Light emitting element driving device and image reading device

Abstract

To provide a light-emitting element drive device and an image reading device which can turn on a light-emitting element more quickly while preventing an excessive current from flowing to the light-emitting element.SOLUTION: A light-emitting element drive device comprises: a light-emitting element which emits light in response to supply of a first current; a first switching element which turns ON / OFF the first current supplied to the light-emitting element; a second switching element which turns ON / OFF a second current not supplied to the light-emitting element; a constant current circuit which performs adjustment such that a total current amount of the first current and the second current becomes constant; and a control circuit which controls ON / OFF of the first switching element and the second switching element. The control circuit turns ON the first switching element in a state that the second switching element is turned ON to flow the second current to the second switching element, and turns OFF the second switching element after turning ON the first switching element to flow the first current to the light-emitting element and the first switching element.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a light emitting element driving device and an image reading device. [Background technology]

[0002] In recent years, there has been a demand for faster imaging of a medium in image reading devices such as scanners that image a medium while conveying the medium or moving an imaging unit. Such image reading devices are provided with, for example, a light emitting element driving device that irradiates the medium with light of each color of RGB, and an imaging unit that images the medium illuminated with light by the light emitting element. In order to image a medium at high speed in an image reading device, it is necessary to quickly switch the color of light irradiated to the medium by the light emitting element driving device.

[0003] A light-emitting element driving device, for example, connects switching elements to light-emitting elements that emit RGB light and a constant current circuit, and switches each switching element ON / OFF to switch each light-emitting element ON / OFF. In such a light-emitting element driving device, there is a possibility that an overcurrent will flow to the light-emitting element at the timing when the switching element is switched from OFF to ON to light the light-emitting element. If an overcurrent flows through the light-emitting element, the amount of light emitted by the light-emitting element may become too large, or the light-emitting element may break down.

[0004] A light-emitting element driving device is disclosed that includes a constant current circuit for supplying a constant current to a light-emitting unit, a main switching element inserted in series between the light-emitting unit and the constant current circuit, and a sub-switching element connected to a node between the switching element and the constant current circuit (Patent Document 1). This light-emitting element driving device turns on the sub-switching element before turning on the main switching element, thereby causing a constant current to flow through the constant current circuit via the sub-switching element, and turns on the main switching element and turns off the sub-switching element at the same time. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-47095 A Summary of the Invention [Problem to be solved by the invention]

[0006] In a light-emitting element driving device, it is desirable to light up a light-emitting element at a higher speed while preventing an overcurrent from flowing through the light-emitting element.

[0007] An object of the present invention is to provide a light emitting element driving device and an image reading device that are capable of turning on a light emitting element at a higher speed while preventing an overcurrent from flowing through the light emitting element. [Means for solving the problem]

[0008] A light-emitting element driving device according to one aspect of the present invention includes a light-emitting element that emits light in response to a first current being supplied thereto, a first switching element that turns the first current supplied to the light-emitting element ON / OFF, a second switching element that turns a second current that is not supplied to the light-emitting element ON / OFF, a constant current circuit that adjusts the total amount of the first current and the second current to be constant, and a control circuit that controls the ON / OFF of the first switching element and the second switching element, wherein the control circuit turns the first switching element ON while turning the second switching element ON to flow a second current through the second switching element, and after turning the first switching element ON, turns the second switching element OFF to flow the first current through the light-emitting element and the first switching element.

[0009] Moreover, an image reading device according to one aspect of the present invention has an imaging unit that images a medium, and a plurality of light-emitting units that each irradiate the medium with light of a different color, each of the plurality of light-emitting units having a light-emitting element that emits light in response to a first current being supplied thereto, a first switching element that turns ON / OFF the first current supplied to the light-emitting element, a second switching element that turns ON / OFF a second current that is not supplied to the light-emitting element, a constant current circuit that adjusts the total amount of the first current and the second current to be constant, and a control circuit that controls the ON / OFF of the first switching element and the second switching element, wherein the control circuit turns ON the first switching element while turning ON the second switching element to flow a second current through the second switching element, and after turning ON the first switching element, turns OFF the second switching element to flow the first current through the light-emitting element and the first switching element. Effect of the Invention

[0010] According to the present invention, the light emitting device and image reading device can light up the light emitting element at a higher speed while preventing an overcurrent from flowing through the light emitting element. [Brief description of the drawings]

[0011] [Figure 1] 1 is a perspective view showing an image reading device 100 according to an embodiment. [Diagram 2] 2 is a diagram for explaining a transport path inside the image reading device 100. FIG. [Diagram 3] FIG. 1 is a diagram for explaining an imaging device 117. [Figure 4] 1 shows an example of a circuit diagram of a red light emitting device 131a. [Diagram 5] 10 is a graph for explaining the amount of current, etc. [Figure 6] 10 is a graph for explaining the amount of current, etc. [Figure 7] 1 is a block diagram showing a schematic configuration of an image reading device 100. FIG. [Figure 8] FIG. 1 is a diagram showing a schematic configuration of a storage device 160 and a processing circuit 170. [Figure 9]10 is a flowchart illustrating an example of the operation of a medium reading process. [Figure 10] FIG. 13 is a diagram showing a schematic configuration of another processing circuit 270. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an image reading device according to one aspect of the present invention will be described with reference to the drawings. However, it should be noted that the technical scope of the present invention is not limited to the embodiment, but extends to the inventions described in the claims and their equivalents.

[0013] FIG. 1 is a perspective view showing an image reading device 100 according to an embodiment.

[0014] The image reading device 100 is an image scanner or the like. The image reading device 100 conveys a medium, which is an original, and captures an image. The medium is paper, cardboard, a card, a booklet, a passport, or the like. The image reading device 100 may be a facsimile, a copier, a multifunction printer (MFP, Multifunction Peripheral), or the like.

[0015] The image reading device 100 includes a lower housing 101, an upper housing 102, a placement table 103, a discharge table 104, an operation device 105, a display device 106, and the like.

[0016] The upper housing 102 is disposed at a position that covers the top surface of the image reading device 100, and engages with the lower housing 101 by a hinge so as to be openable and closable when loading a medium or cleaning the inside of the image reading device 100, for example.

[0017] The placement stage 103 is engaged with the lower housing 101 so that the medium to be transported can be placed thereon. The ejection stage 104 is engaged with the lower housing 101 so that the ejected medium can be held thereon.

[0018] The operation device 105 has an input device such as a button and an interface circuit for acquiring a signal from the input device, accepts an input operation by a user, and outputs an operation signal according to the input operation by the user. The display device 106 has a display including a liquid crystal, an organic EL (Electro-Luminescence), or the like, and an interface circuit for outputting image data to the display, and displays the image data on the display.

[0019] FIG. 2 is a diagram for explaining a transport path inside the image reading device 100. As shown in FIG.

[0020] The transport path inside the image reading device 100 includes a first medium sensor 111, a feed roller 112, a separation roller 113, a first transport roller 114, a second transport roller 115, a second medium sensor 116, an imaging device 117, a first discharge roller 118, and a second discharge roller 119. The number of each roller is not limited to one, and each roller may be multiple. In this case, each roller is arranged side by side at intervals in the width direction perpendicular to the medium transport direction A1. The imaging device 117 includes a first imaging device 117a and a second imaging device 117b arranged opposite each other across the medium transport path.

[0021] The top surface of the lower housing 101 forms a lower guide 101a of the medium transport path, and the bottom surface of the upper housing 102 forms an upper guide 102a of the medium transport path. In Fig. 2, arrow A1 indicates the medium transport direction. In the following, upstream refers to the upstream side of the medium transport direction A1, and downstream refers to the downstream side of the medium transport direction A1.

[0022] The first medium sensor 111 is disposed upstream of the feed roller 112 and the separation roller 113. The first medium sensor 111 has a contact detection sensor and detects whether or not a medium is placed on the placement table 103. The first medium sensor 111 generates and outputs a first medium signal whose signal value changes depending on whether or not a medium is placed on the placement table 103. Note that the first medium sensor 111 is not limited to a contact detection sensor, and any other sensor capable of detecting the presence or absence of a medium, such as a light detection sensor, may be used as the first medium sensor 111.

[0023] The feed roller 112 is provided in the lower housing 101, and feeds the media placed on the placement table 103 from the bottom up. The separation roller 113 is a so-called brake roller or retard roller, and is provided in the upper housing 102, and is disposed opposite the feed roller 112.

[0024] The first transport roller 114 and the second transport roller 115 are disposed downstream of the feed roller 112 and facing each other, and transport the medium fed by the feed roller 112 and the separation roller 113 to the imaging device 117 .

[0025] The second medium sensor 116 is disposed downstream of the first transport roller 114 and upstream of the imaging device 117, and detects a medium transported to that position. The second medium sensor 116 includes a light emitter and a light receiver provided on one side of the medium transport path (for example, the lower housing 101), and a light guide tube provided at a position facing the light emitter and the light receiver across the medium transport path (for example, the upper housing 102). The light emitter is an LED (Light Emitting Diode) or the like, and irradiates light toward the medium transport path. On the other hand, the light receiver is a photodiode or the like, and receives the light irradiated by the light emitter and guided by the light guide tube. When a medium is present at a position facing the second medium sensor 116, the light irradiated from the light emitter is blocked by the medium, and therefore the light receiver does not detect the light irradiated from the light emitter. The light receiver generates and outputs a second medium signal whose signal value changes depending on whether a medium is present or not at the position of the second medium sensor 116, based on the intensity of the received light.

[0026] A reflective member such as a mirror may be used instead of the light guide tube. The light emitter and the light receiver may be disposed opposite each other across the medium transport path. The second medium sensor 116 may detect the presence of the medium using a contact detection sensor that passes a predetermined current when the medium is in contact or when the medium is not in contact.

[0027] The first discharge roller 118 and the second discharge roller 119 are arranged facing each other downstream of the imaging device 117, and transport the medium transported by the first conveying roller 114 and the second conveying roller 115 and imaged by the imaging device 117, and discharge it onto the discharge tray 104.

[0028] The medium placed on the mounting table 103 is transported between the lower guide 101a and the upper guide 102a in the medium transport direction A1 by the rotation of the feed roller 112 in the direction of the arrow A2 in Fig. 2, i.e., the medium feed direction. When transporting the medium, the separation roller 113 rotates in the direction of the arrow A3, i.e., the opposite direction to the medium feed direction. When multiple media are placed on the mounting table 103, the feed roller 112 and the separation roller 113 function to separate only the media placed on the mounting table 103 that is in contact with the feed roller 112. This operates to restrict the transport of media other than the separated media (preventing double feeding).

[0029] The medium is fed between the first conveyor roller 114 and the second conveyor roller 115 while being guided by the lower guide 101a and the upper guide 102a. The medium is fed between the first imaging device 117a and the second imaging device 117b by the first conveyor roller 114 and the second conveyor roller 115 rotating in the directions of the arrows A4 and A5, respectively. The medium read by the imaging device 117 is discharged onto the discharge tray 104 by the first discharge roller 118 and the second discharge roller 119 rotating in the directions of the arrows A6 and A7, respectively. In this way, the feed roller 112, the separation roller 113, the first conveyor roller 114, the second conveyor roller 115, the first discharge roller 118 and / or the second discharge roller 119 function as a conveying unit that conveys the medium.

[0030] FIG. 3 is a diagram for explaining the imaging device 117. As shown in FIG.

[0031] 3, the second imaging device 117b is disposed above the first imaging device 117a and faces the first imaging device 117a. The first imaging device 117a includes a first light transmitting member 121a, a first light emitting device 122a, a first imaging sensor 123a, a first backing member 124a, etc. The second imaging device 117b includes a second light transmitting member 121b, a second light emitting device 122b, a second imaging sensor 123b, a second backing member 124b, etc.

[0032] The first light transmitting member 121a and the second light transmitting member 121b are made of transparent glass. Alternatively, the first light transmitting member 121a and the second light transmitting member 121b may be made of transparent plastic or the like.

[0033] The first light emitting device 122a is provided on the opposite side of the second backing member 124b across the first light transmitting member 121a and the second light transmitting member 121b. The first light emitting device 122a is a light source that irradiates light onto the medium being transported, and includes a red light emitting device 131a, a green light emitting device 132a, and a blue light emitting device 133a. The red light emitting device 131a, the green light emitting device 132a, and the blue light emitting device 133a are each an example of a light emitting element driving device or a light emitting unit, and irradiate red, green, and blue light, respectively, onto the medium transported by the transport unit. That is, the first light emitting device 122a has a plurality of light emitting units that irradiate the transported medium with light of different colors. The first light emitting device 122a irradiates light onto the surface of the medium transported to the position of the imaging device 117 (or the second backing member 124b of the opposing second imaging device 117b when the medium is not being transported).

[0034] Similarly, the second light emitting device 122b is provided on the opposite side of the first backing member 124a across the second light transmitting member 121b and the first light transmitting member 121a. The second light emitting device 122b is a light source that irradiates light onto the medium being transported, and includes a red light emitting device 131b, a green light emitting device 132b, and a blue light emitting device 133b. The red light emitting device 131b, the green light emitting device 132b, and the blue light emitting device 133b are each an example of a light emitting element driving device or a light emitting unit, and irradiate light having red, green, and blue colors onto the medium transported by the transport unit. That is, the second light emitting device 122b has a plurality of light emitting units that irradiate light of different colors onto the transported medium. The second light emitting device 122b irradiates light onto the back side of the medium transported to the position of the imaging device 117 (or the first backing member 124a of the opposing first imaging device 117a when the medium is not being transported).

[0035] The first imaging sensor 123a is an example of an imaging unit, and is provided on the opposite side of the second backing member 124b across the first light transmitting member 121a and the second light transmitting member 121b. The first imaging sensor 123a has a line sensor based on a CIS (Contact Image Sensor) of a life-size optical system type having imaging elements based on CMOS (Complementary Metal Oxide Semiconductor) linearly arranged in the main scanning direction. Each imaging element outputs an electrical signal having a signal value corresponding to each RGB color. The first imaging sensor 123a also has a lens that forms an image on the imaging element, and an A / D converter that amplifies and analog-to-digital (A / D) converts the electrical signal output from the imaging element. The first imaging sensor 123a captures an image of the surface of the medium transported by the transport unit at the imaging position L1, and generates an input image.

[0036] Similarly, the second imaging sensor 123b is an example of an imaging unit, and is provided on the opposite side of the first backing member 124a across the first light transmitting member 121a and the second light transmitting member 121b. The second imaging sensor 123b has a line sensor based on a CIS of a life-size optical system type having imaging elements based on CMOS linearly arranged in the main scanning direction. Each imaging element outputs an electrical signal having a signal value corresponding to each RGB color. The second imaging sensor 123b also has a lens that forms an image on the imaging element, and an A / D converter that amplifies the electrical signal output from the imaging element and performs analog / digital conversion. The second imaging sensor 123b captures an image of the back side of the medium transported by the transport unit at the imaging position L2, and generates an input image.

[0037] The first imaging sensor 123a and the second imaging sensor 123b are one-line monochrome sensors, and identify the component values ​​of each RGB color of the imaging target while switching the color of light irradiated by the first light emitting device 122a and the second light emitting device 122b. The one-line monochrome sensor is less expensive than a three-line color sensor having three line sensors corresponding to each RGB color, and can reduce the amount of color shift. Therefore, by using one-line monochrome sensors as the first imaging sensor 123a and the second imaging sensor 123b, the image reading device 100 can reduce the device cost and suppress the occurrence of color shift in the input image.

[0038] In addition, instead of the CIS line sensor of the life-size optical system type having a CMOS imaging element, a CIS line sensor of the life-size optical system type having a CCD (Charge Coupled Device) imaging element may be used. Also, a reduction optical system type line sensor having a CMOS or CCD imaging element may be used. Also, either one of the set of the first light-emitting device 122a, the first imaging sensor 123a, and the second backing member 124b or the set of the second light-emitting device 122b, the second imaging sensor 123b, and the first backing member 124a may be omitted.

[0039] The first backing member 124a is provided below the first light transmitting member 121a at a position facing the second light emitting device 122b and the second imaging sensor 123b. The surface of the first backing member 124a facing the second imaging sensor 123b has, for example, a white color, and the first backing member 124a functions as a white reference member for correcting images such as shading based on an image signal captured by the first backing member 124a.

[0040] Similarly, the second backing member 124b is provided above the second light transmitting member 121b at a position facing the first light emitting device 122a and the first imaging sensor 123a. The surface of the second backing member 124b facing the first imaging sensor 123a has, for example, a white color, and the second backing member 124b functions as a white reference member for performing image correction such as shading based on an image signal captured by the second backing member 124b.

[0041] Hereinafter, the first light emitting device 122a and the second light emitting device 122b may be collectively referred to as the light emitting device 122. Further, the first imaging sensor 123a and the second imaging sensor 123b may be collectively referred to as the imaging sensor 123.

[0042] FIG. 4 shows an example of a circuit diagram of the red light emitting device 131a of the first light emitting device 122a.

[0043] As shown in FIG. 4, the red light emitting device 131a includes a light emitting element 141, a first switching element 142, a second switching element 143, a constant current circuit 144, a detection circuit 145, a control circuit 146, and the like.

[0044] The light emitting element 141 is connected between the power supply voltage Vcc and the constant current circuit 144. That is, the light emitting element 141 is connected in series to the constant current circuit 144. An input terminal of the light emitting element 141 is connected to the power supply voltage Vcc, and an output terminal of the light emitting element 141 is connected to the first switching element 142. The light emitting element 141 is, for example, a red LED, and irradiates the transported medium with light having a red color. The light emitting element 141 emits light in response to a current being supplied to the light emitting element 141. The larger the amount of current supplied to the light emitting element 141, the larger the amount of light emitted by the light emitting element 141, and the smaller the amount of current supplied to the light emitting element 141, the smaller the amount of light emitted by the light emitting element 141. Hereinafter, the current supplied to the light emitting element 141 may be referred to as a first current.

[0045] The first switching element 142 is connected between the light emitting element 141 and the constant current circuit 144. That is, the first switching element 142 is connected in series to the light emitting element 141 and the constant current circuit 144. The first switching element 142 is, for example, an NPN type transistor, and its collector terminal is connected to the output terminal of the light emitting element 141, its emitter terminal is connected to the input terminal of the constant current circuit 144, and its base terminal is connected to the control circuit 146 via a resistor R1. The first switching element 142 turns on / off a first current supplied to the light emitting element 141 according to a first control signal input from the control circuit 146. When the signal value of the first control signal is equal to or greater than a predetermined threshold value, the first switching element 142 is set to ON, and the first current flows from the power supply voltage Vcc to the constant current circuit 144 via the first switching element 142, thereby causing the first current to flow to the light emitting element 141. On the other hand, when the signal value of the first control signal is less than a predetermined threshold, the first switching element 142 is set to OFF and stops the first current from the power supply voltage Vcc to the constant current circuit 144 via the first switching element 142, thereby preventing the first current from flowing to the light-emitting element 141. The first switching element 142 may be set so that the amount of current between the collector and emitter when the first switching element 142 is set to ON is equal to or less than the rated current of the light-emitting element 141.

[0046] The second switching element 143 is connected between the power supply voltage Vcc and the constant current circuit 144 so as to be connected in parallel to the first switching element 142. That is, the second switching element 143 is connected in series to the power supply voltage Vcc and the constant current circuit 144. The second switching element 143 is, for example, an NPN type transistor, and its collector terminal is connected to the power supply voltage Vcc via a resistor R2, its emitter terminal is connected to the input terminal of the constant current circuit 144, and its base terminal is connected to the control circuit 146 via a resistor R3. The second switching element 143 turns on / off the current supplied to the second switching element according to a second control signal input from the control circuit 146. Hereinafter, the current supplied to the second switching element, i.e., the current not supplied to the light emitting element 141, may be referred to as the second current. When the signal value of the second control signal is equal to or greater than a predetermined threshold value, the second switching element 143 is set to ON, and the second current flows from the power supply voltage Vcc to the constant current circuit 144 via the second switching element 143. On the other hand, when the signal value of the second control signal is less than the predetermined threshold, the second switching element 143 is set to OFF, and the second current from the power supply voltage Vcc to the constant current circuit 144 via the second switching element 143 is stopped.

[0047] The input terminal of the constant current circuit 144 is connected to the emitter terminals of the first switching element 142 and the second switching element 143. The constant current circuit 144 includes an operational amplifier U1, a transistor Tr1, and resistors R4 and R5. The positive power supply terminal of the operational amplifier U1 is connected to a power supply voltage Vcc, and the negative power supply terminal is connected to the ground. A capacitor C1 for preventing fluctuations in the power supply voltage is connected between the connection path between the positive power supply terminal and the power supply voltage Vcc and the ground. The non-inverting input terminal of the operational amplifier U1 is connected to a reference voltage Vref, the inverting input terminal is connected to the emitter terminal of the transistor Tr1, and the output terminal is connected to the base terminal of the transistor Tr1 via a resistor R4. The transistor Tr1 is, for example, an NPN transistor, and its collector terminal is connected to the input terminal of the constant current circuit 144, its emitter terminal is connected to the ground via a resistor R5, and its base terminal is connected to the output terminal of the operational amplifier U1 via a resistor R4. The positive power supply terminal of the operational amplifier U1 is not limited to the power supply voltage Vcc, and may be connected to any power supply voltage different from Vcc.

[0048] The operational amplifier U1 operates so that the potential of the emitter terminal of the transistor Tr1 becomes equal to the output potential of the reference voltage Vref. As a result, the constant current circuit 144 keeps the amount of current flowing from the collector terminal to the emitter terminal of the transistor Tr1 constant, and flows a constant current to the input terminal connected to the first switching element 142 and the second switching element 143. That is, the constant current circuit 144 adjusts the total amount of the first current supplied to the light emitting element 141 and the second current not supplied to the light emitting element 141 to be constant. The constant current circuit 144 is set so that the total amount of the first current supplied from the power supply voltage Vcc via the first switching element 142 and the second current supplied from the power supply voltage Vcc via the second switching element 143 does not exceed the rated current of the light emitting element 141.

[0049] The detection circuit 145 is connected to the constant current circuit 144 and the control circuit 146. The detection circuit 145 is a current sensor, and detects the amount of current supplied to the input terminal of the constant current circuit 144. The detection circuit 145 generates a detection signal whose signal value changes depending on whether the detected amount of current is equal to or greater than a threshold, and outputs the detection signal to the control circuit 146. The threshold is set to, for example, a rated current of the light-emitting element 141. The threshold may be set to a rated current of the first switching element 142. The detection circuit 145 may measure the emitter current, base current, emitter potential, and base potential of the transistor Tr1, and convert them into the amount of current supplied to the input terminal of the constant current circuit 144 to detect the amount of current. The detection circuit 145 is, for example, an AFE (Analog Front End). As the detection circuit 145, an ASIC (Application Specific Integrated Circuit), a DSP (digital signal processor), an LSI (large scale integration), an FPGA (Field-Programmable Gate Array), or the like may be used.

[0050] The control circuit 146 is connected to the base terminal of the first switching element 142 via a resistor R1, and controls ON / OFF of the first switching element 142. The control circuit 146 is also connected to the base terminal of the second switching element 143 via a resistor R3, and controls ON / OFF of the second switching element 143. The control circuit 146 is connected to the detection circuit 145, and controls ON / OFF of the first switching element 142 and the second switching element 143 according to a detection signal from the detection circuit 145. The control circuit 146 generates a first control signal whose signal value changes between an H value that is equal to or greater than a predetermined threshold and an L value that is less than the predetermined threshold, and outputs the first control signal to the base terminal of the first switching element 142, thereby switching ON / OFF of the first switching element 142. The control circuit 146 also generates a second control signal whose signal value changes between an H value that is equal to or greater than a predetermined threshold and an L value that is less than the predetermined threshold, and outputs the second control signal to the base terminal of the second switching element 143, thereby switching ON / OFF of the second switching element 143.

[0051] When the control circuit 146 is instructed by a processing circuit described later to cause the light emitting device 122 to emit light, the control circuit 146 switches the signal value of the second control signal from an L value to an H value at predetermined intervals. The predetermined intervals are set to be the same intervals as the imaging timing of the imaging device 117. Meanwhile, the control circuit 146 switches the signal value of the second control signal from an H value to an L value when a second predetermined time has elapsed since the signal value of the second control signal was switched from the L value to the H value. The second predetermined time is set to be sufficiently shorter than the predetermined interval and longer than the period during which an overcurrent flows when current starts to flow to the constant current circuit 144.

[0052] Furthermore, the control circuit 146 monitors the detection signal input from the detection circuit 145 after switching the signal value of the second control signal from the L value to the H value. The control circuit 146 switches the signal value of the first control signal from the L value to the H value when the signal value of the detection signal becomes a value indicating that the amount of current supplied to the constant current circuit 144 is less than the threshold value. On the other hand, the control circuit 146 switches the signal value of the first control signal from the H value to the L value when a first predetermined time has elapsed since the signal value of the first control signal was switched from the L value to the H value. The first predetermined time is set to a time for which the light emitting element 141 is turned on.

[0053] The control circuit 146 is, for example, an AFE. The control circuit 146 may be, for example, an ASIC, a DSP, an LSI, an FPGA, or the like.

[0054] The circuit of the red light emitting device 131a is not limited to the one described above. For example, the light emitting element 141, the first switching element 142, and the constant current circuit 144 may be arranged in series in any order.

[0055] The green light-emitting device 132a and the blue light-emitting device 133a of the first light-emitting device 122a, and the red light-emitting device 131b, the green light-emitting device 132b, and the blue light-emitting device 133b of the second light-emitting device 122b have the same configuration as the red light-emitting device 131a of the first light-emitting device 122a. That is, the green light-emitting device 132a, the blue light-emitting device 133a, the red light-emitting device 131b, the green light-emitting device 132b, and the blue light-emitting device 133b each have a light-emitting element, a first switching element, a second switching element, a constant current circuit, a control circuit, and a detection circuit. However, the light-emitting element of the red light-emitting device 131b is a red LED, and irradiates the transported medium with red light. On the other hand, the light-emitting element of the green light-emitting device 132a and the green light-emitting device 132b is a green LED, and irradiates the transported medium with green light. Moreover, the light emitting elements of blue light emitting device 133a and blue light emitting device 133b are green LEDs, and irradiate the transported medium with blue light.

[0056] The control circuits of the red light emitting device 131a, the green light emitting device 132a, and the blue light emitting device 133a respectively light up the light emitting elements at different timings. The red light emitting device 131a lights up the light emitting elements when the first imaging sensor 123a generates an electrical signal corresponding to the red component, and turns off the light emitting elements at other timings. The green light emitting device 132a lights up the light emitting elements when the first imaging sensor 123a generates an electrical signal corresponding to the green component, and turns off the light emitting elements at other timings. The blue light emitting device 133a lights up the light emitting elements when the first imaging sensor 123a generates an electrical signal corresponding to the blue component, and turns off the light emitting elements at other timings.

[0057] Similarly, the control circuits of the red light emitting device 131b, the green light emitting device 132b, and the blue light emitting device 133b each turn on the light emitting element at a different timing. The red light emitting device 131b turns on the light emitting element when the second imaging sensor 123b generates an electrical signal corresponding to the red component, and turns off the light emitting element at other timings. The green light emitting device 132b turns on the light emitting element when the second imaging sensor 123b generates an electrical signal corresponding to the green component, and turns off the light emitting element at other timings. The second imaging sensor 123b turns on the light emitting element when the second imaging sensor 123b generates an electrical signal corresponding to the blue component, and turns off the light emitting element at other timings.

[0058] FIG. 5 is a graph for explaining the amount of current in the red light emitting device 131a and the signal values ​​of the first control signal and the second signal.

[0059] 5, graph G11 shows the amount of current supplied to constant current circuit 144, i.e., the total amount of the first current and the second current. Graph G12 shows the current supplied from power supply voltage Vcc to constant current circuit 144 via first switching element 142, i.e., the amount of the first current supplied to light-emitting element 141. Graph G13 shows the current supplied from power supply voltage Vcc to constant current circuit 144 via second switching element 143, i.e., the amount of the second current not supplied to light-emitting element 141. The horizontal axis of each of graphs G11 to G13 shows time, and the vertical axis shows the amount of current.

[0060] On the other hand, graph G14 shows the signal value of the first control signal output from control circuit 146 and input to first switching element 142. Graph G15 shows the signal value of the second control signal output from control circuit 146 and input to second switching element 143. The horizontal axis of each of graphs G14 to G15 indicates time, and the vertical axis indicates the signal value.

[0061] As shown in graphs G14 and G15, before time T1, the signal value of the first control signal and the signal value of the second control signal are set to the L value, and the first switching element 142 and the second switching element 143 are set to OFF. As a result, as shown in graphs G11 to G13, a sufficient amount of current is not supplied to the constant current circuit 144 and the light emitting element 141, and the light emitting element 141 is turned off.

[0062] As shown in graph G15, when turning on the light-emitting element 141, the control circuit 146 first switches the signal value of the second control signal from an L value to an H value at time T1 to switch the second switching element 143 from OFF to ON. As a result, as shown in graph G13, a second current is supplied from the power supply voltage Vcc to the constant current circuit 144 via the second switching element 143, and as shown in graph G11, the second current is supplied to the constant current circuit 144. When the second current starts to flow through the second switching element 143 and the constant current circuit 144, an overcurrent larger than the rated current S of the light-emitting element 141 flows through the second switching element 143 and the constant current circuit 144.

[0063] Thereafter, as shown in graph G13, at time T2, the amount of the second current supplied from the power supply voltage Vcc to the constant current circuit 144 via the second switching element 143 drops to a preset value. As shown in graph G11, when the amount of the current supplied to the constant current circuit 144 becomes less than the rated current S of the light-emitting element 141, the signal value of the detection signal generated by the detection circuit 145 indicates that the amount of the current supplied to the constant current circuit 144 is less than the threshold value. As a result, as shown in graph G14, the control circuit 146 switches the signal value of the first control signal from the L value to the H value, and switches the first switching element 142 from OFF to ON. As a result, as shown in graph G12, the first current is supplied from the power supply voltage Vcc to the constant current circuit 144 via the first switching element 142, and the first current is supplied to the light-emitting element 141.

[0064] At this time, the second current is already being supplied to the constant current circuit 144 via the second switching element 143. The constant current circuit 144 controls the current supplied so that the total current amount does not exceed the rated current S, so that the amount of the first current supplied to the constant current circuit 144 via the first switching element 142 becomes smaller than the rated current S. Therefore, as shown in graph G12, the amount of the first current supplied to the light-emitting element 141 at time T2 is smaller than the rated current S.

[0065] As shown in graph G15, at time T3 when a second predetermined time has elapsed since time T1, the control circuit 146 switches the signal value of the second control signal from H value to L value, and switches the second switching element 143 from ON to OFF. Thereby, as shown in graph G13, the amount of the second current supplied from the power supply voltage Vcc to the constant current circuit 144 via the second switching element 143 becomes 0. As shown in graph G11, the constant current circuit 144 controls the supplied current so that the total amount of current does not exceed the rated current S. Therefore, as shown in graph G12, the amount of the first current supplied to the constant current circuit 144 via the first switching element 142 is smaller than the rated current S.

[0066] As shown in graph G14, at time T4 when a first predetermined time has elapsed since time T2, the control circuit 146 switches the signal value of the first control signal from H value to L value to switch the first switching element 142 from ON to OFF. Thereby, as shown in graph G12, the amount of the first current supplied from the power supply voltage Vcc to the constant current circuit 144 via the first switching element 142 becomes 0, and as shown in graph G11, the amount of the first current supplied to the constant current circuit 144 becomes 0.

[0067] In this way, when switching the first switching element 142 from OFF to ON, the control circuit 146 turns on the first switching element 142 while turning on the second switching element 143 to cause the second current to flow through the second switching element 143. After turning on the first switching element 142, the control circuit 146 turns off the second switching element 143 to cause the first current to flow through the light-emitting element 141 and the first switching element 142.

[0068] As described above, when a current starts to flow through the constant current circuit 144, an overcurrent flows. If an overcurrent flows through the constant current circuit 144 when the first switching element 142 is switched from OFF to ON, a current exceeding the rated current flows through the light emitting element 141. In that case, the amount of light from the light emitting element 141 becomes too large, and the quality of the input image generated by the image sensor 123 may deteriorate. In that case, the light emitting element 141 may also break down.

[0069] On the other hand, the control circuit 146 switches the second switching element 143 from OFF to ON before switching the first switching element 142 from OFF to ON. As a result, an overcurrent generated when a current starts to flow through the constant current circuit 144 flows to the second switching element 143 side, and therefore, the overcurrent is prevented from flowing through the light emitting element 141. Therefore, the control circuit 146 can prevent a deterioration in the quality of the input image generated by the imaging sensor 123 and a failure of the light emitting element 141. In addition, the control circuit 146 switches the first switching element 142 from OFF to ON before returning the second switching element 143 from ON to OFF. As a result, the light emitting element 141 starts to light up early, and the imaging sensor 123 can accumulate a sufficient amount of light in a short time. Therefore, the image reading device 100 can image each line of the medium in a short time, and can image the medium at high speed.

[0070] As described above, the first current is not supplied to the light emitting element 141 from time T1 to time T2, and the amount of the first current supplied to the light emitting element 141 is smaller than the rated current S from time T2 to time T3, so the amount of light emitted by the light emitting element 141 is small. However, the time from time T1 to time T2 and the time from time T2 to time T3 are sufficiently short compared to the entire light emission period of the light emitting element 141 (the time from time T1 to time T4). For example, when the light emission time of the light emitting element 141 is 50 μs, the time from time T2 to time T3 is 5% or less of the entire light emission period. Therefore, the image sensor 123 can accumulate a sufficient amount of light in total from time T1 to time T4.

[0071] Furthermore, when switching the first switching element 142 from OFF to ON, the control circuit 146 turns on the second switching element 143, and then turns on the first switching element 142 when the output current value of the constant current circuit 144 becomes less than the threshold value. This makes it possible for the control circuit 146 to more reliably prevent an overcurrent from flowing through the light-emitting element 141, and more reliably prevent a deterioration in the quality of the image generated by the image sensor 123, and the occurrence of a breakdown in the light-emitting element 141.

[0072] In addition, in each light emitting device, the detection circuit 145 may be omitted. In that case, when switching the first switching element 142 from OFF to ON, the control circuit 146 switches the first switching element 142 ON after a third predetermined time has elapsed since the second switching element 143 was switched ON. The third predetermined time is determined in advance as a time from when the second switching element 143 is switched from OFF to ON until the amount of current in the constant current circuit 144 becomes less than the rated current of the light emitting element 141 or the first switching element 142. In this case, too, the control circuit 146 can light up the light emitting element 141 at a higher speed while preventing an overcurrent from flowing to the light emitting element 141. Furthermore, by omitting the detection circuit 145, the control circuit 146 can suppress an increase in the cost of the device.

[0073] FIG. 6 is a graph for explaining the amount of current in red light-emitting device 131a, the amount of current in green light-emitting device 132a, and the amount of current in blue light-emitting device 133a.

[0074] 6, graph G21 shows the amount of current supplied to the constant current circuit of red light-emitting device 131a. Graph G22 shows the amount of current supplied to the constant current circuit of green light-emitting device 132a. Graph G23 shows the amount of current supplied to the constant current circuit of blue light-emitting device 133a. The horizontal axis of each of graphs G21 to G23 shows time, and the vertical axis shows the amount of current.

[0075] 6, the red light-emitting device 131a switches the signal value of the second control signal from an L value to an H value at time Tr to supply a current to the constant current circuit, and lights up the light-emitting element (red LED) from time Tr to time Tg. The green light-emitting device 132a switches the signal value of the second control signal from an L value to an H value at time Tg to supply a current to the constant current circuit, and supplies a current to the constant current circuit from time Tg to time Tb to light up the light-emitting element (green LED). The blue light-emitting device 133a switches the signal value of the second control signal from an L value to an H value at time Tb to supply a current to the constant current circuit, and supplies a current to the constant current circuit from time Tb to time Tr to light up the light-emitting element (blue LED).

[0076] That is, the control circuits of the red light emitting device 131a, the green light emitting device 132a, and the blue light emitting device 133a each supply a current to each constant current circuit at an exclusive timing, thereby lighting up each light emitting element. This enables the image sensor 123 to capture an image of a medium illuminated with light of each color of RGB and to satisfactorily generate an electrical signal corresponding to each color.

[0077] The control circuit of each light emitting device may switch the signal value of the second control signal from an L value to an H value before times Tr, Tg, and Tb to supply current to the constant current circuit, and switch the signal value of the first control signal from an L value to an H value at times Tr, Tg, and Tb to supply current to the constant current circuit. This allows each light emitting device to keep each light emitting element lit for a longer period of time, making it possible to irradiate light onto the medium satisfactorily.

[0078] FIG. 7 is a block diagram showing a schematic configuration of the image reading device 100. As shown in FIG.

[0079] In addition to the above-mentioned components, the image reading device 100 further includes a motor 151, an interface device 152, a storage device 160, a processing circuit 170, and the like.

[0080] The motor 151 has one or more motors, and rotates the feed roller 112, separation roller 113, first conveyor roller 114, second conveyor roller 115, first discharge roller 118, and second discharge roller 119 to convey the medium in response to a control signal from the processing circuit 170. Note that one of the first conveyor roller 114 and the second conveyor roller 115 may be a driven roller that rotates following the other roller. Also, one of the first discharge roller 118 and the second discharge roller 119 may be a driven roller that rotates following the other roller.

[0081] The interface device 152 has an interface circuit conforming to a serial bus such as USB, and is electrically connected to an information processing device (not shown, for example, a personal computer, a mobile information terminal, etc.) to transmit and receive input images and various information. Also, instead of the interface device 152, a communication unit having an antenna for transmitting and receiving wireless signals and a wireless communication interface device for transmitting and receiving signals through a wireless communication line in accordance with a predetermined communication protocol may be used. The predetermined communication protocol is, for example, a wireless LAN (Local Area Network).

[0082] The storage device 160 includes a memory device such as a random access memory (RAM) or a read only memory (ROM), a fixed disk device such as a hard disk, or a portable storage device such as an optical disk. The storage device 160 also stores computer programs, databases, tables, and the like used for various processes of the image reading device 100. The computer programs may be installed in the storage device 160 from a computer-readable portable recording medium using a known setup program or the like. The portable recording medium is, for example, a compact disc read only memory (CD-ROM), a digital versatile disc read only memory (DVD-ROM), or the like.

[0083] The processing circuit 170 operates based on a program previously stored in the storage device 160. The processing circuit 170 is, for example, a CPU (Central Processing Unit). The processing circuit 170 may be, for example, a DSP, an LSI, an ASIC, an FPGA, or the like.

[0084] The processing circuit 170 is connected to the operation device 105, the display device 106, the first medium sensor 111, the second medium sensor 116, the light emitting device 122, the imaging sensor 123, the motor 151, the interface device 152, the storage device 160, etc., and controls each of these components. The processing circuit 170 performs drive control of the motor 151, irradiation control of the light emitting device 122, imaging control of the imaging sensor 123, etc., acquires an input image, and transmits it to the information processing device via the interface device 152.

[0085] FIG. 8 is a diagram showing a schematic configuration of the storage device 160 and the processing circuit 170. As shown in FIG.

[0086] As shown in Fig. 8, the storage device 160 stores a transport control program 161, a reading control program 162, and the like. Each of these programs is a functional module implemented by software that runs on a processor. The processing circuitry 170 reads each program stored in the storage device 160 and operates according to the read program. In this way, the processing circuitry 170 functions as a transport control unit 171 and a reading control unit 172.

[0087] FIG. 9 is a flowchart showing an example of the operation of the medium reading process of the image reading device 100.

[0088] An example of the operation of the medium reading process of the image reading device 100 will be described below with reference to the flowchart shown in Fig. 9. Note that the flow of the operation described below is executed mainly by the processing circuit 170 in cooperation with each element of the image reading device 100 based on a program stored in advance in the storage device 160.

[0089] First, the transport control unit 171 waits until a user inputs an instruction to read a medium using the operation device 105 and receives an operation signal instructing to read a medium from the operation device 105 (step S101).

[0090] Next, the transport control unit 171 acquires a first medium signal from the first medium sensor 111, and determines whether or not a medium is placed on the placement table 103 based on the acquired first medium signal (step S102). If a medium is not placed on the placement table 103, the transport control unit 171 ends the series of steps.

[0091] On the other hand, when a medium is placed on the placement table 103, the transport control unit 171 drives the motor 151. The transport control unit 171 rotates the feed roller 112, the separation roller 113, the first transport roller 114, the second transport roller 115, the first discharge roller 118 and / or the second discharge roller 119 to transport the medium (step S103).

[0092] Next, the reading control unit 172 waits until the leading edge of the medium passes the position of the second medium sensor 116 (step S104). The reading control unit 172 periodically acquires a second medium signal from the second medium sensor 116, and determines that the leading edge of the medium has passed the position of the second medium sensor 116 when the signal value of the second medium signal changes from a value indicating that the medium is not present to a value indicating that the medium is present.

[0093] Next, the reading control unit 172 controls each control circuit of the light emitting device 122 to start emitting light (step S105).

[0094] Next, the reading control unit 172 causes the image sensor 123 to start capturing an image (step S106).

[0095] Next, the reading control unit 172 waits until the rear end of the medium passes the imaging position of the imaging device 117 (step S107). The reading control unit 172 periodically acquires a second medium signal from the second medium sensor 116, and determines that the rear end of the medium has passed the position of the second medium sensor 116 when the signal value of the second medium signal changes from a value indicating the presence of the medium to a value indicating the absence of the medium. The reading control unit 172 determines that the rear end of the medium has passed the imaging position of the imaging device 117 when a fourth predetermined time has elapsed since the rear end of the medium passed the position of the second medium sensor 116. The fourth predetermined time is set to the time required for the medium to move from the position of the second medium sensor 116 to the imaging position plus a margin.

[0096] Next, the reading control unit 172 causes the image sensor 123 to stop capturing images (step S108).

[0097] Next, the reading control unit 172 controls the light emitting device 122 to cause each control circuit to stop emitting light (step S109).

[0098] Next, the reading control unit 172 acquires an input image from the imaging sensor 123, and outputs the acquired input image by transmitting it to the information processing device via the interface device 152 (step S110).

[0099] Next, transport control unit 171 determines whether or not a medium remains on mounting table 103 based on a medium detection signal obtained from first medium sensor 111 (step S111). If a medium remains on mounting table 103, reading control unit 172 returns the process to step S104, and repeats the processes of steps S104 to S111.

[0100] On the other hand, if there are no media remaining on the mounting table 103, the transport control unit 171 stops the motor 151 (step S112), and ends the series of steps.

[0101] As described above in detail, the light emitting device 122 provides another current path in parallel with the current path between the constant current circuit 144 and the light emitting element 141, and flows a current through the other current path before starting to flow a current through the light emitting element 141. In this way, the light emitting device 122 prevents an overcurrent caused by the constant current circuit 144 from flowing through the light emitting element 141. Furthermore, the light emitting device 122 starts to flow a current through the light emitting element 141 before the amount of current supplied to the other current path becomes zero. In this way, the light emitting device 122 is able to light up the light emitting element 141 at an early stage. Therefore, the light emitting device 122 is able to light up the light emitting element 141 at a higher speed while preventing an overcurrent from flowing through the light emitting element 141.

[0102] In particular, the light emitting device 122 does not need to synchronize the ON / OFF switching timing of the first switching element 142 and the second switching element 143, and the ON / OFF switching timing can be easily designed. This makes it possible for the light emitting device 122 to prevent an overcurrent from flowing through the light emitting element 141 and to light the light emitting element 141 at high speed, while suppressing an increase in development costs.

[0103] In addition, the light emitting device can prevent an overcurrent from flowing to the light emitting element by connecting a resistor and a capacitor between the control circuit and the first switching element to reduce the speed at which the first switching element changes from OFF to ON. However, in that case, it takes a long time for a certain current to be supplied to the light emitting element, and the light emitting device may not be able to emit the amount of light required by the image reading device. The light emitting device 122 can prevent an overcurrent from flowing to the light emitting element 141 without connecting a capacitor between the control circuit 146 and the first switching element 142, and can cause the light emitting element 141 to emit a sufficient amount of light.

[0104] Furthermore, the light emitting device can prevent an overcurrent from flowing through the light emitting element by using a constant current circuit with a high (fast) response speed. However, in that case, the component cost of the constant current circuit increases and power consumption increases. The light emitting device 122 uses an inexpensive constant current circuit 144 with a general response speed to quickly switch the light emitting element 141 on and off. Therefore, the light emitting device 122 can prevent an overcurrent from flowing through the light emitting element 141 and light the light emitting element 141 at high speed while suppressing increases in device cost and power consumption.

[0105] 10 is a diagram showing a schematic configuration of a processing circuit 270 in an image reading device according to another embodiment. The processing circuit 270 is used in place of the processing circuit 170 of the image reading device 100, and executes media reading processing and the like in place of the processing circuit 170. The processing circuit 270 has a transport control circuit 271, a reading control circuit 272, and the like. Each of these units may be formed of an independent integrated circuit, microprocessor, firmware, and the like.

[0106] The transport control circuit 271 is an example of a transport control unit, and has the same function as the transport control unit 171. The transport control circuit 271 receives an operation signal from the operation device 105 or the interface device 152, and a first medium signal from the first medium sensor 111, and drives the motor 151 in response to each of the received signals.

[0107] The reading control circuit 272 is an example of a reading control unit, and has the same function as the reading control unit 172. The reading control circuit 272 receives a second medium signal from the second medium sensor 116, and controls the light emitting device 122 and the imaging sensor 123 based on the received second medium signal. The reading control circuit 272 receives an input image from the imaging sensor 123, and transmits it to the information processing device via the interface device 152.

[0108] As described above in detail, even when the image reading device uses the processing circuit 270, it is possible to prevent an overcurrent from flowing through the light emitting element 141 and to light the light emitting element 141 at a higher speed.

[0109] The image reading device is not limited to a medium conveying device that captures an image of a medium while conveying it, but may be a so-called flatbed type image reading device that captures an image of a medium placed on a glass surface while moving the imaging device 117. In this case, the imaging sensor captures an image of the medium placed on the glass surface, and each light emitting device irradiates light of a different color onto the medium placed on the glass surface.

[0110] Additionally, the light emitting device 122 may be used in any device other than an image reading device. [Explanation of symbols]

[0111] 100 image reading device, 112 feeding roller, 113 separation roller, 114 first conveying roller, 115 second conveying roller, 118 first discharge roller, 119 second discharge roller, 122 light emitting device, 123 imaging sensor, 141 light emitting element, 142 first switching element, 143 second switching element, 144 constant current circuit, 146 control circuit

Claims

1. a light emitting element that emits light in response to a first current being supplied thereto; a first switching element that switches on / off the first current supplied to the light emitting element; a second switching element that switches on / off a second current that is not supplied to the light emitting element; a constant current circuit that adjusts the total amount of the first current and the second current to be constant; a control circuit that controls ON / OFF of the first switching element and the second switching element, the control circuit turns on the first switching element in a state in which the second current flows through the second switching element by turning on the second switching element, and after turning on the first switching element, turns off the second switching element to flow the first current through the light-emitting element and the first switching element; A light emitting element driving device comprising:

2. 2 . The light-emitting element driving device according to claim 1 , wherein the control circuit turns on the first switching element after a predetermined time has elapsed since the second switching element was turned on.

3. 2 . The light-emitting element driving device according to claim 1 , wherein the control circuit turns on the first switching element when an output current value of the constant current circuit becomes less than a threshold value after turning on the second switching element.

4. an imaging unit that images a medium; A plurality of light emitting units each irradiating a medium with light of a different color, Each of the plurality of light emitting units is a light emitting element that emits light in response to a first current being supplied thereto; a first switching element that switches on / off the first current supplied to the light emitting element; a second switching element that switches on / off a second current that is not supplied to the light emitting element; a constant current circuit that adjusts the total amount of the first current and the second current to be constant; a control circuit that controls ON / OFF of the first switching element and the second switching element, the control circuit turns on the first switching element in a state in which the second current flows through the second switching element by turning on the second switching element, and after turning on the first switching element, turns off the second switching element to flow the first current through the light-emitting element and the first switching element; 1. An image reading apparatus comprising:

Images