JPS57199154A - Electromagnetic focusing device for cathode-ray tube - Google Patents

Abstract

PURPOSE:To provide an electromagnetic focusing device for a cathode-ray tube which widely reduces the spiral current loss caused by the electromagnetic induction developing during the operation of focusing correction coils, prevents any reduction of the Q-value of the circuit, and can suppress the loss of consumed electric power. CONSTITUTION:For the purpose of focusing an electron beam accelerated in the direction of the axis (z) of a CRT1 upon the fluorescent screen of the CRT1, an electromagnetic focusing part 10 is provided so that it surrounds a neck 2 of the CRT1. The part 10 is constituted by providing annular focusing correction coils 13 and 14, respectively, on the inner sides of two annular permanent magnets 11 and 12 the central axes of which correspond to the axis (z), providing pole pieces 15 and 16 made of a nonconductive soft magnetic material such as a soft magnetic ferrite on both end surfaces of the annular body consisting of the magnet 11 and the coil 13, and providing pole pieces 17 and 18, in the same manner as above, on both end surfaces of the annular body consisting of the magnet 12 and the coil 14. By constituting the CRT1 in such a way as mentioned above, when the current flowing through the coils 13 and 14 vary, while picturing parabolic wavy form or the like, the pole pieces 15, 16, 17 and 18 made of the above nonconductive soft magnetic material such as a ferrite prevent any generation itself of a spiral current, and widely reduce the spiral-current loss caused by the electromagnetic inductions of the coils 13 and 14. Besides, any reduction of the Q-value of the circuit can be prevented, and operational electric power for the coils 13 and 14 can be reduced.