Multistep focusing electron gun for cathode ray tube

Abstract: A multistep focusing electron gun for a cathode ray tube comprising at least a unipotential auxiliary lens (G5 to G7) and a bipotential major lens (G7, G8). The electron beam passing hole of the middle electrode (G6) of three successive electrodes forming the unipotential auxiliary lens which is supplied with a low potential is in the form of a equilateral parallelogram and has dimensions such that the electron beam passing holes of the electrodes disposed at the front and rear of the middle electrode can be circumscribed. Thus, the intensity of the unipotential auxiliary lens is weakened without reducing the mechanical strength of the electrode, and also the change of the relative position between the electrodes resulted from the structure change is inhibited.

Description: The present invention relates to a multistep focusing electron gun for a cathode ray tube, and more particularly to an electron gun for a color cathode ray tube having an improved unipotential auxiliary lens. A known in-line type electron gun structure is disclosed in GB-A-2019082. The conventional multistep focusing electron gun for a color cathode ray tube comprises a cathode K, a control grid G1, and a screen grid G2 all together constituting a triode section, and also electrodes G4 to G8 constituting an auxiliary lens and a major lens of a main lens system, as shown in FIG.1. In this multistep focusing electron gun having the above constitution, a voltage below 1 KV is supplied to the electrodes G2, G4, and G6, and a voltage below 10 KV is supplied to the electrodes G3, G5, and G6, and a voltage below a maximum 30 KV is supplied to the anode, i.e. the electrode G8. At this time, a first focus voltage of a certain potential is supplied to the electrodes G2, G5, and G7, and a second focus voltage lower than the first focus voltage is supplied to the electrodes G4 and G6. According to the voltage supplying method, a first unipotential static lens is formed by the electrodes G2, G3, and G5, and the second unipotential static lens is formed by the electrodes G5, G6, and G7, and a bipotential static lens is formed by the electrodes G7 and G8. In this conventional multistep focusing electron gun for a cathode ray tube as shown in FIG.2, after thermal electrons emitted from the cathode K is formed to an electron beam by the electrodes G1 and G2, the beam is previously accelerated through the first unipotential static lens and the second unipotential static lens, and is finally focused and accelerated by the bipotential static lens. At this time, the electron beam is gradually diverged while passing the first and second unipotential static lenses, in which the diverging angle  2 of the electron beam in the second unipotential static lens is larger than the diverging angle  1 in the first unipotential static lens. The reason is that an electron beam passing hole H of the electrode G6 among the electrodes G5 to G7 constituting the second unipotential static lens has a diameter equal to those of electron beam passing holes of the electrodes G5 and G7 respectively disposed at the front and at the rear of the electrode G6, and the thickness T of the electrode G6 is relatively thick. Accordingly, this conventional electron gun can not provide a good focus characteristic. In this electron gun, to form an electron beam having a good focus characteristic, the diverging angle of the second unipotential static lens should be reduced. To reduce the diverging angle, the thickness T of the electrode G6 should be reduced or the electron beam passing hole H of the electrode G6 should have a diameter larger than those of electron beam passing holes of the adjacent electrodes G5 and G7 disposed at the front and the rear of the electrode G6. However, there is a limitation in reducing the thickness of an electrode because a thin thickness T of the electrode G6 deteriorates the mechanical strength, thereby being subject to deformation by a compressive force applied when all electrodes are fixed to supporting beads. When the electron beam passing hole H of the electrode G6 is formed to be larger than those of the electrodes which are respectively at the front and at the rear of the electrode G6, the positions can not be exactly set by the guide rod for setting the relative position to be inserted to the electron beam passing hole during a process for assembling electrodes into one structure, thereby deteriorating the precise assembling. It is an object of the present invention to provide a multistep focusing electron gun for a cathode ray tube, whose structure is improved to have a good focus characteristic. According to the present invention there is provided a multistep focusing electron gun for a cathode ray tube, said gun comprising at least three successive electrodes each having a plurality of electron beam passing holes in line with respective ones in each of the other two electrodes for forming a unipotential auxiliary lens and a further electrode having a plurality of electron beam passing holes each in line with a respective one of the holes of said three successive electrodes for forming a bipotential major lens with the last electrode of said three successive electrodes in the direction of electron beam travel, characterised in that each of the electron beam passing holes of the central electrode of said three successive electrodes has the form of an equilateral parallelogram of dimensions to circumscribe the corresponding electron beam passing holes of the other two of said three successive electrodes. Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of the conventional multistep focusing electron gun; Fig. 2 is a cross section view of the electron beam in the conventional electron gun shown in Fig. 1 for 2-dimensionally showing the diverging and focusing states thereof; Fig. 3 is an extracted schematic view of principal parts of the multistep focusing electron gun for a cathode ray tube according to an embodiment of the present invention; FIG.4 is a front view of the electrodes shown in FIG.3 when viewed in the passage of the electron beam; FIG.5 and FIG.6 are front views of the electrodes applicable to other preferred embodiments of the present invention; FIG.7A illustrates the controlling state of the electron beam in the electron gun according to the present invention; FIG.7B is an extracted enlarged illustration of FIG.7A; FIG.8 illustrates controlling state of the electron beam in the electron gun of an embodiment of the present invention by way of equipotential lines; and FIG.9 shows an electron beam section controlled by the electron gun of embodiment of the present invention. Hereinafter, a preferred embodiment of the present invention will be explained with reference to the attached drawings. The electron gun this embodiment having generally the same structure as of the conventional electron gun shown in FIG.1 comprises a cathode, electrodes G1 and G2 all together constituting a triode, electrodes G3 to G7 constituting an auxiliary lens and a major lens of a main lens system, and an anode G8, in which a focus voltage below 10 KV is supplied to the electrodes G3, G5, and G7, a static voltage below 1 KV is supplied to the electrodes G2, G4 and G6, and the anode voltage below 30 KV is supplied to the electrode G8. Accordingly, the first unipotential auxiliary lens is formed by the electrodes G3, G4, and G5, and the second unipotential auxiliary lens is formed by the electrodes G5, G6, and G7, and the major lens is formed by the electrodes G7 and G8. In the electron gun of the present invention, the electrodes G5, G6, and G7 of the second unipotential auxiliary lens which is a characteristic part have the construction shown in FIGs.3 and 4. Each electrode is provided with three electron beam passing holes of in-line type, and the whole beam passing holes of each electrode are disposed in a plane. At this time, the electron beam passing holes H5 and H7 of the electrodes G5 and G7 are in the form of circles having an identical diameter, and the electron beam passing hole H6 of the electrode G6 disposed between the above electrodes is in the form of a square in which the length of sides is as long as the diameter of the electron beam passing holes H5 and H7 of the electrodes G5 and G7 so that the electron beam passing holes H5 and H7 of the electrodes G5 and G7 can be inscribed. Another preferred embodiment of the present invention discloses as shown in FIG.5 that the electrode G6 has an electron beam passing hole H6′ in the form of a rhombus, where the electron beam passing hole H6′ is also sized to circumscribe the electron beam passing holes H5 and H7 of the electrodes G5 and G7 disposed at the front and at the rear of the electrode G6. The other preferred embodiment of the present invention shows that the electrode G6 has two electron beam passing holes H6 in the form of a square at both ends and an electron beam passing hole H6′ in the form of a rhombus at the center, in which the whole electron beam passing holes H6, and H6′ are sized to circumscribe the electron beam passing holes H5 and H7 of the electrodes disposed at the front and at the rear of the electrode G6. The operation of a multistep focusing electron gun for a cathode ray tube of the present invention provided with above described electrode G6 will be explained as follows. The electron beam composed of a cathode K, the electrodes G1 and G2 is previously focused and accelerated by a first unipotential auxiliary lens composed of the electrodes G2, G4, and G5, and a second unipotential auxiliary lens composed of the electrodes G5, G6, and G7, and then is finally accelerated and focused by a bipotential major lens composed of the electrodes G7 and G8 to be landed on a screen. At this time, the square electron beam passing holes H6 and H6′ are larger than the electron beam passing holes H5 and H7 of the electrode G5 and G7 disposed at the front and at the rear of the electrode G6, thereby having a weaker diverging force than that of the first unipotential auxiliary lens formed at the front thereof. Accordingly, the incidence angle of the electron beam entering the major lens is reduced by the second unipotential auxiliary lens of much weaker diverging force, thereby improving the focus characteristic of the electron beam so as to have a desirable electron beam spot on a screen. The detailed descriptions therefor are as follows. Referring to FIG.7, a high potential focus voltage (below 10 KV) is supplied to the electrodes G5 and G7, and a low potential focus voltage (below 1 KV) is supplied to the electrode G6 disposed between the electrodes G5 and G7, so that a unipotential auxiliary lens is formed by the electrodes G5, G6, and G7. Accordingly, the electron beam is decelerated and diverged while passing through the electrodes G5 and G6, and accelerated and focused while passing through the electrodes G6 and G7. When the electron beam is controlled by the electrodes, the electron beam passing hole H6 of the electrode G6 is larger than the electron beam passing holes at the front and at the rear of the electrode G6, thereby preferably decreasing the diverging angle of the electron beam between the electrodes G5 and G6, so as to reduce the desired incidence angle to the major lens of the electron beam. The multistep focusing electron gun according to the present invention, which compensates the deflection astigmation caused by deflection yoke to improve the color purity of the picture of the cathode ray tube as set forth below. As shown in FIG.8, insides of the square type electron beam passing hole H6 of the electrode G6, and the circle type electron beam passing holes H5 and H7 of the electrodes positioned at the front and at the rear of the electrode G6, in which the circle type holes H5 and H7 are inscribed to the square type hole H6, have such potential distributions that are different at the four contacts of the circular holes H5 and H7 and the square hole H6 and around the four corners of the square hole H6. Accordingly, the electron beam B passing the above electrodes is forced in the arrow direction as shown in FIG.8. As a result, the cross sectional form of the electron beam B which have passed the electrodes is extended in the diagonal directions B2 and B3 and is shrunk in the horizontal and vertical directions B1 and B4 to be concaved, as shown in FIG.9. The electron beam B having the above-mentioned cross section passes through the major lens to be finally focused and accelerated. Then, when the electron beam is deflected towards the surroundings of the screen by the deflection yoke, the deflection astigmation of the electron beam by the deflection yoke is compensated by the flare of the beam in the diagonal direction according to the curvature variation of the screen surface, thereby obtaining a uniform beam spot. As shown in FIG.5, the electron beam passing hole H6 of the electrode G6 is formed in such a rhombus that is made by rotating a square by approximataly 45°, and the vertical length is extended longer than the horizontal length. That is, the cross section of the electron beam becomes a longitudinally extended form, so that the deflection astigmation is compensated to improve the resolution in the whole screen when the electron beam B is deflected towards the surroundings of the screen surface by the deflection yoke. In the electron gun of the present invention having a good focus characteristic by controlling an electron beam, the electron beam passing hole H6 of the central electrode G6 is larger than the electron beam passing holes H5 and H7 of the electrodes G5 and G7 disposed at the front and the rear thereof in such a manner that the electron beam passing holes H5 and H7 can be inscribed in the electron beam passing hole H6 of the central electrode G6. Thus, in the present invention, when the electrodes are assembled, the edges of the electron beam passing holes of all of the above electrodes partially or wholly contact the surface of the guide rod inserted through the electrode beam passing holes of the electrodes, thereby keeping the precise relative positions between the electrodes. As described above, the present invention is characterized in that the intensity of the unipotential auxiliary lens is weakened without reducing the mechanical strength of the electrode, and also the change of the relative position between the electrodes resulted from the structure change is inhibited. The present invention is not limited in the above-described preferred embodiment, but is applicable to any other electron gun having at least one unipotential auxiliary lens.

Claims: A multistep focusing electron gun for a cathode ray tube, said gun comprising at least three successive electrodes (G5, G6, G7) each having a plurality of electron beam passing holes in line with respective ones in each of the other two electrodes for forming a unipotential auxiliary lens and a further electrode (G8) having a plurality of electron beam passing holes each in line with a respective one of the holes of said three successive electrodes for forming a bipotential major lens with the last electrode (G7) of said three successive electrodes in the direction of electron beam travel, characterised in that each of the electron beam passing holes of the central electrode (G6) of said three successive electrodes has the form of an equilateral parallelogram of dimensions to circumscribe the corresponding electron beam passing holes of the other two of said three successive electrodes. An electron gun as claimed in claim 1, wherein each of the electron beam passing holes (H6) of the central electrode (G6) is square in shape. An electron gun as claimed in claim 1, wherein each of the electron beam passing holes (H6') of the central electrode (G6) is rhomboidal in shape. An electron gun as claimed in claim 1, wherein each of the electrodes (G5, G6, G7) has three electron beam passing holes, the central beam passing hole of the central electrode (G6) being rhomboidal in shape, and the outer beam passing holes of the central electrode (G6) being square in shape. An electron gun as claimed in claim 4, wherein the height of the central electron beam passing hole (H6') is greater than the width thereof.

Inventors: CHO SEOK-RAE, CHO, SEOK-RAE
Applicants: SAMSUNG DISPLAY DEVICES CO LTD, SAMSUNG DISPLAY DEVICES CO., LTD.
Application year: 1990
Main classification: H01J29
IPC codes: H01J29