Device for suppression of leakage of magnetic flux in display apparatus

Abstract: There is disclosed a device for suppression of leakage flux from a deflecting yoke, which is assembled in a display apparatus having a cathode ray tube (a CRT) for displaying an image by radiation of an electron beam from an electron gun to a fluorescent display surface, and the deflecting yoke having some coils arranged at a rear portion of the CRT and for deflecting the electron beam. The device comprises two cancelling wires (5a, 5b) which are disposed at the inner sides of upper and lower ends in front of the CRT and connected in parallel for generating cancellation flux to negate the leakage flux from the coils, and a control circuit (10) for controlling the cancellation flux generated by the wires in the manner that the leakage flux from a face-plate of the CRT is reduced by regulation of current value supplied to the wires.

Description: The present invention relates to a device for the suppression of the leakaye of magnetic flux from a display apparatus using a cathode ray tube (a CRT), and more particularly relates to a device for reducing the leakage flux from the deflecting yoke having deflecting coils in order to deflect the electron beam, and which yives an adverse influence to other apparatus in front of the display apparatus. As shown in Figure 1, a display apparatus 1 having a CRT 2 uses magnetic fields occurring by coils of a deflecting yoke 3 in order to deflect the electron beam from an electron gun 4. A display apparatus 1 having the above configuration, has a problem that the magnetic flux leaks out of the apparatus by causiny a current to flow in the coils of the yoke 3 and thereby generate a magnetic field. The leakage flux occasionally gives an adverse influence to the operation of other apparatus. Furthermore, it is serious for the leakage flux to give an adverse influence not only to peripheral apparatus but also to persons in front of the display apparatus 1. In conventional devices for reducing the leakage flux, the only known method uses metal plates surrounding the CRT 2 so as to interrupt the magnetic field. However, it is problem that an image plane cannot be covered by metal plates to stop the leaking magnetic flux, because we as viewers could not view pictures, characters, figures, and so on, which are displayed on display apparatus such as TV picture tubes having a CRT, if the image plane were covered. On the other hand, it is possible to cover the image plane by a fine mesh which is made of metal and has transparency. However, the mesh makes it hard to recognize the image and has no effect of interrupting magnetic flux of low frequency. It has been proposed in WO-A-8705437 to place windings spaced apart from but generally along the top and bottom edges of a cathode ray tube and fed by a current varying in synchronism with the current through the deflection coils of the cathode ray tube with the intention of cancelling the leakage field in front of the screen. A similar arrangement is disclosed in EP-A-0235863. Neither of those disclosed the precise positioning of the windings relative to the top and bottom front edge of the cathode ray tube. SUMMARY OF THE INVENTION The object of the present invention is to provide a device for suppressing the unnecessary leakage flux from the display apparatus using a location of the wires which is more effective, without obstructing the image plane of the display apparatus, so as to solve the above problems. The present invention provides a device for the suppression of leakage flux from a deflecting yoke, which is assembled in a display apparatus having a cathode ray tube (CRT) for displaying an image by emission of an electron gun to a fluorescent screen surface, and said deflecting yoke arranged at a rear portion of said CRT and for deflecting said electron beam; comprising two cancelling wires which are arranged parallel to each other and are disposed at positions on the front surface and inside straight lines extrapolated forward from the top and bottom of the screen of said CRT, each of said cancelling wires being disposed in close vicinity to a respective one of the upper and lower peripheries of a front surface of said CRT for generating cancellation flux to negate the leakage flux from deflecting coils of said yoke, and wherein the upper wire extends horizontally along the upper periphery of said fluorescent screen, and the lower wire extends horizontally along the lower periphery of said fluorescent screen; and a control circuit for controlling said cancellation flux generated by said cancelling wires in the manner that said leakage flux from a face-plate of said CRT is reduced by regulation of the current value supplied to said cancelling wires, characterised in that said two cancelling wires are disposed respectively at positions remote from the front surface of said CRT by a predetermined distance d and inside straight lines (L) extrapolated forward from the inner wall of the side envelope at the point where the face plate joins the side envelope of said CRT parallel to the optical axis of CRT. A deflecting current flows in coils of the deflecting yoke in order to deflect the electron beam in the CRT. A part of the magnetic flux occurring in the deflecting yoke by the deflecting current, leaks from the fluorescent character display surface of the display apparatus. In this invention, the wires arranged at the above position receive current supply for controlling the occurrence and quantity of the magnetic flux having a direction opposite to the leakage flux from the fluorescent character display surface. The magnetic flux occurring in the wires by the current supply cancels or negates the leakage flux from the deflecting yoke. As described above, the present invention has the excellent effect that the leakage flux from the fluorescent surface can be reduced without any obstacle being disposed in front of the effective picture area of the CRT. Furthermore, if metal plates shield other surfaces without an image display surface of the CRT, the leakage flux out of the display apparatus can be sharply reduced in all directions. The present invention is effective in reducing the leakage flux to the standards of the VDE - Verband Deutscher Elektrotechniker (Union of Electric Engineers of Germany) of West Germany. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 is a schematic side view showing the leakage flux of a general display apparatus; FIG. 2 is a rear perspective illustration showing a device for suppression of the leakage flux according to an embodiment of the present invention; FIG. 3 is an elevational perspective illustration showing the suppression device shown in FIG. 2; FIG. 4 is a circuit diagram showing an equivalent circuit of the suppression device shown in FIG. 3; FIG. 5 is a schematic side view showing the condition where the leakage flux is reduced by the cancelling magnetic field generated by the device shown in FIGS. 2 to 4; FIGS. 6A and 6B are respectively, a circuit diagram and side view showing the connection and arrangement of cancel coils different from this invention as defined by the claims and corresponding to column II of the table described later; FIGS. 7A and 7B are respectively, a circuit diagram and side view showing the connection and arrangement of cancellation coils further differing from this invention as defined by the claims and corresponding to column III of the table; FIGS. 8A and 8B are a circuit diagram and side view showing cancellation coils of the embodiment of this invention shown in FIGS. 2 to 5; FIG. 9 is a fragmentary view showing a corner portion of a cabinet and a CRT; and FIG. 10 is a fragmentary view showing a corner portion of a cabinet and a CRT having wires different from FIG. 9 (An embodiment not falling within the scope of the claims). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS There will be described in detail preferred embodiments of a device for suppression of leakage flux according to the present invention with reference to the accompanying drawings. FIGS. 2 and 3 are perspective illustrations showing an embodiment according to the present invention. A display apparatus 1 comprises a CRT 2, a deflecting yoke 3, an electron gun 4, cancelling wires 5a and 5b for generating cancellation flux to cancel leakage flux, a variable or fixed inductor 6 connected in parallel with the wires 5a and 5b, and a horizontal deflection circuit 10 for controlling the cancelling wires 5a and 5b. As shown in an equivalence circuit of FIG. 4, the circuit 10 comprises an inductor 11, a battery 12, a diode 13, a capacitor 14, and a switching transistor 15. The circuit 10 also has a flyback transformer for accelerating an electron beam and for obtaining a high voltage. The circuit 10 is connected via the deflecting yoke 3 to a parallel circuit in which the cancelling wires 5a and 5b and the control inductor 6 are connected in parallel. As a horizontal deflection circuit causes the electron beam to swing in a lateral direction, the control circuit 10 generates the cancellation flux in a vertical direction where the scanning direction of the electron beam intersects therewith at substantially a right angle. Therefore, horizontal deflecting coils are arranged at both the top and bottom positions of the electron gun 4. FIGS. 1 and 5 are schematic side views showing the CRT 2 and the leakage flux leaking from the horizontal deflecting coils. FIG. 5 further shows that the cancelling wires 5a and 5b negate or reduce the leakage flux generated by current supplied to the horizontal deflecting coils. The wires 5a and 5b are arranged in front of a face-plate 2a of the CRT 2 as shown in FIG. 3. The upper wire 5a is arranged horizontally at the top end of an effective picture area in the face-plate 2a along the horizontal direction. The lower wire 5b is arranged horizontally at the bottom end of the effective area of picture in the face-plate 2a. As shown in FIG. 9, for effective leakage flux suppression a conductor 5 of the cancelling wires 5a or 5b or both, may be located at a position in front of a glass face plate 2a of the CRT 2 and over the CRT's own display area, i.e. the conductor 5 is located on or below a level L of a side envelope 2b of the CRT 2 in the side view, in other words over the CRT's fluorescent screen 2c, where the conductor 5 is still covered by a mask portion 20a of the cabinet 20 so that it is not visible to a viewer. The level L is a line extrapolated forward from an inner surface of the side envelope 2b at the point where the face plate joins the side envelope of the CRT 2. According to the invention, the conductor 5 is placed a distance d from the glass face plate 2a (see Fig. 9). The deflecting yoke 3 is assembled in the proximity of the electron gun 4 and deflects the electron beam generated therefrom. Namely, the horizontal deflection circuit supplies the deflecting yoke 3 with current for horizontal deflection. At the same time, the current is supplied to the variable or fixed inductance 6 and both the wires 5a and 5b, so that the suppression device of this invention synchronously operates with the deflecting yoke 3. In the side view of the CRT as shown in FIG. 5 to indicate the flux generated by the horizontal deflecting coils and cancelling wires 5a and 5b, solid line arrows denote magnetic flux generated by the deflecting yoke 3 and dotted line arrows denote magnetic flux generated by the wires 5a and 5b. As shown in FIG. 5, the leakage flux generated by the deflecting yoke is negated by the cancellation flux generated by the wires 5a and 5b. The leakage flux from the deflecting yoke changes according to the shape of the display apparatus and the strength of the deflecting current. Accordingly, the current flowing in the wires may be divided and supplied to the inductor 6 which is connected in parallel with wires 5a and 5b, so that it is possible to change the inductance of the circuit and to regulate the strength of cancellation flux. Since the cancellation flux is well-balanced with the leakage flux, the leakage flux can be minimized. The effect of the above embodiment is shown in table denoting the strength of the leakage flux according to the difference of the connection between the deflecting yoke 3 and the cancel wires 5a and 5b as shown in FIGS. 6A, 7A and 8A, and the assembled positions of the wires 5a and 5b as shown in FIGS. 6B, 7B and 8B. In addition to these measurement examples, the value recommended by SSI (Statens Stralskydds Institut) in Sweden is shown in column V of the table, for reference. By the table and FIGS. 6A to 8B, it is better to connect upper and lower wires 5a and 5b in parallel and to arrange them in front of the face-plate 2a and at the top and bottom periphery of a front surface of the CRT 2. If the wires 5a and 5b are disposed at the top and bottom peripheral surfaces as shown in FIG. 7B, a loss of the cancellation flux becomes larger by the great influence of the metal portions such as an explosion protecting belt, a shield, and the like of the display apparatus. Accordingly, it is necessary to supply the cancelling wires 5a and 5b with larger current, so that a loss of the deflecting circuit becomes larger. Furthermore, other components in the display apparatus undergo quite possibly a magnetic adverse effect of such a large current flow to the cancelling wires, resulting a performance degradation of the display apparatus. The cancelling wires 5a and 5b are disposed a predetermined distance d from the front surfaces 2a as shown in Fig. 9 because this is found to be more effective than the location of the wires as shown in Fig 10, which is an embodiment not falling within the scope of the claims. In the suppression device according to the present invention, the cancelling wires are arranged at the upper and lower ends in front of the CRT in order to receive little influence from the metal parts, so that the present invention provide the effect that the value of current flowing the wires becomes extremely small. As shown in FIG. 6B, 7B and 8B, the CRT may be further covered by a shield plate 21 which extends up to front end outer rim of the side envelope of the CRT 2. As shown in FIG. 7B, the shield plate 21 may cover the wires 5a and 5b as well, or may further extend beyond the front surface of the CRT 2 as shown in FIG. 8B.

Claims: A device for the suppression of leakage flux from a deflecting yoke (3), which is assembled in a display apparatus (1) having a cathode ray tube (CRT) (2) for displaying an image by emission of an electron gun (4) to a fluorescent screen surface (2c), and said deflecting yoke arranged at a rear portion of said CRT and for deflecting said electron beam; the device comprising two cancelling wires (5a, 5b) which are arranged parallel to each other with the upper wire extending horizontally along the upper periphery of said fluorescent screen, and the lower wire extending horizontally along the lower periphery of said fluorescent screen; and a control circuit (10) for controlling said cancellation flux generated by said cancelling wires in the manner that said leakage flux from a face-plate (2a) of said CRT is reduced by regulation of the current value supplied to said cancelling wires, characterized in that said two cancelling wires are disposed respectively at positions remote from the front surface of said CRT by a predetermined distance d and inside straight lines (L) extrapolated forward from the inner wall of the side envelope at the point where the face plate joins the side envelope of said CRT parallel to the optical axis of CRT. A device according to any of the preceding claims, wherein a shield plate (21) covers the surfaces of the CRT other than said front surface of the CKT. A device according to claim 2, wherein said shield plate (21) extends forward and beyond said front surface of the CRT to cover said two cancelling wires. A device according to claim 1, wherein said control circuit (10) has a fixed inductance (6) for regulating the value of current supplied to said cancelling wires. A device according to claim 1, wherein said control circuit (10) has a variable inductance (6) for regulating the value of current supplied to said wires. A device according to any of the preceding claims, wherein in the current supplied to said cancelling wires (5a, 5b) flows in the source direction in both of said cancelling wires. A device according to any of the preceding claims, wherein said cancellation wires (5a, 5b) are connected in series with the yoke (3) and the control circuit (10).

Inventors: HAKAMADA YOSHIRO, ONO KIMITERU, HAKAMADA, YOSHIRO, ONO, KIMITERU
Applicants: VICTOR COMPANY OF JAPAN, VICTOR COMPANY OF JAPAN, LIMITED
Application year: 1989
Main classification: H01J29
IPC codes: H04N3, H01J29, G09G1