Sharp AR-160, AR-161 specifications Laser unit, Optical discharge Semiconductor laser, Pwb

AR-161

(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

a. Charge by the Scorotron charger

<1> Function

The Scorotron charger functions to maintain the surface potential of the drum even at all times which. It is used to control the surface potential regardless of the charge characteristics of the photoconduc- tor.

<2> Basic function

A screen grid is placed between the saw tooth and the photoconduc- tor. 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 poten- tial, the current turns to flow to the grid so that the photoconductor potential can be maintained at a stable level.

b. Process controlling

<1> 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 poten- tial and the grid potential of the Scorotron charger.

<2> 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 STOP

0

Print potentioal

	Toner attract	Developing bias
	potential
	2)
	3)
1)	Low

4) High		Drum potential
	Time

<3> 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.

<4> Stop

The reverse sequence takes place.

c. Retaining developing bias at an abnormal occurrence

<1> 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.

<2> 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.

4. Laser unit

The image data sent from the MCU (image process circuit) is sent to the LSU (laser unit), where it is converted into laser beams.

A. Basic structure

The LSU unit is the writing section of the digital optical system.

The semiconductor laser is used as the light source, and images are formed on the OPC drum by the polygon mirror and fθ lens, etc.

The laser beams are passed through the collimator lens, the cylindri- cal lens, the polygon mirror, the fθ lens, and the mirror to form images on the OPC drum in the main scanning direction. The laser emitting PWB is provided with the APC (auto power control) in order to eliminate fluctuations in the laser power. The BD PWB works for measurement of the laser writing start point.

1	Semiconductor laser	Generates laser beams
2	Collimator lens	Converges laser beams in parallel
3	Polygon	Reflects laser beams at a constant
	mirror,polygon motor	rpm
4	BD (Mirror, lens,	Detects start timing of laser
	PWB)	scanning
5	fθ lens	Converges laser beams at a spot
		on the drum.
		Makes the laser scanning speeds
		at both ends of the drum same as
		each other. (Refer to the figure
		below.)
6	Laser emitting PWB	Emits laser beams according to the
		image data.

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