Patent Publication Number: US-6211611-B1

Title: Color cathode-ray tube with resistive spring contact

Description:
FIELD OF THE INVENTION 
     This invention relates to a cathode-ray tube used as a receiving set of television, a computer display, and so on. 
     BACKGROUND OF THE INVENTION 
     In a general cathode-ray tube, a high resistivity film is provided at the midpoint of a feed line from an anode provided for a funnel to an anode of an electron gun in order to lower the maximum instantaneous current at sparking inside a bulb, and to prevent the current from damaging the electron circuit connected to the electron gun, or the like. More specifically, as shown in FIG. 7, a conventional cathode-ray tube comprises a bulb  6  comprising a panel  2  having a phosphor  1  provided on the inner surface and a funnel  5  having conductive films  4   a ,  4   b , and  4   c  provided on the inner wall  3 , an electron gun  7  housed in a neck portion  5   a  of the funnel  5 , and springs  10  having contact portions  9  to electrically connect the conductive film  4   a  on the funnel inner wall  3  with a final electrode  8 . Numeral  24  indicates an anode terminal which applies high voltage to the final electrode  8  of the electron gun  7  through the conductive films  4   a ,  4   b , and  4   c.    
     The specific resistance value of the conductive film  4   a  at the neck portion  5   a  is set to be 0.1-1 Ωcm, the specific resistance value of the conductive film  4   b  at the cone portion  5   b  is set to be no more than 0.1 Ωcm, and the specific resistance value of the conductive film  4   c  between the conductive film  4   a  at the neck portion  5   a  and the conductive film  4   b  at the cone portion  5   b  is set to be 1-10 Ωcm respectively. The contact portions  9  of the springs  10  to contact with the conductive film  4   a  at the neck portion  5   a  are made of a metallic material having high conductivity, such as stainless steel. The above-mentioned construction lowers the maximum instantaneous current generated at a discharge inside the bulb  6 , and protects the circuit parts of a TV set from malfunction or breakage (Tokkai Sho 59-171439). 
     In such a cathode-ray tube, however, the conductive films ( 4   a ,  4   b ,  4   c ) made of different materials are respectively formed on the funnel inner wall  3 , so the cathode-ray tube has problems in connection, such as conductivity failures at the junction A between the conductive films  4   a  and  4   c , and the junction B between the conductive films  4   b  and  4   c , and stripping of the coating film. Another problem is the complicated production process since the different kinds of conductive films ( 4   a ,  4   b ,  4   c ) are respectively formed in a wide range at predetermined regions having different shapes on the inner wall  3  of the funnel  5 . 
     SUMMARY OF THE INVENTION 
     The object of this invention is to provide a cathode-ray tube that solves the problems in connecting different kinds of conductive films at the junctions and that simplifies the production process. 
     A cathode-ray tube of this invention comprises a bulb having a panel provided with a phosphor on the inner surface and a funnel having a conductive film provided on the inner wall, an electron gun housed in the neck portion of the funnel, and a spring that is provided for the final electrode of the electron gun and has a contact portion to electrically connect the conductive film on the funnel inner wall with the final electrode, in which the spring has a part with a specific resistance value greater than that of the conductive film that contacts with the contact portion at the mid-point of the conductive path between the conductive film and the final electrode. 
     As a result, only one kind of conductive film is applied to the funnel inner wall, which resolves the problem of the conventional cathode-ray tube, that is, the problem in the junction of different kinds of conductive films, and the production process can be simplified. Moreover, the maximum instantaneous current at a discharge inside the bulb can be lowered since the part in the spring composing the conductive path at a discharge has a specific resistance value greater than that of the conductive film that contacts with the contact portion. Therefore, malfunction and breakage of the circuit parts of a TV set can be prevented. 
     It is preferable that the portion housing a getter is formed between the phosphor on the inner surface of the panel and the final electrode of the electron gun. 
     This preferable constitution allows a getter flash, and thus, the vacuum level inside the bulb can be improved. Moreover, the entire specific resistance value of the conductive path will not be lowered at a discharge even if the getter adheres to the conductive film on the funnel inner wall at the getter flash so that the specific resistance value of the conductive film is lowered, since the part in the spring has the specific resistance value greater than that of the conductive film that contacts with the contact portion. Therefore, the maximum instantaneous current generated at a discharge inside the bulb can be maintained at a low value even after the getter flash. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view illustrating a cathode-ray tube in a first embodiment of this invention. 
     FIG. 2 is a magnified view illustrating an electron gun in the neck portion of the cathode-ray tube in the first embodiment of this invention. 
     FIG. 3 is a magnified cross-sectional view illustrating the spring portions of the cathode-ray tube in the first embodiment of this invention. 
     FIG. 4 is a plan view illustrating the springs of the cathode-ray tube in the first embodiment of this invention. 
     FIG. 5 is a front view illustrating the springs of the cathode-ray tube in the first embodiment of this invention. 
     FIG. 6 is a graph illustrating the maximum instantaneous current flowing at a discharge inside the bulb of a cathode-ray tube of this invention and that of another cathode-ray tube for comparison. 
     FIG. 7 is a cross-sectional view illustrating a conventional cathode-ray tube. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiment of this invention will be explained below referring to the drawings. 
     As shown in FIG. 1, a cathode-ray tube in a first embodiment of this invention is used for a projection tube, and it comprises a bulb  6  comprising a panel  2  and a funnel  5 , an electron gun  7   a  to emit electron beams, springs  13 , and an anode terminal  24 . The panel  2  has a phosphor  1  provided on the inner surface and the funnel  5  has on its inner wall  3  a conductive film  11  with a specific resistance value of about 0.5 Ωcm, mainly containing graphite and titanium oxide. The electron gun  7   a  is housed in a neck portion  5   a  of the funnel  5 . The springs  13  are provided for the final electrode  19  of the electron gun  7   a  and have contact portions  12  to electrically connect the conductive film  11  on the funnel inner wall  3  with the final electrode  19 . And the anode terminal  24  is provided to a cone portion  5   b  of the funnel  5 , and applies high voltage to the final electrode  19  of the electron gun  7   a  through the conductive film  11 . 
     As shown in FIG. 2, the electron gun  7   a  comprises a cathode  14 , a control electrode  15 , an accelerating electrode  16 , an anodic electrode  17 , a focusing electrode  18 , a final electrode  19  and a getter shielding cylinder  20 , which are sequentially arranged in the direction of emission of electron beams. 
     As shown in FIGS. 3-5, a plurality of the springs  13  are formed at the periphery of a disk metallic plate  13   a  comprising stainless steel whose diameter is the same as the outer diameter of the final electrode  19 . Each spring  13  comprises an elastic arm portion  13   b  comprising stainless steel provided in the direction perpendicular to the surface of the metallic plate  13   a , and a contact portion  12  containing ceramics or paste-like high resistance agents or the like that is formed at the free edge of the elastic arm portion  13   b . The springs  13  are arranged by covering the final electrode  19  with the metallic plate  13   a  and fixing the metallic plate  13   a  to the final electrode  19  through welding or an epoxy-based conductive adhesives in which silver particles are dispersed. The specific resistance value of the contact portions  12  of the springs  13  ranges preferably from 1 to 10 4  Ωcm, and the value is greater than the specific resistance value of the conductive film  11  that contacts with the contact portions  12 . It is specifically preferable that the specific resistance value of the contact portions  12  ranges from 100 to 10 4  Ωcm, when considering the decrease of the maximum instantaneous current generated at a discharge inside the bulb  6 . 
     As shown in FIG. 3, the getter shielding cylinder  20  comprises a housing portion  20   a  and a getter column  20   b . The housing portion  20   a  houses a getter  21  provided between the phosphor  1  on the inner surface of the panel  2  and the final electrode  19  of the electron gun  7   a . The getter column  20   b  is fixed to the metallic plate  13   a  by welding and holds the housing portion  20   a . The getter  21 , which improves the vacuum level inside the bulb  6 , comprises barium materials or the like. 
     The effects of the above embodiment of this invention will be explained below. 
     In a cathode-ray tube of the first embodiment of this invention, the specific resistance value of the contact portions  12  of the springs  13  is greater than that of the conductive film  11  that contacts with the contact portions  12 , so that only one kind of conductive film  11  is formed on the funnel inner wall  3 , and the maximum instantaneous current generated at a discharge inside the bulb  6  can be lowered. In other words, the cathode-ray tube of the first embodiment of this invention comprising a kind of conductive film  11  formed continuously with a uniform composition on the funnel inner wall  3  can resolve the problems in connecting different kinds of conductive films in a conventional cathode-ray tube in which three kinds of conductive films ( 4   a ,  4   b  and  4   c ) are formed on the funnel inner wall  3  as shown in FIG.  7 . In addition, the production process can be simplified because of the reduction of number of the production steps. Moreover, the maximum instantaneous current to be generated is lowered because the contact portions in the conductive path at a discharge inside the bulb  6  have a relatively great specific resistance value. As a result, malfunction and breakage, or some other problems for the circuit parts of a TV set can be prevented even for a projection tube that operates at a high anodic voltage of at least 30 kV. 
     Since the housing portion  20   a  housing the getter  21  is provided between the phosphor  1  on the inner surface of the panel  2  and the final electrode  19  of the electron gun  7   a , the vacuum level inside the bulb  6  can be improved by carrying out a getter flash to evaporate barium. Furthermore, the maximum instantaneous current generated at a discharge can be maintained at a low value even if the getter  21  adheres to the conductive film  11  on the funnel inner wall at the getter flash and the specific resistance value of the conductive film  11  is lowered, since the contact portions  12  with a great and constant specific resistance value are contained in the springs  13  partially composing the discharge path at discharge inside the bulb  6 . As a result, malfunction and breakage, or some other problems for the circuit parts of a TV set can be prevented. 
     The maximum instantaneous current generated at a discharge inside the bulb  6  can be lowered over time since ceramic materials with high abrasion resistance and stable specific resistance value are used for the contact portions  12  of the springs  13 . Also, the portions can be produced in a simple manner as the ceramic materials are formed at a small area of each contact portion  12 . 
     As the plural elastic arm portions  13   b  of the springs  13  are provided at the periphery of the metallic plate  13   a  in the direction perpendicular (axial direction of the tube) to the surface of the metallic plate  13   a , the elastic force can be finely adjusted by the aim length of the elastic arm portions  13   b . As a result, damage to the conductive film  11  on the funnel inner wall  3 , which is caused by the stress from the contact portions  12  of the elastic arm portions  13   b , can be reduced at the time of insertion of the electron gun into the neck portion  5   a  of the funnel  5 . This can also reduce sparks between the electrodes, which are caused by foreign materials that are formed from the stripped conductive film  11 . 
     In the above-mentioned embodiment shown in FIGS. 1-5, the metallic plate  13   a  and the elastic arm portions  13   b  are integrated by using stainless steel, and contact portions  12  are formed by using ceramic materials at the ends of the elastic arm portions  13   b . The springs of this invention are, however not limited to the embodiment, but it is also possible that the elastic arm portions  13   b  and contact portions  12  are formed by using ceramic materials and connected with the metallic plate  13   a . Or all the disk metallic plate  13   a , elastic arm portions  13   b  and the contact portions  12  can be formed with ceramic materials, and arranged by electrically connecting with the final electrode  19 . By preparing not only the contact portions  12  but the metallic plates  13   a  or the like with ceramic materials, more portions in the conductive path at a discharge are made of ceramics having greater specific resistance value, so the maximum instantaneous current can be lowered more remarkably. In general, the resistance at the contact portions of ceramic materials is about 1 MΩ. When the elastic arm portions  13   b  are made of ceramic materials, the general resistance value ranges from 2 to 5M Ω. Therefore, if the maximum instantaneous current cannot be lowered sufficiently by forming only the contact portions  12  with ceramic materials, the elastic arm portions  13   b  serially connected to the contact portions  12  also can be formed with ceramic materials, so that the maximum instantaneous current can be further lowered. At that time, the specific resistance value of the elastic arm portions  13   b  made of ceramic materials is preferably ranging from 1 to 10 4  Ω. 
     When forming the elastic arm portions  13   b  with ceramic materials, applicable ceramic materials include, for instance, zirconia-based ceramics (normal Young&#39;s modulus is about 2-3×10 10 (N/m 2 )) and silicon nitride-based ceramics (normal Young&#39;s modulus is about 3×10 10 (N/m 2 )). In order to provide a suitable material for a spring, the Young&#39;s modulus is raised to 5-30×10 10 (N/m 2 ) by properly adjusting the composition ratio of these ceramic materials before forming the materials into a spring of a desired shape. The arm length of the elastic arm portions  13   b  (In FIG. 5, the half length between the centers of two contact portions  12  contained in a sprino  13 ) ranges from about 3 mm to about 5 mm, though it can be properly determined depending on some factors such as the size of the cathode-ray tube. 
     EXAMPLE 1 
     The following explanation is about this Example describing the effects of his invention. 
     A cathode-ray tube of this invention has the construction as shown in FIGS. 1 and 2, comprising a unipotential type electron gun  7   a  for the neck with diameter of φ29.1 mm as shown in FIG. 2, where the specific resistance value of the conductive film  11  on the funnel inner wall  3  is 0.5 Ωcm, formed with ceramic materials so that the specific resistance value of the contact portions  12  of the springs  13  is 55 Ωcm. 
     For comparison, another cathode-ray tube is also produced in the same manner, except that the contact portions  12  of the springs  13  are formed with stainless steel (SUS304) whose specific resistance value is 7.5×10 −5  Ωcm. 
     The maximum instantaneous current flowing at a discharge inside the bulb was examined by using a method mentioned below. In the examination, the potential of the anodic electrode  17  and the final electrode  19  applied through the anode  24  was determined to be 32 kV, and the other electrodes than these two electrodes ( 17  and  19 ) were determined as the ground potentials. The discharge inside the bulb was artificially generated by irradiating laser beams between the accelerating electrode  16  and the focusing electrode  18 . The maximum instantaneous current was measured with a storage tube oscilloscope by using a current probe. The result is shown in FIG.  6 . The curves X and Y in FIG. 6 respectively indicate the cathode-ray tube of this invention and another cathode-ray tube for comparison. 
     In the cathode-ray tube of his invention, the maximum instantaneous current at a discharge inside the tube was  60 A, namely, it decreased to 43% in comparison with the other cathode-ray tube whose maximum instantaneous current was  140 A. 
     This invention is not limited to the above-mentioned projection tube having a unipotential type electron gun for neck with a diameter of φ29.1 mm, but similar effects can be obtained by using projection tubes having a unipotential type electron gun for neck with a diameter of φ36.5 mm, an electromagnetic focusing type electron gun or bipotential type electron gun; monochrome cathoderay tubes, color cathode-ray tubes, and so on. 
     The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limitative, the scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.