JP2715680B2 - Grid bias circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a grid bias circuit, and more particularly to a grid bias circuit for applying a bias voltage to a grid electrode of a charger such as a PPC (Plain Paper Copy) or an LBP (Laser Beam Printer).

[0002]

2. Description of the Related Art In recent years, as shown in FIG.
The charging device 3 is provided with a grid electrode 5 in addition to the shield electrode 4 in order to make the drum current Id flowing from the charging device 3 to the charging device 3 uniform. The grid bias circuit 1 connected to the shield electrode 4 and the grid electrode 5
Is configured by connecting a varistor 6 and a variable resistor 7 in series.

In the grid bias circuit 1, the grid voltage Vg is optimally set by adjusting the variable resistor 7, and the drum current Id and the grid current Ig are indirectly determined. Since the operating current Ih is made constant by the output current of the high-voltage power supply 8, Ih = Id +
Ig = constant.

[0004]

In the conventional grid bias circuit 1, when the discharge state changes due to, for example, temperature, the balance between the grid current Ig and the drum current Id is lost, and the drum current Id becomes too small or too large. This causes a difference in image quality. That is, even if the knob for density adjustment is at the same position, a difference in shading occurs, resulting in a copy error, and it is necessary to readjust it and recopy. As described above, there is a problem that sufficient performance cannot be obtained in terms of environmental resistance.

Therefore, a main object of the present invention is to provide a grid bias circuit capable of stabilizing a drum current.

[0006]

According to the present invention, a predetermined constant operating current supplied from a constant current high voltage power supply (40) is provided.
(Ih) a grid bias circuit for applying a bias voltage to the grid electrode (34) of the charger (32 ) for charging the photoconductor drum (30) with a variable constant voltage circuit (12) connected to the grid electrode; and it detects the potential of the photosensitive drum includes a voltage control means for providing a control signal to the variable constant voltage circuit (22), variable
The variable voltage circuit is a power supply whose voltage is controlled by a control signal.
Source (Vc) and the power supply (Vc) and the grid electrode (34).
Series connection of two resistors (14,20)
Base and grid electrodes connected to series connection points (34)
A transistor (16) having an output electrode connected to
A grid bias circuit.

[0007]

When the drum current Id exceeds a predetermined value, for example, the potential of the photosensitive drum detected by the potential sensor also increases. At this time, a control signal for increasing the grid voltage is sent from the voltage control means to the variable constant voltage circuit .
Supplied to power . Therefore, the output voltage of the power supply
The voltage applied to the base of the transistor
A grid that flows through the transistors to become larger
The current Ig increases. On the other hand, since a constant current is supplied from the high-voltage power supply, there is a relation of Ig + Id = Ih = constant, and Id = Ih-Ig. Therefore, if the grid current Ig increases, the drum current Id decreases and returns to the original predetermined value.

On the other hand, when the drum current Id is smaller than a predetermined value, the potential of the photosensitive drum detected by the potential sensor is also smaller. At this time, a control signal for reducing the grid voltage is sent from the voltage control means to the variable constant voltage circuit.
Given to the power supply . Therefore, the output voltage of that power supply
And the voltage applied to the base of the transistor
Is small, so the grid flowing through the transistor
The current Ig decreases. Then, the drum current Id increases from the above relationship and returns to the original predetermined value.

According to the present invention, the grid voltage, that is, the grid current is increased or decreased in accordance with the increase or decrease in the potential of the photosensitive drum, and the drum current is maintained at a predetermined value. Even if it occurs, the optimum drum current is always maintained, so that good image quality performance of copying can be guaranteed and environmental resistance can be improved. Moreover, the grid
The resolution of the current (grid voltage) change is the resistance value of the two resistors
Can be determined by the ratio of
It becomes possible.

The above objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.

[0011]

Referring to FIG. 1, a grid bias circuit 10 of this embodiment includes a variable constant voltage circuit 12. The variable constant voltage circuit 12 includes an external control voltage Vc. The − side of the external control voltage Vc is grounded, and the + side is connected to the base of a PNP transistor 16 via a resistor 14. The emitter of the transistor 16 is grounded, and a plurality of series-connected zener diodes 18 and a resistor 20 are connected in series between the base and the collector of the transistor 16. The Zener diodes 18 connected in series are connected so that the cathode thereof is on the collector side of the transistor 16 and used for guaranteeing the breakdown voltage of the transistor 16.

Here, the external control voltage Vc and the grid voltage Vg have the relationship shown in FIG. That is, when changing the external control voltage Vc within the range of Vc ₁ to Vc _2, grid voltage Vg in response to it Vg ₁ ~
It varies linearly in the range of vg _2. Variable constant voltage circuit 12
Is connected to a voltage control means 22. Voltage control means 22
Is constituted by a potential sensor 24 and an amplifier 26 which amplifies a detection signal from the potential sensor 24 and applies the amplified signal to an external control voltage Vc.

The series connection point 28 of the zener diode 18 and the resistor 20 of the grid bias circuit 10 thus formed is connected to the grid electrode 34 and the shield electrode of the charger 32 arranged near the outer surface of the photosensitive drum 30. 36
Connected to. In addition, the discharge electrode 38 of the charger 32 is connected to a high-voltage power supply 40 that has a constant current. On the other hand, the potential sensor 24 is arranged near the outer surface of the photosensitive drum 30.

In the operation, when the drum current Id is larger than a predetermined value, the potential of the photosensitive drum 30 detected by the potential sensor 24 is also increased. The potential sensor 24 outputs a detection signal to increase the external control voltage Vc according to the detected potential of the photosensitive drum 30. This detection signal is amplified by the amplifier 26 and applied to the external control voltage Vc. The external control voltage Vc increases as the potential of the photosensitive drum 30 increases, and is applied to the variable constant voltage circuit 12. Then, according to the relationship shown in FIG. 2, as the external control voltage Vc increases, the grid voltage Vg also increases. Therefore, the grid current Ig also increases, and Id = Ih-I
The drum current Id decreases from the relation of g. in this way,
When the drum current Id has exceeded the predetermined value, the function of the grid bias circuit 10 suppresses the increase in the drum current Id, and automatically returns the drum current Id to the original predetermined value.

On the other hand, when the drum current Id decreases below a predetermined value, the potential of the photosensitive drum 30 detected by the potential sensor 24 also decreases. At this time, the potential sensor 24
Outputs a detection signal to reduce the external control voltage Vc. This detection signal is amplified by the amplifier 26 and applied to the external control voltage Vc. The external control voltage Vc decreases as the potential of the photosensitive drum 30 decreases, and is applied to the variable constant voltage circuit 12. Then, according to the relationship shown in FIG. 2, the grid voltage Vg decreases in accordance with the decrease in the external control voltage Vc. Therefore, the grid current Ig also decreases, and Id = Ih-I
The drum current Id increases from the relation of g. in this way,
Conversely, when the drum current Id decreases below the predetermined value, the function of the grid bias circuit 10 suppresses the reduction of the drum current Id and automatically returns the drum current Id to the original predetermined value.

As described above, the shunt condition for the environmental change is automatically corrected, a stable drum current Id, that is, a surface potential of the photosensitive drum 30 is always obtained, and the image quality performance can be kept constant. Also,

[0017]

(Equation 1)

Thus, if Ra / Rf is determined, ΔVg / Δ
Vc (amplification factor) is determined. The smaller the value of ΔVg / ΔVc, the better the resolution. Therefore, ΔVg / ΔV
If Ra / Rf is determined so that c becomes small, fine control can be performed by adjusting the grid voltage Vg, and the performance of the charger 32 can be improved. The variable range of the grid voltage Vg, that is, Vg1 to Vg in FIG.
g2 is set in consideration of not expanding more than necessary.

Further, in the grid bias circuit 10 of this embodiment, since a combination of the transistor 16 and the zener diode 18 is used, it is not necessary to use an element such as a varistor having a large voltage tolerance, that is, a large variation as in the prior art. . Further, in this embodiment, a PNP transistor is used as the transistor 16, but it goes without saying that an NPN transistor can be used for the same configuration. Further, the high voltage power supply 40 may be a constant voltage.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is a graph showing a relationship between an external control voltage and a grid voltage.

FIG. 3 is a circuit diagram showing a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Grid bias circuit 12 ... Variable constant voltage circuit 22 ... Voltage control means 30 ... Photoconductor drum 32 ... Charger 34 ... Grid electrode

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukio Maba 2-26-10 Tenjin, Nagaokakyo-shi, Kyoto Murata Manufacturing Co., Ltd. (56) References JP-A-3-25460 (JP, A) JP-A Sho 58-215669 (JP, A)

Claims (1)

(57) [Claims] A bias is applied to a grid electrode (34) of a charger (32) for charging a photosensitive drum (30) with a predetermined constant operating current (Ih) supplied from a constant current high voltage power supply (40). A grid bias circuit for applying a voltage, comprising: a variable constant voltage circuit (12) connected to the grid electrode; and a voltage control means for detecting a potential of the photosensitive drum and supplying a control signal to the variable constant voltage circuit ( 22) , wherein the variable constant voltage circuit controls the voltage by the control signal.
Power supply (Vc) controlled, the power supply (Vc) and the grid electrode
Series connection of two resistors (14,20) connected between
And a base connected to the series connection point of the two resistors
And an output electrode connected to the grid electrode (34).
A grid bias circuit, comprising: