A method of, and device for, reducing magnetic stray fields of a cathod ray tube

Abstract: The invention relates to a method and device reducing magnetic stray fields. A current loop (4) is arranged at a dis­ tance from a stray field generating means (3) and is sup­ plied with a current having a time function substantially coinciding with the time function of the current in the stray field generating means (3). By means of this arrangement the electrical stray field to a high extent may be neutralized in a region beyond the position of the current conductor (4) regarded in a direction from the stray field generating means (3). The effect on technical equipment and living be­ ings of undesired magnetic stray field is reduced to a sub­ stantial extent by means of the invention.

Description: The invention relates to a cathode ray tube arrangement. In magnetic field generating coils, such as deflection coils of cathode ray tubes, undesired magnetic stray fields are generated. These stray fields may have a prejudicial influence upon the operation of the adjacent equipment. It has been discovered, for example, that the magnetic field from a power supply unit may disturb the operation of an adjacent record carrier disc in a disc station. Some investigations of the influence of magnetic fields on human beings and animals have been interpreted in such a way that injuries could be caused by the magnetic field from, for example, a cathode ray tube. It is an object of invention to provide a reduction of the stray field at a distance from a cathode ray tube arrangement. According to a first embodiment of the invention there is provided a cathode ray tube arrangement, comprising an envelope including a neck connected to a faceplate, a deflection unit being mounted on the envelope, and compensating means for reducing a stray field originating from the deflection unit, the compensating means comprising a current conductor system arranged in the vicinity of the faceplate, the cathode ray tube arrangement comprising means for applying to the current conductor system a current having a time function and strength for reducing the stray field, the current conductor system when the current is applied to the current conductor system, generating a compensating field for reducing the stray field at a certain distance in front of the faceplate, said the current conductor system comprising upper and lower horizontal conductor sections arranged in the vicinity of the cathode ray tube faceplate, said current conductor system not comprising degaussing coils provided on the cone of the envelope. According to a second embodiment of the invention there is provided a cathode ray tube arrangement, comprising an envelope including a neck connected to a faceplate, a deflection unit being mounted on the envelope, and compensating means for reducing a stray field originating from the deflection unit, the compensating means comprising a current conductor system arranged in the vicinity of the faceplate, the cathode ray tube arrangement comprising means for applying to the current conductor system a current having a time function and strength for reducing the stray field, the compensating means, when the current is applied to the current conductor system, , generating a compensating field for reducing, at a certain distance in front of the faceplate, the stray field deriving from the field deflection field. It is remarked that European patent application EP-A-220 777 which European patent application is comprised in the prior art under art. 54(3) EPC describes a cathode ray tube arrangement, comprising an envelope including a neck connected to a faceplate, a deflection unit being mounted on the envelope, and compensating means for reducing a stray field originating from the deflection unit, the compensating means comprising a current conductor system, the cathode ray tube arrangement comprising means for applying to the current conductor system a current having a time function and strength for reducing the stray field, the compensating means, when the current is applied to the current conductor system, generating a compensating field for reducing the stray field at a certain distance in front of the faceplate. The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective, diagrammatic view of a cathode ray tube, Figure 2 illustrates stray fields and reducing magnetic fields in a vertical plane, Figures 3a to 3d are embodiments of the connection of the current conductor to the deflection coils of a cathode ray tube, and Figure 4 are comparative graphs of measured magnetic fields in front of a cathode ray tube with and without the use of the magnetic field reduction current conductor. The cathode ray tube 1 shown in Figure 1 is of conventional type. A deflection unit 3 is located on a neck 2 of the cathode ray tube 1. A stray field reduction current conductor 4 is arranged in the vicinity of a face plate 5 of the cathode ray tube 1. The conductor 4 can be attached to or carried by the faceplate 5. The current conductor 4 is coupled to the deflection unit 3 in order to be applied with a current which has substantially the same variation with time, hereinafter, termed the time function as the current applied to the coils 6a to 6d (Figures 3a to 3d) of the deflection unit 3. Optionally the current supply to the conductor may be via intermediate couplings. As shown in Figure 1 a section 4a of the current conductor 4 is attached to or in close proseunity with the upper front edge of the faceplate of the cathode ray tube and another section 4b is attached to or in close proseunity with the lower front edge of the cathode ray tube faceplate. The current conductor 4 may consist of one revolution or loop as shown in Figure 1. However, the current conductor 4 may consist of a multiplicity of revolutions or loops if this is made necessary because, for example, of the high strength of the cathode ray tubes stray field or the electrical characteristics of the tube. By locating the current loop as shown in Figure 1, it is possible to obtain a very effective reduction of the stray field generated in the deflection coils of the deflection unit 3 during the line deflection. By means of magnetic field lines Figure 2 shows the presence of the stray field generated in the deflection unit by means of the deflection coils and the reducing magnetic field generated by the current conductor in a vertical plane transverse to the front edge of the cathode ray tube. The deflection field has been denoted by H d (t) and the field reduction magnetic field has been denoted by H a (t). As is apparent from Figure 2, the stray field generated by the deflection unit has its highest strength closest to the deflection coils 6a, 6b. The magnetic field generated by the horizontal sections 4a, 4b of the current conductor has its highest strength adjacent to the front edge of the cathode ray tube 1. The strength of the reduction magnetic field is adapted in such a way that its field strength in the vertical direction some distance in front of the cathode ray tube is of substantially the same order of magnitude as the stray field at the same point, i.e. H a (t) = -H d (t). It is to be noted that the deflection field at the said point consists of the stray field. The above arrangement enables the strength of the reduction magnetic field to be much lower than the strength of the deflection field in a point adjacent to the deflection unit, i.e., |H a (t)| << |H d (t)|. This is of great importance to the operation of the cathode ray tube and means that the introduced reduction magnetic field does not in any substantial degree affect the deflection field but its influence on the normal operation of the cathode ray tube is quite negligable. Figures 3a to 3d show examples of ways in which the current conductor 4 may be coupled electrically to the deflection unit and arranged with respect to the face plate 5 of the cathode ray tube. The terminals 7a, 7b, 7c and 7d denote the normal connecting terminals of the deflection unit. The current conductor 4 according to Figure 3a is connected in series with deflection coils 6a, 6b and has two horizontal sections 4a, 4b attached to or in close proximity with the upper and lower edges, respectively, of the face plate. In the embodiment according to Figure 3b, the deflection coils 6a, 6b are provided with individual compensation. The deflection coil 6a is coupled in series with an upper horizontal current conducting section 4a and the deflection coil 6b is coupled in series with a lower horizontal current conducting section 4b. In the embodiment according to Figure 3c there are provided horizontal current conducting sections 4a, 4b as well as vertical current conducting sections 4c, 4d, all of which are attached to or in close proximity with the edges of the face plate 5 of the cathode ray tube. The current conducting sections 4a, 4b are coupled in series with deflection coils 6a, 6b while the current conducting sections 4c, 4d are coupled in series with the deflection coils 6c and 6d. The embodiment according to Figure 3d shows a controlled current source 8 arranged between the deflection coils 6a, 6b and the current conducting section 4a, 4b. The current conducting sections 4a, 4b in this case consist of a plurality of revolutions or loops. By means of the arrangement described above with reference to the Figures 3a to 3d a current may be applied to the current conductor 4 in a simple way, the current having a time function which substantially coincides with the time function of the current through the deflection coils 6a, 6b. Figure 4 is a graph showing the results of measurements performed on a test arrangement. On the horizontal axis, the abscissa, of the graph, the distance from the cathode ray tube has been indicated, while the vertical axis, the ordinate, indicates the measured magnetical field in nT (nanotesla). The vertical magnetic field in front of the cathode ray tube has been measured at different distances from a cathode ray tube without the presence of the magnetic field reduction current conductor 4, the upper curve 10, and in the presence of the magnetic field reduction current conductor, the lower curve 12. A substantial reduction of the magnetic field may be observed. At a distance of 0.4 m from the front surface of the cathode ray tube, for example, the difference between a previously known cathode ray tube and a cathode ray tube provided with a current conductor 4 is approximately 100 nT. It is also to be noted that by means of the method in accordance with the invention the measured magnetic field only is about one tenth of the original field on the said distance of 0,4 m. As stated above the measurements shown in Figure 4 were made on the vertical magnetic field, the y-direction (see Fig. 1). Reductions of the field in the x-direction and the z-direction (see Fig. 1) have also been measured. Also in these directions it has been observed some reduction of the measured magnetic field even if it is less pronounced. The reduction field may, as stated above, be utilized to reduce the magnetic stray field deriving from the line deflection field. However, the method in accordance with the invention may also be used to reduce other stray fields deriving from, for example, the picture scan.

Claims: A cathode ray tube arrangement, comprising an envelope including a neck (2), connected to a faceplate (5), a deflection unit (3) being mounted on the envelope, and compensating means for reducing a stray field (H d (T)) originating from the deflection unit, the compensating means comprising a current conductor system arranged in the vicinity of the faceplate (5), the cathode ray tube arrangement comprising means for applying to the current conductor system a current having a time function and strength for reducing the stray field, the current conductor system, when the current is applied to the current conductor system, generating a compensating field (H a (T)) for reducing the stray field at a certain distance in front of the faceplate (5), said current conductor system comprising upper and lower horizontal conductor sections (4a, 4b) arranged in the vicinity of the cathode ray tube faceplate (5), said current conductor system not comprising degaussing coils provided on the cone of the envelope. A cathode ray tube arrangement as claimed in claim 1, characterized in that the current conductor system is attached to or carried by the faceplate. A cathode ray tube arrangement (1), comprising an envelope including a neck (2), connected to a faceplate (5), a deflection unit (3) being mounted on the envelope, and compensating means for reducing a stray field (H d (T)) originating from the deflection unit, the compensating means comprising a current conductor system arranged in the vicinity of the faceplate (5), the cathode ray tube arrangement comprising means for applying to the current conductor system a current having a time function and strength for reducing the stray field, the current conductor system, when the current is applied to the current system, generating a compensating field (H a (T)) for reducing the stray field at a certain distance in front of the faceplate (5), the stray field being derived from the field deflection field. A cathode ray tube arrangement as claimed in claims 1, 2 or 3, characterized in that the current conductor system comprises left and right vertical current conductor sections (4c, 4d) arranged in the vicinity of the cathode ray tube faceplate (5). A cathode ray tube arrangement as claimed in claim 1 or 2, characterized in that the horizontal conductor sections consist of a plurality of loops. A cathode ray tube arrangement as claimed in claims 1 to 5, characterized in that the current conductor system (4) is coupled to the deflection unit (3).

Inventors: KEVIUS BRUNO, KEVIUS, BRUNO
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1987
Main classification: H01J29
IPC codes: H01J29