Patent Publication Number: US-6339691-B1

Title: Image forming apparatus with a constant-current power supply

Description:
BACKGROUND OF THE INVENTION 
     There has been a image forming apparatus to irradiate a laser beam to a photoconductive drum of an image-carrying body from an exposing unit to form an electrostatic latent image on the surface of the photoconductive drum, make the electrostatic latent image visible by a developer (toner), and transfer the image onto a paper sheet. 
     In such a image forming apparatus, a photoconductive drum is rotatably provided below the exposing unit. A charging unit, a developing unit, a transferring unit, and a removing unit are disposed one after another, facing the surface of the photoconductive drum. 
     The charging unit charges the surface of the photoconductive drum with electrostatic charges. A laser beam emitted from the exposing unit is irradiated to the surface of the photoconductive drum to form an electrostatic latent image on the drum. The electrostatic latent image is made to appear as an visible image by a developer (toner) supplied from the developing unit. 
     The paper sheet is sent between the photoconductive drum and the transferring unit according to the rotation of the photoconductive drum. The transferring unit transfers the visible image (the image by the developer) on the photoconductive drum onto the sent sheet. The removing unit removes the charges of the sheet after passing through the transferring unit. 
     The charging unit comprises a metal case, and a electrifying wire provided in the metal case. Application of a D.C. voltage between the electrifying wire and the photoconductive drum causes electric discharge between the wire and the photoconductive drum to electrify the surface of the photoconductive drum. Moreover, there is caused electric discharge between the electrifying wire and the metal case. At the discharge, a direct current flows on a path between the electrifying wire and the photoconductive drum, and the total of both the direct currents becomes the operation current on the charging unit. 
     The removing unit comprises a metal case and a removing wire provided in the metal case. Application of a D.C. voltage between the removing wire and the metal case causes electric discharge between the removing wire and the metal case to remove the charges (that is, the charges caused on the paper sheet by the transferring unit) on the surface of a paper sheet passing on the removing unit through the transferring unit. And, when there is no sheet on the removing unit, or when the resistance of a sheet on the unit is small even in the presence of a sheet, the electric discharge is also caused between the removing wire and the photoconductive drum. At the electric discharge, a direct current flows through a path between the electrifying wire and the metal case, and a direct current is passed through a path between the electrifying wire and the photoconductive drum. The total of both the direct currents becomes an operation current of the removing unit. 
     Incidentally, the operation current of the removing unit is considerably small by a factor of about {fraction (1/10)}. Moreover, the polarity of the output of the removing unit is the same as that of the output of the charging unit. 
     Then, it is conceivable to use one power supply circuit in common both for the removing unit and for the charging unit to reduce the number of the parts and the cost. 
     However, it is necessary to keep the operation current of the charging unit at a predetermined value at any time. A fluctuation in the operation current of the charging unit causes density irregularities of copied image, particularly when copying photographic images with half tone density. 
     BRIEF SUMMARY OF THE INVENTION 
     Considering the above circumstances, the object of the invention is to offer a reliable image forming apparatus to use one power supply circuit in common both for an electrifying unit and for a removing unit, and keep the operation current of the charging unit at a predetermined value at any time. 
     An image forming apparatus of the invention is to form an electrostatic latent image on an image-carrying body, develop the latent image, and transfer the latent image on to a paper sheet, and comprises: 
     charging unit to electrify the surface of the image-carrying body with electrostatic charges; 
     an exposing unit to form an electrostatic latent image on the surface of an image-carrying body by irradiation of light on the surface of the image-carrying body after passing through the charging unit; 
     an developing unit to develop the electrostatic latent image formed by the exposing unit; 
     a transferring unit to transfer the image developed by the developing unit on the sheet; 
     a removing unit to remove the sheet after passing through the transferring unit; 
     a constant-current power supply circuit to supply operation currents to the charging unit and the removing unit, respectively, and keep the total of the operation currents at a predetermined value; 
     a correction circuit to detect a fluctuation in the operation current of the removing unit, and correct the predetermined value of the constant-current power supply circuit according to the detection result. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 shows an external appearances of an image forming apparatus according to one embodiment. 
     FIG. 2 shows a configuration of an inside of an image forming apparatus according to one embodiment. 
     FIG. 3 shows a configuration of a photoconductive drum and the principal parts around the drum according to one embodiment. 
     FIG. 4 shows a block diagram of an electric circuit according to one embodiment. 
     FIG. 5 shows a waveform of a signal on a constant-current power supply circuit in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of the invention will be described below. 
     FIG. 1 shows an external appearances of an image forming apparatus, for example, an electronic copier. 
     An original table  2  of transparent glass is provided on the upper part of an main body  1 . An automatic document feeder (ADF)  3  is openably provided on the original table  2 . A sheet of original set on the upper part of the feeder  3  is automatically taken in one by one, and sent to the table  2 . 
     A control panel  4  is provided adjacent to the original table  2  as an operation part in the upper part of the body  1 . The control panel  4  with a display unit  5  as displaying means is provided to set starting and stopping of copying operation, the number of copies to be copied, a copying magnification, and a copying size. 
     A tray  6  is provided to receive paper sheets discharged after copying on the side of the main body  1 . A plurality of paper sheet feeding cassettes  7  holding paper sheets for copying is provided in the lower part of the body  1 . 
     FIG. 2 shows a configuration in the main body  1 . 
     A carriage  11  is provided below the underside of the original table  2  in a reciprocating manner. An exposing lamp  12  is provided in the carriage  11 . All over the surface of the original table  2  is optically scanned by the reciprocation of the carriage  11  while lighting of the lamp  12 . 
     An reflected light image of the original put on the table  2  is obtained by the optical scanning, and the image is projected on a line sensor  17  of a CCD (charge coupled device) type line sensor (hereafter, called as CCD sensor) through reflecting mirrors  13 ,  14 ,  15 , and a lens block  16  for changing the magnification. The CCD sensor  17  outputs an image signal at the voltage level corresponding to the received light. The image signal is sent to an exposing unit  27 . The exposing unit  27  emits laser beam corresponding to the image signal. 
     A photoconductive drum  20 , serving as an image-carrying body, is rotatably provided in the vicinity of the exposing unit  27 . A charging unit  21 , a developing unit  22 , a transferring unit  23 , a first removing unit  24 , a cleaner  25 , and a second removing unit  26  is disposed one after another around the photoconductive drum  20 . A laser beam emitted from the above exposing unit  27  is irradiated on the surface of the photoconductive drum  20 , passing between the charging unit  21  and the developing unit  22 . 
     Each paper sheet feeding cassette  7  is provided in the lower part of the main body  1 . Each paper sheet feeding cassette  7  holds many paper sheets P for copying. A pickup roller  31  to take a paper sheet P out one by one is provided in each paper sheet feeding cassette  7 . 
     A paper sheet P is taken out one by one from any one of the paper sheet feeding cassettes  7  at copying. The sheet P taken out is separated from the paper sheet feeding cassette  7  with a separating unit  32  to the corresponding resist rollers  33 , and is waiting for good timing with the rotation of the photoconductive drum  20 . Each resist rollers  33  sends the sheet P between the transferring unit  23  and photoconductive drum  20 , according to the rotation of the photoconductive drum  20 . 
     The photoconductive drum  20  rotates in the direction of the arrow shown in the figure while copying. The charging unit  21  charges the surface of the photoconductive drum  20  with electrostatic charges. An electrostatic latent image is formed on the photoconductive drum  20  by the electrification, and application of the laser beam from the exposing unit  27  to the photoconductive drum  20 . 
     The developing unit  22  supplies a developer to the photoconductive drum  20 . The electrostatic latent image on the photoconductive drum  20  is made to appear as an visible image by supplying the developer. The transferring unit  23  transfers the appeared image on the photoconductive drum  20  onto the sheet P sent from each resist roller  33 . The removing unit  24  removes charges on the sheet P after transferring. The sheet P after passing through the removing unit  24  is sent to a fixing device  40  by a carrying belt  34 . 
     The fixing device  40  is provided with a heating roller  41  and a pressure roller  42 , and fixes the image by the developer on the sheet P by heating with the heating roller  41 , while carrying the sheet P under holding it between the both rollers. The sheet P is discharged to the tray  6  by a carrying roller after passing through the fixing device  40 . 
     FIG. 3 shows the photoconductive drum  20  and the principal parts around the drum. 
     The charging unit  21  comprises a metal case  21   c , and an electrifying wire  21   w  provided in the metal case  21   c . The metal case  21   c  and the electrifying wire  21   w  are extended along the axial direction of the photoconductive drum  20 . The metal case  21   c  is grounded similarly to the photoconductive drum  20 . The electrifying wire  21   w  is connected to the output terminal  100   b  of a constant-current power supply circuit  100  described below. 
     A D.C. voltage V 4  generated between the output terminal  100   b  and the ground is applied between the electrifying wire  21   w  and the photoconductive drum  20 , so that electric discharge between the electrifying wire  21   w  and the surface of the photoconductive drum  20  is generated. The surface of the photoconductive drum  20  are charged to an positive potential by the electric discharge. And the electric discharge is also caused between the electrifying wire  21   w  and the metal case  21   c . At the electric discharge, a direct current Itg flows through a path between the electrifying wire  21   w  and the photoconductive drum  20 , and a direct current Itc is passed through a path between the electrifying wire  21   w  and the metal case  21   c . The total of the direct currents Itg and Itc becomes an operation current It of the charging unit  21 . 
     There is a resistance Rtg between the electrifying wire  21   w  and the photoconductive drum  20  on the path where the direct current Itg flows. There is a resistance Rtc between the electrifying wire  21   w  and the metal case  21   c  on the path where the direct current Itc flows. 
     The removing unit  24  comprises a metal case  24   c , and a removing wire  24   w  provided in the metal case  24   c . The metal case  24   c  and the removing wire  24   w  are extended along the axial direction of the photoconductive drum  20 . The metal case  24   c  is grounded similarly to the photoconductive drum  20  and the metal case  21   c . The removing wire  24   w  is connected to the output terminal  100   b  of a constant-current power supply circuit  100  described below. 
     A D.C. voltage V 4  between the output terminal  100   b  and the ground is applied between the removing wire  24   w  and the metal case  24   c , so that electric discharge between the removing wire  24   w  and the metal case  24   c  is caused. The charges (charges charged to an positive potential by the transferring unit  23 ) on a paper sheet P passing on the removing unit  24  are removed by the electric discharge. And, when there is no paper sheet P on the removing unit  24 , or when the resistance Rp of a paper sheet P on the unit is small even in the presence of a paper sheet P on the removing unit  24 , the electric discharge is also caused between the removing wire  24   w  and the photoconductive drum  20 . At the electric discharge, a direct current Idc flows through a path between the removing wire  24   w  and the metal case  24   c , and a direct current Idg is passed through a path between the removing wire  24   w  and the photoconductive drum  20 . The total of the direct currents Idc and Idg becomes an operation current Id of the removing unit  24 . Moreover, the total of the operation current Id of the removing unit  24  and the current It of the charging unit  21  becomes an operation current Io of the constant-current power supply circuit  100 . 
     There is a resistance Rdc between the removing wire  24   w  and the metal case  24   c  on the path where the direct current Idc flows. There is a resistance Rdp between the metal case  24   c  and a paper sheet P, resistance Rp of the sheet P, and a resistance Rpg between the sheet P and the photoconductive drum  20 , between the removing wire  24   w  and the photoconductive drum  20  on the path where the direct current Idg flows. 
     A current detection resistance  91  of a correction circuit  90  is inserted and connected between the metal case  24   c  and the ground, on the path where the direct current Idc flows. FIG. 4 shows the configuration of the correction circuit  90  and the constant-current power supply circuit  100 . 
     The constant-current power supply circuit  100  comprises a switching circuit  50 , a driving circuit  60 , a rectifying circuit  70 , and a current control circuit  80 . 
     The switching circuit  50  comprises a transformer  51  having a primary coil  51   a  and a secondary coil  51   b , a capacitor  52  composing a resonance circuit together with the primary coil  51   a  of the transformer  51 , and a switching element  53  to excite the resonance circuit. And ON-OFF control operation of the switching element  53  according to a driving signal Vb output from the driving circuit  60  causes conversion of a D.C. power-supply voltage Vc supplied from the outside to an A.D. voltage of a predetermined level, and outputs the alternating voltage from the secondary coil  51   b  of the transformer  51 . 
     The driving circuit  60  acts according to a ON-signal (logic “0” signal) input from the input terminal  100   a  to output a pulsing driving signal Vb with a changing duty ratio according to a level of the output voltage V 3  of the current control circuit  80 . 
     The rectifying circuit  70  rectifies the alternating voltage output from the switching circuit  50  to output the voltage to the output terminal  100   b.    
     The current control circuit  80  comprises a series circuit with resistances  81 ,  82  under application with the D.C. power-supply voltage Vc; a series circuit with resistances  83 ,  84  connected to the output terminal  100   b  through the rectifying circuit  70 ; an amplifying circuit  85  to amplify the difference between a voltage V 2  caused on the series circuit and a reference voltage V 1  on the resistance  82 ; and a series circuit with a resistance  86  for application of the D.C. power-supply voltage Vc to a series circuit with the resistance  83  and the adjusting resistance  84 , and a diode  87 . The interconnection point between the resistance  86  and the diode  87  is connected to the input terminal  100   a.    
     When an OFF signal (logic “1” signal) is input to the input terminal  100   a , the D.C. power-supply voltage Vc is applied to the series circuit with the resistance  83  and the adjusting resistance  84 . Thereby, the voltage V 2  caused on the series circuit with the resistance  83  and the adjusting resistance  84  is forced to be in a high-level state to have the output voltage V 3  of the amplifying circuit  85  in a high-level state. 
     The higher level of the output voltage V 3  of the amplifying circuit  85  causes the smaller duty ratio of the driving signal Vd output from the driving circuit  60 , as shown in FIG.  5 . In case of an OFF signal (logic “1” signal) input to the input terminal  100   a , the duty ratio of the driving signal Vd becomes 0% to have the switching circuit  50  in a not-driven state. 
     In case of an ON signal (logic “0” signal) input to the input terminal  100   a , the application of the D.C. power-supply voltage Vc to the series circuit with the resistance  83  and the adjusting resistance  84  is released. Thereby, the voltage V 2  caused on the circuit with the resistance  83  and the adjusting resistance  84  changes according to the direct current Io (the total of the operation current Itc of the charging unit  21  and the operation current Id of the removing unit  24 ) output from the output terminal  100   b  through the rectifying circuit  70 , that is, the output current Io of the constant-current power-supply circuit  100 . 
     For example, when the output current Io is increased, the level of the voltage V 2  is increased. When the level of the voltage V 2  is increased, the level of the output voltage V 3  of the amplifying circuit  80  is increased, and the duty ratio of the driving signal Vd changes in the decreasing direction. When the duty ratio of the driving signal Vd changes in the decreasing direction, the level of the output voltage of the switching circuit  50  decreases, and, accordingly, the output current Io changes in the decreasing direction. 
     When the output current Io is decreased, the level of the voltage V 2  is decreased. When the level of the voltage V 2  is decreased, the level of V 3  on the amplifying circuit  85  is decreased, and the duty ratio of the driving signal Vd is changed in the increasing direction. When the duty ratio of the driving signal Vd is changed in the increasing direction, the level of the output voltage of the switching circuit  50  is increased, accordingly, the output voltage of the constant-current power supply circuit  100  is increased, and the output current Io is changed in the increasing direction. 
     The operation of the current control circuit  80  keeps the output current Io at a predetermined value, for example, 800 μA with a 720 μA of the rated operation current It of the charging unit  21 , and a 80 μA of the rated operation current Id of the removing unit  24 . Moreover, a resistance Rj is provided on the path where the operation current Id flows, in order to decrease the operation current Id of the removing unit  24  to a rated value of 80 μA. The reference value of the direct current Idc flowing between the removing wire  24   w  of the removing unit  24  and the metal case  24   c  becomes 72 μA. 
     However, the problem of such constant-current power supply circuit  100  is that the operation current Id of the removing unit  24  depends on the presence of a paper sheet P on the removing unit  24 , and the kind of the paper sheet P (having different resistance Rp). 
     For example, when a paper sheet P with an infinite resistance Rp is on the removing unit  24 , and the path of the direct current Idg is interrupted, the operation current Id decreases, followed by that the output current Io decreases. When the output current Io is decreased, the output voltage of the constant-current power supply circuit  100  is increased by the current control circuits  80  to adjust the output current Io in the increasing direction. The adjustment corrects the decrease in the operation current Id of the removing unit  24 . However, when the output current Io is adjusted in the increasing direction, the operation current It of the charging unit  21  has a larger value than the rated value (=720 μA). 
     Though the fluctuation in the operation current Id of the removing unit  24  does not have much influence on image forming, there has been a problem that there is irregularities in the density of the copied image, when there is the fluctuation in the operation current It of the charging unit  21 , particularly, when a photographic image with half-tone density is copied. 
     In order to solve the above problem, the correction circuit  90  is adopted. As mentioned above, the current detection resistance  91  of the correction circuit  90  is inserted and connected between the metal case  24   c  of the removing unit  24 . 
     The correction circuit  90  detects the fluctuation in the operation current Id of the removing unit  24  with the current detection resistance  91  to correct the predetermined value (800 μA) of the constant-current power supply circuit  100  according to the detection results, and comprises an amplifying circuit  92  to amplify a voltage V 5  caused on the current detection resistance  91  with a gain of “1”; a series circuit with resistances  93 ,  94  applied with the D.C. power-supply voltage Vc; an amplifying circuit  95  to amplify a reference voltage V 6  caused on the resistance  94  with a gain of “1”; a resistance  96  applied with the output voltage of the amplifying circuit  95 ; and a differential amplifying circuit  97  to output a voltage at the level corresponding to the difference between the voltage V 7  caused on the resistance  96  and the output voltage of the amplifying circuit  92 . The output voltage of the differential amplifying circuit  97  is added to the current control circuit  82  as a correction voltage V 8 . As the current control reference voltage V 1  is caused on the resistance  82 , the level of the reference voltage V 1  is made up according to the level of the correction voltage V 8 . 
     For example, when a paper sheet P with an infinity of resistance Rp is on the removing unit  24 , and the path of the operation current Id is interrupted, the operation current Id decreases, followed by that direct current Idc flowing between the removing wire  24   w  and the metal case  24   c  also decreases to have the rise in the Voltage V 5  caused on the current detection resistance  91 . The rise in the voltage V 5  is corresponding to the decrease in the operation current Id. When the voltage v 5  rises, a correction voltage V 8  of a negative level corresponding to the rise is output from the differential amplifying circuit  97 . The reference voltage V 1  of the current control circuit  80  decreases, according to the correction voltage V 8  of a negative level. 
     When the reference voltage V 1  decreases, the output voltage V 3  of the current control circuit  80  rises, and the duty ratio of the driving signal Vd changes in the decreasing direction. The change in the duty ratio of the driving signal Vd in the decreasing direction causes the fall in the output voltage level of the switching circuit  50 , followed by that the decrease in the output voltage of the constant-current power supply circuit  100  causes the change in the output current Io in the decreasing direction. 
     For example, when the operation current Id decreases by 8 μA, the output current Io becomes 792 μA by the operation of the correction circuit  90 , though the output current Io is, usually, increased to 800 μA by increase in the direction to correct the decrease of 8 μA. 
     Therefore, the operation current It of the charging unit  21  is always kept at a rated value (=720 μA), regardless of the presence of a paper sheet P on the removing unit  24 , and the kind of the paper sheet P (causes different resistance Rp). Thereby, even when photographic images with half tone density are copied, it is possible to produce good copies with no density irregularities. 
     Moreover, the operation current It to the charging unit  21  and the operation current Id to the removing units  24  may be supplied by one common constant-current power supply circuit  100  to reduce the number of parts and the cost. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.