FERRITE SHEET FOR CORRECTING MAGNETIC FIELD PRODUCED BY DEFLECTION YOKE

Abstract: A ferrite sheet used to finely adjust the magnetic field which is produced and controlled by the deflection yoke of a cathode-ray tube. The sheet is simply bonded to an appropriate portion of the surface of the yoke via pressure sensitive adhesive. The layer of the adhesive which is formed on one surface of the sheet contains substantially no solvent.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferrite sheet stuck on a deflection yoke placed around the neck of a cathode-ray tube for use in a television receiver or a display unit to finely adjust and correct the magnetic field set up and controlled by the yoke and, more particularly, to an improvement over such a ferrite sheet. 2. Description of the Prior Art A ferrite sheet for correcting the magnetic field produced by a deflection yoke has been heretofore fabricated in the manner described below. A plastic such as vinyl chloride is mixed with a soft ferrite such as Ni-Zn ferrite or Mn-Zn ferrite. The mixture is then molded into a sheet to form the body of the ferrite sheet. A pressure sensitive adhesive layer is formed on one surface of the body of the sheet. Then, a sheet of release paper is attached to the surface of the adhesive layer. The present applicant has already filed Japanese Utility Model application No. 35078/1987 for this ferrite sheet. In this field-correcting sheet, the pressure sensitive adhesive layer formed on one surface of the body of the sheet may attack the insulating layer coated on the surface of each turn of the winding, thus impairing the insulation. It is inferred that the organic solvent contained in the pressure sensitive adhesive attacks and dissolves the insulating layer of the winding, lowering the insulation. Especially, where a base material which is impregnated with the pressure sensitive adhesive or to which the adhesive is applied exists in the adhesive layer, a solvent is indispensable to allow the pressure sensitive adhesive to permeate the base material. Further, the foregoing drawback becomes more conspicuous, because the solvent tends to remain in the base material. SUMMARY OF THE INVENTION In view of the foregoing problems with the prior art ferrite sheet, it is an object of the invention to provide a ferrite sheet free of these problems. It is a more specific object of the invention to provide a field-correcting ferrite sheet which is for use with a deflection yoke and has pressure sensitive adhesive applied to the rear surface of the body of the ferrite sheet, the insulation between the individual turns of the yoke being not attacked by the adhesive. Therefore, the insulation can be maintained, and the deflection yoke can be used while retained in normal state. The above objects are achieved by a ferrite sheet according to claim 1. The pressure sensitive adhesive layer contains substantially no organic solvent. More specifically, the body of the ferrite sheet is molded and has a given size. A layer of a pressure sensitive adhesive having substantially no organic solvent is formed on one surface of the body. The pressure sensitive adhesive via which the field-correcting ferrite sheet is bonded to the surface of the winding of the deflection yoke contains hardly any organic solvent or other similar substance which can attack the insulating layer of the winding. For this reason, the solvent contained in the pressure sensitive adhesive does not attack the material of the insulating layer. Hence, the insulation between the neighboring turns of the winding does not deteriorate. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic representation of a deflection yoke; Fig. 2 is an enlarged cross section of one turn of the winding that forms the deflection yoke shown in Fig. 1; Fig. 3 is a cross-sectional view of a field-correcting ferrite sheet according to the invention; Fig. 4 is a cross-sectional view of a winding on which the ferrite sheet shown in Fig. 3 is stuck; and Fig. 5 is a view schematically illustrating a series of experiments performed to examine the effects of the ferrite sheet shown in Fig. 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, there is shown a deflection yoke according to the invention. This yoke, generally indicated by numeral 1, is placed around the neck of a cathode-ray tube (not shown). A sheet 3 made of a ferrite is stuck on an appropriate portion of the surface of a winding 2 to correct the magnetic field produced by the yoke 1. The sheet 3 is fabricated in the manner described below. First, a soft ferrite such as Ni-Zn ferrite or Mn-Zn ferrite is mixed with a plastic such as polyvinyl chloride. The mixture is molded into a sheet, which is then cut into a desired size to form the body of the ferrite sheet. A layer 4 of pressure sensitive adhesive is formed on one surface of the body. In accordance with the invention, the adhesive layer 4 does not contain any base material consisting of nonwoven fabric, Japanese paper, polyester film, film of foamed polyurethane but consists only of a pressure sensitive adhesive which does not substantially contain any solvent or other component that attacks insulating materials. The pressure sensitive adhesive can be a known pressure sensitive adhesive made from an acrylic resin or a rubber. Fig. 2 is an enlarged cross section of the winding 2 that constitutes the deflection yoke on which the field-correcting ferrite sheet is stuck. The winding 2 comprises a copper wire 6, an insulating layer 5 made from a synthetic resin such as urethane resin or polyester, and a self-fusing layer 7 made of a nylon. The layer 7 acts to bond together neighboring turns of the windings. This field-correcting ferrite sheet fabricated in this way can be directly stuck on an appropriate portion of the surface of the deflection yoke 1. If necessary, a sheet of release paper can be placed on the pressure sensitive adhesive layer 4. When the ferrite sheet is used, the sheet of release paper is peeled off, and then the ferrite sheet is stuck on an appropriate portion of the surface of the deflection yoke 1. The novel ferrite sheet for correcting the magnetic field does not contain any base material consisting of a sheet of nonwoven fabric, Japanese paper, fabric, or a synthetic resin. Rather, only a pressure sensitive adhesive is applied to the body of the ferrite sheet. Therefore, it is unlikely that the insulating layer 5 of the winding 2 forming the deflection yoke 1 is attacked. Consequently, the insulation between the neighboring turns of the winding 2 does not deteriorate. This completely prevents the turns of the copper wire 6 from causing short between them. As shown in the cross section of Fig. 4, thickness d of the pressure sensitive adhesive layer 4 is set larger than the radius r of each turn of the winding 2. When the field-correcting ferrite sheet 3 is stuck on the surfaces of the turns of the winding 2 of the deflection yoke 1, the pressure sensitive adhesive is inserted into the spaces between the turns of the winding 2 more deep than the radius r of each turn. Hence, the winding 2 can be bonded to the sheet 3 with certainty. The insulating performance of the novel field-correcting ferrite sheet 3 was compared with the insulating performance of a conventional field-correcting ferrite sheet including a base material of nonwoven fabric. The results are shown in the Table below. For this comparison a series of experiments was made as follows. As shown in Fig. 5, two windings 8 were twisted together to form a coil. Two portions of the coil were wrapped in two field-correcting ferrite sheets 9 and fixed. Simultaneously, the surfaces of the sheets were coated with acetate tape 10. This coil was placed in a constant temperature oven the inside of which was maintained at 110°C. The coil was heated for 24 hours. Then, it was taken out of the oven. High voltages were applied between the windings 8 of the coil without removing the ferrite sheets 9 to perform a withstand voltage test. The coil was heated by the oven to take account of the environment under which the deflection yoke is actually used. The high voltages were impressed at a room temperature of 28°C at a normal pressure. A 60Hz AC power supply was employed as the power source of the high voltages. insulating material Experiment 1 (no base material) Experiment 2 (with base material) Experiment 3 (no ferrite sheet) Experiment 4 (no ferrite sheet) urethane resin 13.2-14.0 KV 12.5-13.2 KV 13.8-14.5 KV 13.5-14.5 KV polyester resin 13.1-14.5 KV 10.4-12.0 KV 13.0-14.7 KV 12.5-13.0 KV Referring to the Table above, Experiment 1 was made using the novel field-correcting ferrite sheet. A pressure sensitive adhesive made of an acrylic resin was directly applied to one side of a sheet of polyvinyl chloride containing 81% by weight of a soft ferrite. Experiment 2 was made using a layer of a pressure sensitive adhesive containing a base material of nonwoven fabric as the pressure sensitive adhesive layer of the aforementioned field-correcting ferrite sheet. Experiments 3 and 4 were made using only coils fabricated by twisting together windings; no ferrite sheet for correcting the magnetic field was employed. Under these conditions, in Experiment 3 the coil was heated, while in Example 4 it was not heated. As can be seen from the Table, the ferrite sheet consisting simply of the pressure sensitive adhesive sheet (Experiment 1) are quite similar in resistance to voltage to the structures having no ferrite sheet (Experiments 3 and 4). The resistance to voltage of the structure having a base material of nonwoven fabric (Experiment 2) is lower. This lower resistance to voltage is estimated to be caused by the solvent contained in the nonwoven fabric. In reality, three experiments were made for each of the structures shown in the Table, and the average value was calculated and shown in the Table. As described thus far, the novel field-correcting ferrite sheet of a deflection yoke which uses as its pressure sensitive adhesive layer a pressure sensitive adhesive layer containing substantially no organic solvent. Therefore, when this sheet is stuck on the surface of the deflection yoke, the insulating layer of the winding of the yoke is not attacked. Consequently, the yoke does not malfunction after it is used for many years.

Claims: A ferrite sheet for correcting the magnetic field produced by a deflection yoke, said ferrite sheet comprising: a molded sheet ferrite having a given size; and a layer of a pressure sensitive adhesive containing substantially no organic solvent, the layer being formed upon one surface of the sheet ferrite. A ferrite sheet for correcting the magnetic field produced by a deflection yoke as set forth in claim 1 , wherein said layer of the pressure sensitive adhesive contains no base material. A ferrite sheet for correcting the magnetic field produced by a deflection yoke as set forth in claim 1 or 2, wherein said pressure sensitive adhesive consists of an acrylic resin or a rubber. A ferrite sheet for correcting the magnetic field produced by a deflection yoke as set forth in any one of claims 1 - 3, wherein the thickness of the layer of the pressure sensitive adhesive is set larger than the radius of each turn of the winding which constitutes the deflection yoke.

Inventors: SAKAUCHI, TAKASHI, YAMASAKI, HIROKA
Applicants: KANEGAFUCHI KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1988
Main classification: H01J29
IPC codes: H01J29