Toner transferring device

An apparatus for transferring toner images which is capable of executing optimum discharging control even where the environment, the material of the paper, and the kind of paper may be changed. First, the load current is detected when the transferring roller 11 is pressed to a transferring drum 6. The detected load current is converted to a characteristics detecting signal S6 at a signal converting section 109. An environment is selected on the basis of the characteristics detecting signal S6. Next, the load current flowing to the transferring roller 11 is detected when an AC discharger 14 is stopped. The detected load current is converted to characteristics detecting signal S6 at the signal converting section 109. A voltage control signal is selected based on the converted characteristics detecting signal S6, the environmental information, and the paper kind. A power source for discharging 22 is controlled by the selected voltage control signal so that the transferring becomes optimum. Accordingly, even if the kind of paper and environment is changed, optimum transferring and high transferring efficiency can be maintained.

BACKGROUND OF THE INVENTION 
The present invention relates to an electrophotographic printer such as a 
laser printer and an apparatus for electrophotography such as an 
electrophotographic copying machine. Specifically, the present invention 
relates to optimum transfer control of a transferring means for 
transferring toner images to paper. 
Conventionally, a transferring means for transferring toner images in the 
above described kind of apparatus for electrophotography transfers toner 
images at a toner image carrier to paper by passing paper as a recording 
medium through an interval between a transferring roller, whereon a 
transferring voltage is charged, and the toner image carrier. 
Transferring means of the above described type has a problem that 
transferring performance varies depending on changes in resistant values 
of the recording medium and transferring rollers in accordance with change 
in an environmental condition. FIG. 11 is a graph indicating a 
transferring characteristic relating to humidity which is one of the 
environmental conditions. A transferring voltage with which an optimum 
transferring efficiency can be obtained under an environmental condition 
at room temperature and normal humidity (20.degree. C., 60%) is set, and 
the transferring efficiency at the transferring voltage is designated as 
100%. And, transferring efficiencies which are determined at various 
humidities such as a low humidity on clear day and a high humidity on 
rainy day are indicated. The characteristic graph reveals that 
transferring efficiency decreases remarkably at humidities shifted from 
the environmental humidity for which the transferring voltage is set. 
In consideration of the above problem, conventional transferring means of 
the above described type prevents the decreasing of the transferring 
efficiency by detecting resistant values of the recording medium and by 
controlling the transferring voltage charged to a transferring roller as 
disclosed in JP-A-55-28081 (1980), JP-A-2-300774 (1990), and JP-A-4-190381 
(1992). Further, JP-A-64-40867 (1989) discloses a method wherein a 
discharging voltage is controlled in accordance with a transferring 
current which is determined under a condition wherein both the 
transferring device and the discharger are concurrently operated. 
However, currently, a demand for recording to various kinds (such as 
materials and sizes) of recording media by electrophotographic printers or 
electrophotographic copiers is increasing, and such a problem has been 
created that the control in the conventional transferring means can not 
satisfy the demand sufficiently. 
Especially, a transferring defect caused in a case using paper for an 
over-head projector (OHP) at high temperature and high humidity is one of 
the problems. 
The paper for OHP is processed for anti-static treatment by decreasing 
surface resistance to prevent the paper from becoming incapable of being 
feed by mutual electrostatic adhering when the paper is fed from a paper 
cassette. The above processing causes another problem such as extremely 
decreasing the transferring efficiency by flowing current from a charging 
device such as a transferring roller through the paper for OHP to the AC 
discharger at a high humidity condition and lowering the voltage at 
transferring points (refer to FIG 12). 
Further, a method which uses needle electrodes for discharging electricity 
on paper without using high AC has the problem that discharging to the 
needle electrodes causes the lowering of the voltage at the transferring 
points and thus the transferring efficiency can not be improved. 
Furthermore, another method, wherein a discharging voltage is controlled in 
accordance with a measured transferring current when both a transferring 
voltage and the discharging voltage are concurrently supplied, has 
problems that a measured transferring current is influenced by the 
discharging voltage and results of the measurement can not be obtained 
exactly, transfer can not be performed stably, and paper-wrapping is 
caused as a result of insufficient discharging of electricity. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an apparatus for 
electrophotography having a transferring device capable of maintaining a 
high transferring efficiency whenever the environmental conditions charge 
or the kind of recording media is altered. 
The above object is realized with an apparatus for electrophotography 
having a transferring condition measuring means which determines electric 
characteristics of a transferring means and having a discharging voltage 
controlling means which controls the discharging means by controlling the 
voltage and charging time of the discharging means based on the electric 
characteristics determined by the transferring condition measuring means. 
When a toner image is transferred to a sheet of paper, operation of the 
discharging means is stopped before a top end of the paper reaches at the 
discharging means to prevent the lowering of the voltage at the 
transferring points by opening or restricting current circuits from the 
transferring means to the discharging means. However, intermediate 
stopping of the discharging process under a low temperature and a low 
humidity condition causes adhering of the paper to a toner carrier and 
touching of the paper to a paper guide. 
In consideration of the above described problem, the transferring condition 
measuring means determines electric characteristics of the whole 
transferring means and paper first under a condition wherein the 
discharging means does not operate, and the discharging condition is 
controlled based on the environmental changes, such as temperature and 
humidity, and the kind of paper, such as material and/or size which are 
determined previously by the transferring condition measuring means. 
Because the AC discharger operates to neutralize the charge of the paper, a 
voltage at the transferring roller is transferred to the AC discharger 
through the paper depending on the resistance of the paper. Accordingly, 
the voltage at the transferring section is lowered and transferring 
efficiency is decreased. However, the current flowing from the 
transferring roller, the paper, and the AC discharger to ground can be 
stopped by terminating operation of the AC discharger, and consequently, 
optimum transcription wherein a constant voltage is supplied to the paper 
can be realized. Further, the paper can be separated from the transferring 
medium smoothly, and preferable transcription with small toner spread can 
be realized.

DETAILED DESCRIPTION 
One of the embodiments of the present invention is explained for a laser 
color printer, an apparatus for color image electrophotography, relating 
to the present invention. 
Referring to the drawings, FIG. 10 is a schematic vertical section of the 
laser color printer relating to the present invention. 
An exposing belt 1 rotating at a constant velocity around guide rollers 
2a-2d is charged uniformly by a charger 3. Static latent images 
corresponding to each color in regular order are formed on the exposing 
belt 1 (a static latent image corresponding to a color is formed per a 
rotation of the exposing belt in the present embodiment) by being exposed 
to laser light 4a which is generated from an exposing device 4 in 
accordance with image signals of each color. Further, toner images of each 
color are formed on the exposing belt 1 by selecting a development machine 
using color developer corresponding to one of four development machines 
5Y, 5M, 5C, and 5K and by developing the latent images by each development 
machine in regular order. To ensure precise positioning of the laser light 
4a, the exposing device 4 is provided to expose the exposing belt 1 with 
the laser light 4a at a position where the guide roller 2a is located. The 
toner image of each color at the surface of the exposing belt 1 is 
transcribed to surface of a transferring drum 6, which contacts the 
surface of the exposing belt 1 and rotates synchronously with the exposing 
belt 1. Thus one by one each of colors are transferred in a superimposing 
manner to form a color toner image at the surface of the transferring drum 
6. The transferring drum 6 has a structure (not shown in FIG. 10) to be 
supplied with a designated transferring voltage when the toner images at 
the surface of the exposing belt 1 are transcribed. 
Paper 8, recording medium, piled up in a paper cassette 7 is withdrawn and 
transferred to a resist roller 10 by a paper feed roller 9. After 
straightening the paper and adjusting the transferring timing of the paper 
with color toner images at the surface of the transferring drum 6, the 
paper 8 is transferred to the position of the transferring roller 11. 
The transferring roller 11 composing a part of the transferring means 
transcribes the color toner images at the surface of the transferring drum 
6 to the paper by making the transferred paper 8 contact with the 
transferring drum 6 and applying mechanical pressing force, and by 
transferring voltage from a face of the paper opposite to the contacting 
face. The transferring roller 11 is separated from the transferring drum 6 
by a transferring roller transferring device 12 so as not to operate while 
color toner images are being formed at the surface of the transferring 
drum 6 by transferring toner images of each color at the surface of the 
exposing belt 1. The transferring roller 11 is transferred by the 
transferring roller transferring device 12 to contact with the 
transferring drum 6 at a point of time to transcribe the color toner 
images to the paper after completing formation of the color toner images 
at the surface of the transferring drum 6. The transferring voltage 
supplied to the transferring roller 11 is generated by a power source for 
transcription 13, which can change the output voltage arbitrarily and 
continuously, in accordance with a duty ratio of an input signal and the 
like. 
An AC discharger 14 generates an AC corona voltage by being supplied with 
AC voltage from a power source for the AC discharger shown in FIG. 1. The 
corona voltage eliminates static by neutralizing a residual electric 
charge remaining at the back face of the paper 8 whereon the color toner 
images are transcribed so as to facilitate separation of the paper 8 from 
the transferring drum 6. The paper 8 separated from the transferring drum 
6 is sent to a fixing device 15, and the color toner images are fixed 
thermally at the surface of the paper 8 while the paper 8 passes through 
the fixing device 15. The paper 8 whereon the color toner images have been 
thermally fixed are extruded to a paper outlet tray 17 by an extruding 
roller 16. 
A belt cleaner 18 eliminates residual toner remaining at the surface of the 
exposing belt 1 after transferring toner images formed at the exposing 
belt 1 to the transferring drum 6. A drum cleaner 19 eliminates residual 
toner remaining at the surface of the transferring drum 6 after 
transferring color toner images formed at the surface of the transferring 
drum 6 to the paper. The drum cleaner 19 is separated from the 
transferring drum 6 so as not to operate while color toner images are 
being formed at the surface of the transferring drum 6 by transferring 
mono-color toner images repeatedly. The drum cleaner 19 is transferred to 
contact with the transferring drum 6 after transferring the completed 
color toner images to the paper 8. 
A control apparatus 100 controls the above described composing members in 
accordance with input signals from an operating panel 50 and an 
information processing apparatus which is explained later. 
FIG. 1 is a block diagram indicating details of the control apparatus 100 
of the color laser printer. The control apparatus 100 mainly comprises CPU 
101, ROM 102 which stores control programs for the CPU 101, and RAM 103 
which operates as a work memory necessary for executing the control 
programs by CPU 101. 
An input/output interface (I/F) 104 intermediates communication of the CPU 
101 with the operation panel 50 and the information processing apparatus 
200 such as a word processor and a personal computer, and transmits 
printing data generated by the information processing apparatus 200 to an 
exposure control section 105. The exposure control section 105 generates 
laser light 4a by controlling the exposing apparatus 4. 
A mechanism control section 106 comprises a group of controllers for 
controlling a group of mechanisms of electrophotographical processing, 
supplies a motor drive instructing signal S1, a transferring roller 
pressure driving signal S2 which operates the transferring roller 
separating apparatus 12, transferring pressure controlling signal S3 for 
controlling generating voltage of the transferring power source 13, and AC 
discharger controlling signal S5 for controlling operation of the AC 
discharger 14; and receives characteristics detecting signal S6 including 
determined electric characteristics of the transferring roller 11 and the 
others. 
A motor control section 107 is composed so as to control a motor 20 which 
drives the exposing belt 1, the paper feed roller 9, and the fixing device 
15 and the others; generates driving signal S7 in accordance with the 
motor drive instructing signal S1 from the mechanism control section 106; 
and receives a detecting signal S8 for detecting an amount of angular 
displacement generated from an encoder 21 which is directly connected to 
the motor 20. Further, the motor control section 107 transmits the 
detecting signal S8 from the encoder 21 to the CPU 101 via an interrupt 
control section 108 as an angular displacement signal S9 for the exposing 
belt 1. The motor 20 is connected to the guide roller 2b of the exposing 
belt 1 directly, and to the paper feed roller 9 and the fixing device 15 
by a gear mechanism via a clutch. 
A signal converter 109 comprises, as shown in FIG. 2, a converting resistor 
109a which determines the load current flowing through the transferring 
roller 11 and converts it to a voltage signal, a group of voltage dividing 
resistors 109b for generating a group of standard reference voltages to 
perform an AD conversion of the above voltage signal, a group of reference 
comparators 109c for supplying converting signal 109d by comparing the 
above voltage signal with the group of standard reference voltages, and a 
selector 109e for supplying characteristics detecting signal S6 of n bits. 
The CPU 101 performs a control processing for calculating an optimum 
transferring voltage by receiving the above characteristics detecting 
signal S6 via the mechanism control section 106 and making the mechanism 
control section 106 generate transferring voltage signal S3 for making the 
transferring power source 13 generate the optimum transferring voltage. 
Information distinguishing the kind of medium, such as paper standard A/B, 
can be obtained as an input information supplied by an operator via the 
operating panel 50. But, an automatic detecting machine for distinguishing 
kind of paper can be installed at a paper feeding section. 
Referring to FIGS. 3 and 4, a time chart and a flow chart, respectively, 
control of color toner images transcription to the paper 8 of the above 
described laser color printer is explained hereinafter. 
The control of the transcription starts after the completion of the 
formation of the color toner images at surface the of the transferring 
drum 6. At timing TO when the control of the transcription starts, a motor 
rotating process 401 is performed so as to start operation of the motor by 
sending the motor drive instructing signal S1 from the mechanism control 
section 106 to the motor control section 107. 
At timing T1, judgement whether printing is for the first page or not is 
performed at the processing step 450. If the first page case, the step 
transfers to the transferring roller pressing process 451; if not the 
first case, the step transfers to the paper feed starting process 402. 
In case of the first page, operation of the transferring roller 
transferring device 12 starts, the transferring roller pressure driving 
signal S2 is generated at the mechanism control section 106, and the 
transferring roller pressing process 451 is performed at timing T1. 
Further, the charging process of the transferring voltage for measurement 
452 for generating the transferring voltage control signal S3 from the 
mechanism control section 106 is performed so as to supply a transferring 
voltage for measuring the environment to determine the environmental 
condition at the moment. 
The load current determining process 453 starts at timing T2. The 
determined load current is converted to characteristics detecting signal 
S6 by the signal conversion section 109, and then sent to the mechanism 
control section 106. The load current judging process 454 judges whether 
the characteristics detecting signal S6 is normal or abnormal, and goes to 
the process 455 if normal. If abnormal, the processing for error 411 is 
executed, and subsequently, the motor terminating process 420 which is 
explained later is performed to complete the transferring control. When 
the characteristics detecting signal S6 is normal, an environment 
corresponding to the characteristics detecting signal S6 is selected in 
accordance with a data table shown in FIG. 5 which is previously stored in 
the ROM 101 and transferring voltage for measurement, which is explained 
later, is determined. 
FIG. 6 is a graph indicating environmental characteristics. For instance, 
when the characteristics detecting signal S6 has a value X1, the 
environment is judged as low temperature and low humidity by referring to 
the data table shown in FIG. 5, and the transferring voltage for 
measurement is decided to be V1. As shown in FIG. 7, the optimum 
transferring voltage for measurement under an environment varies depending 
on the environmental condition, and more preferable transferring control 
can be performed by selecting the environment at the present step. 
At timing T3, the transferring roller separating process 456 to separate 
the transferring roller 11 from the transferring drum 6 is performed. 
At timing T4, the paper feed starting process 402 for rotating paper feed 
roller 9 to pull out a sheet of paper 8 from the paper cassette 7 wherein 
the paper are piled, and the pulled paper is transferred to the resist 
roller 10. 
At timing T5, paper feeding is stopped at the paper feed terminating 
process 403, and paper transfer starting process 404 for straightening 
paper and adjusting transferring timing of the paper with color toner 
images at the surface of the transferring drum 6 is performed. Further, 
the transferring roller pressing process 405 is performed for making the 
mechanism control section 106 generate the transferring roller separation 
driving signal S2 which operates the transferring roller separating 
apparatus 12 so that the transferring roller 11 contacts pressingly with 
the transferring drum 6. Furthermore, the kind of paper judging process 
406 is performed for judging the kind of paper 8 such as PPC paper or OHP 
paper based on input information set in the operation panel 50 or the 
detecting signal from the detector. 
At timing T6, transferring voltage for measurement loading process 408 is 
started for making the mechanism control section 106 generate transferring 
voltage control signal S3 so as to charge the transferring voltage for 
measuring electric characteristics of the paper 8. The timing is just 
before a timing when a top end of paper 8 which is transferred from the 
resist roller reaches at an interval between the transferring drum 6 and 
the transferring roller 11. At timing T7, the transferring voltage for 
measurement corresponding to the transferring voltage control signal S3 
which is selected at the above described process 455 is supplied to the 
transferring roller 11. 
At timing T6, load current determining process 409 starts. The timing when 
the load current is determined is a timing later than the paper feed 
starting process 402 by a fixed time t1, within t2 for stabilizing period 
of the transferring voltage for measurement, and within p1 which is the 
non-printing region of the top end portion of the paper 8. The determined 
load current is converted to the characteristics detecting signal S6 
corresponding to the kind of the paper 8 by the signal conversion section 
109, and is sent to the mechanism control section 106. At the process 410, 
the characteristics detecting signal S6 is judged whether it is normal or 
abnormal, and goes to the process 412 if normal. If abnormal, the 
processing for error 411 is executed, and subsequently, the motor 
terminating process 420 which is explained later is performed to complete 
the transferring control. 
When the characteristics detecting signal S6 is normal, the optimum 
transferring voltage generating process 412 starts at timing T8. At the 
process 412, data for generating optimum transferring voltage 
corresponding to the characteristics detecting signal S6 and data for the 
charging time of the AC discharger are selected from the data table shown 
in FIG. 8 which is previously stored in the ROM 101. By changing the 
transferring voltage control signal S3 generated from the mechanism 
control section 106 based on the data of the optimum transferring voltage, 
the transferring voltage given from the power source for the transcription 
13 to the transferring roller 11 becomes optimum. 
FIG. 8 indicates the data table composed in ROM 102. When paper 8 is the 
paper for PPC, the transferring voltage control signal S3 is prepared by 
referring to voltage data in various environments for paper in a group A, 
and charging time for the AC discharger is determined by the charging time 
data. When paper 8 is the paper for OHP, the transferring voltage control 
signal S3 is prepared by referring to voltage data in various environments 
for paper in a group B, and charging time for the AC discharger is 
determined by the charging time data. 
Timing T9 is a timing when transcription of the color toner images starts, 
the process 413 for making the mechanism control section 106 generate AC 
discharging control signal S5 which puts the AC discharger in an operating 
condition is performed, and the transferring managing process 414 is 
started. In accordance with the above process, the color toner images at 
the surface of the transferring drum 6 are transcribed to the paper 8. The 
paper 8 transcribed with the color images is electrically eliminated by AC 
discharger 14, separated from the transferring drum 6, and sent to the 
fixing apparatus 15. The color toner images transcription starting timing 
T9 is a timing later than the paper feed starting process 402 by a fixed 
time t3, within t4 for stabilizing period of the optimum transferring 
voltage, and prior of a boundary between the non-printing region and the 
printing region of the paper 8. 
When the transcription starting process is completed, time count starts at 
the color toner images transcription starting timing T9, and the process 
415 which changes AC discharging control signal S5 so as to terminate 
operation of the AC discharger 14 at a time TA which is obtained from the 
discharger charging time table is executed. 
The time TA is set, when the paper 8 for OHP is used, as a time to reach 
the AC discharger under a high temperature and high humidity condition, 
and as a time equivalent to full length of the paper 8 so as to prevent 
the transferring drum from being adhered with the paper 8 under conditions 
other than high temperature and high humidity. When the paper for ordinary 
use is used, the time TA is set as a time equivalent to full length of the 
paper 8 under all conditions. Therefore, the end of operation of the AC 
discharger becomes later than timing T10 (the timing for the paper feed 
terminating process 416) depending on the value of the time TA, and it 
becomes the same as the timing T11 in some cases. 
At timing T10, the paper feed terminating process 416 is executed, and the 
resist roller 10 is stopped. 
The timing T11 is a timing when the transferring of color toner images from 
the transferring drum 6 to the paper 8 is completed. When the transferring 
managing process 414 detects the timing T11, the process 417 for changing 
transferring voltage control signal S3 to terminate generation of the 
transferring voltage, and the process 418 for changing transferring roller 
separation driving signal S2 to separate the transferring roller 11 from 
the transferring drum 6 are performed. 
At timing T12, the completion judging process 419 for judging continuation 
or termination of printing based on a signal from input/output interface 
104 is executed, and if the case is continuing the printing, returns to 
first printing judging process 450 and goes to transcription controlling 
process for a next page. If the case is terminating the printing, the 
process 420 for changing the motor drive instructing signal S1 to 
terminate operation of the motor 20 is performed, and completes the 
transferring control process. 
In accordance with the above described transferring voltage control and AC 
discharger charging time control, optimum measuring voltage for each 
environmental condition can be determined based on transferring 
environment measured at start of printing. And transferring voltage and 
the AC discharger charging time are determined based on electric 
characteristics of the paper which is measured with the above optimum 
measuring voltage. Therefore, a high transferring efficiency can be 
maintained even if materials of the paper used for a laser color printer 
are varied such as paper for PPC or paper for OHP. 
Further, electric characteristics of the paper 8 are measured by detecting 
load current flowing through the transferring roller 11 under a condition 
when operation of the AC discharger 14 is stopped. Therefore, noises in 
determining very weak load current can be decreased, and accuracy of 
determination and reliability can be increased. 
Referring to FIG. 13, the second embodiment of the present invention is 
explained hereinafter. 
In the second embodiment, operation of the AC discharger does not stop 
completely on the middle of the discharge, but repeats on-off of the 
discharging operation with short cycles for a certain period. 
The detecting method for the environmental condition and paper condition is 
the same as the method in the first embodiment. 
By repeating on-off of the discharging operation with short cycles for a 
certain period, such advantages are realized that rapid change of images 
caused by changing of the AC discharger operation from on to off can be 
moderated. 
Referring to FIGS. 14 and 15, the third embodiment of the present invention 
is explained hereinafter. 
In the third embodiment, voltage for an AC discharger is controlled 
depending on the transferring condition and the kind of paper by providing 
a power source for AC discharger 22 which can control voltage of the AC 
discharger at plural steps (three steps in the present embodiment) and by 
storing a data table for voltage control in the ROM 102. 
The detecting method for the environmental condition and paper condition is 
the same as the method in the first embodiment. 
By controlling the voltage for the AC discharger at plural steps, uniform 
discharge through the paper becomes possible and advantages such that 
decreasing of fluctuation of images at printing plane can be realized. 
Referring to FIGS. 16 and 17, the fourth embodiment of the present 
invention is explained hereinafter. 
In the fourth embodiment, a needle type electrode 23 is provided instead of 
the AC discharger 14, and discharge can be performed by discharging 
operation of the transcribed paper 8 to the needle type electrode 23. 
A switching circuit 24 such as relays is connected to the needle type 
electrode 23. The needle type electrode can be connected to ground in 
accordance with the signal S5 from the control apparatus 100. 
The detecting method for the environmental condition and paper condition is 
the same as the method in the first embodiment. 
When transferring images at surface of the transferring drum 6, the needle 
type electrode 23 is connected to ground first. Therefore, discharge is 
caused at the top end of the paper 8, and the discharged paper is 
separated from the transferring drum 6. Subsequently, after a time TA 
which is designated based on an environmental condition, the needle type 
electrode 23 is disconnected from ground, and the discharge is stopped. 
Accordingly, the flowing current from the transferring roller 11 to ground 
via the paper 8 and the needle type electrode is interrupted, and a 
voltage is charged to the paper. 
In accordance with the present embodiment, use of a power source for AC 
high voltage can be avoided by using the needle type electrode, and 
advantages such as no ozone generation are realized. 
In the above embodiments, explanations were performed regarding to a 
transferring type wherein a transferring drum 6 was provided and a color 
toner image was formed by overlapping toner of each color at the surface 
of the transferring drum 6. However, the present invention can naturally 
be applied to another transferring type such as a type wherein the 
transferring drum is not provided, but the color toner image is formed at 
the surface of a photosensitive body (photosensitive belt or 
photosensitive drum) and the formed color toner image is transferred to 
paper. Further, a transferring belt can be used instead of the 
transferring drum. 
The present invention was explained above taking a color printer for an 
example, but the present invention can naturally be applied to general 
mono-color printers and copying machines by using appropriate sensors for 
temperature and humidity, and by controlling the AC discharger. 
A means for controlling the discharger can change discharging control 
characteristics which is based on measured electric characteristics of the 
paper depending on a change of environmental condition and the kind of the 
paper (material and/or size). Therefore, even if papers of various 
materials such as PPC paper or OHP paper and/or of various sizes are used, 
optimum discharging for the used paper can be performed, and a high 
transferring efficiency can be maintained regardless of the material and 
size of the paper. 
Further, measurement of electric characteristics of the paper is performed 
by detecting load current flowing through the transferring roller in a 
condition when the AC discharger stopped. Therefore, noises in determining 
very weak load current can be decreased, and the accuracy of the 
determination and reliability can be increased.