Abstract:
A color image forming apparatus which, in a sequential order for each of plural colors of a composite color image, charges plural photosensitive bodies, exposes electrostatic latent images on the plural photosensitive bodies, develops latent images on the photosensitive bodies into color images, and transfers the color images to sequentially form images of each of the plural colors one on another to form the composite color image and transcribing the composite color image onto a printing medium. The color image forming apparatus includes: plural charging units which perform the charging process for each of the plural colors; plural exposing units which perform the exposing process for each of the plural colors; plural developing units which perform the developing process for each of the plural colors; plural transfer units which perform the transfer process for each of the plural colors; and a power supply unit which supplies power to the plural charging units, the plural developing units, and the plural transfer units.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims the benefit of Korean Application No. 2002-17905, filed Nov. 19, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a single pass type color image forming apparatus, and more particularly to a color image forming apparatus capable of supplying power to a plurality of developing devices, transfer devices and charging devices with a single power supply device.  
           [0004]    2. Description of the Related Art  
           [0005]    Generally, for a high speed image formation, a single pass type color image forming apparatus is provided with a plurality of color image forming units which are arranged along the traveling direction of the transfer belt to form a color image. Such a single pass type color image forming apparatus is sometimes called a tendem type color image forming apparatus. The plurality of color image forming units generally represent cyan (C), magenta (M), yellow (Y) and black (K) color images. The color image forming apparatus may be, for example, a color printer or a color photocopier that can reproduce color image on the printing medium.  
           [0006]    [0006]FIG. 1 shows a conventional single pass type color image forming apparatus.  
           [0007]    Referring to FIG. 1, the color image forming apparatus includes a plurality of color image forming units, a transfer belt  50 , a paper transfer unit  40  and a plurality of power supply units.  
           [0008]    The plurality of color image forming units are arranged along the traveling direction (arrow) of the transfer belt  50  for forming cyan (C), magenta (M), yellow (Y) and black (K) images. The color image forming units respectively includes photosensitive bodies  10 C,  10 M,  10 Y,  10 K for transcribing the image onto the transfer belt  50 , transfer units  30 C,  30 M,  30 Y,  30 K arranged opposite to the photosensitive bodies  10 C,  10 M,  10 Y,  10 K with respect to the transfer belt  50  disposed therebetween, and developing units  20 C,  20 M,  20 Y,  20 K for developing an electrostatic latent image formed on the photosensitive bodies  10 C,  10 M,  10 Y,  10 K with a developer such as toner or ink. On one side of each photosensitive body  10 C,  10 M,  10 Y,  10 K is formed charging unit  12 C,  12 M,  12 Y,  12 K for charging the surface of the photosensitive body  10 C,  10 M,  10 Y,  10 K, and on one side of each developing roller  21 C,  21 M,  21 Y,  21 K is formed a feeding roller  22 C,  22 M,  22 Y,  22 K for feeding a developer to the developing roller  21 C,  21 M,  21 Y,  21 K.  
           [0009]    An image of respective colors is formed on the transfer belt  50  by the plurality of color image forming units as the respective color image are sequentially formed on one another, and the complete image is transcribed by the paper transfer apparatus  40 .  
           [0010]    The paper transfer apparatus  40  transcribes the color image from the transfer belt  50  onto a printing medium  49  fed from a printing medium feeding unit (not sown) and then fixes the image.  
           [0011]    A power supply apparatus supplies power to the respective units and includes separate power supply units that supply power to each of the units. Accordingly, provided to the power supply apparatus are: a plurality of first power supply units  24 C,  24 M,  24 Y,  24 K,  23 C,  23 M,  23 Y,  23 K,  14 C,  14 M,  14 Y,  14 K for supplying power to the developing rollers  21 C,  21 M,  21 Y,  21 K, feeding rollers  22 C,  22 M,  22 Y,  22 K and charging rollers  12 C,  12 M,  12 Y,  12 K; second power supply units  32 C,  32 M,  32 Y,  32 K for supplying power to the plurality of transfer rollers  30 C,  30 M,  30 Y,  30 K; and a third power supply unit  46  for supplying power to the paper transfer apparatus  40 . In other words, at least 16 separate power supply units are required. Each of the power supply units supplies a different voltage level suitable for the corresponding process performed by the unit to which it is associated, and the level of the voltage is adjustable with respect to each of the power supply units.  
           [0012]    As described above, in a conventional single pass type color image forming apparatus, since separate power supply units are respectively required for each of the rollers and transfer rollers of the color image forming unit, the size of the color image forming apparatus tends to be large and costly to manufacture. As a result, there has been a demand for a simplified color image forming apparatus with a simplified power supply unit, tending to be smaller and less costly to manufacture.  
         SUMMARY OF THE INVENTION  
         [0013]    Accordingly, it is an aspect of the present invention to provide a single pass type color image forming apparatus having a power supply apparatus of simple construction for supplying power to the respective parts, which is compact-sized and manufactured at reduced cost.  
           [0014]    Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.  
           [0015]    According to an aspect of the present invention, there is provided A color image forming apparatus which, in a sequential order for each of plural colors of a composite color image, charges plural photosensitive bodies, exposes electrostatic latent images on the plural photosensitive bodies, develops latent images on the photosensitive bodies into color images, and transfers the color images to sequentially form images of each of the plural colors one on another to form the composite color image and transcribing the composite color image onto a printing medium. The color image forming apparatus includes: plural charging units which perform the charging process for each of the plural colors; plural exposing units which perform the exposing process for each of the plural colors; plural developing units which perform the developing process for each of the plural colors; plural transfer units which perform the transfer process for each of the plural colors; and a power supply unit which supplies power to the plural charging units, the plural developing units, and the plural transfer units.  
           [0016]    A charging power supply unit may branch an output from a single charging power transforming unit and may supply the branched power.  
           [0017]    A developing power supply unit may supply the power to the plural developing units. The developing power supply unit may include a single developing power transforming unit, and plural developing voltage dropping members disposed between the developing power transforming unit and the respective developing units.  
           [0018]    The plural developing units may include feeding rollers that supply a developer, and feeding voltage dropping members may be disposed between the feeding rollers and the plural developing voltage dropping members.  
           [0019]    The color image forming apparatus may include a transfer power unit that supplies power to the plural transfer units. The transfer power unit may include a single transfer power transforming unit, and plural transfer voltage dropping members may be disposed between the transfer power transforming unit and the respective transfer units.  
           [0020]    The developing voltage dropping members may be zener diodes.  
           [0021]    The power transforming unit is a DC-DC converter that controls the output voltage by controlling pulse width.  
           [0022]    According to another aspect of the present invention, there is provided a color image forming apparatus which forms a composite color image on a printing medium by transferring and fixing a composite color image from a transfer medium onto the printing medium, including: plural photosensitive bodies arranged on a side of and in contact with the transfer medium in an order, for bearing an image on a surface thereof; plural transfer units each disposed opposite to the side of the transfer medium on which the photosensitive bodies are arranged, for transferring the image from the surface of each of the plural photosensitive bodies onto the transfer medium by pressing the transfer medium into contact with the surfaces of the plural photosensitive bodies; plural charging units each disposed on a side of each of the plural photosensitive bodies, for charging the surface of each of the plural the photosensitive bodies; plural developing units each disposed on a lower side of each of the plural photosensitive bodies, for developing electrostatic latent images formed on the surface of each of the plural the photosensitive bodies; and a power transforming unit for modulating externally-supplied power to a power suitable for the plural transfer units, the plural charging units and the plural developing units, and which controls an output voltage in accordance with an operation environment; and plural voltage dropping members each disposed between the power transforming unit and the plural transfer units and between the power transforming unit and the developing units, for dropping a voltage output from the power transforming unit into a power suitable for the plural transfer units and the plural developing units.  
           [0023]    The plural developing units may include developer feeding rollers for feeding the developers, and feeding voltage dropping members may be disposed between the plural developer feeding rollers and the plural voltage dropping members.  
           [0024]    According to still another aspect of the present invention, there is provided an image forming device which includes: a plurality of photosensitive bodies, one for each color of a composite color image; a plurality of charging units which charge a surface of each of the plurality of photosensitive bodies so that an electrostatic latent image is formable thereon; a plurality of developing units which develop electrostatic latent images formed on the surfaces of the plurality of photosensitive bodies; a plurality of transfer units which transfer developed electrostatic latent images onto a transfer medium; a charging power supply unit which powers the plurality of charging units; a developing power supply unit which powers the plurality of developing units; and a transfer power supply unit which powers the plurality of transfer units.  
           [0025]    According to yet another aspect of the present invention, there is provided an image forming device, including: a plurality of photosensitive bodies, one for each color of a composite color image; a plurality of charging units which charge a surface of each of the plurality of photosensitive bodies so that an electrostatic latent image is formable thereon; a plurality of developing units which develop electrostatic latent images formed on the surfaces of the plurality of photosensitive bodies; a plurality of transfer units which transfer developed electrostatic latent images onto a transfer medium; and a power transforming unit, which powers the plurality of charging units, the plurality of developing units, and the plurality of transfer units. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments taken in conjunction with the accompanying drawings in which:  
         [0027]    [0027]FIG. 1 is a schematic view showing the structure of a conventional color image forming apparatus;  
         [0028]    [0028]FIG. 2 is a schematic view showing a color image forming apparatus according to a first embodiment of the present invention;  
         [0029]    [0029]FIG. 3A and 3B are schematic views showing the transfer power supply apparatus of the color image forming apparatus of FIG. 2 according to an embodiment of the present invention;  
         [0030]    [0030]FIG. 4 is a block diagram showing the power transforming apparatus of the transfer power supply apparatus of FIGS. 3A and 3B;  
         [0031]    [0031]FIGS. 5A and 5B are schematic views respectively showing the developing power supply apparatus of the color image forming apparatus of FIG. 2 according to an embodiment of the present invention;  
         [0032]    [0032]FIG. 6 is a schematic view showing the charging power supply apparatus of the color image forming apparatus of FIG. 2; and  
         [0033]    [0033]FIG. 7 is a schematic view showing the color image forming apparatus according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.  
         [0035]    Throughout the description, the same parts with the parts of the conventional apparatus shown in FIG. 1 will be referred to by the same reference numerals, and where possible, the redundant description will be omitted.  
         [0036]    Referring to FIGS. 2 and 6, the color image forming apparatus according to a first embodiment of the present invention includes 4 photosensitive bodies  1 C,  10 M,  1 Y,  10 K, transfer medium  50 , 4 transfer units  30 C,  30 M,  30 Y,  30 K, 4 developing units  20 C,  20 M,  20 Y,  20 K, 4 charging units  12 C,  12 M,  12 Y,  12 K, a paper transfer unit  40 , a transfer power transforming unit  36 , a developing power transforming unit  26  and a charging power transforming unit  16 .  
         [0037]    In an image forming operation, the surfaces of each of the photosensitive bodies  10 C,  10 M,  10 Y,  10 K is charged by an associated charging units  12 C,  12 M,  12 Y,  12 K, and an electrostatic latent image is formed on the photosensitive bodies  10 C,  10 M,  10 Y,  10 K by the laser light emitted from an associated exposure unit (not shown). The electrostatic latent images on each of the photosensitive bodies  10 C,  10 M,  10 Y,  10 K is developed by the developer fed from associated developing units  20 C,  20 M,  20 Y,  20 K, and the developed images are transcribed onto the transfer medium  50  by the transfer units  30 C,  30 M,  30 Y,  30 K, respectively. In order to represent color of the image, 4 photosensitive bodies  10 C,  10 M,  10 Y,  10 K are provided for developing a color image with 4 colors of developers. In other words, the 4 photosensitive bodies  10 C,  10 M,  10 Y,  10 K, each corresponding to the color developers of cyan C, magenta M, yellow Y and black K, are arranged in turn on the transfer medium  50  in the traveling direction (arrow of FIG. 2), and accordingly, respectively colored images are formed on the transfer medium  50  on one another.  
         [0038]    After sequentially forming the respective color images of the photosensitive bodies  10 C,  10 M,  10 Y,  10 K on one another and thus completing a given image, the complete color image is transferred onto the paper transferring unit  40 . The transfer medium  50  is a transfer belt which is driven by a driver roller  52 . In this embodiment, the transfer belt  50  is disposed to rotate clockwise (arrow of FIG. 2). However, it is to be understood that the transfer medium may be another medium and may be driven in another direction and by other means.  
         [0039]    The transfer units  30 C,  30 M,  30 Y,  30 K are formed on the inner side of the transfer medium  50 , opposing the photosensitive bodies  10 C,  10 M,  10 Y,  10 K, respectively. The transfer units  30 C,  30 M,  30 Y,  30 K respectively transcribe the developed image from the surface of each of the photosensitive bodies  10 C,  10 M,  10 Y,  10 K onto the transfer medium  50 . More specifically, the transfer units  30 C,  30 M,  30 Y,  30 K correspond to the 4 photosensitive bodies  10 C,  10 M,  10 Y,  10 K, and different levels of power are supplied to the respective transfer units  30 C,  30 M,  30 Y,  30 K to transfer the respective developed color images from the surface of the photosensitive bodies  10 C,  10 M,  10 Y,  10 K onto the transfer medium  50  one after another (i.e., sequentially). The first transfer unit  30 C in the advancing direction of the transfer medium  50  has the least voltage level, and the voltage level increases toward the last transfer unit  30 K which the highest level of voltage.  
         [0040]    The developing units  20 C,  20 M,  20 Y,  20 K are disposed on the lower sides of the photosensitive bodies  10 C,  10 M,  10 Y,  10 K, developing electrostatic latent images of the surfaces of the photosensitive bodies  10 C,  10 M,  10 Y,  10 K with a proper developer such as toner or ink. The developing units  20 C,  20 M,  20 Y,  20 K each include a developing rollers  21 C,  21 M,  21 Y, and  21 K disposed to rotate in a direction opposite to that of the photosensitive bodies  10 C,  10 M,  10 Y,  10 K, respectively, and a feeding rollers  22 C,  22 M,  22 Y, and  22 K, that feed the developer onto the developing rollers  22 C,  22 M,  22 Y, and  22 K, respectively. Albeit not shown, the developing units  20 C,  20 M,  20 Y,  20 K are connected to a developer container.  
         [0041]    The developing units  20 C,  20 M,  20 Y,  20 K develop electrostatic latent images of the 4 photosensitive bodies  10 C,  10 M,  10 Y ,  10 K with C, M, Y and K developers, respectively.  
         [0042]    The charging units  12 C,  12 M,  12 Y,  12 K are disposed on one side of each of the photosensitive bodies  10 C,  10 M,  10 Y,  10 K, respectively, and charge the surface of the photosensitive bodies  10 C  10 M,  10 Y,  10 K with a level of electricity so that an electrostatic latent image is formable on the surface of each of the photosensitive bodies  10 C,  10 M,  10 Y,  10 K by an exposure unit (not shown).  
         [0043]    The paper transfer unit  40  transfers the color image from the transfer medium  50  onto the printing medium  49  fed from the printing medium feeding unit (not shown) for fusing, and includes a paper transfer roller  42  and a paper transfer backup roller  44 . The paper transfer unit  40  is supplied with the electricity from a power supply unit  46 .  
         [0044]    The transfer power transforming unit  36  and a plurality of transfer voltage dropping members  38 C,  38 M,  38 Y,  38 K cooperate to supply appropriate power to the transfer units  30 C,  30 M,  30 Y,  30 K.  
         [0045]    The transfer power transforming unit  36  transforms externally-supplied DC, or AC power into a DC power that is suitable to operate the transfer units  30 C,  30 M,  30 Y,  30 K. AC-DC conversion is used as the transfer power transforming unit  36  when the external power is AC, while DC-DC conversion is used when the external power is DC.  
         [0046]    [0046]FIG. 4 illustrates one example of the DC-DC converter employed in this embodiment. Referring to FIG. 4, a DC-DC converter includes a pulse width control unit  62 , a voltage transforming unit  63 , a high voltage output unit  64 , a rectifying unit  65  and a voltage recognition circuit  66 . DC power, being input to the DC input unit  61 , is converted into high DC voltage as it passes through the pulse width control unit  62 , the power transforming unit  63 , the high voltage output unit  64 , and the converted voltage is fed to the transfer unit  30  to the rectifying unit  65 . The level of output DC voltage is detected at the voltage recognition circuit  66  and returned to the pulse width control unit  62 , and the pulse width control unit  62  being informed of the detected voltage level modulates the pulse width so that appropriate output can be obtained.  
         [0047]    Turning now to FIG. 3 a , power from the transfer power transforming unit  36  is branched to connect to the transfer units  30 C,  30 M,  30 Y,  30 K. Between the transfer power transforming unit  36  and the respective transfer units  30 C,  30 M,  3 Y,  30 K are the transfer voltage dropping members  38 C,  38 M,  38 Y,  38 K. The transfer voltage dropping members  38 C,  38 M,  38 Y,  38 K are electrical elements that adjust the input voltage to the respective transfer units  30 C,  30 M,  30 Y,  30 K to render the voltage suitable for the transfer units  30 C,  30 M,  30 Y,  30 K. The transfer voltage dropping member may be, by way of non-limiting example, a zener diode. Since voltage is applied to the 4 transfer units  30 C,  30 M,  30 Y,  30 K in different levels, 4 transfer voltage dropping members  38 C,  38 M,  38 Y,  38 K, i.e., one transfer voltage dropping member for each transfer unit, are employed. Alternatively, as shown in FIG. 3B, 3 transfer voltage dropping members may be used instead of 4, by fixing the power level from the transfer power transforming unit  36  to the transfer unit  30 C where the highest voltage is applied, and then gradually dropping the voltage level to the remaining transfer units  30 M,  30 Y,  30 K from the fixed level by predetermined intervals.  
         [0048]    The developing power transforming unit  26  and a plurality of developing voltage dropping members  28 C,  28 M,  28 Y,  28 K cooperate to supply appropriate power to the developing rollers  21 C,  21 M,  21 Y,  21 K.  
         [0049]    The developing power transforming unit  26  converts externally-supplied DC or AC power to a DC power suitable to operate the developing unit, and may be constructed in the same manner as that of the transfer power transforming unit  36 , but with the different level of DC voltage from the transfer power transforming unit  36 .  
         [0050]    A power output from the developing power transforming unit  26  is branched to connect to the 4 developing rollers  21 C,  21 M,  21 Y,  21 K (FIG. 5A), and between the developing power transforming unit  26  and the respective developing rollers  21 C,  21 M,  21 Y,  21 K are formed the developing voltage dropping members  28 C,  28 M,  28 Y,  28 K. The developing voltage dropping members  28 C,  28 M,  28 Y,  28 K are electrical elements that adjust the input voltage from the developing power transforming unit  26  to the developing rollers  21 C,  21 M,  21 Y,  21 K to a suitable voltage level for the respective developing rollers  21 C,  21 M,  21 Y,  21 K The developing voltage dropping members may be, by way of non-limiting example, zener diodes. Since voltage is applied to the 4 developing rollers  21 C,  21 M,  21 Y,  21 K in different levels, 4 developing voltage dropping members  28 C,  28 M,  28 Y,  28 K, i.e., one developing voltage dropping member for one developing unit, are employed. Alternatively, as shown in FIG. 5B, 3 developing voltage dropping members may be used instead of 4, by fixing the power level from the developing power transforming unit  26  to the developing roller  21 C where the highest voltage is applied, and then gradually dropping the voltage level to the remaining developing rollers  21 M,  21 Y,  21 K from the fixed level by desired intervals.  
         [0051]    On one side of each developing roller  21 C,  21 M,  21 Y,  21 K is formed a feeding roller  22 C,  22 M,  22 Y,  22 K, respectively, and the power branch from the developing voltage dropping member  28 C,  28 M,  28 Y,  28 K is respectively applied to the feeding roller  22 C,  22 M,  22 Y,  22 K. Between the respective feeding rollers  22 C,  22 M,  22 Y,  22 K and the developing voltage dropping members  28 C,  28 M,  28 Y,  28 K are formed the feeding voltage dropping members  29 C,  29 M,  29 Y,  29 K respectively, for dropping the voltage output from the developing voltage dropping members  28 C,  28 M,  28 Y,  28 K to desired levels. Since same level of voltage is supplied between the developing rollers  21 C,  21 M,  21 Y,  21 K and the feeding rollers  22 C,  22 M,  22 Y,  22 K, the same parts of same specification may be used for the feeding voltage dropping members  29 C,  29 M,  29 Y,  29 K.  
         [0052]    Referring to FIG. 6, a power output from the charging power transforming unit  16  is branched to the 4 charging units  12 C,  12 M,  12 Y,  12 K. Since the charging units  12 C,  12 M,  12 Y,  12 K each require substantially similar level of voltage to charge the photosensitive bodies  10 C,  10 M,  10 Y,  10 K, the same level of voltage is applied to the respective charging units  12 C,  12 M,  12 Y,  12 K. Accordingly, unlike the transfer power supply unit or the developing power supply unit, the charging power supply unit does not require a voltage dropping member. However, if voltage is applied to the charging units in different levels, the voltage dropping members may be provided to the charging power supply unit.  
         [0053]    According to the second embodiment of the present invention, as shown in FIG. 7, necessary power is supplied to the 4 transfer units  30 C,  30 M,  30 Y,  30 K, 4 developing units  20 C,  20 M,  20 Y,  20 K, and 4 charging units  12 C,  12 M,  12 Y,  12 K by using a single power transforming unit  70 . The level of voltage to the respective units is adjusted by the voltage dropping members  38 C,  38 M,  38 Y,  38 K,  28 C,  28 M,  28 Y,  28 K, and  72  which are installed upstream of the respective units. Because the power transforming unit  70  and the voltage dropping members  38 C,  38 M,  38 Y,  38 K,  28 C,  28 M,  28 Y,  28 K, and  72  are identical to the transfer power transforming unit and the transfer voltage dropping member described above, detailed description thereof will be omitted.  
         [0054]    The power supply process to the respective units of the color image forming apparatus constructed as above according to the first embodiment present invention will be described.  
         [0055]    First, power supply to the 4 developing units will be described.  
         [0056]    As shown in FIG. 5A, the output from a single developing power transforming unit  26  is branched four ways to connect to the 4 developing voltage dropping members  28 C,  28 M,  28 Y,  28 K and then to the developing rollers  21 C,  21 M,  21 Y,  21 K. Accordingly, the power outputted from the developing power transforming unit  26  is dropped at the developing voltage dropping members  28 C,  28 M,  28 Y,  28 K and applied to the developing rollers  21 C,  21 M,  21 Y,  21 K in the reduced levels, respectively. Also, the feeding rollers  22 C,  22 M,  22 Y,  22 K, which feed developer to the developing rollers  21 C,  21 M,  21 Y,  21 K, are supplied with the power which is branched from the power flowing through the developing voltage dropping members  28 C,  28 M,  28 Y,  28 K to the developing rollers  21 C,  21 M,  21 Y,  21 K and passed through the feeding power dropping members  29 C,  29 M,  29 Y,  29 K. Accordingly, output power from the developing power transforming unit  26  is applied to the developing rollers  21 C,  21 M,  21 Y,  21 K in different levels after being respectively reduced by the feeding power dropping members  29 C,  29 M,  29 Y,  29 K.  
         [0057]    The relation of the developing power transforming unit  26  and the developing voltage dropping members  28 C,  28 M,  28 Y,  28 K will be described.  
         [0058]    Due to different charge to mass ratios (Charge/Mass) of the cyan, magenta, yellow and black color developers, voltage is also required to be supplied in different levels for the developing of the color developers. In other words, developing voltage varies depending on the respective colors. The respective voltage levels for the respective color developers are maintained at a desired level under a general environment. However, with variation in the environment, the developing voltage also needs to vary. More specifically, the reference of the developing voltage varies in accordance with the changes in the environment, while the gaps between the respective developing voltage levels are maintained constant. In order to satisfy these requirements, the reference of the developing voltage is varied by varying the developing power transforming unit  26 , while appropriately adjusting the gaps of voltage levels of the respective color developers with the developing voltage dropping members  28 C,  28 M,  28 Y,  28 K. Describing it more detail, with the reference Vd of the developing voltage, and the gaps ΔV1, ΔV2, ΔV3, ΔV4 of voltage levels of the respective color developers, voltages applied to the cyan, magenta, yellow and black developing rollers  21 C,  21 M,  21 Y,  21 K are, respectively, Vd+ΔV1, Vd+ΔV2, Vd+ΔV3, Vd+ΔV4. Reference Vd is adjusted by the developing power transforming unit  26 , and voltage gaps ΔV1, ΔV2, ΔV3, ΔV4 are maintained by the developing voltage dropping members  28 C,  28 M,  28 Y,  28 K.  
         [0059]    The voltage, which is applied to the feeding rollers  22 C,  22 M,  22 Y,  22 K, is from the developing rollers  21 C,  21 M,  21 Y,  21 K via the feeding voltage dropping members  29 C,  29 M,  29 Y,  29 K, and accordingly the supplied voltage corresponds to the voltage of the developing rollers  21 C,  21 M,  21 Y,  21 K, which has dropped at the feeding voltage dropping members  29 C,  29 M,  29 Y,  29 K. Since the 4 feeding voltage dropping members  29 C,  29 M,  29 Y,  29 K are same in size, the gaps of the voltage supplied to the feeding rollers  22 C,  22 M,  22 Y,  22 K are maintained same as in the developing rollers  21 C,  21 M,  21 Y,  21 K of the 4 developing units.  
         [0060]    Next, power supply to the 4 transfer units  30 C,  30 M,  30 Y,  30 K will be described.  
         [0061]    As shown in FIG. 3A, an output power from the transfer power transforming unit  36  is branched four ways to connect to the 4 transfer units  30 C,  30 M,  30 Y,  30 K via the transfer voltage dropping members  38 C,  38 M,  38 Y,  38 K, respectively. The 4 transfer units  30 C,  30 M,  30 Y,  30 K transfer developed cyan, magenta, yellow and black images from the photosensitive bodies  10 C,  10 M,  10 Y,  10 K onto the transfer belt  50 , and the 4 transfer units  30 C,  30 M,  30 Y,  30 K include a transfer roller. With the developer coated on the transfer belt  50 , voltage level increases accordingly. Or by the transfer electric field, the transfer belt  50  is charged to some extent. Accordingly, the level of the voltage for transferring the respective color developers from the photosensitive bodies  10 C,  10 M,  10 Y,  10 K varies. The levels of transferring voltage are maintained relatively constant, while the reference thereof varies with changes of environment. Also, the reference transfer voltage is adjusted by the transfer power transforming unit  36 , while the transfer voltage of the respective colors is adjusted by the transfer voltage dropping members  38 C,  38 M,  38 Y,  38 K.  
         [0062]    An output power from the single charging power transforming unit  16  is branched four ways to connect to the 4 charging units  12 C,  12 M,  12 Y,  12 K. Although the 4 photosensitive bodies  10 C,  10 M,  10 Y,  10 K vary in thickness in their layers, the voltage required for the charging of surface is not necessary different. Accordingly, voltage is applied to the charging units  12 C,  12 M,  12 Y,  12 K in the same level for charging the 4 photosensitive bodies  10 C,  10 M,  10 Y,  10 K. However, since the charging voltage for charging the surface of the photosensitive bodies  10 C,  10 M,  10 Y,  10 K varies with the changes in the environment, the charging voltage is adjusted by using the charging power transforming unit  16 .  
         [0063]    According to the second embodiment of the present invention, since power is supplied to the entire color image forming apparatus from a single power transforming unit  70 , the voltage for overall operation is adjusted by the power transforming unit  70 , while the specific voltages to the developing units  20 C,  20 M,  20 Y,  20 K, the transfer units  30 C,  30 M,  30 Y,  30 K and the charging units  12 C,  12 M,  12 Y,  12 K are adjusted by using the respective voltage dropping members  28 C,  28 M,  28 Y,  28 K,  38 C,  38 M,  38 Y,  38 K, and  72 . Accordingly, voltage of suitable levels are supplied to the respective units even with the single power transforming unit  70 .  
         [0064]    As described above, in the color image forming apparatus according to second embodiments of the present invention power required for the plural transfer units, developing units and charging units is supplied respectively through a single transfer power supply unit, a single developing power supply unit and a single charging power supply unit, or, the power required for overall operation of the image forming apparatus is supplied through a single power supply unit. As a result, the number of parts related to power supply is reduced, and the manufacturing cost can be reduced.  
         [0065]    Although a few preferred embodiments of the present invention have been shown and described, the present invention is not limited to the disclosed embodiments. Rather, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.