Sharp AR-5132 Service Manual page 15

Characteristics of organic photoconductors (OPC)
•
Can be formed into various shapes (drum, sheet, belt)
•
High insulation in a dark place. (Acceptability and retainability of
charges)
•
Light weight
•
Stable against humidity and temperature
•
Safe and clean to the environment (harmless)
•
Weak in wear by friction
•
Weak in durability against light and ozone
(4) Characteristics of photoconductors
The important characteristics of photoconductors are as follows:
1. Photo sensitivity
3. Acceptance potential
5. Residual potential
[Photo sensitivity]
It is determined by the attenuation speed of the potential when ex-
posed with light.
[Spectrum characteristics]
The sensitivity of photoconductors differs depending on the kind and
the waveform of light.
Amorphous silicon
Se:Te
1.0
0.8
0.6
0.4
0.2 OPC for
analog
400 500 600
Wavelength (nm)
Spectrum sensitivity
Relationship between color and waveform
Human eyes can feel the lights with waveform of 380nm to 780nm.
These are called "Visible lights." The light whose waveform is shorter
than that is called "Ultraviolet light." The light whose waveform is
longer than that is called "Infrared light."
The figure below shows the relationship between lights and
waveforms.
Blue green
Ultraviolet
350 400 450 500 550
2. Spectrum characteristics
4. Charge retainability
6. Fatigue
OPC for digital
700 800 900
Red Infrared
600 650 700 750 800
[Acceptance potential]
The dark resistance of the photoconductor layer decreases as the
electric field applied between layers increases.
When the photoconductor is charged, the electric field is formed to a
high level and the resistance of the layer decreases to restrict the
charging amount of the photoconductor. The potential of the
photoconductor at that time is called the acceptance potential, which
serves as an important factor to determine the potential contrast. The
photoconductor is generally charged to a potential slightly lower than
the acceptance potential in order to avoid applying an electrical strain
to the photoconductor.
[Charge retainability]
The retaining time of electrostatic latent images on the photoconduc-
tor is determined by the speed of decrease in the potential in a dark
place. That is, it is measured with the time for the photoconductor
potential to decrease to the half of the initial level. This retainability of
electrical charge makes a problem when the interval time between
exposure and developing is longer. In the machines where a series of
operations of charging, exposure, and developing are automated, the
interval between the processes is short enough and there is no
problem.
[Residual potential]
When the charged photoconductor is exposed, the potential is rapidly
attenuated at first then slowly. The potential where this slow attenua-
tion starts is called the residual potential. The lower the residual
potential is, the greater the voltage contrast is. Therefore, the lower
residual potential is desirable.
[Fatigue]
When the photoconductor is charged and exposed repeatedly, it is
fatigued. Fatigue of the photoconductor results in increase in attenua-
tion speed of the photoconductor potential and decrease in the
retainability of charges.
In the above, the necessary characteristics for the photoconductors
are described. In an actual machine, when charging is repeated by
the charger, dust and dirt or splashed toner may be attached to the
saw tooth. These are not resulted from uneven charging, and they
should be removed by cleaning.
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