JP3435852B2 - High-voltage residual prevention spot generation circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spot generating circuit for preventing a CRT from remaining at a high voltage in an electronic apparatus using the CRT. 2. Description of the Related Art In recent years, CRTs used for television receivers, monitor displays of personal computers, etc.
There is a demand for a screen having a larger effective display screen area than ever before. In a CRT, a high voltage of several tens of kV needs to be applied between an anode and a cathode. However, even after the power is turned off by a television receiver or the like, electric charges remain on the CRT, and a charge is applied between the anode and the cathode. When a voltage of about 20 kV or more remains, thermionic emission occurs in which electrons are emitted from the cathode to the anode regardless of the potential of the cathode. Further, since the electron beam collides with the phosphor screen even though the deflection is stopped, the central portion of the phosphor screen is burned. [0003] This is also due to the life of the cathode, that is, the CRT.
In order to shorten the life of a television, a method of detecting a power-off in a television receiver or the like and discharging the high voltage by illuminating the entire screen at the moment when the screen disappears has been used. For example, Japanese Patent Application Laid-Open No. 4-252668 discloses an example of a conventional spot generating circuit. Hereinafter, a conventional high-pressure residue prevention spot generating circuit will be described.
FIG. 4 is a circuit diagram of a conventional spot generating circuit for preventing residual high voltage. In FIG. 4, 1 is an R signal drive circuit,
2 is a G signal drive circuit, 3 is a B signal drive circuit,
Each signal of R, G, B is amplified and the signal is sent to the cathode of CRT4. Reference numeral 5 denotes a DC power supply for outputting the voltage V1, which is a drive power supply for the drive circuits 1 to 3. Reference numeral 6 denotes a DC power supply for outputting a voltage V2, for example, a DC power supply for driving a signal system of a television receiver. [0006] The DC power supply 6 is assumed to have its voltage lowered as the power of the television receiver is turned off. Reference numeral 7 denotes a diode, 8 denotes a PNP transistor, 9 denotes a base resistance of the PNP transistor 8, and a switch circuit 10 is constituted by three elements 7 to 9. Diodes 11 to 13 are connected in a forward direction from the collector of the transistor 8 to the respective inputs of R, G and B. A capacitor 14 is connected from the DC power supply 6 via the diode 7. Reference numeral 15 denotes a flyback transformer, 16 denotes an anode cap, 17 denotes a DC power supply for generating a high voltage V3, and 18 denotes a horizontal output circuit. The high pressure generated by the flyback transformer 15 is supplied to the anode of the CRT through the anode cap 16. Here, the operation of the switch circuit 10 will be described. During normal operation of the television receiver, a voltage is supplied from the DC power supply 6. At this time, since the capacitor 14 is connected from the DC power supply 6 via the diode 7, the voltage VC14 between the terminals of the capacitor 14 rises to a voltage represented by the following equation. VC14 = V2-Vf where Vf: forward voltage drop of diode 7. Next, when the power of the television receiver is turned off, the voltages of DC power supplies 5 and 6 start to drop. Further, since the supply of the DC voltage 17 is also stopped, the high voltage generated in the flyback transformer 15 also decreases, but the anode potential remains high due to the large anode capacity of the CRT. The output voltage V2 of the DC power supply 6 is
4 begins to fall below the terminal voltage VC14, the emitter voltage VE of the transistor 8 becomes the terminal voltage V
Fixed to C14. When V2 further decreases and the base-emitter voltage VBE of the transistor 8 becomes about 0.7 V or more, the transistor 8 is turned on, and the collector voltage VC of the transistor 8 increases. The output voltage V2 of the DC power supply 6 is R, G, B
Since the voltage is higher than the maximum voltage of each signal output, a transistor 8 is connected to each of the R, G, and B signal terminals via diodes 11 to 13.
Current flows from the collectors of the respective terminals, and the voltage of each signal terminal increases. The higher the voltage, the higher the luminance of each of the R, G, and B inputs. Therefore, the cathode potential of the CRT is reduced by the output that is inverted and amplified by each of the drive circuits 1 to 3, and the charge accumulated in the capacitor 14 is discharged. During this period, the entire screen glows white, and a large amount of beam current flows, whereby charges accumulated in the CRT are released, and the anode potential decreases. However, in the above-described conventional configuration, the current flowing out of the capacitor 14 cannot be limited. Therefore, the transistor 8 is turned on at the moment when the base-emitter voltage VBE of the transistor 8 exceeds 0.7V. 8, the collector current starts to flow rapidly. Accordingly, R,
Since the input voltages of G and B increase sharply, the potential of the cathode also sharply decreases. At that moment, the screen emits white light and a beam current flows. At this time, the amount of charge released per unit time is substantially proportional to the beam current and the area of the CRT. For this reason, in a CRT having a large effective area, if a large amount of beam current is applied at a time, the amount of charge emitted from the CRT per unit time increases, and the high voltage drops sharply. When the high voltage of the CRT is lowered, the space potential difference between the CRT and the vicinity of the CRT increases, and when the space potential exceeds a certain level, a sound called an electrostatic sound is generated. Since the volume generated per unit area is constant, the total volume of the sound increases as the surface area of the CRT increases, and when it exceeds a certain level, it may cause discomfort in a quiet place. Further, the base-emitter voltage VBE of the transistor 8 tends to decrease as the temperature rises. Conversely, since hfe tends to be large, when the temperature of the television receiver is high, such as when the television is turned on for a long time as compared with the room temperature, the current flows from the collector even if the base current of the transistor 8 is constant. Since the current increases, the voltage rise gradient of each of the R, G, and B signal inputs increases, and as a result, the spatial potential difference between the CRT and its vicinity increases, and the electrostatic noise also increases. In order to solve the above-mentioned conventional problems, the present invention reduces the gradient of the output voltage rise of the switch circuit when discharging the electric charge remaining on the CRT so that the beam current does not suddenly flow. It provides a means. In order to solve the above-mentioned problems, a high-pressure residual prevention spot generating circuit according to the present invention comprises a CR
In an electronic device having a T, a DC power supply that is always energized when receiving an image, a diode connected to the DC power supply, and an emitter connected to the DC power supply via the diode and a base connected to the DC power supply. First
A switch circuit composed of a transistor connected through a resistor, a capacitor charged by the DC power supply through the diode, and an emitter disposed between the capacitor and an emitter of a transistor constituting the switch circuit. A second resistor provided, three diodes respectively connected in a forward direction from the collector of the transistor to each of the RGB signal lines, and the switch circuit is configured.
Of the transistor to be turned on and each of the RGB signal lines
Of three diodes connected in the forward direction
The configuration with the insert disposed to zener diode between the connection point, in the respective RGB signal lines
The peak value of each input voltage of R, G, B is kept almost constant.
The switch circuit detects the drop in the DC power supply voltage, and according to the drop in the DC power supply voltage, disconnects the capacitor from the DC power supply by the diode, and conducts the transistor to charge the capacitor. Charge is discharged to each of the RGB signal lines via the second resistor. According to the above means, by limiting the current flowing out of the capacitor, the slope of the voltage rise of the R, G, B outputs can be reduced, and as a result, the electrostatic noise is suppressed. can do. Further, according to the above-mentioned means, by keeping the current flowing out of the switch circuit constant, it is possible to prevent the magnitude of the electrostatic noise from being influenced by the surrounding temperature conditions. (Embodiment 1) First, a high voltage residual prevention spot generating circuit according to a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a high-voltage residual preventing spot generating circuit according to a first embodiment of the present invention. FIG. 1 differs from FIG. 4 of the conventional example in that a resistor 19 is provided between the emitter of the transistor 8 and the capacitor 14. The operation of the high-pressure residue prevention spot generating circuit configured as described above will be described below. First, the voltage V2 of the DC power supply 6 decreases, and the base-emitter voltage VBE of the transistor 8 decreases by about 0.7.
When the voltage exceeds V, the transistor 8 is turned on, and the electric charge accumulated in the capacitor 14 flows out as the collector current of the transistor 8. Here, the current is limited by the resistor 19, and the voltage rise of each of the R, G, and B inputs becomes gentle, so that the high voltage also becomes gentle. As a result, by discharging the charge of the CRT, no high pressure remains and at the same time, C
The spatial voltage gradient between the RT tube surface and the vicinity of the CRT becomes smaller,
Generation of electrostatic noise is suppressed. Although the resistor 19 is provided between the emitter of the transistor 8 and the capacitor 14 in FIG. 1, the emitter of the transistor 8 and the capacitor 14 are directly connected, and the collector of the transistor 8 and the diodes 11 to 13 are connected.
A resistor may be provided between the anode and the anode. (Embodiment 2) Next, a description will be given of a spot generating circuit for preventing a high voltage residue according to a second embodiment of the present invention with reference to the drawings. FIG. 2 is a circuit diagram of a high-voltage residual preventing spot generating circuit according to a second embodiment of the present invention. FIG. 2 differs from FIG. 1 of the first embodiment in that the collector of the transistor 8 and the diodes 11 to 11 are different.
13, a Zener diode 20 is provided, the cathode of the Zener diode 20 is connected to the collector side of the transistor 8, and the anode is connected to the diodes 11 to 1
3 is connected to the anode side. In the circuit of the second embodiment, if the values of the resistors 9 and 19 are set so as to perform a desired operation when the ambient temperature of the transistor 8 is room temperature, and when the ambient temperature rises significantly from room temperature, the transistor 8 Is larger than that at normal temperature, so that a large collector current of the transistor 8 flows. Then, the voltage rise of the R, G, and B signal inputs becomes large, the beam current suddenly flows, and the high voltage drops in a short time, so that the potential gradient between the CRT tube surface and the vicinity of the CRT becomes large, and the electrostatic capacity becomes large. The sound becomes louder. Further, when the ambient temperature is considerably lower than the room temperature, the collector current of the transistor 8 decreases because the hfe of the transistor 8 becomes smaller than that at the room temperature. Then, since the voltage rise of each of the R, G, and B signal inputs becomes small, the beam current cannot flow sufficiently, and the charge remains on the CRT. As a result, there is a problem that the high pressure does not decrease. The second embodiment of the present invention solves the above-mentioned problem, and the transistor hfe according to the change of the ambient temperature.
It is an object of the present invention to correct the temperature characteristics of the above and to achieve both the prevention of the residual high pressure and the generation of the electrostatic noise regardless of the ambient temperature. FIG. 3 shows the relationship between the Zener current Iz of the Zener diode and the Zener voltage Vz. The Zener voltage Vz of the Zener diode is shown in FIG.
As shown in the figure, the characteristic increases as the Zener current Iz increases. Therefore, by selecting a Zener diode having such a characteristic as to correct the increase of each input voltage of R, G and B accompanying the increase of the collector current due to the temperature characteristic of hfe of the transistor 8, the R, G and B are selected. The peak value of each input voltage can be kept substantially constant, and both prevention of high-voltage residual charge and prevention of generation of electrostatic noise can be achieved regardless of the ambient temperature. In FIG. 3, VZR is a standard Zener voltage of the Zener diode. Although the resistor 19 is provided between the emitter of the transistor 8 and the capacitor 14 in FIG. 2, as in the first embodiment, the emitter of the transistor 8 and the capacitor 14 are directly connected, and the collector of the transistor 8 is connected. A resistor may be provided between the diode and the anodes of the diodes 11 to 13. As described above, according to the present invention, in an electronic device having a CRT, when the operation of the device is stopped,
By discharging the charge remaining on the RT, it is possible to prevent a high pressure from remaining, and it is possible to suppress electrostatic noise generated near the CRT tube surface when discharging the charge remaining on the CRT. Further, according to the present invention, it is possible to realize an excellent spot generation circuit for preventing high-pressure residue, which can prevent both high-pressure residue and electrostatic noise regardless of the ambient temperature.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a high-voltage residual preventing spot generating circuit according to a first embodiment of the present invention. FIG. 2 is a high-voltage residual preventing spot generating circuit according to a second embodiment of the present invention. FIG. 3 is a current-voltage characteristic diagram of a Zener diode according to a second embodiment of the present invention. FIG. 4 is a circuit diagram of a conventional high voltage residual prevention spot generating circuit. 2 G signal drive circuit 3 B signal drive circuit 4 CRT 5 Drive circuit drive power supply 6 Low voltage power supply 7 Diode 8 PNP transistor 9 Base resistor 10 Switch circuit 11 Diode 12 Diode 13 Diode 14 Capacitor 15 Flyback transformer 16 Anode cap 17 High voltage generation DC power supply 18 Horizontal output circuit 19 Resistance

Claims (1)

(1) In an electronic device having a CRT, a DC power supply that is always energized when receiving an image, a diode connected to the DC power supply, and an emitter connected via the diode. A switch circuit configured by a transistor connected to the DC power supply and having a base connected to the DC power supply via a first resistor; a capacitor charged by the DC power supply via the diode;
A second resistor inserted and disposed between the capacitor and an emitter of a transistor constituting the switch circuit;
Three diodes respectively connected in a forward direction from the collector of the transistor to each of the RGB signal lines ;
The collector of the transistor constituting the switch circuit and the above
Three connected in the forward direction to each of the RGB signal lines
Of the diode inserted between the common connection point of
And the switch circuit detects a drop in the DC power supply voltage, and disconnects the capacitor from the DC power supply by the diode in accordance with the drop in the DC power supply voltage, and conducts the transistor to make the capacitor The high-charge residual prevention spot generating circuit is characterized in that the electric charge charged in the spots is discharged to the RGB signal lines via the second resistor.