Patent Publication Number: US-6985680-B2

Title: Image forming apparatus

Description:
This application claims priority from Japanese Patent Application Nos. 2003-107056 filed Apr. 10, 2003 and 2003-116254 filed Apr. 21, 2003, which are incorporated hereinto by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus, more particularly to an electro-photography type image forming apparatus for forming images while charging an image carrier. 
   2. Description of the Related Art 
   An electro-photography type image forming apparatus is designed, as known generally, to uniformly charge a surface of a photoconductor drum as being a drum type electro-photographic receptor. Conventionally, however, the corona electrifying method, characterized by having a corona occurring when a high voltage is applied to a thin corona discharging wire act on the surface of the photoconductor drum, has been commonly employed method. Recently, however, the contact charging method, advantageous in terms of the low-pressure process, the low ozone generation, the low cost, etc is getting popular. This method is characterized, for example, by that the charge roller, as being a charging roller member, is made to come into contact with the surface of the photoconductor drum thereby to apply a voltage to the charge roller to electrify the photoconductor drum. 
   The voltage to be applied to the charge roller may be DC voltage alone, but the AC voltage may also be applied so that the discharge to the positive and the negative can be made alternately for uniformly charging. For instance, it is known that charging the member to be charged can be made evenly when, for example, an oscillating voltage, obtained by superposing the AC voltage with a DC voltage (DC offset bias), such AC voltage having a peak-to-peak voltage equal to or higher than a discharge starting threshold voltage (charging start voltage) available when an AC voltage is applied. 
   When a sinusoidal AC voltage is applied to the charge roller, there occurs a resistive load current to flow in a resistive load between the charge roller and the photoconductor drum, a capacitive load current to flow in a capacitive load between the charge roller and the photoconductor drum, and a discharging current to flow between the charge roller and the photoconductor drum. The sum of these currents will flow in the charge roller. It is empirically known that an amount of the discharging current should be kept equal to or greater than a predetermined amount in order to maintain the discharge stable. 
     FIG. 1  shows a characteristic of the current Ic flowing through the charge roller when the charging voltage Vc is applied to the charge roller. In this case, Vc on the x-axis represents a peak value of the AC voltage, while the charging current Ic on the y-axis represents an effective value of the alternating current. 
   Gradually increasing the amplitude of the charging voltage VC causes the charging current to flow. Where the charging voltage is equal to or lower than the predetermined voltage Vh, the amplitude of the AC voltage is substantially in proportion to the charging current. This is because the discharge current will not flow where the resistance load current and the capacitive load current are in proportion to the voltage amplitude and the voltage amplitude is relatively small. Then, as the applied voltage is raised further, the discharge starts at the predetermined voltage (Vh), and the charging current Ic to the voltage amplitude comes off proportionality relation to flow in a value larger by the value of the discharging current, Is. In order to obtain a stable charge, it is sufficient to set the charging voltage to a level at which the value of the discharging current Is becomes larger than the predetermined value. 
   However, there have occurred cases where the increase in the amount of the discharge to the photoconductor drum not only accelerates the deterioration thereof such as the damage to the surface of the photoconductor drum but also causes the formation of abnormal image owing to the effect of the high-temperature and high-humidity environment coupled with the products formed during the discharging. Thus, in order to obtain a stable charge as well as to resolve such problem, it is necessary to minimize the discharge to be generated on the positive side and on the negative side alternately by applying the minimum necessary voltage. 
   Actually, the relationship between the voltage applied to the photoconductor drum and the value of discharge is not always constant but varies with the thickness of the photosensitive layer or dielectric, the material of the charging member and the changing condition of the environment such as the condition of the air. In the low-temperature and low-humidity environment, the material become dry and the resistance thereof become hard to increase, and thus it becomes necessary to apply the perk-to-peak voltage equal to or higher than a certain level. When the charging operation is carried out in a high-temperature and high-humidity environment regardless of that the operating voltage is set to the minimum voltage suiting the charging operation for obtaining a uniform charge in a low-temperature and low-humidity environment, the materials are apt to become too humid to cause a fall of resistance and resulting excessive discharging. Then, such an increase in the amount of discharge can give rise to the problems such as the poor image forming, the fusion of the toner, the cracking on the surface or the shortening of the life of the photoconductor drum. 
   Besides, it is also known that the fault caused by the change in the level of discharge is resulted also from the causes such as the variation of the quality occurring during manufacturing process, the variation of the resistance value owing to the contamination, the variation of the electrostatic capacity with the laps of time, the variation of the characteristic of the high-voltage generator and so on, in addition to the previously mentioned cause resulting from the variation of the environmental condition. 
   In order to prevent the changes in the discharge level, “Discharging Current Control Method” has been proposed (Refer to Japanese Patent Application Laid-open No. 2001-201921). In this method, the AC voltage to be applied to the charge member is made variable; the AC values are sampled respectively by the current sampling means at least at two voltage levels, namely, a voltage level lower than the voltage Vh at which the discharge starts and another voltage level equal to or higher than the voltage Vh; the optimal voltage for the optimal level of discharge is calculated to determine the level of the AC voltage to be applied to the charging member. 
   In  FIG. 1 , those points indicated by the circles and the corresponding letters, A, B, C and D, represent the points at which (the voltages) are sampled. The characteristics of the charging AC voltage Vc within the range, wherein the discharging current will not occur, and the characteristic of the charging current Ic are measured by sampling (the voltages) at the voltage levels, A and B, which are lower than the voltage Vh, at which the discharge starts. Similarly, two points, C and D, are sampled to measure the characteristic of the applied AC charging voltage Vc and the characteristic of the charging current Ic, within the range where the discharging current will not occur. Since the difference in characteristic between the above-mentioned two voltages corresponds to the discharging current Is, the level of the charging AC voltage, required for obtaining the discharging current of predetermined level, is calculated on the basis of the relationship between the above-mentioned two characteristics, and the level of the charging AC voltage is controlled according to the result of such calculation, thereby controlling the variation of the magnitude of the discharge. 
   However, the conventional discharge control method is considered to have the problems as set forth below. 
   (1) The sampling error by the current sampling means, if occurs, adversely affects the accuracy to a considerable extent in controlling the discharging current. 
   As discussed previously, in the conventional discharging current control method, the discharging current is calculated on the bases of the two relationships namely, the relationship between the characteristic of the discharging AC voltage Vc, sampled at the points (points A and B in  FIG. 1 ), lower than the discharge starting voltage Vh, and the characteristic of the discharging current Ic, and the relationship between the characteristic of the discharging AC voltage Vc, sampled at another point, lower than the discharge starting voltage Vh, and the characteristic of the discharging current Ic. However, the levels of the charging currents at the points A an B differ largely from the levels of the charging currents at the points C and D, and so the occurrence of the sampling error can cause a substantial error of the calculated discharging current. This has been a drawback to the optimal control of the discharging current. 
   (2) Another drawback to the conventional method is that the continuous printing operation can cause the variation of the charging current magnitude. When carrying out the printing operation in the continuous printing mode, the temperature around the photoconductor drum rises to cause the change in the relationship between the applied voltage to the charge roller and the discharging current and the resulting change in the value of the discharging current. This entails the problem such as the inability for optimal discharging current control. In order to overcome such a problem, it can be devised to stop the printing operation or a predetermined period at predetermined intervals during the printing operation in the continuous printing mode to let the charging AC voltage fall to a level below the discharge starting voltage Vh to sample the level of the alternating current thereby to enable the level of the discharging current to be reset to the optimal level. It has been found, however, that this method cannot be an effective solution, since this method entails the slowdown of the printing speed of the image forming apparatus. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to provide an image forming apparatus capable of providing a uniform charge being free of the problems such as the poor image forming by maintaining a highly accurate predetermined intensity of the discharge regardless of the variation of the characteristic of the charging member resulting from the change in the environmental condition and the manufacturing process, also capable of providing a predetermined charge with high accuracy without causing the slowdown of printing speed, the poor image forming or the like regardless of the variation of the characteristic of the charging member during the continuous printing operation, and further capable of stably maintaining a high image quality and a high (product) quality for a long period of time. 
   An embodiment of the present invention provides an apparatus for charging an image carrier and for transferring a latent image formed on the image carrier to form an image onto a recording medium. The apparatus comprises generating means for generating an AC voltage, a charge member whereto AC voltage from the generating means is applied, control means for controlling the generating means so as to flow a constant current according to a control value through a current path from the generating means to the charge member, first output means for outputting an information according to an peak value of the AC voltage applied to the charge member, and second output means for outputting an information according to changes in the AC voltage applied to the charge member. The control means sets the control value based on outputs from the first output means and from the second output means when the AC voltage generated by the generating means has a peak value equal to or higher than a discharge starting voltage of the image carrier. 
   Another embodiment of the present invention provides an apparatus for charging an image carrier and for transferring a latent image formed on the image carrier to form an image onto a recording medium. The apparatus comprises generating means for generating an AC voltage, a charge member whereto AC voltage from the generating means is applied, control means for controlling the generating means so as to flow a constant current according to a control value through a current path from the generating means to the charge member, first output means for outputting an information according to an peak value of the AC voltage applied to the charge member, and second output means for outputting an information according to the AC voltage applied to the charge member when the AC voltage is in a predetermined phase. The control means sets the control value based on outputs from the first output means and from the second output means when the AC voltage generated by the generating means has a peak value equal to or higher than a discharge starting voltage of the image carrier. 
   The apparatus according to the present invention accomplishes producing the discharge at a constant level and with high accuracy regardless of the variation of the characteristic of the charging member resulting from the environmental condition or the manufacturing condition, providing a uniform charge without causing problems such as the deterioration of the image carrier, the fusion of the toner, poor image formation or the like, continuing the printing operation without causing the slowdown of the operating speed, and further providing a uniform charge regardless of the contamination of the charging member and the variation of the environmental condition, for the maintaining the high quality of the image and the high quality of the apparatus over a long period of time. 
   Further, the apparatus according to the present invention accomplishes producing a constant discharge with high accuracy. 
   The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing the characteristics of the AC charging voltage and the charging current for the charging current control in the conventional image forming apparatus; 
       FIG. 2  is a diagram illustrating the compositions of the image forming apparatus as a first through seventh embodiments of the present invention; 
       FIG. 3  is a circuit diagram showing the charging high voltage output circuit of the image forming apparatus as the first embodiment of the present invention; 
       FIG. 4A  through  FIG. 4C  are illustrative diagrams showing the waveform of the charging AC voltage for the first embodiment of the present invention; 
       FIG. 5  is a sequence diagram of the printing operation in the first embodiment of the present invention; 
       FIG. 6  is a diagram showing the characteristics of the charging AC voltage and the charging current in the first embodiment; 
       FIG. 7  is a flowchart of the pre-rotation process in the first embodiment; 
       FIG. 8A  and  FIG. 8B  are the diagrams showing the characteristic of the detect signal in the pre-rotation process of the first embodiment; 
       FIG. 9A  and  FIG. 9B  are the diagrams showing the characteristics of the detect signals for the printing process of the first embodiment; 
       FIG. 10  is a flowchart of the printing process of the first embodiment; 
       FIG. 11  is a circuit diagram showing the charging high-voltage output circuit of the image forming apparatus as the second embodiment of the present invention; 
       FIG. 12  and  FIG. 13  are the circuit diagrams showing the zero crossing signal in the second embodiment; 
       FIGS. 14A and 14B  are the diagrams showing the characteristics of the detect signals in the first embodiment; 
       FIG. 15  is a processing flowchart of the second embodiment; 
       FIGS. 16A and 16B  are diagrams showing the characteristics of the detect signals for the pre-rotation process of the third embodiment; 
       FIG. 17  is a flowchart of the pre-rotation process of the third embodiment; 
       FIG. 18  is a diagram representing the characteristic of the detect signals before and after the printing process of the third embodiment; 
       FIGS. 19A and 19B  are diagrams showing the characteristics of the detect signals in the third embodiment; 
       FIG. 20  is a flowchart representing the processes of the fourth embodiment; 
       FIG. 21  is a charging high-voltage output circuit of the image forming apparatus as the fifth embodiment; 
       FIGS. 22A through 22C  are charging AC voltage waveform diagrams illustrating the charging characteristics, of which  FIG. 22A  represents the waveform at the time when the charging is not present;  FIG. 22B , the waveform at the time when the charging is present; and  FIG. 22C , the waveform at the time when the charging is present in the case of the fifth embodiment (with choke coil); 
       FIG. 23  is a characteristic diagram representing the charging AC voltage vs. the charging current in the fifth embodiment; 
       FIGS. 24A and 24B  are diagrams illustrating the charging high-voltage control processes by the charging high-voltage output circuit according to the fifth embodiment; 
       FIG. 25  is a flowchart illustrating an example of the charging control process according to the fifth embodiment; 
       FIG. 26  is the charging high-voltage circuit diagram of the image forming apparatus according to the sixth embodiment; 
       FIG. 27  is a charging high-voltage output circuit diagram according to the seventh embodiment; 
       FIG. 28  is a sectional view illustrating the construction of the high-voltage transformer of the image forming apparatus according to the seventh embodiment; and 
       FIG. 29  is an equivalent circuit of the high-voltage transformer shown in  FIG. 28 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   (The First Embodiment) 
   The first embodiment of the present invention will be described in the following ref erring to pertinent drawings.  FIG. 2  shows the composition of the laser beam printer  100  according to the present embodiment and the 2 nd  through 7 th  embodiments of the present invention. 
   The laser beam printer  100  comprises a deck  101  for storing the printing sheets P, an in-deck printing sheet detecting sensor  102  for finding the presence or absence of the printing sheets, a sheet size sensor  103  for detecting the size of the printing sheet P in the deck  101 , a pick-up roller  104  for taking the printing sheet out of the deck  101 , a printing sheet feed roller  105  for transferring the printing sheets P picked up by the pick-up roller  104 , and a retard roller  106 , constituting a pair with the sheet feed roller  105  to prevent the transfer of overlapped sheets. 
   On the downstream side of the printing sheet feed roller  105 , there are provided a feed sheet sensor  107  for detecting the transfer condition of the feed sheet coming from the deck  101  byway of a reverse turn means, a feed sheet transfer roller  108  for transferring the printing sheet towards further downstream side, a registration roller  109  for synchronized transfer of the printing sheets, and a pre-registration sensor  110  for detecting the condition of the printing sheet P to be transferred to the registration roller  109 . Further, on the downstream side of the registration roller  109 , there are provided a process cartridge  112  for forming the toner image on a photoconductor drum  1  according to the laser beam coming from a laser scanner  111 , a transcribing roller  113  for transcribing the toner image formed on the photoconductor drum  1  onto the printing sheet P, and a discharging needle  114  for removing the charge on the recording sheet P to facilitate separation of (the recording sheet) from the photoconductor drum  1 . 
   On further downstream side of the discharging needle  114 , there are provided a transfer guide  115 , a pair of a fixing roller  117 , containing a halogen heater  116  inside for heating, and a pressure roller  118 , a fixed image carrier sheet sensor for ejection  119 , and a 2-way action flapper  120  for switching the destination of the printing sheet P transferred from the fixing means to either a sheet ejecting means or a turnaround means. On further downstream side, there are provided an ejected sheet sensor  121  for detecting the condition of the transfer of the ejected sheets from the sheet ejection means and a pair of sheet ejection rollers  122 . 
   Further, on turnover side of the reverse turn means, designed for reversing the printed side of the sheet so as to be transferred back to the image forming means for having the other side thereof used for another printing, there are provided a pair of reverse turn roller pair  123 , designed for returning the recording sheet P by turning in normal direction or reverse direction, a returning action sensor  124  for detecting the transfer of the sheet to the reverse turn roller pair  123 , a D-shape section roller  125  for transferring the printing sheet P to a horizontal registration means (not shown) for registering the printing sheets with respect to the horizontal direction, a 2-way sensor  126  for detecting the transfer condition of the printing sheet P by the reverse turn means, and a transfer roller pair  127  for transferring the printing sheet P from the reverse turn means to the paper feed means. 
   The laser scanner  111  comprises a laser unit  129  for emitting the laser light modulated according to the image signal transmitted from an external apparatus  128 , the combination of a polygon mirror  130  and a scanner motor  131  for scanning the photoconductor drum with the laser light coming from the laser unit  129 , imaging lens group  132  and a turn-back mirror  133 . The processing cartridge  112  comprises the photoconductor drum  1 , the charge roller  2  and a development roller  134 , as being charging members, a toner container  135 , etc., which are essential for the known electro-photographic process and is designed for being detachable from the laser beam printer  100 . Further, the high-voltage power source  3  incorporates other high-voltage circuits besides the charging high-voltage circuit, which will be described later. The high-voltage circuit supplies necessary voltages to the development roller  234 , the transcribing roller  113  and the discharging needle  114 . 
   A main motor  136  supplies the electric power to various parts A printer controller  4  comprises an MPU (microprocessor)  5 , incorporating a RAM 5   a , a ROM 5   b , a timer  5   c , a digital input/output (I/O) port  5   d , an analog/digital conversion (A/D) input port  5   e , a digital/analog (D/A) output port  5   f , and various input/output control circuits (not shown). The printer controller  4  controls the laser beam printer  100 . The printer controller  4  is connected with the external apparatus  128 , such as the personal computers or the like, through an interface  138 . 
   The charging high-voltage control will be described referring to the diagram of the charging high-voltage circuit of  FIG. 3 . The charging high-voltage output circuit generates a charging high voltage consisting of a high AC voltage superimposed on a DC voltage and is outputted from an output terminal  200 . The output terminal  200  is connected with the charge roller  2  being in contact with the photoconductor drum  1 . 
   The base of a transistor  239  is connected with the I/O port  245   d  of a CPU  245  through a base resistor  238 ; the base resistor  238  is connected with a pull-up resistor  260 ; an emitter is grounded; a collector is connected with the output terminal of an operation amplifier  265  through a diode  240  and also connected with a pull-up resistor  237 . Hence, when a clock pulse (PRICLK) is outputted from an I/O port  245   d  of the CPU  245 , the transistor  239  is made to perform a switching action through a pull-up resistor  260  and a base resistor  238 . 
   The switching action of the transistor  239  causes an amplified clock pulse, having an amplitude corresponding to the output of an operational amplifier  265 , to be outputted. 
   This clock pulse is inputted to a filter circuit  235  to cause the filter circuit  235  to output sine wave of mainly +12V level. The filter circuit  235  comprises a capacitor  242 , resistors  223  through  232 , capacitors  216  through  220 , and operational amplifiers  217  and  220 . 
   The power of the sinusoidal output from the filter circuit  235  is amplified by a push-pull high-voltage transformer drive circuit  205  and inputted to the primary winding of a high-voltage transformer  204  through the capacitor  210  and a choke coil  2100  to cause a high AC voltage of the sine wave to be generated by the secondary winding. 
   One of the terminals of the high-voltage transformer  204  is connected with a DC high-voltage generating circuit  247  through a resistor  246 , while the other terminal thereof is connected with an output terminal  200  through a protective output resistor  203 . The high-voltage bias provided by superimposing the high AC voltage, generated in the secondary winding, on the high DC voltage, supplied from a high DC voltage generating circuit  247 , is outputted from an output terminal  200  through a protective output resistor  203  and supplied to the charge roller  2 . 
   Then, the function of the current detector of the AC high-voltage circuit will be described. The AC current, generated-by driving the previously mentioned AC high-voltage generating circuit, flows through a capacitor  248  in a fashion that the half wave in the direction of an arrow A flows through a diode  250  while the half wave in the direction of an arrow B flows through a diode  249 . The half wave in the direction of an arrow A, passed through the diode  250 , is inputted to an integrating circuit, comprising the operational amplifier  256 , a resistor  253  and a capacitor  252 , to be converted to a DC current. The characteristic of the voltage V 1  at the output terminal of the operational amplifier  256  can be expressed by the following equation.
 
 V   1 =( Rs×I mean)+ Vt   (1)
 
where Imean=Mean value of the half wave of the alternating current; Rs=Resistance value of the resistor  253 ; Vt=the non-inversion voltage inputted to the operational amplifier  256 . The output of the operational amplifier  256  is inputted to the non-inversion input (terminal) thereof to be compared with the level of the current control signal PRICNT inputted to the inversion input (terminal). The AC value is set by the current control signal PRICNT. When the output voltage V 1  from the operational amplifier  265  is larger than the current control signal PRICNT, the output of the operational amplifier  265  increases. As mentioned previously, as the output of the operational amplifier  265  increases, the amplitude of the clock pulse inputted to the filter circuit  235  increases to cause the high AC voltage to rise.
 
   With such a composition of the system, the level of the high AC voltage is controlled so that the alternating current is controlled to a value corresponding to the current control signal PRICNT. That is, the constant current control is effected corresponding to the current control signal. 
   Next, an explanation will be made as to the voltage sampling means of the charging high-voltage output circuit. The charging high-voltage output circuit comprises two sets of voltage detecting circuits, namely, a voltage detecting circuit  201  and a voltage detecting circuit  202 . 
   The voltage detecting circuit  201  detects the peak voltage of the charging AC voltage. The charging output voltage is made to drop to a lower voltage level by being divided by a capacitor  271  and a resistor  273  and inputted to a non-inversion input terminal. The operational amplifier  281 , constituting a voltage follower, is driven by both the positive and negative power sources A voltage having both the positive and negative polarities are inputted to the input terminal of the operational amplifier  281  to output the voltage having a positive polarity and the voltage having a negative polarity to the output terminal thereof. The same applies to operational amplifiers  278  and  1003 , which will be described later. 
   Further, the impedance of the capacitor  271  is set so as to be sufficiently smaller than the sum of the impedance of the resistor  272  and the impedance of the resistor  273  so that the phase difference measured between the both ends of the capacitor  271  becomes sufficiently small. Further, since the DC high voltage is interrupted by the capacitor  271 , only the AC component is inputted to the non-inversion input terminal of the operational amplifier  281 . The AC voltage passes through the operational amplifier  281 , and is converted to a DC voltage corresponding to the peak value of the charging AC voltage by means of a peak hold circuit comprising a diode  288 , a capacitor  289  and a resistor  290 , to be inputted, as a detect signal PRIVS, to an analog input terminal of a CPU  245 . 
     FIGS. 4A and 4B  represent the relationship between the charging AC waveform and the value sampled by the voltage detecting circuit  201 .  FIG. 4A  represents the case where the AC waveform is the sine wave. In this case, Vp 1  is sampled by a voltage detecting circuit  201 . On the other hand,  FIG. 4B  shows a distorted AC waveform; the time t 1  in  FIG. 4B  coincides with the time t 1  shown in  FIG. 4A ; the time t 2  in  FIG. 4B  coincides with the time t 2  in  FIG. 4A  (In  FIG. 4B ), the broken line represents the waveform, a sine wave, whose peak is distorted and peak value (Vp 2 ) thereof is lower than the normal peak value Vp 1  having no distortion. In such a case, the value of Vp 2  is detected by the voltage detecting circuit  201 . 
   Given that the peak value of the charging AC voltage is Vp, and the voltage of a diode  288  descending in normal direction is Vf, the level of the sampled signal PRIVS can be given by the following-expression:
 
 PRIVS=λ×Vp−Vf   (2)
 
where λ is a constant dependent on the resistors  272  and  273  and the capacitor  271  and can be given by the following expression:
 
λ=2×π× f×C   271 ×( R   272 + R   273 )× v{ 2×π× f×C   271 ×( R   272 + R   273 )} 2 +1/1+{2×π× f×C   271 ×( R   272 + R   273 )} 2   ×R   273 / R   273 + R   272   (3)
 
   In the Eq. (3), R 272 =the resistance value of the resistor  272 ; R 273 =resistance value of the resistor  273 ; C 271 =capacitance of the capacitor  271 . The same rule applied in the later equations. Also, note that symbol f represents the frequency of the charging AC high voltage. 
   The voltage detecting circuit  202  detects the peak value of the differential waveform of the charging AC voltage waveform. 
   The charging output voltage is differentiated by the differentiating circuit, comprising the capacitor  275  and the resistor  276 , and the differential voltage is inputted to the non-inversion terminal of the operational amplifier  278 . Where the impedance of the capacitor  275  is set sufficiently larger than the impedance of the resistor  276 , the AC voltage equivalent to the differential value of the charging AC voltage can be supplied to the input terminal of the operational amplifier  278  that constitutes the voltage follower. This AC voltage, passing through the operational amplifier  278 , is converted to the DC voltage corresponding to the peak value of the differential value of the charging AC voltage by means of the peak hold circuit, and then is inputted, as the detect signal PRIDV, to the input terminal  245  of the CPU  245 . 
     FIG. 4C  shows the relationship between the charging AC waveform and the instantaneous voltage detect signal by the voltage detecting circuit  202 . Further,  FIG. 4C  shows the differential waveform of the AC voltage waveform ( FIG. 4B ); in  FIG. 4C , the time t 1  is identical with the time t 1  in  FIG. 4A  and  FIG. 4B ; the time t 2  in  FIG. 4C  is identical with the time t 2  in  FIG. 4A  and  FIG. 4B . The broken line represents the form of the sine wave. In  FIG. 4B , the waveform is distorted in the area near the peak thereof, while in the case of the waveform shown in  FIG. 4C , the distortion ranges whole the sine wave. On the other hand, however, in the case of the waveform shown in  FIG. 4C , the value of the portion being free of the distortion coincides with the value of Vp 1  of the waveform of  FIG. 4B . In other words, even when the charging AC waveform is distorted as in the case of  FIG. 4B , the voltage detecting circuit  202  is capable of detecting the voltage identical with the value, Vp 1 , which is the value of the waveform being free of the distortion. Where the peak value of the differential value of the charging AC voltage is gives as Vp′, the level of the detect signal PRIDVS of the voltage detecting circuit  202  can be given by the following expression.
   PRIDVS=φ×Vp′−Vf   (4) 
where Vf=voltage of the diode  284  descending in normal direction. φ is a value dependent on the resistor  276  and the capacitor  275  and can be given by the following expression.
 φ=2×π× f×C   275 × R   276 × v (2×π× f×C   275 × R   276 ) 2 +1/1+(2×π× f×C   275 × R   276 ) 2   (5) 
   The values of the voltage detecting circuit  201  and the values of the resistors  273  and  276 , and the capacitors  271  and  275 , which constitute the voltage detecting circuit  201  and  202 , are set so that the constant value φ and the constant value λ become equal to each other, thereby making sampling range of the sampling value PRIVS and the sampling range of the sampling value PRIDVS coincide with each other. 
   Next, the charging high voltage control process during the printing operation of the image forming apparatus according to the present embodiment will be described. 
     FIG. 5  is a diagram representing the sequence of the printing operation of the present image forming apparatus When the main power source  100  of the apparatus is turned on, the fixing device executes the pre-multi-rotation process, i.e., a series of processes including the process for being warmed up to the predetermined temperature until reaching the standby state. Then, when the command for the start of the printing operation is received from an external apparatus  128  such as a personal computer, the image forming apparatus enters the pre-rotation process as the preparative step for the predetermined printing operation is carried out, and then enters the printing process for printing images on the printing sheets by means of a series of electro-photographic processes. When the image forming apparatus is set to the repetitive printing mode, the predetermined pre-printing processing preceding the printing of the next sheet is carried out before entering the printing process for the second printing sheet and on. After-the printing process for the last (Nth) sheet is completed, the image forming apparatus undergoes the backward rotation process and re-enters the standby state. 
   In the case of the image forming apparatus according to the present embodiment, the processing for determining the charging high AC voltage level is carried out during the pre-multi-rotation period and printing process or pre-printing process, and the result of such process is applied in controlling the charging high AC voltage during the printing operation. 
     FIG. 6  represents the characteristics of the charging alternating current Ic (on y-axis), the peak value of the charging AC voltage and the peak value of the differential value of the charging AC voltage (x-axis) at the time when the charging high AC voltage is applied to the charge roller. In  FIG. 6 , the characteristic line, LINE-A, represents the peak value of the charging AC voltage and the characteristic of the charging alternating current, while the characteristic line, LINE-B, represents the peak value of the differential value of the charging alternating current and the characteristic of the charging alternating current. The charging alternating current Ic is given in terms of the average current value of the half-wave of the charging alternating current. The peak value of the charging AC voltage is sampled by the previously described voltage detecting circuit  201 , while the peak value of the differential value of the charging AC voltage is sampled by the previously described voltage detecting circuit  202 . 
   As the charging AC voltage to the charge roller is raised, both the charging alternating currents represented by the characteristic lines, LINE-A and the LINE-B, increase linearly in proportion to the two AC voltages. The area (defined by the LINE-A and the LINE-B) corresponds to an area being free of discharging (non-discharging area), wherein only the nip current flows according to the resistive load and the capacitive load between the charge roller and the photoconductor drum. If the AC voltage is raised further, the area (defined by the LINE-A and The-LINE-B) becomes the discharging area (discharge producing area) to cause the charging current, composed of the sum of the nip current and the discharging current, to flow. 
   On the boundary between the non-discharging area and the discharging area, the value of the charging current is discontinuous and varies almost linearly with respect to the AC high voltage level within respective areas. On the other hand, the characteristic line, LINE-B, varies continuously and linearly within both the areas with respect to the high AC voltage level, irrespective of the presence or absence of the discharging. The difference in the characteristic between the characteristic line, LINE-A, and the characteristic line, LINE-B, results from the distortion of the waveform of the charging AC voltage occurring with the start of the discharge. When the charging AC voltage exceeds the charging start voltage, the discharge occurs at the time when the peak of the AC voltage nears to cause the discharge current to flow. The discharge current rises abruptly to flow instantaneously. 
   When the discharge current flows in the high-voltage transformer  204  provided for generating the charging AC voltage, the voltage drop occurs between the output terminals of the high-voltage transformer  204  owing to the effect of the leakage inductance of the high-voltage transformer  204 , causing the distortion of the output voltage waveform. In this case, the waveform is as given in  FIG. 4B . The distortion occurred to the charging AC voltage causes the difference in the peak value between the charging AC voltage and the differential value of the charging AC voltage and the resulting difference between the characteristic lines, LINE-A and LINE-B. 
   Irrespective of the presence or the absence of the discharge, the characteristic represented by the characteristic line, LINE-B, varies linearly to the level of the high AC voltage and presents a linear characteristic similar to the characteristic of the nip current and according to the resistive load and the capacitive load between the charge roller and the photoconductor drum, except the case of the discharging current. Hence, as in  FIG. 6 , the difference between the characteristic line, LINE-A, and the characteristic line, LINE-B, corresponds to the discharging current Is. 
   In the process of the charging high voltage control according to the present embodiment, the two characteristics represented by the characteristic lines, LINE-A, and the characteristic line, LINE-B, are sampled respectively as the bases whereon the charging current Ic, with which the predetermined value of the charging current can be obtained, is calculated, and, on the basis of the result of this calculation, the charging high AC voltage is controlled during the printing operation. In calculating the line characteristics, the characteristic represented by the characteristic line, LINE-A, is calculated by the samplings at the points αa and βa, while the characteristic represented by the characteristic line, LINE-B, is calculated by the sampling at the points αb and αb. All these 4 points are to be set within the area where the discharge occurs. A series of processes for determining the level of the charging high AC voltage level will be described in the following. 
   (1) Process during Pre-Rotation Period 
   Prior to the shift of the process of the image forming apparatus from the standby state to the printing process, the level of the charging high AC voltage is determined during the pre-rotation process. The series of the processes during the pre-rotation period will be described referring to  FIG. 7 . 
   In the step S 702 , the charging high DC voltage in the high DC voltage generating circuit  247  is turned on. Then, the samplings are made at the four points at the steps S 703  through S 708 .  FIG. 8A  and  FIG. 8B  show the sampling points respectively. In  FIG. 5A  and  FIG. 8B , the points αa, βa, αb and βb correspond to the points αa, βa, αb and βb in  FIG. 6  respectively. 
   First, the samplings are made at the points αa and αb at the steps S 703  through S 705 . In the step S 703 , the current control signal PRICNT (on the y-axis) is set to Vc 1 , and the charging AC voltage drive signal PRION is set to the LOW level to output the charging AC voltage. Subsequently, at the step S 704 , the sampling value PRIVS (on the x-axis in  FIG. 8A ) sampled by the voltage detecting circuit  201  is read. In this process, the inputted value is represented by Va 1 . Further, at the step S 705 , the instantaneous voltage detect signal PRIS (on the x-axis in  FIG. 8B ) is sampled by the voltage detecting circuit  202 . In this process, the read value is represented by Vb 1 . 
   In the steps S 706  through S 708 , the samplings are made at the points βa and βb. In the step S 706 , the setting of the current control signal PRICNT is altered from Vc 1  to Vc 2  to alter the output level of the charging AC voltage. In the step S 707 , the sampled value PRIVS sampled by the voltage detecting circuit  201  is read. In this process, the read value is Va 2 . In the step S 708 , only the instantaneous voltage detect signal PRIDVS sampled by the voltage detecting circuit  202  is read. In this process, the read value is Vb 2 . 
   Subsequently, the characteristics of the characteristic lines, LINE-A and LINE-B, are calculated at the previously used 4 points (S 709 ). Assuming that the characteristics represented by the characteristic lines, LINE-A and the LINE-B, can be approximated respectively by the linear equations as are given below, the constants α, β, γ and θ are calculated with respect to the 4 sampling points.
 
 PRICNT=α×PRIVS+β   (6)
 
 PRICNT=γ×PRIDVS+θ   (7)
 
   Next, the value, Vc 0 , of the current control signal PRICNT is calculated by using the above Eqs. (6) and (7) (S 710 ). As discussed previously, the difference between the characteristic lines, i.e., LINE-A and LINE-B, corresponds to the discharging current. Assuming that the amplitude (range) of the current control signal PRICNT is ΔVc, Vc 0  can be expressed by the following equation.
 
 Vc   0   =ΔVc/α−γ+α×θ−β×γ/α−γ   (8)
 
   In this case, the amplitude (range) ΔVc of the current control signal PRICNT corresponding to the predetermined discharging current value is previously stored in the ROM 245 b of the CPU 245 . Subsequently, at step S 711 , the current control signal PRICNT is set to the Vc 0 , calculated at the step S 710 , while the charging AC voltage is set to the value for the printing operation, to complete the series of processes. When these processes are completed, the processing enters the process for the printing of the first sheet. 
   (2) Process in Printing 
   Described in the forgoing is concerned with the processing starting from the standby state to the process for determining the charging high AC voltage level that is required for starting the printing operation for the printing of the first sheet. In the image forming apparatus according to the present embodiment, in carrying out the continuous printing, even after entering the printing process, the processing for determining the high AC charging voltage level is repeated so that the setting of the AC voltage level can be corrected according to the result of the processing. This processing is made each time when the continuous printing of 50 sheets is finished starting from the standby state. The necessity of this processing is determined on the basis of the count made by the counter for counting the number of printed sheets incorporated into the CPU 245 . 
   The corrective processing for the high AC charging voltage level during the period of the printing process will be described referring to  FIG. 10 . For the corrective processing, similarly to the case of the corrective processing during the previously mentioned pre-rotation process period, the sampling is made at the 4 points to re-detect the characteristics represented by the LINE-A and the LINE-B to thereby calculate the value of the current control signal PRICNT at which the value of the current for discharging coincides with the predetermined value. 
     FIG. 9A  and  FIG. 9B  are the diagrams showing the sampling points on the characteristic lines, LINE-A and LINE-B. First, at the steps S 1002  and S 1003 , the current control signal PRICNT (on the y-axis) is set to the present value Vc 0  to make the sampling. In this case, the present value Vc 0  means the set value calculated in the process for determining the AC voltage level, which has been carried out immediately before carrying out the present processes. The value PRIVS (on the x-axis in  FIG. 9A ) sampled by the voltage detecting circuit  201  and the value PRIDVS (on the x-axis in  FIG. 9B ) sampled by the voltage detecting circuit  202 . These values are read as Va 1 ′ and Vb 1 ′ respectively. 
   Subsequently, at steps S 1004  through S 1006 , the value of the current control signal PRICNT is increased by Vk to Vc 0  before carrying out the sampling. In other words, the sampling is made in the state where AC charging voltage is set higher than the present value. The value PRIVS sampled by the voltage detecting circuit  201  and the value PRIDVS sampled by the voltage detecting circuit  202  are read respectively as Va 2 ′ and Vb 2 ′. 
   The reason for that the current control signal PRICNT is sampled at the point where the value of the charging current is higher than the present value Vc 0  is to prevent the occurrence of the poor image owing to the change in the level of the AC charging voltage by raising the level of the charging current on the photoconductor drum. If the sampling is made at the point where the value of the charging current is lower than the present value Vc 0 , there is the possibility that the charge on the photoconductor drum becomes too low to cause the formation of the poor image. 
   Subsequently, similarly to the processing carried out in the pre-rotation process, the characteristics of the characteristic lines, LINE-A and LINE-B, are calculated (S 1007 ), and the value Vc 0  of the current control signal PRICNT is calculated (S 100 B) to thereby obtain the value coinciding with the predetermined value of the current for discharging. Subsequently, at step S 1009 , the setting of the current control signal PRICNT is altered from Vc 0  to Vc 0 ′ to alter the output level of the AC charging voltage thereby completing a series of processes. After completing the necessary processes, the counter for counting the number of the printed sheets is reset, and the same processes are repeated from the point at which the printing of 50 sheets is completed. 
   As described in the foregoing, in the control of the high charging voltage according to the present embodiment, the peak value of the differential value of the AC charging voltage is measured, and the nip current is sampled by using the measured value of the AC charging voltage. By employing such composition for the system, it becomes possible to sample the nip current within the discharging range, thereby enabling the level of the current for discharging to be controlled with high accuracy. In this way, it becomes possible to obtain a uniform charge without giving rise to a problem such as the deterioration of the photoconductor drum or the formation of poor image, irrespective of the variation of the characteristic of the charging member during the manufacturing process or the change in environmental condition. Further, at the time of the continuous printing operation, it becomes possible to reset the level of the current for discharging without causing the level of the charging current to become lower than the present level, and also, at the time of the continuous printing operation, it becomes possible to obtain a uniform charge without causing the deterioration of the photoconductor drum or the formation of poor image, irrespective of the change in the environmental condition or the variation of the characteristic of the charging member occurring during the manufacturing process. 
   (The Second Embodiment) 
   The second embodiment of the present invention will be described in the following. In the first embodiment, the nip current is sampled by sampling the peak value of the differential value of the AC charging voltage. In the second embodiment, however, the nip current is sampled on the basis of the phase deviation in the predetermined phase interval. 
     FIG. 11  is the charging high-voltage output circuit in the image forming apparatus as the second embodiment, and the basic composition the circuit is similar to the circuit of the first embodiment. 
   The second embodiment differs from the first embodiment in that the voltage detecting circuit  202 , as being the differential voltage detecting circuit which is found in the first embodiment, is not provided, that a zero crossing detecting circuit  1009  for sampling the zero crossing point, at which the witching between the positive polarity and the negative polarity of the alternating current waveform takes place, is provided, and that a circuit for sampling the instantaneous value of the AC charging voltage is provided. 
   The AC charging voltage is connected with a comparator  1003  through a capacitor  1001  and resistors  1002 ,  1005 ,  1004  and  1007 . The capacitance of the capacitor  1001  is set to a value so that the impedance becomes sufficiently low to the combined resistance of the resistors  1002 ,  1004 ,  1005  and  1007 . Hence, the phase shift at the two ends of the capacitor  1001  is (relatively) small, so that, for the non-inverted input and the inverted input to the comparator  1300 , the AC signal having the phase which is identical with the output terminal is inputted. When the AC charging voltage has a positive polarity, the potentiality of the inverted input to the comparator  1003  becomes equal to or higher than the potentiality of the non-inverted input to make the output 0V, whereas when the same has a negative potentiality, the potentiality of the inverted input to the comparator  1003  becomes lower than the non-inverted input to make the output 5V. 
   A diode  1006  prevents the potentiality of the comparator  1003  from becoming lower than the predetermined potentiality. The output of the comparator  1003 , as being the detect signal PRIZERO of the zero crossing detecting circuit  1009 , is connected with CPU 245 . The sampled signal PRIZERO is inputted to the external interruption terminal of the I/O port of the CPU  245  where the interruption occurs at the falling edge of the input signal. 
     FIG. 12  is a timing chart representing the waveform of the AC charging voltage and the zero crossing detect signal PRIZERO. At the time point where AC charging voltage has a negative polarity, the voltage of the zero crossing detect signal PRIZERO becomes 5V which corresponds to the HIGH level of the CPU 245 . At the time point when the polarity of the AC charging voltage is switched from a negative polarity to a positive polarity, the voltage of the zero crossing detect signal PRIZERO is switched to 0V. In other words, the system is in a state so that the zero crossing timing of the AC charging voltage can be read by the CPU  245 . Further, the internal timer of the CPU  245  is used to sample the timing after laps of the predetermined time length φ from the timing for the fall of the zero crossing detect signal PRIZERO. 
   In the image forming apparatus according to the second embodiment, the time φt corresponding to the time, at which the AC charging voltage is equivalent to 30 deg. from the phase of the AC charging voltage circuit, is sampled to sample the level Vt of the AC charging voltage at that time. The magnitude of the φt is to be set so that the distortion will not occur within the range of the φt in consideration of the magnitude of the distortion that can cause the distortion of the waveform of the AC charging voltage. 
   φt can be expressed by the following equation where the frequency is given as f.
 
φ=1/ f× 30/360  (9)
 
   Further, Vt is sampled by means of the instantaneous voltage detect signal PRIDVS to be inputted to A/D input port  245   f  of the CPU  245 . 
   The instantaneous voltage detect signal PIRVS is a signal corresponding to the instantaneous value of the AC charging voltage and is obtained by converting the AC charging voltage, which has been divided by means of the capacitor  271 , the resistor  272  and the resistor  273 , through a voltage follower, comprising an operational amplifier  1013 , and a diode  1011 . The diode  1011 , having a characteristic identical with the characteristic of a diode  288  in the voltage detecting circuit  201 , is used so that the instantaneous voltage detect signal PRIVS and the detect signal PRIVS, to be applied to the voltage detecting circuit  201  have identical sampling ranges. 
   The relationship between the AC charging voltage waveforms φt and Vt is shown in  FIG. 13 . In  FIG. 13 , the broken line is a sine wave whose peak value is Va 1 . Similarly to the case in the first embodiment, the portion near the peak of the wave is distorted to make peak value Va 2  thereof lower than the peak value Va 1 . However, the distortion of the waveform is not present within the range of φ 1 . Since φt is the timing of the sine wave at the point of 30 deg, the relationship between the voltage at 30 deg. and the peak value Va 1  of the sine wave can be expressed by the following equation.
 
 Vt =SIN(30 deg.)× Va   1 =0.5 ×Va   1   (10)
 
   That is, the double value of the voltage level Vt at the timing of φt becomes the peak value Va 1  of the sine wave. 
   Since the characteristic of the peak value of the sine wave and the characteristic of the alternating charging current are identical with the characteristic of the characteristic line, LINE-B, in  FIG. 6  of the first embodiment, the nip current can be measured by using the double value of the Vt. In controlling the alternating charging current in the second embodiment, the Vt is measured; the characteristic of the nip current is measured on the basis of the double value of the Vt; and the value of the current for discharging is controlled to the predetermined value according to the steps similar to those in the case of the first embodiment. 
   Next, a series of steps for determining the high AC charging voltage level the pre-rotation stage in the second embodiment are shown in  FIG. 14A  and  FIG. 14B . The basic steps corresponding to the series of the processes are similar to those of the first embodiment but differ only in the sampling process of the characteristic line, LINE-B. 
   In  FIG. 15 , the bias of the direct charging current is turned on, and then the samplings are made at 4 points, i.e., αa, αb, βa and βb shown in  FIG. 6 .  FIG. 14A  shows the characteristics of the detect signal PRIVS (on x-axis) and the charging current control signal PRICNT (on y-axis), while  FIG. 14B  shows the characteristics of the PRIRVS×2 (on the x-axis), the double value of the instantaneous voltage detect signal PRIRVS, and the current control signal PRICNT (on the x-axis). 
   First, the samplings at the points, αa and αb, are made at the steps, S 1503  through S 1505 . In step S 1503 , the current control signal PRICNT is set to Vc 1 , and the charging AC voltage drive signal PRION is set to LOW level to output the charging AC voltage. Subsequently, at step S 1504 , the value PRIVS sampled by the voltage detecting circuit  201  is read. In this case, the value to be read is Va 1 . Further, at step  1505 , the instantaneous voltage detect signal PRIRVS is read, and the double value thereof is set to Vt 1 . 
   In the steps S 1506  through S 1408 , the samplings are made at the points, βa and βb. In the step S 1506 , the set value of the current control signal PRICNT is altered from Vc 1  to Vc 2  to alter the output level of the AC charging voltage. In the step S 1507 , the value of PRIVS sampled by the voltage detecting circuit  201  is read. In this case, the value to be read is Va 2 . In the step S 1508 , the sampled instantaneous voltage signal PRIRVS is read, and the double value of the read value is set as Vt 2 . 
   Subsequently, the processing proceeds to step  1509  to calculates the characteristics of the characteristic lines, LINE-A and LINE-B, by using the 4 points at which the samplings have been made according to the previously described processes. Assuming that the characteristic lines, LINE-A and LINE-B, can be approximated by the linear equations as are given below respectively, the constants, α, β, γ and θ are calculated on the bases of the four sampling points.
 
 PRICNT=α×PRIVS+β   (11)
 
 PRICNT =( PRIRVS× 2)+θ  (12)
 
   Next, from the Eqs. (11) and (12), the Vc0, the value of the current control signal PRICNT, with which the discharging current value coincides with predetermined value (S 2610 ). 
   Similarly to the case of the first embodiment, where the range of the current control signal PRICNT, corresponding to the predetermined discharging current, is given as ΔVc, the Vc 0  can be expressed by the following equation.
 
 Vc   0   =ΔVc/α−γ+α×θ−β×γ/α−γ   (13)
 
   Subsequently, at the step S 1511 , the current control signal PRICNT is set to Vc 0 , which has been calculated at the step S 1510 , to thereby set the AC charging voltage to the value for the printing operation to finish a series of processes. After completing these processes, the processing proceeds to the printing process for the first sheet. Further, previously, the example of the application of the processing (in the case of the first embodiment) to the processing during the pre-rotation process; however, the processing during the printing operation process in the case of the first embodiment can also be applied to the processing in the present embodiment. 
   As described in the foregoing, in the case of the control of the high-voltage for charging according to the second embodiment, the deviation of the AC voltage-in the predetermined section is measured so that the measured deviation value can be applied in sampling the nip current. With the system composed in this way, it becomes possible to sample the nip current within the range wherein the discharge occurs to thereby making it possible to control the discharging current with high accuracy. Hence, irrespective of the variation of the environmental condition or the variation of the characteristic of the charging member occurred during the manufacturing process, it becomes possible to obtain a uniform charge without giving rise to the problems such as the deterioration of the photoconductor drum or the poor image formation. Furthermore, in the continuous printing operation, it becomes possible to reset the magnitude of the discharging current to prevent the magnitude of the charging current from falling below the level of the present charging current, whereby a uniform charge can be obtained without entailing the problems such as the deterioration of the photoconductor drum or poor image formation while being free of the variation of the environmental condition or the variation in the characteristic of the charging member owing to the manufacturing process. 
   (The Third Embodiment) 
     FIG. 16A  shows the relationship among the peak value of the AC charging voltage, the peak value (on the x-axis) of the differential value of the AC charging voltage and the charging current Ic (the y-axis). The process of the charging high voltage control will be described referring to  FIG. 16A . In the present embodiment, in order for the high charging voltage to be controlled to the predetermined value, Iac 1 , the charging AC voltage is applied accordingly, and the processing as is described in the following are executed. 
   First, the peak value Vac 1 , corresponding to the intersecting point a between the characteristic line, LINE-A, (representing the peak value of the charging AC voltage) and the straight line, including the point of the peak value Vac 1  of the AC charging voltage, is sampled by the voltage detecting circuit  201 , while the peak value Vac 1 ′ of the differential value of the AC value for charging, corresponding to the intersecting point a′ between the characteristic line, LINE-B, (representing the peak value of the differential value of the AC charging voltage) and the straight line, including the point at which the charging current becomes Iac 1 , is sampled by the voltage detecting circuit  202  Next, the charging current Ic is varied until the sampled value Vac 1 ′ sampled by the voltage detection circuit  202  becomes equal to the initial sampled value Vac 1  by the voltage detecting circuit  201 . 
   Then, the charging current Iac 1 , at which the level of the actual charging current Is′ coincides with the predetermined charging current Is, is calculated so that the high AC charging voltage during the printing operation can be controlled on the basis of the Iac 1 ; the Is′ is the value corresponding to the difference between the value of the intersecting point, a, between the straight line, representing the charging current Iac 1 , and the characteristic line, LINE-A, and the value of the intersecting point, b, between the straight line, representing the charging current Iac 1 ′ and-the characteristic line, LINE-B. All these three points, a, a′ and b, are set within the range of charging. A series of processes for determining the AC high charging voltage level will be described in detail in the following. 
   (1) Process during Pre-rotation Period 
   When the operation of the image forming apparatus proceeds to the printing process from the standby state, the processing for determining the level of AC high charging voltage will be made during the pre-rotation period. The series of processes during the pre-rotation period will be described referring to  FIG. 17 . 
   First, at step S 1702 , the high DC charging voltage means is turned on. Then, by undergoing the processes at the steps S 1703  through S 1708 , the value of the charging current Ic is determined at the point where the peak value of the charging AC voltage becomes equal to the peak value of the differential value of the charging AC voltage (at the point where the value Vac 1 ′ sampled by the voltage detecting circuit  202  becomes equal to the value Vac 1  sampled by the voltage detecting circuit  201 ). In the case of the processing shown in  FIG. 17 , “the point at which a value becomes equal to” means the case where the value of the difference is less than 0.03V. 
   In the step S 1703 , the current control signal PRICNT is set to Vc 1 , while the charging AC voltage drive signal PRION is set to LOW level, to output the charging AC voltage. Further, the initial value of the current control signal PRICNT is set as Vc 1 ′=Vc 1  at the time when the peak value Vac 1 ′ of the differential value of the charging AC voltage is approximated to the peak value Vac 1  of the charging AC. In this stage, the value of the Vc 1  is set to the value that is sufficiently larger than the value of the charging current to be set finally so that the value can be controlled only for the direction of lowering in the later step S 1707 . 
   Following the setting of various parameters, at the step S 1704 , the value of PRIVS (the peak value Vac 1  of the charging AC voltage), sampled by the voltage detecting circuit  201 , is read, and, at the step SL 705 , the instantaneous voltage detect signal PRIDVS (the peak value Vac 1 ′ of the differential value of the charging AC voltage) sampled by the voltage detecting circuit  202 , is read. Then, the processes of the steps S 1705  through S 1708  are repeated until the difference between the peak value Vac 1  of the charging AC voltage and the peak value Vac 1 ′ of the differential value of the charging AC voltage is reduced by 0.1V to less than 0.03V, and the value of Vac 1 ′ is read to the current control signal PRICNT. 
     FIG. 16B  shows the relationship between the peak value of the AC voltage/the peak value (on the y-axis) of the differential value of the AC voltage and the current control signal PRICNT (on the x-axis). Assuming that the value of the charging current satisfying the value required at the step S 1706  is Iac 1 ′, the value of the controlling current corresponding to Iac 1  is Vc 1 , while the current control signal corresponding to the current control signal is Vc 1  is Vc 1 ′. Hence, the difference Vis in voltage of the current control signal to the actual charging current Is′ is set as Vis−Vc 1 −Vc 1 ′. 
   Next, the processes of the steps S 1709  through S 1715  are executed to determine the value of the charging current at which the difference (in charging current) between the value of the characteristic line, LINE-A, and the value of the characteristic line, LINE-B, coincides with the predetermined value. First, at the step S 1709 , the sampled voltage, Vis=Vc 1 −Vc 1 ′, to the actual discharging current is determined, and, at the step S 1710 , the sampled voltage difference Vs, corresponding to the predetermined discharging current Is, is compared with Vis. When the value of the actual charging current (Is′=Iac 1 −Iac 1 ′) is larger than Is, that is, when Vs−Vis&gt;0, the processing proceeds to step S 1711 , where whether Vis is larger than Vs by more than 0.03V is checked. At this stage, when (Vis) is found to be larger than (Vs) it should be (i.e., when Vis−Vs&lt;0.03V does not hold), the processing proceeds to step S 1712  to reduce the value of the Vc 1  by 0.1V, and the processes from the step S 1703  on will be repeated. 
   On the other hand, when the actual charging current (Iac 1 −Iac 1 ′) is smaller than Is and Vs&gt;Vis, the processing proceed from step S 1710  to step S 1714  through the step S 1713 . More particularly, when Vs−Vis&gt;0, the processing proceeds to the step S 1713  to examine whether Vs−Vis=0 or not, and when Vs−Vis≠0, whether Vs is larger than Vis by 0.03V or more is checked at the step S 1714 . In this stage, when (the Vs) is larger (when Vs−Vis&lt;0.03V does not hold), the processing proceeds to the step S 1715  to increase the value of the Vc 1  by 0.1V, and then the processes from the step S 1703  and on are repeated. 
   In the step S 1713 , when Vs is equal to Vis, or when the difference between Vis and Vs found to be less than 0.03V at the step S 1711  or S 1714 , the charging current Iac 1  will be outputted on the basis of the current control signal PRICNT=Vc 1 , as being the definite value, and the processing proceeds to the printing operation for the first sheet. 
   In the third embodiment, the minimum control range of Vc 1  is defined to be 0.1V, and the control range is set to 0.03V, (approximately the double value) of the minimum control range, but any values closer to 0 may be chosen for the minimum control range depending on the actual composition of the circuit and the processing speed and thus the control range is not limited to the values applied in the third embodiment. Further, the process for controlling Vc 1  characterized by that the value of Iac 1  is reduced starting from the value having a sufficiently large magnitude, but the control of the Vc 1  may be started from sufficiently small value. 
   (2) Process in Printing 
   What has been discussed in the foregoing is concerned with the process for determining the level of the high charging AC voltage in starting the printing operation for the first sheet. 
   In carrying out the printing operation continuously, the charging characteristic is apt to vary from the initial state thereof owing to the effect of the change in the temperature of the charge roller or the contamination on the surface thereof.  FIG. 18  shows, for example, the characteristics of the charging alternating current (on the y-axis), the peak value of the charging AC voltage and the peak value (on the x-axis) of the differential value of the charging AC voltage respectively at the point before the printing operation and at the point after printing 500 sheets. In  FIG. 18 , the thin solid line (representing the peak value of the differential value of the charging AC voltage) and the thin alternate long and short dash line (representing the peak value of the charging AC voltage) show the initial characteristics of these factor, while the thick solid line and the thick alternate long and short dash line represent the characteristics of the same factors after the continuous printing operation. 
   In the third embodiment, the previously mentioned current control signal PRICNT is controlled by the defined value of Vc 1 , so that, as shown in  FIG. 18 , as the inclinations of the characteristic lines, LINE-A and LINE-B, become smaller than the inclinations of the initial characteristics lines, the value of actual charging current Is′ increases to Is″. In other words, when the peak voltage Vac 1 ′ of the differential value of the charging AC voltage is made equal to the peak voltage Vac 1  of the charging AC voltage, the charging current Iac 1 ′ becomes smaller than Iac 1 ′. Hence, the actual discharging current Is″ after the continuous printing operation becomes larger than the actual discharging current Is′ at the initial stage of the printing operation. 
   Thus, in the case of the continuous printing operation according to third embodiment, even after the processing has entered the printing process, the level of the high AC charging voltage is determined again to correct the setting of the AC voltage on the basis of the re-determined AC voltage level. Such setting adjustment processing in the third embodiment is made each time the continuous printing of 50 sheets is finished starting from the standby state and the subsequent start of the printing operation. The timing (for such re-determination of the charging voltage) will be determined on the basis of he reading of the counter for counting the number of the printed sheets incorporated into the CPU 245 . The high AC charging voltage during the process of the printing operation is adjusted by the processing similar to the processing shown in  FIG. 17 . 
   After adjusting processing is completed, the counter for counting the number of printed sheets is reset, and the same adjusting processing for the high AC charging voltage is repeated each time the continuous printing of 50 sheets has finished. The interval of such adjusting process need not be limited to the interval for the continuous printing of 50 sheets and thus any other interval based on the number of the printed sheets may be chosen in consideration of the reading of the counter. 
   As discussed in the foregoing, in the case of the control of the charging high voltage according to the third embodiment, the nip current is sampled on the basis of the measured peak value of the differential value of the high AC charging voltage. With the system composed as described in the foregoing, it becomes possible to sample the nip current within the range of the discharge, so that the charging current can be controlled with high accuracy. Hence, it becomes possible to obtain a uniform charge without giving rise to the problems such as the deterioration of the photoconductor drum or the poor image formation or the like, irrespective of the change in the environmental condition or the variation of the characteristic of the charging member owing to the manufacturing process. Further, during the continuous printing operation, not only the resetting of the charging current can be made without increasing the charging current from the present level but also, even during the continuous printing operation, a uniform charge can be attained without giving rise to the problems such as the deterioration of the photoconductor drum or the poor image formation, irrespective of the change in the environmental condition or the variation of the characteristic of the charging member owing to the manufacturing process. 
   Furthermore, according to the third embodiment, each time the printing of the predetermined number of sheets is finished, the resetting of the charging voltage level is repeated to control the charging voltage, so that, even when the condition of the image forming apparatus varies depending on the operating condition thereof, it is possible to always keep the photoconductor drum charged optimally. 
   (The Fourth Embodiment) 
   The fourth embodiment of the present invention will be described in the following. In the first embodiment and the third embodiment, the peak value of charging AC voltage and the peak value of the differential value of the charging AC voltage are detected, and the discharging current is determined directly from the difference between the value of the charging current at the time when the peak value of the differential value of the charging AC voltage is equalized with the peak value of the charging AC voltage, and the value of the charging current obtained in this way is controlled to a constant level. 
   In the fourth embodiment, the peak value of the charging AC voltage, controlled with the predetermined charging current, and the peak value of the differential value of the charging AC voltage are obtained respectively, and the value of the charging current is controlled on a real-time basis by calculating the actual discharging current Is′ on the bases of the similar relationships shown in  FIG. 19A  and  FIG. 19B . The description of the high charging AC voltage output circuit is omitted here, since being similar in composition to the diagrams of the first and the third embodiments shown in  FIG. 3 . 
   In  FIG. 19A , when the AC voltage is outputted so that the predetermined charging current value Ica 2  can be obtained, the triangle ABC and the triangle BDE are similar to each other, since these triangles include the alternate-interior angles θ being equal to each other and the right angles. The base of the triangle ABC coincides with the difference (Vac 2 ′−Vac 2 ) between the peak value Vac 2 ′ of the differential value of the charging AC voltage and the peak value Vac 2  of the charging AC voltage, whereas the base DE of the triangle BDE coincides with the peak value Vac 2 ′ of the differential value of the charging AC voltage. Further, the height AC of the triangle ABC coincides with the actual discharging current Is′, while the height BD of the triangle BDE coincides with the charging current Iac 2 . 
   Hence, from these relationships the actual discharging current Is′ can be obtained by the following equation.
 
 Is ′=(1 −Vac   2 / Vac   2 ′) Iac   2   (14)
 
   Next, the processing for controlling the actual discharging current Is′ to the predetermined value Is, referring to the flowchart given in  FIG. 20 . 
   When the operation of the image forming apparatus proceeds to the printing process from the standby state, the processing for determining the level of the high AC discharging voltage is carried out during the pre-rotation period. 
   When the above processing is started, the DC high charging voltage is turned on at the step S 2002 . Then, at steps S 2003  through S 2005 , the value Ic of the charging current is obtained; the value of the Ic, for this purpose, is required to be at the level for equalizing the peak value of the charging AC voltage with the peak value of the differential value of the charging AC voltage (i.e., when the Vac 1 ′ sampled by the voltage detecting circuit  202  becomes equal with the initial value Vac 1  sampled by the voltage detecting circuit  201 ). In the processing shown in  FIG. 20 , “the value for equalizing” means, similarly to the case of the third embodiment, the value at which the difference becomes less than 0.03V, but the equalizing value of Ic is not limited to this value as mentioned previously. 
   In the step S 2003 , the current control signal PRICNT is set to Vc 2 , and the charging AC voltage driving signal PRION is set to LOW level to output the charging AC voltage. In this stage, the value of Vc 2  is finally set to a value that is sufficiently larger than the charging current value. Subsequently, at the step S 2004 , the value PRIVS (the peak value Vac 2  of the charging AC voltage) sampled by the voltage detecting circuit  201  is read. Further, at the step S 2005 , the instantaneous voltage signal PRIDVS (the peak value Vac 2 ′ of the differential value of the charging AC voltage) sampled by the voltage detecting circuit  202  is read. 
     FIG. 19B  shows the relationship between the peak value of the AC voltage/the peak value (on the x-axis) of the differential value of the AC voltage and the current control signal PRICNT (y-axis).  FIG. 19B  shows that, when Vac 2 , as being the value of the current control signal PRCNT, is inputted, the AC peak voltage, i.e., Vac 2  is applied to the charge roller to cause the charging current having the value of Iac 2  to flow, and that the peak value of the differential value of the AC voltage becomes Vac 2 ′. 
   Where the fall of the voltage in normal direction of the diode  288  and the diode  284  is given as Vf, since the triangle FGH and the triangle IJG are similar, the difference Vis′ of the charging current control voltage corresponding to the actual discharging current Is′ can be obtained by the following equation.
 
 Vis′=Vac   2   ′−Vac   2   /Vac   2   ′+Vf×Vc   2   (15)
 
   In this stage, at the step S 2006 , the Vis is compared with the voltage difference Vs corresponding to the predetermined discharging current Is, and, when the absolute value of the difference is less than 0.03V, the processes of the steps S 2004  through S 2006  are repeated. 
   On the other hand, when the absolute value of the difference between Vs and Vis′ is larger than 0.03V, the processing proceeds to the step S 2007  to compare Vs with Vis′ in magnitude. When the actual discharging current Vis′ is smaller than the predetermined discharging current Vs (i.e., Vs−Vis′&gt;0), the processing proceeds to the step S 2008  to increase the input value Vc 2  of the current control signal PRICNT by 0.1V, and then returns to the step S 2004 . On the other hand, when the actual discharging current Vis′ is larger than the predetermined discharging current Vs (i.e., Vs−Vis′&lt;0), the processing proceeds to the step S 2009  to reduce the value of Vc 2  by 0.1V, and then returns to the step S 2004 . 
   As discussed above, by controlling the input value Vc 2  of the current control signal PRICNT on the real time basis, the level of the discharging current to flow in the charge roller not only can be kept to the predetermined value from the start of the charging control but also can be controlled even during the printing operation, so that the stable charging control can be made at all times. 
   In the fourth embodiment, the minimum control range of the Vc 2  is set to 0.1V, while the minimum control range at the step S 2006  is doubled to be set to 0.03V, but these control ranges can be selectively varied to as close as to 0V depending on the actual circuit composition and the processing speed and thus need not be limited to the values given in the fourth embodiment. 
   (The Fifth Embodiment) 
     FIG. 21  shows the high AC charging voltage output circuit according to the fifth embodiment of the present invention. The present embodiment differs from the first embodiment in that the present embodiment does not comprise the voltage detecting circuit  202  but comprises a choke coil  100  provided between the primary winding side of a high-voltage transformer  204  and a capacitor  210 . 
   In the present embodiment, when the level of the charging AC voltage exceeds the discharge starting voltage, the discharging current occurs in addition to the nip current. The sum of the nip current and the discharging current flows in the charge roller  2 . In this case, even when the current in the primary winding of the high-voltage transformer increases instantaneously, the voltage drops on both the ends of the choke coil  2100 , so that the input voltage to the high-voltage transformer  204  drops. Consequently, the waveform of the charging AC voltage fed to the charge roller  2  is modified thereby altering the characteristic of the discharging current corresponding to the charging high AC voltage to be applied. 
   The waveforms of the nip current and the discharging current, at the time when the charging AC voltage whose peak value is equal to or higher than the discharge starting voltage is applied, are shown in  FIG. 22C . Insertion of the choke coil  2100  brings about the increase in the distortion of the waveform of the charging AC voltage and the rise of the level of the discharging current Is. The discharging current Is flows during the same period as the period τb wherein the distortion of the charging AC voltage occurs. 
   For comparison, the waveform of the nip current and the waveform of the discharging current where the discharging is not present, that is, in the range wherein the peak value of the charging AC voltage is lower than the discharge starting voltage, are shown in  FIG. 22A . In this range, the nip current flows only when being in correspondence to the resistive load and the capacitive load between the charge roller  2  and the photoconductor drum  1 . 
   Further, for comparison, the waveform of the nip current and the waveform of the discharging current where the choke coil  2100  is not inserted in the circuit of  FIG. 21  are shown in  FIG. 22B . The discharging current Is flows and the distortion occurs within the peak of the AC voltage when the peak value Vb of the AC voltage becomes higher than the discharge starting voltage. This occurs because of that the discharging current flows at the peak value of the AC voltage not only causing the current to flow abruptly and instantaneously in both the secondary winding and the primary winding of the high-voltage transformer  204  but also causing the drop of the output from the high-voltage transfer  204 . Such drop of the voltage is caused by the leakage inductance component occurring in the primary winding and the secondary winding of the high-voltage transformer  204 . 
   The distortion having the duration of τa occurs in the vicinity of the peak of the charging AC voltage, and the waveform of the nip current is distorted accordingly. The discharging current Is flows during the same period of time with the time period of τa(&lt;τb). 
   Next, the effect of the insertion of the choke coil  2100  will be described in more detail referring to  FIG. 23 .  FIG. 23  shows the characteristics of the charging AC voltage and the charging alternating current, wherein the x-axis represents the peak value of the AC voltage, while they-axis represents the charging current Ic in terms of the average half-wave current value. 
   In  FIG. 23 , the line indicated by LINE-C represents the characteristic line (hereinafter referred to as “characteristic line, LINE-C”) representing the characteristic line in the case where the choke coil is inserted; the line indicated by LINE-B represents the characteristic line (hereinafter referred to as “characteristic line, LINE-B”) in the case where the choke coil  2100  is not inserted; the line indicated by LINE-A represents the characteristic line in the non-discharging range (hereinafter referred to as “characteristic line, LINE-A”). The points indicated as A, B and C correspond to the characteristics in the states as are shown in  FIG. 22A ,  FIG. 22B  and  FIG. 22C  respectively. 
   Within the range where the peak value of the AC voltage is lower than the discharge starting voltage Vh, the characteristics of the characteristic lines, LINE-B and LINE-C are equal to each other. However, within the range where (the peak value of the AC voltage is) equal to or higher than the discharge starting voltage Vh, the characteristic of the characteristic LINE-C differs from the characteristic of the characteristic LINE-B owing to (the presence or absence) of the choke coil  2100 . In other words, the characteristic LINE-C is deviated from the characteristic LINE-A more than does the characteristic LINE-B. 
   Since there occurs a large difference between the characteristic LINE-C representing the case where the choke coil  2100  is inserted and the characteristic LINE-A in the discharging range, the insertion of the choke coil  2100 , as in the case of the present embodiment, brings about the effect such as the increase in the magnitude of the discharge relative to the applied AC voltage. 
     FIG. 24A  shows the characteristic of the charging alternating current Is (on the y-axis) and the peak value of the charging AC voltage (on the x-axis) in case wherein the charging high AC voltage is applied to the charge roller  2 .  FIG. 24B  shows the characteristics of the voltage detect signal PRIVS (on the x-axis) and the current control signal PRICNT (on the y-axis) corresponding to those shown in  FIG. 24A . In  FIG. 24A  and  FIG. 24B , the characteristic line, LINE-A, represents the characteristic line within the non-discharging range, while the characteristic line, LINE-B, represents the characteristic line within the charging range. 
   In the charge control according to the present embodiment, the equations for calculating the characteristics to be represented by the characteristic line, LINE-A, and the characteristic to be represented by the characteristic line, LINE-B, are derived in order for the value of the charging current at which the discharging current having the predetermined value to be calculated to determine the level of the charging high AC voltage applicable to the printing operation. 
     FIG. 25  is a flowchart showing an example of a series of processes for determining the level of the charging high AC voltage. In the DC high voltage generating circuit  247 , the charging high AC voltage is turned ON (S 2502 ); after applying the predetermined DC bias to the charge roller  2 , the characteristic of the characteristic line, LINE-A, is calculated at steps S 2503  through S 2508 . 
   (1) Deriving Equation Representing Characteristic Line, LINE-A 
   The characteristic of the characteristic line, LINE-A, within the non-discharging range is calculated by sampling the point A 1  and the point A 2  within the non-discharging range given in  FIG. 24A . First, the level of the charging current control signal PRICNT is set to Vc 1  (S 2503 ), and the charging alternating current ON signal PRION is switched to LOW level to apply AC voltage to the charge roller  2  (S 2504 ). Then, the voltage detecting signal PRIVS at this time is detected and the sampling of A 1  point is made (S 2505 ) At this point, the value of the voltage detect signal PRIVS is set to Vt 1 . 
   Subsequently, the value of the charging current control signal PRICNT is switched to Vc 2  (S 2506 ), and the voltage detect signal PRIVS is sampled to sample the A 2  point (S 2507 ). In this case, the value of the voltage detect signal is set to Vt 2 . On the bases of the 2 points (i.e., the point A 1  and the point A 2 ) within the non-discharging range, y=fa (x), the characteristic formula, representing the characteristic of the characteristic line, LINE-A, is derived (S 2508 ). Where b is given as a constant, y=fa (x) can be approximated by the equation given below. 
             y   =       f   ⁢           ⁢     a   ⁡     (   x   )         =       a   ⁢           ⁢   x     +   b               (   16   )             
 
(2) Deriving Equation Representing Characteristic Line, LINE-B
 
   In the steps S 2509  through S 2513 , the characteristic of the characteristic line, LINE-B is calculated. The characteristic of the characteristic line, LINE-B, is calculated by sampling the points A and B within the discharging range shown in  FIG. 24A . First, the level of the charging current control signal PRICNT is switched to Vc 3  (S 2509 ), and the voltage detect signal PRIVS at that time is sampled, the sampling of the point B will follow. In this case, the value of the voltage detect signal PRIVS is set to Vt 3  (S 2510 ). 
   The value of the charging current control signal PRICNT is switched to Vc 4  (S 2511 ), and the voltage detect signal PRIVS is sampled to be followed by the sampling of the point C. In this processing, the value of the voltage detect signal PRIVS is set to Vt 4  (S 2512 ). On the bases of 2 points, i.e., point B and point C, y=fb(x), the equation representing the characteristic of the characteristic line, LINE-B, is derived (S 2513 ). Where c and d are given as constants, y=fb(x) can be expressed by the following equation. 
             Y   =       f   ⁢           ⁢     b   ⁡     (   x   )         =       c   ⁢           ⁢   x     +   d               (   17   )             
 
In this case, the constants, c and d, differ largely and respectively from the constants, a and b, in the characteristic equation of the characteristic line, LINE-A. This results from the fact that there occurs a large difference in the characteristic between the characteristic line, LINE-A and the characteristic line, LINE-B.
 
(3) Determination of Charging Current Control Value
 
   The level, Vc(cnt), of the charging current control signal PRICNT, at which the discharging current coincides with the predetermined value, is calculated (S 2514 ). The discharging current corresponds to the difference between the characteristic line, LINE-B, and the characteristic line, LINE-A. In  FIG. 24A , when the target value of the discharging current is set to Is, the target discharging current value Is can be obtained by controlling the charging current value to Ic (cnt). Hence, the level, Vc (cnt), of the charging current control signal PRICNT can be calculated by using the two characteristic equations, namely, y=fa (x) and y=fb (x), which have been derived by the processes described previously. 
   Where the range of the charging current control value PRICNT is set to ΔK, the value, Vt (cnt), of the voltage detect signal PRIVS, at which the target control value Is can be obtained, can be expressed by the following equations
 
 Vt ( cnt )=( d−b+ΔK /( a−c )  (18)
 
   Vc (cent) can be expressed by the following equation.
 
 Vc ( cnt )={ c ( d−b+ΔK )/( a−c )}+ d   (19)
 
(4) Setting Level of Charging Current for Printing Operation
 
   The level of the charging current is switched to the level for the printing operation. The charging current control signal PRICNT is set to the value represented by Eq. (19) for the above-mentioned switching processing (S 2515 ) thereby to complete the series of processing (S 2516 ). 
   The optimal value of the charging current is obtained by proceeding through the above-mentioned series of processes, and the processing proceeds to the process of printing. 
   In the present embodiment, in controlling the charging current, there occurs difference (or errors) between the target control value Is of the discharging current and the actually available discharging current value owing to the variations of conditions occurring in the charging high voltage output circuit. The factors most responsible for the occurrence of such difference are the processes executed at the steps S 2503  through S 2514 . For example, there are control errors occurring with the charging current control signals PRICNT (Vc 1 , Vc 2 , Vc 3  and Vc 4 ) and the sampling errors occurring with the charging voltage detect signal PRIVS (Vt 1 , Vt 2 , Vt 3  and Vt 4 ). 
   The occurrence of the control error and the sampling error such as those discussed above cause the occurrence of the errors in the process for calculating the charging voltage vs. charging alternating current characteristic (i.e., the characteristic lines, LINE-A and LINE-B) and the resulting error of the discharging current control value. The effects of such control error and the sampling error are inversely proportional to the difference between the characteristic line, LINE-A, and the characteristic line, LINE-B. 
   However, since the charging high voltage output circuit of the image forming apparatus according to the present embodiment comprises a choke coil  2100  provided on the primary winding side of the high-voltage transformer  204 , the difference in the characteristic between the characteristic lines, LINE-A and LINE-B, can be increased. Hence, the previously mentioned effects of the control error and the sampling error on the actual discharging current are small. In other words, there is a large difference in characteristic between the charging AC voltage vs. the charging alternating current characteristic (the characteristic line, LINE-A) in the non-discharging range and the charging AC voltage vs. charging alternating current characteristic (the characteristic line, LINE-B) in the charging range, so that the charging alternating current for obtaining the desired charging current can be sampled with high accuracy. 
   Further, because of that the magnitude of the discharge relative to the charging AC voltage can be increased, the value of the necessary charging AC voltage, necessary for obtaining desired discharging current, can be reduced than that required conventionally, thereby enabling the sizes of the charging high voltage output circuit and the size of the image forming apparatus to be reduced. 
   As discussed in the foregoing, in the image forming apparatus according to the present embodiment, the predetermined charging current is applied to the charging high voltage output circuit, and the resulting charging voltage is sampled not only to sample the charging AC voltage vs. charging alternating current characteristic (the characteristic line, LINE-A) in the non-discharging range and the charging AC voltage vs charging alternating current characteristic (the characteristic line, LINE-B) in the charging range but also to calculate the value of the charging alternating current, thereby obtaining the desired value of the charging alternating current for the optimal control. However, the optimal control of the charging alternating current value can also be realized by the system other than the present system. In other words, an alternative system may be employed so that the predetermined charging voltage is applied; the resulting charging current is sampled; the charging AC voltage vs. charging alternating current characteristic (the characteristic line, LINE-A) in non-charging range and the charging AC voltage vs. charging alternating current characteristic (the characteristic line, LINE-B) in the charging range are sampled; the value of the charging alternating current, at which the desired charging current is obtained for the optimal control. 
   (The Sixth Embodiment) 
   An example of the charging high voltage output circuit in the image forming apparatus according to the present embodiment is shown in  FIG. 26 . The present embodiment differs from (the fifth) embodiment with respect to the location of the choke coil. More specifically, in the fifth embodiment, the choke coil  2100  is provided on the primary winding side of the high-voltage transformer  204 , while, in the case of the present embodiment, the choke coil  2600  is provided on the secondary winding side of the high-voltage transformer  204 . 
   In the charging high voltage output circuit shown in  FIG. 26 , the choke coil  2600  provided on the secondary winding side of the high voltage transformer  204  practically performs the same function as that of the choke coil  2100  (in the case of the fifth embodiment) provided on the primary winding side of the high voltage transformer  204 . Hence, because of a large difference in characteristic between the charging AC voltage vs. charging alternating current characteristic (the characteristic line, LINE-A) in the non-discharging range and the charging AC voltage vs. charging alternating current characteristic (the characteristic line, LINE-B) in the charging range, the value of the charging alternating current, with which the desired charging current can be obtained, can be sampled with high accuracy. 
   (The Seventh Embodiment) 
   The charging high voltage output circuit in the image forming apparatus according to the present embodiment is shown in  FIG. 27 . This charging high voltage output circuit differs from the fifth and the sixth embodiments in the composition of the output component designed for adjusting the waveform of the charging AC voltage. In the present embodiment, the high-voltage transformer  2700  of the charging high voltage output circuit is adopted as a substitute for the choke coil  2100  and the high-voltage transformer  204  in the fifth embodiment and also as a substitute for the high-voltage transformer  204  and the choke coil  2600  in the sixth embodiment. 
   For the high-voltage transformer  2700 , one having the construction shown in  FIG. 28  is adopted to reduce (the interdependency) between the primary winding side and the secondary winding side. 
   The high-voltage transformer  2700  comprises an EI-type core consisting of E-type core  2801 A and I-type core  2801   b . The E-type core comprises three parallel arranged parts,  2806   a ,  2806   b  and  2806   c . The part  2806   a  is provided with the primary winding  2802 , while the part  2806   c  is provided with the secondary winding  2803 . The central part  2806   b  is not provided with any winding. Reference symbols  2804   a  and  2804   b  indicates the input terminal of the primary winding  2802 . The reference symbols  2805   a  and  2805   b  indicates the output terminal of the secondary winding  2803 . 
   The flow of the current in the primary winding  2802  causes the magnetic flux φ to be produced in the part  2806  of the E-type core thereby forming the magnetic loop M 1  passing the central part  2806   b  of the core and the magnetic loop M 2  passing the marginal part  2806   c . Hence, the magnetic loop M 2  intersects the secondary winding  2803 . 
   The equivalent circuit of a high-voltage transformer  2700  is shown in  FIG. 29 . In  FIG. 29 , an input terminal  2905   a  is connected with one end of the primary winding of the transformer  2902  through an inductance element  2903 , while an input terminal  2905   b  is connected with the other terminal of the primary winding of the transformer  2902 . An output terminal  2906   a  is connected with one end of the secondary winding of the transformer  2902  through an inductance element  2904 , while an output terminal  2906   b  is connected with the other end of the transformer  2902 . The primary winding side and the secondary winding side of the transformer  2902  is highly (interdependent with each other). 
   As described in the foregoing, in the charging high voltage output circuit according to the present embodiment, the high-voltage transformer  2800  is equivalent to the transformer  2902  having an inductance element  2903  provided on the primary winding side thereof and an inductance element  2904  provided on the secondary winding side thereof. Thus, since the operation of the charging high voltage output circuit according to the present embodiment is substantially (similar) to the operations of the corresponding circuits according to the fifth embodiment and the sixth embodiment, in the present embodiment too, there is a large difference in characteristic between the charging AC voltage vs. the charging alternating current characteristic (the characteristic line, LINE-A) in the non-charging range and the charging AC voltage vs. charging alternating current characteristic (the characteristic line, LINE-B) in the charging range, so that the charging alternating current, with which the desired charging current can be obtained, can be sampled with high accuracy. 
   In the foregoing, the present invention relating to an image forming apparatus and the embodiments thereof have been described; however, besides the charging process of the image carrier, other processes relating to the charging control apparatus and the charging control are also available. 
   The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspect, and it is the intention, therefore, in the apparent claims to cover all such changes and modifications as fall within the true spirit of the invention.