Sharp AR-168S Service Manual page 34

Step-7: Optical discharge (Semiconductor laser)
Before the drum rotation is stopped, the semiconductor laser is
radiated onto the drum to reduce the electrical resistance in the OPC
layer and eliminate residual charge, providing a uniform state to the
drum surface for the next page to be printed.
When the electrical resistance is reduced, positive charges on the
aluminum layer are moved and neutralized with negative charges on
the OPC layer.
Semiconductor laser
Charge by the Scorotron charger
Function
The Scorotron charger functions to maintain uniform surface potential
on the drum at all times, It control the surface potential regardless of
the charge characteristics of the photoconductor.
Basic function
A screen grid is placed between the saw tooth and the
photoconductor. A stable voltage is added to the screen grid to
maintain the corona current on the photoconductor.
As the photoconductor is charged by the saw tooth from the main
corona unit, the surface potential increases. This increases the current
flowing through the screen grid. When the photoconductor potential
nears the grid potential, the current turns to flow to the grid so that the
photoconductor potential can be maintained at a stable level.
Process controlling
Function
The print pattern signal is converted into an invisible image by the
semiconductor laser using negative to positive (reversible) developing
method. Therefore, if the developing bias is added before the drum is
charged, toner is attracted onto the drum. If the developing bias is not
added when the drum is charged, the carrier is attracted to the drum
because of the strong electrostatic force of the drum.
To avoid this, the process is controlled by adjusting the drum potential
and the grid potential of the Scorotron charger.
Basic function
Voltage added to the screen grid can be selected, high and low. To
make it easily understood, the figure below shows voltage transition at
the developer unit.
START
0
Toner attract
potential
2)
3)
1) Low
4) High
STOP
Print potential
Developing bias
Drum potential
Time
AR-168S/168D COPY PROCESS 6 - 4
Start
1) Because the grid potential is at a low level, the drum potential is at
about -400V. (Carrier may not be attracted though the carrier is
pulled towards the drum by the electrostatic force of -400V.
2) Developing bias (-400V) is applied when the photoconductor
potential is switched from LOW to HIGH.
3) Once developing bias (-400V) is applied and the photo conductor
potential rises to HIGH, toner will not be attracted to the drum.
Stop
The reverse sequence takes place.
Retaining developing bias at an abnormal occurrence
Function
The developing bias will be lost if the power supply was removed
during print process. In this event, the drum potential slightly abates
and the carrier makes deposits on the drum because of strong static
power. To prevent this, the machine incorporates a function to retain
the developing bias for a certain period and decrease the voltage
gradually against possible power loss.
Basic function
Normally, the developing bias voltage is retained for a certain time
before the drum comes to a complete stop if the machine should stop
before completing the normal print cycle. The developing bias can be
added before resuming the operation after an abnormal interruption.
Therefore, carrier will not make a deposit on the drum surface.