JPH027469B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a developing bias device for an electrostatic recording device;
In particular, the present invention relates to an electrostatic recording apparatus that performs electrostatic recording by developing an electrostatic latent image formed on a recording medium (for example, a photosensitive drum) with toner by jumping development.

In the one-component development method in which development is performed only with toner without using carrier particles, jumping development (see, for example, Japanese Patent Application Laid-Open No. 18656/1983) is used to obtain uniform toner charging and development with gradation. It will be done. Normally, in such one-component jumping development, the distance between the photosensitive drum and the developing sleeve is large, so the characteristic curve of the photosensitive drum potential and development density shows a steep rise as shown by the solid line A in Figure 3, and the gradation changes. To avoid this, by applying a strong alternating current electric field to the developing sleeve and causing the toner to reciprocate between the drum and the sleeve, the toner can be created with a smooth rise as shown by the broken line in Figure 3, and the gradation can be improved. Achieves high-quality development.

However, if the frequency of the AC developing bias is kept constant, the contrast cannot be strengthened for images with few intermediate tones, resulting in poor image reproducibility. Furthermore, for images with many halftones, there is a drawback that the background portion of the document tends to overlap.

The present invention aims to eliminate the above-mentioned drawbacks, namely, latent image forming means 2 to 5 for forming a latent image of a document on a recording medium 1, and a latent image formed by the latent image forming means. means 7 and 8 for developing, and means 9 to supply a developing bias voltage consisting of an AC bias voltage and a DC bias voltage to the developing means.
17, the supply means selectively supplies an AC bias voltage of a first frequency corresponding to an image with many halftones and an AC bias voltage of a second frequency corresponding to an image with few halftones. and further includes means 16a for determining the frequency of the AC bias voltage supplied by the supply means, and the supply means further varies the value of the DC bias voltage to be supplied according to the determination result by the determination means. The purpose of the present invention is to provide an electrostatic recording device that can be used.

Next, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows an embodiment of a developing bias device for an electrostatic recording device equipped with a surface potential sensor. For example, a photosensitive drum 1 is composed of three layers, an insulating layer, a photoconductive layer, and a conductive layer from the surface. A primary charger 2 is arranged around the photosensitive drum 1 to charge the entire surface of the photosensitive drum 1. Next to the primary charger 2, a secondary charger 3 and a full-surface exposure lamp 5 are arranged adjacent to each other in the drum rotation direction. The secondary charger 3 removes the charge on the photosensitive drum exposed to the reflected light beam 4 from the original (not shown) according to the amount of exposure, and forms an electrostatic latent image of the original on the photosensitive drum 1. . The electrostatic latent image thus formed is fully exposed by a full-surface exposure lamp 5 in order to increase sensitivity, resulting in an electrostatic latent image with even better gradation. Thereafter, the toner is transferred to a developing device 7 and developed with toner via a developing roller 8.

A bias voltage is applied to the developing sleeve 8 constituting the developing device 7 via a line 11a in order to perform jumping development. The bias voltage consists of an AC bias voltage and a DC bias voltage, and the AC bias voltage is generated via a sine wave oscillation circuit 9a or 9b, an amplifier 10, and a step-up transformer 11.
The sine wave oscillation circuits 9a and 9b are selected by the switch 16a, and each selected sine wave generation circuit oscillates a sine wave of a predetermined frequency. It is amplified by distortion and then boosted by a step-up transformer 11.
This step-up transformer 11 is a transformer with a step-up ratio of around 1:100, and converts the sine wave into an unsaturated and distortion-free transformer.
Boost the voltage to about 500V _pp to 2000V _pp . The AC bias voltage boosted in this way is applied to the developing roller 8 via the line 11a.

The output of the amplifier 10 is sent to an automatic gain adjustment circuit 17, compared with a reference value, and then fed back to the gain adjustment input of the amplifier 10. Automatic gain adjustment circuit 17
The LED 1 connected to the amplifier 10 lights up in accordance with the magnitude of the output of the amplifier 10, and if the output is small, the LED 1 turns off or goes dark, indicating an abnormality in the circuit.

On the other hand, a wire 1 is connected to one end of the secondary side of the step-up transformer 11.
The output of the DC-DC inverter 14a is connected through 1b, and a DC bias of about -500V to +500V is applied. A voltage from the surface potential control circuit 13 is applied to the DC-DC inverter 14a. A surface potential sensor 6 installed immediately before the developing device 7 measures the potential of the latent image on the photosensitive drum 1, and measures the potential of the latent image formed under a constant amount of light (hereinafter referred to as V _SL ).
is detected by the surface potential sensor 6, converted to 1/300 of the latent image potential by the surface potential measuring circuit 12, and inputted to the surface potential control circuit 13. This control circuit 13 is
Arithmetic control is performed so that, for example, a DC voltage of (V _SL +100)V is output from the output of the DC-DC inverter 14a.

In addition to the DC-DC inverter 14a, there is also a DC-
A DC inverter 14b is connected and similarly controlled by the surface potential control circuit 13. DC-DC
The inverter 14b is driven by a switch 16b interlocking with the switch 16a, and the sine wave oscillation circuit 9
When b is selected, +
A potential of 5V is sent, and the DC-DC inverter 14b generates a predetermined DC voltage (for example, 50V).
Therefore, the AC bias voltage obtained from the sine wave oscillation circuit 9a or 9b and the DC bias voltage obtained from the DC-DC inverters 14a, 14b are applied to the developing roller 8 in a superimposed manner.

FIG. 2 shows sine oscillation circuits 9a, 9b and an amplifier 1.
0, an automatic gain adjustment circuit 17, and a DC-DC inverter 14b. The sine wave oscillation circuit 9a includes a differential amplifier Q1 and a capacitor C.
3. It has resistors R4 and R5, which constitute a pseudo inductance, and this output is connected to a differential amplifier Q'1.
, and the oscillation frequency ₁ is
If R ₄ =R ₅ =R' and the parallel capacitance of capacitors C1 and C2 is C', then f ₁ =1/ ^2π√'3'2 .

The sine wave oscillation circuit 9b is configured similarly to 9a, and the sine wave oscillation circuit 9a oscillates a sine wave of 800 to 1500 Hz by adjusting the capacitor C2, and the sine wave oscillation circuit 9b adjusts the capacitor C7. By doing so, it is configured to generate a sine wave voltage of 200 to 600Hz. Note that V1,
V2 is a power supply voltage.

The outputs of these sine wave oscillation circuits 9a and 9b are input to a diode switch composed of diodes D2 to D5. When the switch 16a is switched to the upper side in FIG. 1 and the switch terminal J1 is grounded, the diodes D2 and D3 are made conductive, and the diodes D4 and D5 are cut off, so that the sine wave oscillation circuit 9a
The output of is input to the amplifier circuit 10. On the other hand, when switch 16a is turned down and switch terminal J1 is opened, V2 (+24 volts) is applied via resistor R54.
is applied to the connection point of each diode, D
2 and D3 are cut off, D4 and D5 are made conductive, and the output of the sine wave oscillation circuit 9b is input to the amplifier circuit 10.

The amplifier circuit 10 is composed of a differential amplifier Q5, an emitter follower Q6, and final stage drivers Q7 and Q8.
of the automatic gain adjustment circuit 17 connected to the resistor R18.
The amplification factor is determined by the resistance value between the source and drain of FETQ4. The output of differential amplifier Q5 is connected to the base circuit of emitter follower Q6 and the collector of transistor Q9 via resistor R20.
Transistor Q9 is a control switch for developing bias AC voltage, and when connector terminal J2 is opened,
A voltage of V2 is applied to the base circuit of transistor Q9 via resistor R1, transistor Q9 becomes conductive, and the output of differential amplifier Q5 is applied to resistors R20 and R2.
At the intersection of 1, the voltage becomes zero V, and the AC bias becomes 0. When the terminal J2 is grounded, the transistor Q9 is opened and the output of the differential amplifier Q5 is directly applied to the emitter follower Q6, so that an AC output voltage of a predetermined frequency is generated at the emitters of the transistors Q7 and Q8.

A control signal is sent to the above-mentioned connector J2 from the sequence controller of the recording apparatus, and the terminal J2 is opened to set the AC bias to 0 except during development. The capacitor C23 inserted between the base and emitter of the transistor Q9 is connected to the transistor Q9.
This is to smooth the rise and fall during switching of step 9 and eliminate overshoot linking that occurs on the secondary side of the step-up transformer during on/off switching.

The output of amplifier 10 is connected to resistor R31 and capacitor C.
13 to the primary side of the step-up transformer 11. The step-up transformer 11 is a laminated type transformer made of silicon steel plates with high saturation magnetic flux density and has a step-up ratio of 1:100.
It is configured to have flat frequency characteristics in the frequency range of 200 to 1500 Hz.
One end of the secondary winding of the step-up transformer 11 is DC-
(+) side output of DC inverter 14b (J3 terminal)
connected to. The (-) side output (J4 terminal) of the DC-DC inverter 14b is connected to the output of the DC-DC inverter 14a (not shown in Figure 2), and this output is at (V _SL +100V) during development. ) voltage is controlled. When the switch terminal J1 is opened, the sine wave oscillation circuit 9b is selected, and at the same time, the transistor Q10 of the electronic switch 18 becomes conductive (the connection point between the resistors R9 and R54 is connected to the base of Q10 via the resistor R25). A self-excited inverter including transistors Q11 and Q12, capacitor C17, and coil T3 starts oscillating, and J
3. Generate 50V DC voltage between J4. Further, when the switch terminal J1 becomes the ground potential, the transistor Q10 is cut off, power is no longer supplied to the primary side of the DC-DC inverter 14b, and the voltage between J3 and J4 becomes 0V. Note that the terminal J5 is connected to the developing sleeve 8. Resistance R50, R51, R
52 and R58 serve as both a bleeder resistor and an attenuator that generates a monitoring voltage of 1/100 of the AC output at the intersection of R52 and R58.

The automatic gain adjustment circuit 17 rectifies the output of the amplifier 10 into a voltage doubler using diodes D14 and D8, and converts it into a DC voltage. The differential amplifier Q13 converts that voltage and the terminal voltage of the constant voltage diode ZD1 into a volume VR.
It is compared with a reference voltage obtained by dividing by 1, and the error voltage is amplified. The error voltage changes the gate voltage of FETQ4, changes the resistance value between the drain and source of FETQ4, controls the open circuit gain of amplifier 10, and keeps the output of the amplifier constant. A part of the rectified output is connected to a light emitting diode via resistor R33.
A current is passed through LED 1 to light up the display to indicate whether or not normal AC output is being output. Also
A light emitting diode LED2 is also connected in series to the bleeder resistor R44 on the secondary side of the DC-DC inverter 14b, and displays whether the inverter is being driven and outputting a normal voltage.

In the developing bias device configured as described above, charging and exposure are started according to a predetermined sequence, and a latent image formed on the photosensitive drum 1 is transferred to the developing device 7.
When approaching , a control signal is sent from the sequence controller of the recording device to ground terminal J2. This turns off transistor Q9 and resistor R2
An AC bias voltage is obtained at the intersection of 1 and R20. For example, in the case of an image with many intermediate tones, such as a photograph, the switch 16a is turned downward. At this time, the terminal J1 is opened, the low frequency sine wave oscillation circuit 9b is selected, and an AC bias voltage of 200 to 600 Hz is obtained, and the switch 16a is turned upward for an original with few halftones. As a result, the terminal J1 is grounded and the high frequency sine wave oscillation circuit 9b is selected to obtain an AC bias voltage of 800 to 1500 Hz. These AC bias voltages are amplified by an amplifier 10 and applied to the primary side of a step-up transformer 11, so that an AC bias voltage boosted by 1:100 is obtained on the secondary side.

On the other hand, a DC bias voltage of (V _SL +100V) is generated at the output of the DC-DC inverter 14a, and when the switch 16a is turned down, the DC-DC
Inverter 14b operates and terminals J3 and J4
Since a voltage of 50V is generated, step-up transformer 11-2
A DC bias voltage of (V _SL +150V) is generated on the next side.

Therefore, the developing sleeve 8 is equipped with a sine wave oscillation circuit 9a.
Alternatively, the alternating current bias voltage from 9b and the direct current bias voltage from DC-DC inverter 14a or 14b are applied in a superimposed manner to perform jumping development. If no AC bias is applied, the image will have a strong contrast as shown in Figure 3 A, but if a high frequency AC bias is applied, the gradation will be well reproduced as shown in B. When the DC bias is increased from 100V to 150V, the gradation is better reproduced as shown in C, and when the DC bias is increased from 100V to 150V, the characteristics become as shown in D. Development with good reproducibility and low fog can be obtained. Therefore, for documents with many halftones, a low-frequency AC bias voltage and a large DC bias voltage are applied, and for originals with few halftones, a high-frequency AC bias with a low DC bias is selected. Of course, in order to simplify the structure, the magnitudes of the AC bias and DC bias can be made constant.

Furthermore, the output of the amplifier 10 is the automatic gain adjustment circuit 1
7 and is automatically adjusted, so the oscillation circuit 9
Even if the outputs of a and 9b vary or fluctuate, a stable output can always be obtained. Further, the automatic gain adjustment can be performed using the output of the step-up transformer 11 as well as the output of the amplifier 10 as an input.

In addition, instead of switching the switches 16a and 16b manually, the original is automatically scanned in advance before the normal copying and recording process, and it is determined whether there are many line copies or halftones, and the magnitude and frequency of the bias voltage are automatically adjusted. You can make it switch.

Furthermore, three or more sine wave oscillation circuits with different frequencies are provided, one of which is selected as the AC bias voltage, and the level of the DC-DC inverter is switched in plural or continuously to generate the DC bias voltage. Of course, it can also be made variable.

FIG. 4 shows an embodiment in which no surface potential sensor is provided. In this embodiment, the same parts as in FIG. 1 are given the same reference numerals, and their explanations are omitted. In this example, a suitable fixed bias of 15 is applied to the other end of the secondary winding of the step-up transformer.
A, 15b are connected, and the fixed bias 15a is set to 1 by the switch 16b.
By making it smaller than 5b, the same effect as the embodiment shown in FIG. 1 can be obtained.

As described above in detail, the following effects can be obtained with the apparatus according to this embodiment.

1 This embodiment does not use the conventional method of extracting only the reference frequency from the rectangular wave by resonance of the LC to create a sine wave, so there is no resonance loss and the power consumption is very low.
There is no electromagnetic vibration sound.

2. Frequency switching is easy. The conventional example is a large capacitance C or L for frequency and resonance of a square wave oscillation circuit.
In contrast, in this embodiment, it is only necessary to change the oscillation frequency of the sine wave oscillation circuit, so the frequency can be easily changed using a volumetric volume or a variable capacitor.

3. No adjustment of the resonant circuit is required.

4. By changing the DC bias superimposed on the AC voltage according to the latent image potential, development with excellent gradation and no fogging can be realized.

5. Even when a plurality of sine wave generation circuits with different frequencies are switched, there is no need to adjust the amplitude of each oscillation circuit, and a stable output amplitude can always be obtained.

6. It is possible to easily and reliably display whether the outputs of the DC bias and AC bias of the developing bias are normal or abnormal.

As explained above, according to the present invention, the frequency of the AC developing bias can be changed depending on whether the image has many or few halftones, and when the frequency of the AC bias is changed, the DC bias voltage can be changed. By changing the value in accordance with the frequency of the AC bias, it is possible to improve the gradation for images with many intermediate tones, and prevent the background part of the document from overlapping. The contrast can be increased for images with little tone.

[Brief explanation of drawings]

FIG. 1 is a layout diagram of a first embodiment of the device of the present invention, FIG. 2 is a detailed circuit diagram of the circuit shown in FIG. 1, and FIG. 3 is a characteristic diagram showing concentration characteristics due to changes in bias voltage. FIG. 4 is a layout diagram of a second embodiment of the present invention. DESCRIPTION OF SYMBOLS 1...Photosensitive drum, 2...Primary charger, 3...Secondary charger, 5...Full surface exposure lamp, 7...Developer, 8...Developing roller.

Claims (1)

[Scope of Claims] 1. A latent image forming means for forming a latent image of a document on a recording medium, a means for developing the latent image formed by the latent image forming means, and an alternating current bias voltage and a direct current bias for the developing means. and means for supplying a developing bias voltage consisting of a voltage, and the supplying means has an AC bias voltage of a first frequency corresponding to an image with many halftones and an AC bias voltage of a second frequency corresponding to an image with few halftones. The bias voltage is selectively supplied, and further includes means for determining the frequency of the AC bias voltage supplied by the supply means, and the supply means further supplies the AC bias voltage according to the determination result by the determination means. An electrostatic recording device characterized in that the values of DC bias voltages are different.