GB2265492A

GB2265492A - Cathode ray tube screening arrangement

Info

Publication number: GB2265492A
Application number: GB9305296A
Authority: GB
Inventors: Tetsuya Yamaguchi
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1992-03-18
Filing date: 1993-03-15
Publication date: 1993-09-29
Anticipated expiration: 2013-03-15
Also published as: GB2265492B; KR960011770B1; DE4308491A1; JPH05266829A; KR930020550A; GB9305296D0

Description

2265492 CATHODE RAY TUBE

This invention relates to a cathode ray tube and more particularly to a cathode ray tube having an alternating field shielding function.

FIG. 2 of the accompanying drawings is a cross-sectional view of a conventional cathode ray tube (hereinafter also called CRT) disclosed in, for example, Japanese Patent Publication No. Sho 57-47544.

As shown in FIG. 2, this conventional CRT has a vacuum enclosure 11 which is composed of a display window 12, a cone portion 14 and a neck portion 15 in which electron gqns are mounted.

On the inner surface of the display window 12, a display screen 13 is located. In a CRT to be used in an ordinary color television, the display screen 13 has many lines or dots, three in a set, of fluorescent regions.

In the neck portion 15, three elect ron guns 16, 17, 18 are mounted. Electron beams 19, 20, 21 emitted 1 - A from these electron guns pass through an aperture 22 in a shadow mask 23 and then hit the fluorescent regions of the display screen 13 to generate colors individually. These three electron beams 19, 20, 21 are deflected concurrently in both frameand line-deflection directions using a deflecting coil unit 24.

The deflecting coil unit 24 small-diameter connecting portion is mounted around a between the neck portion 15 and the cone portion 14. A surface of a vacuum enclosure located at a lower part of the deflecting coil 24 is normally made of glass which constructs the enclosure itself, and its resistance is C>CP.. However, in the specific case, a conductive layer 25 having a resistance of 10a - 10-2-0SI per unit area, which is located outside the cone portion 14 and is electrically connected with a low-resistance layer 27, is provided over the surface of the vacuum enclosure for the purpose of anti-static protection.

The operation of this conventional CRT will now be described. For the operative principle of the CRT, the electron beams 19, 20, 21 emitted from the electron guns mounted in the neck portion 15 is electromagnetically deflected by the deflecting coil unit 24, and a high - 2 A voltage is applied to the display screen 13 located on the inner surface of the display window 12 to accelerate the electron beams, thus generating energy to excite the fluorescent surface to emit light.

Although the conductive layer 25 having a resistance of 10a to 10-'LOSI per unit area is formed in the lower region of the deflecting coil unit 24, it is provided for the purpose of serving as an electrostati shield against a potential generated between the deflection coil 24 and the vacuum enclosure 11. Further although the conductive layer 25 is electrically connected with the low-resistance layer 27 and is earthed, substantially adequate insulation is assured between the conductive layer 25 and the deflection coil unit 24 since the conductive layer 25 has considerably high resistance.

However, adverse effects of electromagnetic waves on the human body have recently come into question. Regarding the display screen (display monitor) of CRT, there has been a concern that the alternating field generated chiefly by the deflecting coil unit would give an adverse effect to a human body. As a result of this situation, in an effort to prevent the human body from - 3 c A any bad influence of electromagnetic waves emitted from the display monitor, in 1991 at least the Sweden National Board of Measurement and Test (MPR-II) and Sweden Central Labor Council (TCO) made standards concerning allowable electromagnetic waves leaking from the display monitor. The content of the standards is shown in TABLE 1.

TABLE 1

ELF Band VLF Band 5Hz - 2kHz 2kHz - 400kHz MPR-II below 25V/m below 2.5V/m Measurement Condition 20#C, 20% humidity at position 50cm. from CRT face TCO below 1OV/m below 1.OV/m 20#C, 21% humidity at position 30cm from CRT face As is apparent from TABLE 1, according to the standards of Sweden Central Labor Council (TCO), allowable electromagnetic waves for the alternating field of 2kHz to 400kHz (i.e., VLF band) of electromagnetic waves emitted from the display monitor is prescribed to be less than 1.0 V/m, and allowable electromagnetic waves for the alternating field of 5Hz to 2kHz (i.e., ELF band) is prescribed to be less than 10 V/m.

4 A Against the alternating field, particularly of the

VLF band of 2kHz to 400kHz, which passes through the display window of CRT and might give a bad influence upon the body of a viewer, shielding has been difficult to achieve.

With the foregoing problems in view, it is an object of this invention to provide a cathode ray tube having a function of shielding an alternating field in the VLF band of 2kHz to 400kHz, out of the electromagnetic waves emitted from a display monitor, which might pass through the face panel of CRT to give adverse effects to a human body.

According to the invention, there is provided a cathode ray tube comprises: an electron gun for emitting an electron beam; a deflecting coil unit for deflecting the electron beam emitted from the electron gun; and a conductive layer covering a lower region of the deflecting coil unit and electrically grounded, the conductive layer having a resistance of less than 10-01L per unit area. It is preferable to insert an insulating layer between the conductive layer and the deflecting coil unit.

4 With this CRT, partly since jamming radiation is electrostatically shielded by the conductive layer located in a lower region of the deflecting coil unit and having a resistance of less than 10-0-fL per unit area and partly since the conductive layer is electrically grounded, it is possible to form a field shield for shielding the alternating field in the VLF band which might pass through the display window of CRT to give adverse effect to a human body. Said field leaks from the deflecting yoke through the vacuum enclosure and the display window. In order to ensure the effect of the present invention, it is preferable that a shadow mask having a shield effect should be disposed inside the display window.

The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Fig. 1 is a cross-sectional view of a cathode ray tube according to one embodiment of this invention; and Fig. 2 is a crosssectional view of a conventional cathode ray tube.

A preferred embodiment of this invention will now be described with reference to the accompanying drawing. Fig. 1 shows a cathode ray tube embodying the invention.

4 As shown in FIG. 1, the cathode ray tube, like the conventional cathode ray tube, has a vacuum enclosure 11 which is composed of a display window 12, a cone portion 14, and a neck portion 15 in which electron guns described below are mounted. On the inner surface of the display window 12, a display screen 13 is located. The display screen 13 the display screen 13 has many lines or dots, three in a set, of fluorescent regions.

In the neck portion 15, three electron guns 16, 17, 18 are mounted. Electron beams 19, 20, 21 emitted from these electron guns pass through an aperture 22 in a shadow mask 23 and then hit the fluorescent regions of the display screen 13 to generate colors individually. These three electron beams 19, 20, 21 are deflected concurrently in both frame- and line-deflection directions using a deflecting coil unit 24.

The deflecting coil unit 24 is mounted around a small-diameter connecting portion between the neck portion 15 and the cone portion 14. A vacuum enclosure region located in a lower part of the deflecting coil 24 is covered by a conductive layer 34 having a resistance of less than 10'_(1 per unit area and is e lectrically connected with a low-resistance layer 27 located outside 4 the cone portion 14. The conductive layer 34 is covered with a diaphragm- like insulating layer 33. Further, the low-resistance layer 27 is connected to a frame and hence electrically grounded.

The operation of this CRT will now be described. For the operative principle of the CRT, like the conventional CRT, the electron beams 19, 20, 21 emitted from the electron guns mounted in the neck portion 15 is electromagnetically deflected by the deflecting coil unit 24, and a high voltage is applied to the display screen 13 located on the inner surface of the display window 12 to accelerate the electron beams, thus generating energy to excite the fluorescent surface to emit light.

In the CRT of this embodiment, the alternating field leaking from the deflecting coil unit 24 is shielded by the conductive layer 34, which is formed in a lower region of the deflecting.coil unit 24 and has a resistance of less than 10-3-D- per unit area. Thus, partly since the alternating field generated from the deflecting coil unit 24 converges at the low-resistance conductive layer 34 and partly since the conductive layer 34 is electrically connected to the low-resistance- - 8 A 1 layer 27 and hence electrically grounded via a moderately low resistance, it is possible to adequatel3 shield the alternating field (VLF band), which might give adverse effects to a human body.

With the insulating-film-coated wires used in th conventional deflecting coil unit 24, since the deflecting coil unit 24 and the conductive layer 34 are located very close to each other, an adequate degree of insulation cannot be expected. Consequently, in the cathode ray tube of this embodiment, a diaphragm-like insulating layer 33 is formed between the deflecting coil unit 24 and the conductive layer 34 so that a flashover between them is prevented.

With the cathode ray tube of this invention, since a conductive layer having a resistance of less than 10-OSI per unit area is formed in a lower region of the deflecting coil unit and is electrically grounded, it is possible to adequately shield the alternating field, out of electromagnetic waves emitted from the deflecting coil unit, which might pass through the display window of CRT to give adverse effects to a human body. -

9 - A

Claims

1. A cathode ray tube comprising:

an electron gun for emitting an electron beam; a deflecting coil unit for deflecting the electron beam emitted from the electron gun; and a conductive layer covering a lower region of said deflecting coil unit and electrically grounded and having a resistance of less than 10 31L per unit area.

2. A cathode ray tube according to claim 1, further comprising a diaphragm-like insulating layer inserted between said conductive layer and said deflecting coil unit.

3. A cathode ray tube according to claim 1 or 2, further comprising:

a shadow mask having an aperture through which the electron beam passes; and a display screen having fluorescent regions hit by the electron beam which has passed through the aperture and generating individual colors.

4. A cathode ray tube according to claim 1, 2 or 3, wherein the conductive layer is connected to a low resistance layer on the cone part of the cathode ray tube.

5. A cathode ray tube according to claim 1, 2, 3 or 4, wherein the conductive layer is on the neck part of the tube to be under the deflection coils.

6. A cathode ray tube constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in Fig. 1 of the accompanying drawings.

7. A video display apparatus including a cathode 5 ray tube according to any one of the preceding claims.