Sharp AL-2051 Service Manual download pdf (Page 35)

AL-2051 COPY PROCESS 6 - 4

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.

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 photocon-

ductor. 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) devel-

oping method. Therefore, if the developing bias is added before

the drum is charged, toner is attracted onto the drum. If the devel-

oping 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 transi-

tion at the developer unit.

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 car-

rier 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 conduc-

tor 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.

Semiconductor laser

START STOP

Print potential

Toner attract

potential

Low

High

Drum potential

Developing bias

Time

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Sharp AL-2051 Service Manual Page 35

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