Abstract:
A method of, and a device for, reducing the magnetic stray field generated in the vicinity of the faceplate of a cathode ray tube by the deflection current in the deflection coils of the tube. A current loop (4) is arranged at a distance from the deflection coils and is supplied with current having a time function substantially coinciding with the time function of the deflection current. The stray field is thereby neutralized to a large extent in a region extending beyond the position of the current conductor (4) in a direction away from the faceplate.

Description:
This is a continuation of application Ser. No. 021,869, filed Mar. 4, 1987, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method of reducing magnetic stray fields, and to a device for reducing magnetic stray fields near a cathode ray tube. 
     2. Description of the Related Art 
     In magnetic field generating coils, deflection coils of cathode ray tubes, power supplies and other devices, undesired magnetic stray fields are generated. These stray fields may have a prejudicial influence upon the operation of adjacent equipment. It has been discovered, for example, that the magnetic field from a power supply unit may disturb the operation of an adjacent compact disc reproducing apparatus. Some investigations of the influence of magnetic fields on human beings and animals have been interpreted as showing that injuries could be caused by the magnetic field from, for example, a cathode ray tube. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a reduction of the stray field produced at a distance from a magnetic field generating means. 
     According to the present invention there is provided a method of reducing a magnetic stray field, characterized in that a current having substantially the same time function as that of the current supplied to the stray field generating means is supplied to a current conductor which is positioned at a distance from the stray field generating means. The current in such conductor thereby generates a magnetic field which neutralizes the stray field at least in a region situated beyond the current conductor relative to the stray field generating region. 
     A device for carrying out the method when the magnetic stray field occurs near a cathode ray tube and originates from the deflection coils of the deflection unit is characterized in that a current conductor is arranged in the vicinity of the face plate of the cathode ray tube, the current conductor being supplied with a current having a time function corresponding substantially to the time function of the stray field generating current. 
     In one embodiment of the device, the current conductor has a horizontal section arranged in the vicinity of the upper front edge of the cathode ray tube and another horizontal section arranged in the vicinity of the lower front edge of the cathode ray tube. Magnetic measurements made with this embodiment has shown a high reduction of the magnetic field in front of the cathode ray tube. Another feature of this embodiment is that the arrangement of the current conductor is easy to implement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective, diagrammatic view of a cathode ray tube, 
     FIG. 2 illustrates stray fields and reducing magnetic fields in a vertical plane, 
     FIGS. 3a to 3d are embodiments of the connection of the current conductor to the deflection coils of a cathode ray tube, and 
     FIG. 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. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The cathode ray tube 1 shows in FIG. 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 supplied with a current which has substantially the same variation with time, hereinafter termed the time function, as the current supplied to the coils 6a to 6d (FIGS. 3a to 3d) of the deflection unit 3. Optionally the current supply to the conductor may be via intermediate couplings. As shown in FIG. 1 a section 4a of the current conductor 4 is attached to or in close proximity with the upper front edge of the faceplate of the cathode ray tube and another section 4b is attached to or in close proximity 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 FIG. 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 FIG. 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 FIG. 2 shows the presence of the stray field generated in the deflection unit by 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 stray deflection field has been denoted by H d  (t) and the reduction magnetic field has been denoted by H a  (t). As is apparent from FIG. 2, the stray field generated by the deflection unit has its highest strength closest to the deflection coils 6a, 6b. The reduction 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 adjusted so that its field strength in the aforesaid vertical plane at some distance in front of the cathode ray tube is of substantially the same order of magnitude as the stray field at the same distance, i.e. H a  (t)=-H d  (t). It is to be noted that the deflection field at the said distance consists of the stray field. The above arrangement results in the strength of the reduction magnetic field being much lower than the strength of the deflection field in a location adjacent to the deflection unit, i.e., |H a  (t)|&lt;|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 operation of the deflection field and that its influence on the normal operation of the cathode ray tube is quite negligable. 
     FIGS. 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 FIG. 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 FIG. 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 FIG. 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 FIG. 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 FIGS. 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. 
     FIG. 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 is only about one tenth of the original field at the said distance of 0.4 m. 
     As stated above the measurements shown in FIG. 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 that there is 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.