Built-in Resistor for a Cathode Ray Tube

The present invention relates in general to cathode ray tube (CRT) structural arrangements. More specifically, the invention is directed to the structure of a resistor built into the CRT envelope.

In a color CRT a focusing potential of 5 KV to 8 KV is required. A high potential slightly lower than the anode potential, such as 25 KV to 30 KV, is also required in a mask focusing color CRT. Such focusing or high potential voltage are provided to respective electrodes through stem pins mounted on a neck of the tube envelope. To reduce deflection power consumption a diameter of a neck of the tube envelope is narrowed. With the neck narrowed, the pins of the CRT are close to one another. The use of high potential voltages causes the problem of withstanding high voltage at the stem portion of the CRT and also complicates the stem structure.

To solve this problem, a CRT having a resistor built in the tube envelope was proposed in Japanese Utility Model Nos. 48-21561 and 55-38484 and U.S. Pat. Nos. 3,932,786 and 4,143,298. The teachings of these documents are hereby incorporated by reference as if fully set forth herein.

The resistor divides the applied potential, such as an anode potential applied from an anode button provided on a funnel of the tube envelope, into an intermediate high potential such as a focusing potential. The resistor built in the neck of the tube envelope must be made small because of the small space available the neck. The resistor must have a very high resistance value, for example 500.times.10.sup.6 ohms, because a high potential of 25 KV to 30 KV is applied to the resistor and a power consumption in the resistor must be low. Thus, the resistor must have a small volume and a high resistance value. However, it is difficult to control a value of the resistance of such resistor in a mass production.

A known resistor of such type comprises a ceramic insulating substrate and a zigzag shaped resistive layer printed on the substrate. The resistive layer is made of a mixture of ruthenium oxide and glass. However in production, such resistors have resistance values in a range of about .+-.10% depending upon the production process. So, a divided potential provided by such resistors also varies from one resistor to another. Consequently a cathode ray tube having a built-in resistor must have some voltage adjusting means, such as a variable resistor, provided outside of the tube envelope. Even if the adjusting means is utilized, when the deviation from nominal value of the resistance value is large its deviation can not be fully adjusted or compensated by the adjusting means. Further, the built-in resistor set outside of the tube must also have very high resistive value, for example 100 to 500.times.10.sup.6 ohm. Mass production of such high resistance variable resistors is difficult.

A primary object of the present invention is to provide a resistor having a selectable resistive value.

Therefore the present invention provides a built-in resistor having "built-in" means for adjusting its resistance value so that a desired resistance value can be obtained despite normal manufacturing tolerances. It includes a "main resistor" portion and an "additional" resistor for providing adjustment.

Even if the resistive value of the main resistor is deviant from the desired or nominal value as a result of the production process, the resistive value of the built-in resistor can be adjusted by adjusting the additional resistor. Thereby a divided potential obtained becomes within a preferably adjustable range. A fine adjustment of the divided potential can be carried out by a variable resistor serially connected to the additional resistor, which variable resistor is provided outside the tube envelope. If the resistance value of the additional resistor is proper, the divided potential can be made precise without any further variable resistor.

The invention will be described in detail with reference to the accompanying drawings, wherein:

FIG. 1 is a cross section of a color cathode ray tube utilizing a resistor according to the present invention;

FIG. 2 is a cross section of a neck portion of cathode ray tube having a built-in resistor according to the present invention;

FIG. 3 is a perspective view of a resistor according to the present invention;

FIG. 4 is a schematic diagram of equivalent circuit of the embodiment of the present invention shown in FIG. 2;

FIG. 5 is a perspective view of another embodiment of the present invention;

FIG. 6 is a cross section of a neck portion of cathode ray tube having the built-in resistor shown in FIG. 5;

FIG. 7 is a schematic diagram of equivalent circuit of the embodiment of the present invention shown in FIG. 6;

FIG. 8 is a perspective view of an alternative embodiment of the present invention;

FIG. 9 is a cross section of a further embodiment of the present invention; and

FIG. 10 is a schematic diagram of equivalent circuit of the embodiment shown in FIG. 9.

Referring now to FIG. 1, a color cathode ray tube (CRT) is shown. A tube envelope 2 has a panel 4, a funnel 6 and a neck 8. An electron gun assembly 10 for emitting three electron beams 11 is disposed in the neck 8. A phosphor screen 12, which emits three different colors red, green and blue, is coated on an inner surface of the panel 4. In the vicinity of the phosphor screen 12 a color selection electrode 14 is provided. An inner conductive layer 16 is coated on an inner surface of the funnel 6. An anode button 18, which is in electrical contact with the inner conductive layer 16, is mounted on the funnel 6.

The anode button is supplied with an anode potential from an anode power source 19. A deflection yoke 20 is mounted on an outer surface of the funnel 6. Electron beams 11 emitted from the electron gun assembly 10 are deflected by yoke 20 and pass through an aperture 15 of the color selection electrode 14. Then, the electron beams impinge the phosphor screen 12 causing it to emit color light which, in turn, causes a color image to be displayed on panel 4.

FIG. 2 shows the detail of a neck portion of the cathode ray tube. In neck 2 of the tube envelope, the electron gun assembly 10 is disposed. The electron gun assembly 10 includes a pair of insulating glass beads 22 and 24, a plurality of grid electrodes 26, 28, 30 and 32 attached to the glass beads 22 and 24 and a cathode structure 34 attached to the glass beads 22 and 24. The cathode structure 34 emits three electron beams, which are focused and accelerated by the grid electrodes 26, 28, 30 and 32 and then strike the blue, red and green phosphors coated on an inner surface of a face plate of the tube envelope. Grid electrodes 26, 28, 30 and 32 are arranged along the electron beams travelling direction and each electrode has three apertures corresponding to the three electron beams. First and second grid electrodes 26 and 28 are plate like electrodes and positioned in the vicinity of the cathode structure 34. The third electrode 30, which is a focusing electrode, has two cup-like electrodes 30a and 30b with openings facing each other. The fourth electrode 32 has also two cup-like electrodes 32a and 32b. On cup-like electrode 32b facing the phosphor screen a cup shape convergence electrode 36 is mounted. Convergence electrode 36 has three apertures for passing the three electron beams, respectively. Three bulb spacers 38 are attached to the convergence electrode 36. One end of each bulb spacer 38 is abutting an inner surface of neck 2 and holds the electron gun assembly 10 in the neck 2. The cathode structure side of the gun assembly 10 is held to stem pins 38 by lead wires (not shown) connecting between the cathode structure 34 and the grid electrodes 26 and 28. The bulb spacers 38 are in electrical contact with the inner conductive layer 16. The inner conductive layer 16 is in electrical contact with anode button 18, so the anode potential is applied to the convergence electrode 36 and to fourth electrode 32.

A built-in resistor 42 of the present invention is positioned in the vicinity of the glass bead 22. Referring to FIG. 3 the built-in resistor is shown in detail. Resistor 42 has a ceramic insulating substrate 44, a zigzag shaped resistive layer 46 provided on the substrate 44. The resistive layer 46 is made of a mixture of ruthenium oxide and glass. At one end of the substrate is provided a terminal 48 and near the other end of the substrate there is provided another terminal 50. A resistive value between terminals 48 and 50 is very high, preferably 500 to 5000.times.10.sup.6 ohm. Another terminal 52 is positioned between terminals 48 and 50. A main resistor is formed between the terminals 48 and 50. Two other terminals 54 and 56 are provided near the second terminal 50. An additional resistor is formed between terminals 50 and 56. Terminals 48, 50, 52, 54 and 56 are made of a resistive material having a lower resistance than that of resistive layer 46, such as a relatively low resistive material including ruthenium oxide as its chief constituent, or made of conductive paint including silver or gold. Surfaces of resistive layer 46 and substrate 44 are covered by an insulating coating 58 such as, for example, glass except for the terminals 48, 50, 52, 54 and 56. A first connector 60 and an end connector 62 made of elastic conductive material are attached to the terminals 48 and 56, respectively. Further, other connectors 64, 66 and 68 are attached to the terminals 52, 50 and 54 respectively. Resistive values among terminals 48, 50, 52, 54 and 56 are denoted by R.sub.1, R.sub.2, R.sub.3 and R.sub.4 shown in FIG. 3.

Referring back to FIG. 2, connector 60 is welded to the convergence electrode 36 and the end connector 62 is welded to a stem pin 38a, thereby the resistor 42 is held in neck 2. Terminal 48 is electrically connected to the anode button 18 mounted on the funnel 4 through the first connector 60, the convergence electrode 36, the bulb spacers 38 and the inner conductive layer 16. The connector 64 is electrically wired to the third electrode 30 and other connectors 66 and 68 are wired to stem pins 38b and 38c respectively.

Referring now to FIG. 4, there is shown a schematic diagram of an equivalent circuit formed in an operation of the color cathode ray tube. Eb denotes the anode voltage applied to the resistor 42, VR denotes a variable resistor outside of the CRT and R.sub.0 denotes a resistive value between the terminal 52 and a ground GND. A resistive value R.sub.5 can be adjusted by selecting the stem pins 38a, 38b and 38c to be connected to the variable resistor VR. A potential of the terminal 52 is determined by a division ratio between R.sub.0 and R.sub.1 when a current of the third grid electrode is neglected. A resistive value of R.sub.5 can be widely changed so that the potential of terminal 52, which is connected to the third grid electrode, can be widely adjustable. Therefore, even if the resistive value of main resistor is largely deviated from the designed value depending upon the production process, selecting the stem pins 38a, 38b and 38c makes it possible to adjust the potential of the third grid electrode 30 within a preferable and adjustable voltage range. The divided potential at terminal 52 can be finely adjusted to the predetermined potential by the variable resistor VR.

In designing the built-in resistor the variety of the resistive depending upon the mass-production is considered. When the focusing potential is required to be 30% of the anode potential Eb, a divisional ratio of R.sub.2 /(R.sub.1 +R.sub.2) is designed to be 0.2. An embodiment of the built-in resistor is designed so that R.sub.1 +R.sub.2 is 1000.times.10.sup.6 ohm, R.sub.2 is 200.times.10.sup.6 Ohm, R.sub.3 is 100.times.10.sup.6 ohm and R.sub.4 is 100.times.10.sup.6 Ohm. When the divisional ratio is varied to about 0.3, no additional resistor means is utilized in the tube envelope. When the divisional ratio is varied to about 0.1, R.sub.3 and R.sub.4 are utilized as additional resistor means. When a divisional ratio of about 0.2 is obtained R.sub.3 is utilized as additional resistor means. In each case the variable resistor VR of several megaohm to 30.times.10.sup.6 Ohm is utilized for fine adjustment outside the tube envelope.

In the prior art without additional resistor means, a variable resistor provided outside the tube envelope must have a resistive value of 200 to 300.times.10.sup.6 Ohm to compensate a deviation of the built-in resistor. However such high resistive variable resistor is difficult to produce and can not be obtained at low price in a commercial market. On the contrary the variable resistor utilized in the present invention is easy to produce and is obtained at low price.

According to the present invention, the variable resistor outside the tube envelope can be omitted. Generally, a cathode ray tube is designed to have a focusing potential which is 30% of the anode potential. The focusing potential must be set within a range of 29% to 31% of anode potential so as to not degrade the focus characteristics on a screen. Even if a severe quality control is done in the production process of the built-in resistor, the deviation of the divisional ratio is more than 1%. In the present invention to compensate the deviation the additional resistive means is applied to the built-in resistor. In this case the additional resistive means compensates the deviation of about 1% and such deviation can be compensated easily by additional resistive means without the variable resistor outside the tube envelope. Such embodiment is designed so that R.sub.1 +R.sub.2 is 1000.times.10.sup.6 Ohm, R.sub.2 is 290.times.10.sup.6 Ohm, R.sub.3 is 10.times.10.sup.6 Ohm and R.sub.4 is 10.times.10.sup.6 Ohm. In this case the variable resistor is not necessary outside the tube envelope so that a cathode ray tube having this built-in resistor is a convenience for TV set maker. However a severe quality control of the production process of the built-in resistor is necessary and a cost of the built-in resistor becomes high compared to the afore-mentioned embodiment. The greater the number of the terminals provided on the additional resistor means, the finer the adjustment.

FIGS. 5 and 6 show another embodiment of the present invention. In this embodiment a resistive value of the resistor is preset before the resistor is built in the neck. Only one stem pin is used for connection to the terminal of the additional resistor means. Referring now to FIG. 5, resistor 42 has additional resistor having a plurality of fine adjusting terminals 70 near the end of subtrate 44. The fine adjusting terminals 70 have smaller pads than the other terminals 48 and 52 because many terminals 70 are provided in a small area. One of the fine adjusting terminals is selected and electrically connected to the end terminal 53 positioned at the end of the substrate 44. One of the fine adjusting terminals is selected to provide a predetermined potential to the third terminal 52. For this purpose a resistive value R.sub.1 between the first and third terminals 48 and 52 and a resistive value R.sub.5 between the fine adjusting terminals 73 and the third terminal 52 are measured. A small connector 72 is attached to the selected terminal and is wired to the end connector 62. The resistor 42 is attached and electrically connected to the electron gun assembly 10 in a same manner described with respect to the first embodiment. The third terminal is also wired to the third grid electrode 30.

FIG. 7 shows an electrical equivalent circuit of this embodiment. In this embodiment many fine adjusting terminals are provided and the built-in resistor can be fine adjusted to a desired resistance value so that the variable resistor provided outside the tube envelope can be omitted. Of course the variable resistor can be utilized for further fine adjustment by a television set manufacture who utilizes the CRT including the resistor of the present invention. Furthermore only one stem pin is required for the resistor, so that gaps between the stem pins can be wide compared to the first embodiment and therefore can withstand even greater voltages. It can be understood that the end terminal and end connector 53 and 62 can be omitted and the small connector 72 is directly wired to the stem pin 38a when the resistor 42 is mounted in the neck in some other manner.

FIG. 8 shows a further embodiment of the present invention. In this embodiment the resistive layer 74 of the additional resistor 75 is not covered by insulating layer 58. Resistive layer 74 is made of relatively lower resistive material than the left portion. Resistive layer 74 has a "line" shape, rather than a zigzag shape. A connector 76 is slidably attached to the resistive layer 74 and is fixed at a suitable position. In this embodiment resistive layer 74 is made of a material having lower specific resistance than that of the resistive layer of the main resistor to lengthen the additional resistor portion. This makes it easy to adjust the slidable adjustment. A change of the value of resistor 42 is continuous and the resistive value can be adjusted very fine.

FIG. 9 shows an alternative embodiment of the invention. The built-in resistor 42 includes a main resistor 80 and additional resistors 82 and 84, which are separate from each other. The main resistor 80 comprises an insulating substrate, a resistive layer printed on the insulating substrate, first, second and third terminal and an insulating layer covering the resistive layer and the surface of the insulating substrate. This structure is the same as the resistor shown in above-mentioned embodiments except that an additional resistor is not formed on the substrate of the main resistor. In this embodiment a plural of individual resistors 82 and 84 are provided as an additional resistor means. The individual resistors have different resistive value from each other to enable the compensation of the deviation of the resistive value of the main resistor. Additional resistors 82 and 84 are connected between the second terminal of the main resistor and stem pins 38a and 38b respectively. The second terminal is wired to stem pin 38a without any additional resistor.

FIG. 10 is a schematic diagram of an equivalent circuit of the FIG. 9 embodiment.

According to the present invention the deviation of the resistive value, which is caused by the production process, can be compensated and the divisional potential can be set within a compensatable range by the additional variable resistor. The tolerance of the resistive value of the resistor becomes generous and a production yield can be improved. This reduces the manufacturing price of the cathode ray tube. If a severe quality control of the production process is carried out, the additional variable resistor can be eliminated.

Other embodiments and modification of the present invention will be apparent to those of ordinary skill in the art having the benefit of the teaching presented in the foregoing description and drawings. It is therefore, to be understood that this invention is not to be unduly limited and such modifications are intended to be included within the scope of the appended claims.