1 according to the first embodiment in that a control unit 403 and a toner density sensor 418 instead of the control unit 3 and the toner density sensor 18 are provided. Although they differ in the points they have, other points are configured similarly. The control unit 403 (FIG. 1) has substantially the same configuration as the control unit 3 according to the first embodiment, but performs control different from that of the first embodiment with respect to the toner density sensor 418. (Details will be described later).

  The toner density sensor 418 (FIG. 3) is different from the toner density sensor 18 according to the first embodiment in that a color light receiving circuit 445C and a monochrome light receiving circuit 445M replace the color light receiving circuit 45C and the monochrome light receiving circuit 45M. Although different, other configurations are the same.

  The color light receiving circuit 445C and the monochrome light receiving circuit 445M are independent of each other and have the same circuit configuration as the color light receiving circuit 45C and the monochrome light receiving circuit 45M (FIG. 5) according to the first embodiment. . Therefore, hereinafter, the color light receiving circuit 445C will be described as an example. For convenience of description, the color light receiving circuit 445C and the monochrome light receiving circuit 445M are also collectively referred to as a light receiving circuit 445 below.

  As shown in FIG. 10 corresponding to FIG. 9, the color light receiving circuit 445C is different from the color light receiving circuit 345C in the fourth embodiment in that a capacitor 465, an analog switch circuit 466, and an inverter 467 are used instead of the resistor 365. Although they are different in that they are provided, they are configured similarly in other respects.

  The capacitor 465 replaces the resistor 365 in the fourth embodiment, and is provided between the connection points N21 and N24 via the connection points N25 and N26. This capacitor 465 has a capacitance of Cs [μF].

  The analog switch circuit 466 includes an N-MOS (Metal Oxide Semiconductor) transistor 466N and a P-MOS transistor 466P, and both source terminals and drain terminals are connected to each other. Hereinafter, each of the source terminal and the drain terminal is referred to as a contact. These two contacts are connected to connection points N25 and N26, respectively.

  Further, the color light receiving circuit 445C receives the supply of the start signal ST from the control unit 403 (FIG. 1), and supplies this to the connection point N27. This connection point N27 is connected to the gate terminal of the N-MOS transistor 466N and also connected to the gate terminal of the P-MOS transistor 466P via the inverter 467.

  Therefore, when the start signal ST goes high, the color light receiving circuit 445C turns on both the P-MOS transistor 466P and the N-MOS transistor 466N to make the two contacts conductive, and the connection points N25 and N26 At the same potential. As a result, the capacitor 465 is in a state in which the charge stored so far is released, that is, in a discharged state.

  On the other hand, when the start signal ST becomes low level, the color light receiving circuit 445C turns off both the P-MOS transistor 466P and the N-MOS transistor 466N and disconnects the two contacts. As a result, the capacitor 465 is in a state where electric charge can be stored, that is, a state where charging is possible.

  As described above, in addition to the configuration common to the color light receiving circuit 345C (FIG. 9) according to the fourth embodiment, the color light receiving circuit 445C transfers electric charge to the capacitor 465 between the connection points N21 and N24 according to the start signal ST. The state is controlled between a storable state and a state where electric charges are released.

That is, the color light receiving circuit 445C switches the state of the analog switch circuit 466 on or off based on the start signal ST supplied from the control unit 403,
While storing or discharging the electric charge in the capacitor 465, a color light receiving signal is generated.

  For example, as shown as a waveform diagram in FIG. 11A, in the color light receiving circuit 445C (FIG. 10), the start signal ST supplied from the control unit 403 (FIG. 1) is initially at a low level. At this time, it is assumed that the light emitting circuit 44 (FIGS. 3 and 4) is not supplied with the power supply voltage VDD and the light emitting diode 54 does not emit light as shown in FIG.

  Therefore, in the color light receiving circuit 445C (FIG. 10), the photo-induced current Is is not generated by the photodiode 61 as shown in FIG. Further, at this time, in the color light receiving circuit 445C (FIG. 10), since the charge state of the capacitor 465 is unknown, the value of the voltage Vout1 of the color light receiving signal is undefined. At this time, the control unit 403 does not acquire a color light receiving signal from the color light receiving circuit 445C.

  When the predetermined toner concentration detection program is executed, the control unit 403 raises the start signal ST from a low level to a high level at time t0, and does not continuously supply the power supply voltage VDD to the light emitting circuit 44 and emits light. The state where the diode 54 is turned off is continued. At this time, in the color light receiving circuit 445C, since the state of the analog switch circuit 466 is switched from off to on, the capacitor 465 is discharged, the voltage Vcs becomes 0 [V], and the connection points N21, N24, N25, and N26 are all connected. It has the same potential. As a result, the value of the voltage Vout1 of the color light receiving signal is reset to 0 [V].

  Thereafter, the control unit 403 lowers the start signal ST from a high level to a low level at a time point t1 after a lapse of a short time (FIG. 11A). As a result, in the color light receiving circuit 445C (FIG. 10), the state of the analog switch circuit 466 is switched from ON to OFF, so that the capacitor 465 can be charged. However, at this time, since the light emitting diode 54 is turned off (FIG. 11B), the photo-induced current Is is not generated (FIG. 11C), and the voltage Vout1 of the color light receiving signal is 0 [V]. (FIG. 11D).

  Further, at the predetermined time t2, the control unit 403 starts supplying the power supply voltage VDD to the light emitting circuit 44 (FIG. 11B). In response to this, the photodiode 61 of the color light receiving circuit 445C generates a photo-induced current Is (FIG. 11C), and supplies this to the connection point N21 (FIG. 10).

  Then, the capacitor 465 is gradually charged by supplying the photo-induced current Is, and the voltage Vcs gradually increases from 0 [V]. Thus, in the color light receiving circuit 445C, the voltage Vout1 of the color light receiving signal gradually increases (FIG. 11D).

  Eventually, the control unit 403 stops supplying the power supply voltage VDD to the light emitting circuit 44 when a predetermined period T has elapsed from the time point t2 and the time point reaches the time point t3 (FIG. 11B). In response, the photodiode 61 of the color light receiving circuit 445C does not generate the photo-induced current Is (FIG. 11C).

  Then, the capacitor 465 is no longer charged and maintains the value of the voltage Vcs as it is. Thus, in the color light receiving circuit 445C, the voltage Vout1 of the color light receiving signal is set to the voltage VT1 (FIG. 11D).

  Here, the potential V1 at the connection point N24 of the color light receiving circuit 445C is calculated in the same manner as when the equation (13) is calculated in the fourth embodiment, and the following equations (17) and (18) are used. And (19) can be calculated.

  V1 = Vout1 × Rb / (Ra + Rb) (17)

  Vcs = (1 / Cs) × ∫Isdt (18)

  V1 = Vref1-Vcs (19)

  When rearranging the equations (17) to (19) and assuming that the resistance value Ra = 0 [Ω] and the reference voltage Vref1 = 0 [V] to simplify the calculation, the following equation (20) Expression can be obtained.

  Vout1 = (− 1 / Cs) × ∫Isdt (20)

  As can be seen from the equation (20), the voltage Vout1 of the color light receiving signal changes linearly with the passage of time (FIG. 11D), and at time t3, becomes the following equation (21).

  Vout1 ≒ (−1 / Cs) × Is × T (21)

  That is, in the color light receiving circuit 445C, the value of the voltage Vcs is set to a sufficiently large value by charging the capacitor 465 with the minute light-induced current Is over the period T, and accordingly, the voltage Vout1 of the color light receiving signal is sufficiently increased. The voltage VT1 can be as large as the voltage VT1.

  For example, in the color light receiving circuit 345C (FIG. 9) according to the fourth embodiment, the resistance value Rf of the resistor 365 needs to be set to an extremely large value in order to make the minute photo-induced current Is a signal level that can be observed. However, such a resistor having a resistance value is poor in availability and may lead to an increase in manufacturing cost.

  On the other hand, in the color light receiving circuit 445C (FIG. 10) according to the fifth embodiment, by setting the period T appropriately, such a small photo-induced current Is can be sufficiently charged in the capacitor 465, and As shown in equation (18), the voltage Vcs can be expressed as a value of a magnitude that can be easily measured. In other words, in the color light receiving circuit 445C, by accumulating the charge of the photo-induced current Is in the capacitor 465 over an appropriate period T, a sufficient amount corresponding to the integrated amount of the amount of light received by the photodiode 61 is obtained. It is possible to generate a voltage Vcs of an appropriate magnitude.

  Also, at this time, in the color light receiving circuit 445C, even if noise is included in the photo-induced current Is, the value of the obtained voltage Vcs can be leveled by charging the capacitor 465 as an electric charge. It is possible to reduce the influence of this noise on the voltage Vout1 of the color light receiving signal. That is, as compared with the first and fourth embodiments, the color light receiving circuit 445C can reduce the influence of noise included in the photo-induced current Is and can increase the light detection accuracy.

  Incidentally, the control unit 403 acquires the color light receiving signal having the voltage Vout1 from the color light receiving circuit 445C, detects the toner density, and then detects the toner density again. By repeating this, the electric charge of the capacitor 465 is discharged each time.

  In other respects, the color light receiving circuit 445C according to the fifth embodiment can provide the same operation and effects as those of the fourth embodiment.

  According to the above configuration, in the toner density sensor 418 of the image forming apparatus 401 according to the fifth embodiment, in the color light receiving circuit 445C, the capacitor 465 is charged with the photo-induced current Is generated by the photodiode 61, A color light receiving signal having a voltage Vout1 corresponding to the voltage Vcs generated at both ends is generated. Thus, the color light receiving circuit 445C has a relatively simple circuit configuration using only one operational amplifier 62, and even if the photo-induced current Is is very small, the color light receiving circuit 445C accurately reflects the size of the light-induced current Is. A signal can be generated.

[6. Other Embodiments]
In the first embodiment described above, the case where the diode 53 (FIG. 4) is used for the light emitting circuit 44 and the transistor 153 (FIG. 7) is used for the light emitting circuit 144 in the second embodiment has been described. However, the present invention is not limited to this, and another semiconductor element having a PN junction may be used for the light emitting circuit 44 and the like. The point is that a constant output voltage Vo from the three-terminal regulator 51 is applied to the semiconductor element and the resistor 52 connected in series, and the forward voltage decreases as the temperature increases. It is only necessary that the voltage applied to the device and the flowing current can be increased. The same applies to the third to fifth embodiments.

  In the above-described first and second embodiments, the constant output voltage Vo is generated by the three-terminal regulator 51, and in the third embodiment, the constant reference voltage Vref is generated by the shunt regulator 251. Mentioned the case. However, the present invention is not limited to this, and a constant voltage may be generated by various regulators. The same applies to the fourth and fifth embodiments.

  Further, in the first embodiment described above, the photodiode 61 in the color light receiving circuit 45C (FIG. 5) is not affected by temperature, that is, has no temperature dependency, and has a size corresponding to the amount of received light. The case where the photo-induced current Is is generated has been described. However, the present invention is not limited to this. For example, the photodiode 61 has a positive temperature coefficient, and as shown in FIG. May be provided.

  In this case, as shown in FIG. 12B corresponding to FIG. 6D, the LED drive current (that is, the forward current If) increases as the temperature Ta increases, but the degree of the increase is slightly reduced. Keep it down. Thus, as shown in FIG. 12 (C) corresponding to FIG. 6 (E), it is possible to provide a negative temperature characteristic such that the light emission amount is slightly reduced with an increase in the temperature Ta. As a result, the temperature dependency of the photodiode 61 can be canceled, and as shown in FIG. 12D corresponding to FIG. 6F, the output voltage (that is, the voltage Vout1 of the color light receiving signal) is changed to the temperature Ta. Irrespective of the value, the characteristic can be maintained to be substantially constant. The same applies to the second to fifth embodiments.

  Further, in the above-described first embodiment, only one diode 53 is arranged between the output terminal 51o of the three-terminal regulator 51 and the connection point N1 in the light emitting circuit 44 (FIG. 4). The case where only one is provided has been described. However, the present invention is not limited to this, and an arbitrary number of diodes 53 and an arbitrary number of light emitting diodes 54 may be provided. For example, in a light emitting circuit 544 shown in FIG. 13, two diodes 53 are connected in series, and two light emitting diodes 54 are connected in series. In the light emitting circuit 544, since the resistance voltage Vr applied to the resistor 52 is a value obtained by subtracting the sum of the two forward voltages Vf from the output voltage Vo, the amount of change in the voltage according to the temperature change is determined by the first voltage. Can be doubled with respect to the embodiment. Further, in this case, since the two light emitting diodes 54 are connected in series, the light emission amount can be doubled as compared with the first embodiment. The same applies to the second to fifth embodiments.

  Further, in the above-described first embodiment, the case where the light emitting diode 54 is connected to the output terminal 51o side of the three-terminal regulator 51 in the light emitting circuit 44 (FIG. 4) has been described. However, the present invention is not limited to this, and the light emitting diode 54 may be connected to the input terminal 51i side of the three-terminal regulator 51, for example, as in a light emitting circuit 644 shown in FIG. The same applies to the fourth and fifth embodiments.

  Further, in the above-described second embodiment, the case where the output terminal 51o of the three-terminal regulator 51 is directly connected to the base terminal of the transistor 153 in the light emitting circuit 144 (FIG. 7) has been described. However, the present invention is not limited to this. For example, as in a light emitting circuit 744 shown in FIG. 15, a resistor 755 is connected between the output terminal 51o and the base terminal of the transistor 153, and between the resistor 755 and the transistor 153. A resistor 756 may be connected between the provided connection point N41 and the ground. In this case, the voltage V41 with respect to the ground at the connection point N41 can supply the voltage divided from the output voltage Vo to the base terminal of the transistor 153 by the ratio of the resistance values of the resistors 755 and 756. Compared to the embodiment, it is possible to cope with the temperature coefficient of the light emitting diode 54 more widely.

  Further, in the above-described fourth embodiment, a resistor 365 is connected between the connection points N21 and N24 in the color light receiving circuit 345C (FIG. 9). In the fifth embodiment, the color light receiving circuit 445C (FIG. The case where the capacitor 465 is connected between the connection points N21 and N24 in 10) has been described. However, the present invention is not limited to this. For example, various other elements such as coils and combinations thereof may be connected between the connection points N21 and N24. In this case, any element that allows a relatively small amount of current to pass can be used.

  Further, in the above-described fifth embodiment, as shown in FIG. 11, the control unit 403 supplies the power supply voltage VDD to the light emitting circuit 44 and controls the light emitting diode 54 only during the period T from time t2 to time t3. The case where light is emitted and the capacitor 465 is charged with the charge of the photo-induced current Is in the color light receiving circuit 445C has been described. However, the present invention is not limited to this. For example, as shown in FIG. 16 corresponding to FIG. 11, in addition to the period T from time t2 to time t3, the period T from time t4 to time t5 and the time T6 from time t6 to time t7 The light emitting diode 54 may emit light also in the period T up to.

  In this case, since the charging of the capacitor 465 has been increased from once to three times, the voltage Vout1 of the color light receiving signal can be increased to the voltage VT3 corresponding to three times the voltage VT1, and the weak light-induced current The magnitude of Is can be more easily and accurately detected. Further, in this case, the light emitting diode 54 is not continuously emitted, but is intermittently emitted, that is, blinking is repeated a plurality of times. , And a significant change in characteristics due to this. The number of repetitions of the period T during which the light emitting diode 54 emits light is not limited to three, but may be two or four or more. In addition to the same period T each time, each period may have a different length.

  Further, in the above-described first embodiment, as the toner density sensor 18 (FIG. 3), the light emitting unit 42, the color light receiving unit 43C, and the monochrome light receiving unit 43M are provided inside the sensor housing 41, and the integrated toner density sensor 18 is provided. The case of configuring as a sensor has been described. However, the present invention is not limited to this. For example, the light emitting unit 42, the color light receiving unit 43C, and the monochrome light receiving unit 43M may be independently attached to the attachment member 40. In this case, the angles and positions of the respective optical axes may be appropriately adjusted. The same applies to the second to fifth embodiments.

  Further, in the above-described first embodiment, in the toner density sensor 18 (FIG. 3), the optical axis XE and the optical axis XM are symmetrical with respect to the normal line 14N of the conveyor belt 14, and the light emitting unit 42 is sandwiched. The case where the color light receiving unit 43C is arranged on the opposite side to the monochrome light receiving unit 43M so that the optical axis XC forms an angle φ with the optical axis XE has been described. However, the present invention is not limited to this, and the optical axis XE of the light emitting section 42 and the optical axis XM of the monochrome light receiving section 43M may be set to any angle, and the optical axis XC may be set to any angle with respect to the optical axis XE. It may be arranged. The same applies to the second to fifth embodiments.

  Further, in the first embodiment described above, a case has been described in which the toner density sensor 18 detects the density of the toner attached to the transport belt 14. However, the present invention is not limited to this. For example, the toner density sensor 18 is disposed near the photoconductor drum 38 in the image forming unit 16 (FIG. 2), and the density of the toner adhering to the surface of the photoconductor drum 38 is measured. For example, the density of the toner at various places may be detected. The same applies to the second to fifth embodiments.

  Further, in the above-described first embodiment, the case where color printing is performed by providing the image forming units 16 corresponding to cyan, magenta, yellow, and black in the image forming apparatus 1 (FIG. 1) has been described. However, the present invention is not limited to this. For example, only the black image forming unit 16 may be provided in the image forming apparatus 1 and only monochrome printing may be performed. In this case, the color light receiving section 43C can be omitted from the toner density sensor 18. Alternatively, the image forming apparatus 1 may be provided with an image forming unit 16 corresponding to a special color such as white or clear (transparent), and may perform printing using the special color. In this case, the light receiving unit 43 and the light emitting unit 42 corresponding to the special color may be provided as appropriate. The same applies to the second to fifth embodiments.

  Further, in the first embodiment described above, the case where the present invention is applied to the image forming apparatus 1 including the MFP has been described. However, the present invention is not limited to this, and may be applied to various electronic apparatuses having a function of forming a toner image by an electrophotographic method and fixing it on paper, such as a copying machine and a facsimile machine. The same applies to the second to fifth embodiments.

  Furthermore, the present invention is not limited to the above-described embodiments and other embodiments. That is, the present invention covers an embodiment obtained by arbitrarily combining some or all of the above-described embodiments and the other embodiments described above, and an embodiment in which a part is extracted. It is.

Further, in the above-described fourth embodiment, the toner density sensor 18 as a light detecting device having a light emitting unit 42 as a light emitting unit and a light receiving unit 43 as a light receiving unit is configured. The case where the photodiode 61 as an element, the voltage dividing resistors 363 and 364 as a series connection circuit, the operational amplifier 62 as an operational amplifier, and the resistor 365 as a conversion element have been described. However, the present invention is not limited to this, and constitutes a light detecting device having a light emitting unit having various other configurations and a light receiving unit. The light receiving unit is provided with a light receiving element having other various configurations, and a series connection circuit. , An operational amplifier and a conversion element.

  INDUSTRIAL APPLICABILITY The present invention can be used in an MFP that prints by forming a toner image by an electrophotographic method and fixing the toner image on a sheet.

  1, 101, 201, 301, 401... Image forming apparatus, 3, 403... Control unit, 14... Transport belt, 16... Image forming unit, 18, 118, 218, 318, 418. , 38 photosensitive drum, 42 light emitting unit, 43 light receiving unit, 43C color light receiving unit, 43M monochrome light receiving unit, 44, 144, 244 light emitting circuit, 45, 345, 445 ... Light receiving circuit, 45C, 345C, 445C ... Color light receiving circuit, 45M, 345M, 445M ... Monochrome light receiving circuit, 51 ... Three terminal regulator, 51g ... Ground terminal, 51i ... 52, a resistor, 53, a diode, 54, a light emitting diode, 60C, a current-voltage converter, 61, a photodiode, 62, an operational amplifier, 3 Variable resistor, 64 resistor, 70C amplifier, 152 emitter resistor, 153, 256 transistor, 251 shunt regulator, 252, 255 resistor, 363, 364 partial voltage Resistance, 365: Resistance, 465: Capacitor, 466: Analog switch circuit, Ib: Base current, Ic: Collector current, Ie: Emitter current, If: Forward current, Ik: Cathode current, Io Output current, Ir Reference current, Is Photoinduced current, Iss Ground terminal current, L1 Detection light, N1, N21, N24 Connection point, ST Start signal, T Period, Tc temperature coefficient, VDD power supply voltage, VT1 voltage, Vbe voltage between base and emitter, Vf forward voltage, Vo output voltage, Vcs Vo1, Vout1, Vout2 ...... voltage, Vr ...... resistor voltage, Vref ...... reference voltage, Vref1, Vref2 ...... reference voltage, W ...... conveying path.

Claims (15)

A light detection device having a light emitting unit that emits light and a light receiving unit that receives the light,
The light emitting unit emits a current determined by a differential voltage value obtained by subtracting a potential difference generated in a semiconductor element having a semiconductor PN junction from an output voltage of a regulator and a resistance value of a resistor to which an output current from the regulator is supplied. Supplying the light to the diode to emit light;
The light receiving unit,
A light receiving element having a first terminal and a second terminal, receiving the light and generating a photo-induced current;
A plurality of voltage dividing resistors connected in series at the connection point;
An inverting input terminal is connected to the first terminal of the light receiving element, a non-inverting input terminal is connected to the second terminal of the light receiving element, and an output terminal is connected to ground via the plurality of voltage dividing resistors. By doing so, an operational amplifier that amplifies the photo-induced current,
A conversion element that has one end connected to the inverting input terminal and the other end connected to the connection point, and converts a current into a voltage. The photodetector according to claim 1, wherein the conversion element is a resistor. The photodetector according to claim 1, wherein the conversion element is a capacitor. The photodetector according to claim 3, further comprising: a switch circuit that connects or disconnects between the one end and the other end of the conversion element. The light emitting unit repeats the blinking of the light a plurality of times,
The light receiving unit, after repeating the blinking of the light in the light emitting unit a plurality of times, sets an output voltage obtained from an output terminal of the operational amplifier to a value representing a detection result of the light. 5. The photodetector according to 4. The switch circuit repeats the blinking of the light in the light emitting unit a plurality of times, and after the output voltage obtained from the output terminal of the operational amplifier is used as a detection result of the light, the one end and the one end of the conversion element. The photodetector according to claim 5, wherein the other end is connected. The output voltage of the regulator and the resistance value of the resistor in the light emitting unit are selected so as to maintain a constant light amount of the light emitted from the light emitting diode regardless of a temperature change. The light detection device according to claim 1 . The light receiving element of the light receiving unit has a temperature dependency of changing the photo-induced current with a change in temperature,
The output voltage of the regulator and the resistance value of the resistor in the light emitting unit are changed by changing a light amount of the light emitted from the light emitting diode with a temperature change, so that a temperature of the light induced current in the light receiving unit is changed. The photodetector according to claim 1 , wherein the photodetector is selected so as to cancel the dependence. The photodetector according to claim 1 , wherein the semiconductor element is made of silicon. The photodetector according to claim 1 , wherein the semiconductor element is a diode. The photodetector according to claim 1 , wherein the semiconductor element is a transistor. The light detecting device according to claim 1 , wherein the light emitting diode is connected to an output terminal of the regulator. The light detecting device according to claim 1 , wherein the light emitting unit supplies the current to a plurality of the light emitting diodes. The light emitting diode emits the light when supplied with a current determined by a resistance value and a differential voltage value obtained by subtracting a total value of potential differences generated by a plurality of the semiconductor elements from the output voltage. Item 2. The photodetector according to item 1 . A photodetector according to any one of claims 1 to 14 , and an image forming unit that forms an image with a developer.
The image forming apparatus, wherein the light detecting device irradiates the image formed by the image forming unit with the light from the light emitting unit, and receives light reflected in the image by the light receiving unit. .

Images