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

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a device for the suppression of the leakage 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 gives an adverse influence to other apparatus in front of the display apparatus.

[0002] As shown in FIG. 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.

[0003] A display apparatus 1 having the above configuration, has a problem that the magnetic flux leaks out of the apparatus by causing 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.

[0004] 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 display apparatus such as TV picture tubes having a CRT could not display a picture, character, figure, and so on, if the image plane were covered.

[0005] 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.

SUMMARY OF THE INVENTION

[0006] The object of the present invention is to provide a device for suppressing the unnecessary leakage flux from the display apparatus without obstructing the image plane of the display apparatus, so as to solve the above problems.

[0007] In order to achieve the above object, a device for the suppression of the leakage flux according to the present invention, which is provided in a display apparatus for displaying images by deflecting an electron beam by magnetic field generated by a deflecting yoke, and by causing a fluorescent character display surface to emit light by the deflected electron-beam, comprises a pair of electric wires which are respectively arranged side by side at the upper and lower ends out of the effective picture area and which are connected to a deflecting yoke in parallel, and a controller for causing current flowing in the yoke to flow in the wires and for controlling the current flowing in the wires.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] 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

[0012] 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 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 and corresponding to column III of this 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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 of the CRT 2.

[0019] The conductor 5 may be placed against the glass face plate 2a (see FIG. 10) or remote therefrom (see FIG. 9) by a distance d as this location is found to be more effective than the former.

[0020] 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 the variable or fixed inductance 6 and is supplied to both the wires 5a and 5b, so that the suppression device of this invention synchronously operates with the deflecting yoke 3.

[0021] 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.

[0022] 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.

[0023] 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 Strálskydds Institut) in Sweden is shown in column V of the table, for reference.

TABLE

Case	Condition of cancelling wires 5a and 5b		dB/dt (mT/s)	Cancellation flux of wires (mT/s)
	Connection	Assembled position
I	Cancelling wires are not provided to CRT, & whole configuration is shown in FIG. 1		-100	--
II	Upper & lower wires are connected in series as shown in FIG. 6A	Condition of FIG. 6B	+74	+174
III	Upper & lower wires are connected in series as shown in FIG. 7A	Condition of FIG. 7B	-85	+15
IV	Upper & lower wires are connected in parallel as shown in FIG. 8A	Condition of FIG. 8B (The same as FIGS. 2 to 5)	-13	+87
V	Value recommended by SSI (Sweden)		1dB/dt1 < 25	--

[0024] 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.

[0025] Through the cancelling wires 5a (5b) is disposed at position a predetermined distance d from the front surface 2a as shown in FIG. 9, the present invention is not restricted in this configuration. As shown in FIG. 10, the cancelling wire 5a (5b) may be disposed at position against the front surface 2a of the CRT 2.

[0026] 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.

[0027] 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

1. 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 emission of an electron beam from 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 connected in parallel which are disposed respectively in close vicinity to top and bottom peripheries of a front surface of said CRT, and for generating cancellation flux to negate the leakage flux from deflecting coils of said yoke; 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 current value supplied to said wires.

2. A device according to claim 1;
wherein said two wires are disposed respectively at positions remote from the front surface of said CRT with a predetermined distance.

3. A device according to claim 1;
an upper wire of said wires is disposed horizontally along the top periphery of said fluorescent screen, and
a lower wire of said wires is disposed horizontally along the bottom periphery of said fluorescent screen.

4. A device according to claim 3;
wherein said upper and lower wires are disposed respectively at front end outer rims of a side envelope of the CRT so as not to cover said fluorescent screen thereof.

5. A device according to claim 1;
wherein a shield plate covers other surfaces than said front surface of the CRT.

6. A device according to claim 5;
wherein said shield plate extends forward and beyond said front surface of the CRT to cover said cancelling wires.

7. A device according to claim 1;
wherein said control circuit has a fixed inductance for regulating value of current supplied to said wires.

8. A device according to claim 1;
wherein said control circuit has a variable inductance for regulating value of current supplied to said wires.

9. A device according to claim 1;
wherein said two wires are disposed at positions a predetermined distance from a front surface of said CRT and on the inside of lines extrapolated forward from a side envelope of said CRT.

10. A device according to claim 1;
wherein said two wires are disposed at positions on a front surface of said CRT and inside of lines extrapolated forward from a side envelope of said CRT.

Drawing