Leakage field decreasing device for CRT display

Abstract: The invention provides means and device for effectively decreasing the electric field irradiated from the front surface of a CRT display, and this device cancels the leakage field composed of a deflection component and a high voltage ripple component irradiated in the front direction of the CRT, by generating an inverse electric field of leakage field composed of the deflection component and high voltage ripple component, and this inverse electric field is obtained by detecting the alternating-current voltage generated in the portion connected with the anode of the CRT by capacitive coupling, inverting and amplifying the detected voltage, and applying it to inverse field generating means installed near the front surface of the CRT.

Description: The present invention relates to a CRT display device for decreasing leakage field irradiated in the front direction of a CRT display. Recently, as a result of wide distribution of computer appliances, the CRT display comes to be used widely at home and in office. Accordingly, adverse effects of electric field or magnetic field leaking from the CRT display on the human health and other electric appliances are much discussed. Outline of operation of a well-known CRT display is described below. A deflection yoke is driven in a deflection circuit, and deflects an electron beam passing through the CRT in the horizontal and vertical directions. On the other hand, a high voltage output after being rectified in a horizontal synchronous period from a high voltage generating circuit containing FBT (flyback transformer) is supplied to an anode of the CRT through an anode cable as an acceleration voltage of CRT. The CRT is coated with a metal backing inside of the cone-shaped glass wall of the CRT, and inside of the front glass panel, and the anode is electrically connected to the metal backing and the shadow mask disposed behind inside of the front panel of the CRT. The metal backing and Aquadaque formed outside of the CRT compose a capacitor, and the above high voltage charges the capacitor. On the other hand, the beam current of the CRT is controlled by a video signal entering from the video circuit, and the CRT displays the picture. The beam current flows while displaying the picture, but does not flow in the horizontal and vertical flyback time. The capacitor composed of the glass wall of the CRT is charged up in the horizontal and vertical flyback time because beam current does not flow, but while displaying the picture, that is, while beam current is flowing, it is discharged. Therefore, due to fluctuations of the beam current, the anode voltage generates ripple voltages in the horizontal and vertical period. A prior art is described below by referring to Fig. 9. Conventionally, in operation of the CRT display, the leakage field induced to the front direction of the CRT 1 has been explained as follows. One of principal components of leakage field is the component formed by the voltage generated in a deflection yoke 2 (hereinafter called deflection component D). Other one is the component formed by ripple voltages of the high voltage (hereinafter called high voltage ripple component V) from all constituent parts connected to the anode (hereinafter called the anode potential band) such as anode cable, CRT shadow mask 4, and metal coated parts 5 inside of the CRT. During operation of the CRT display, the leakage field induced to the front direction of the CRT display is a synthesis of deflection component D and high voltage ripple component V. As means for decreasing the leakage field from the CRT display, it may be first considered to shield effectively by metal plate or the like. This means may be executed easily on the side and back surfaces of the CRT 1, but metal plate cannot be applied on the front surface of the CRT 1 because of the purpose of use of the display. It is hence proposed to mount a transparent panel on the surface of the CRT 1 or the front surface of the CRT 1, and coat its surface with a see-through conductive film, thereby enhancing the shielding effect. This means of applying a conductive coating on the surface of the CRT 1 has, however, its problems in the cost and productivity. For example, in Japanese Laid-open Patent No. 6-189323, the following method is disclosed as the means of decreasing the leakage field induced to the front direction of the CRT 1. A lead wire is wound around the front surface of the CRT 1, and it is formed as inverse field generating means 6. The voltage showing an inverse polarity of the voltage generated in the deflection yoke 2 is picked up from a deflection circuit 7, and it is used as an inverse polarity field generating voltage. This inverse polarity field generating voltage is applied to the inverse field generating means 6, and the leakage field ahead of the CRT 1 is canceled. EP-A1-0 630 036 describes a cathode ray tube display having reduced electric field emissions. Such a cathode ray tube (CRT) comprises an element for detecting modulations in the final anode voltage of the CRT. The signal from this element is phase and gain corrected and amplified before being provided to an emission means for radiating a cancelling electric field which is dependent on the modulations detected by said element. As mentioned above, principal irradiation components of the leakage field from the CRT display front surface comprise deflection component D from the deflection yoke 2 and high voltage ripple component V from the anode potential band, and these combined leakage fields cannot be canceled by the deflection component D alone. That is, the leakage field due to the high voltage ripple component V was not decreased at all, and effective decrease of leakage field was not attained. As the means for solving the problems, means for canceling the leakage field is disclosed, for example, in Japanese Laid-open Patent No. 4-315741. It is proposed to provide individual measures for the deflection component D from the deflection yoke 2 and the high voltage ripple component V from the anode potential band. In both measures, by making use of the voltage obtained from the internal circuit of the CRT display, a voltage for generating a canceling field is obtained. Against the deflection component D, the voltage taken out from the FBT 8 is utilized. This voltage is applied to the inverse field generating means 6 through a phase adjusting circuit 9. Moreover, against the high voltage ripple component V, the high voltage delivered through a high voltage generating circuit 10 from the FBT 8 is divided by resistance R1, R2, and a divided voltage is obtained, and applied to the inverse field generating means 6 through the phase adjusting circuit 9 and inverting circuit 11. In the conventional means, the voltage applied to the inverse field generating means 6 was obtained by processing one from the deflection circuit 7 and the other from the high voltage generating circuit 10, which was very troublesome. Moreover, since the acceleration voltage of the CRT display is a high voltage close to 30 kV. When dividing the acceleration voltage by resistance for supplying voltage, the resistance value of the resistor must be high for decreasing an electric power consumption and other reasons. In particular, the resistance value of the resistance R1 must be about 1000 Mohms to 100 Mohms. Owing to the withstand voltage and other reasons, the shape of the resistor is large in size, Therefore, the resistor is susceptible to effects of the surrounding electric field. In addition, the divided voltage waveform is deformed by the distribution capacity of the resistors. As a result, in the voltage divided by resistors, an unexpected voltage waveform is superposed, and favorable results are not obtained. That is, the obtained voltage waveform was large in difference from the expected alternating-current voltage waveform, and the leakage field could not be decreased sufficiently. In the light of the above problems, it is the object of the invention to decrease the leakage field irradiated from the front surface of the CRT display by effective and inexpensive means. This is achieved by the features of claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a structural diagram of a leakage field decreasing device of a CRT display ; Fig. 2 is a block diagram of an electric circuit of the leakage field decreasing device in Fig. 1; Fig. 3 is a perspective exploded view showing the structure of inverse field generating means of the leakage field decreasing device in Fig. 1; Fig. 4 is a perspective exploded view of other structure than the one in Fig. 3 showing a structure of inverse field generating means of the leakage field decreasing device in Fig. 1; Fig. 5 is a diagram of irradiation patterns explaining the difference in the leakage field decreasing effect in the prior art and in Fig. 3 and Fig. 4; Fig. 6 is a sectional view seen from the CRT front side, showing a structure of detector of the leakage field decreasing device in Fig. 1; Fig. 7 is a sectional view of other structure than the one in Fig. 6 showing a structure of detector of the leakage field decreasing device in Fig. 1 according to the present invention; Fig. 8 is a sectional view of other structure than the one in Fig. 6 or Fig. 7, showing a structure of detector of the leakage field decreasing device in Fig. 1; and Fig. 9 is a structural diagram of essential parts of a conventional CRT display device. DETAILED DESCRIPTION OF THE INVENTION Exemplary Embodiment Fig. 1 is a structural diagram of parts of a leakage field decreasing device of CRT display not forming part of the present invention. Same constituent components as in the prior art in Fig. 9 are identified with same reference numerals. The operation of the leakage field decrease in the constitution shown in Fig. 1 is described. A metal plate (not shown) is disposed at the side and back surfaces of a CRT 1, and this metal plate effectively shields the leakage field in the side and back directions of the CRT 1. A deflection yoke 2 is driven by a deflection circuit 7, and electron beams passing through the CRT 1 are deflected in the horizontal and vertical directions. At this time, the voltage generated at the deflection yoke 2 is induced in the anode potential band, and is irradiated in the front direction (arrow A) of the CRT 1, through a shadow mask 4 composed of a conductive material. This component is a deflection component D. On the other hand, the high voltage delivered from a high voltage generating circuit 10 is applied to the anode potential band containing the shadow mask 4 through an anode cable 3. At the same time, a video signal from a video circuit 12 enters the CRT 1, and hence the beam current is modulated. The high voltage output from the high voltage generating circuit 10 fluctuates by the changes of beam current, that is, load fluctuations, and generates a ripple voltage. This ripple voltage is irradiated in the front direction (arrow A) of the CRT 1. This component is a high voltage ripple component V. Therefore, the alternating-current component of the anode potential band is a superposition of the deflection component D on the high voltage ripple component V. It is thus understood that the leakage field irradiated in the front direction of the CRT 1 is formed by the alternating-current voltage generated in the shadow mask 4. Accordingly, a detector 13 is provided, and the alternating-current voltage waveform in the anode potential band including the shadow mask 4 is detected by capacitive coupling. In this detection, it is necessary to prevent induction of unexpected field near the output obtained in the detector. The detected alternating-current voltage is inverted and amplified to a specified voltage level in an inverting amplifier 14, and the inverted and amplified voltage is applied to inverse field generating means 6 installed near the front surface of the CRT 1. This inverse field generating means 6 is composed of metal plate or coil-shaped lead wire, and is insulated so as not to contact with the surface of the CRT 1, being disposed between the CRT 1 and bezel 15. As a result, an inverse electric field generated by the inverse field generating means 6 works to cancel the leakage field from the front surface of the CRT 1. That is, the inverse electric field generated from the inverses field generating means 6 works to cancel the synthesized leakage field of the deflection component D and high voltage ripple component V of the leakage field irradiated in the front direction of the CRT 1, by the synthesized inverse field of deflection component D' and high voltage ripple component V' in the same phase and in reverse polarity of the synthesized leakage field. As a result, the synthesized leakage field is decreased, and the slightly remaining deflection component and high voltage ripple component are indicated by d and v in Fig. 1. Fig. 2 is a block diagram of an electric circuit in Fig. 1 for canceling this leakage field. Structural examples of an inverse field generating means 6 shown in Fig. 1 are shown in perspective views in Fig. 3 and Fig. 4. Fig. 3 shows the inverse field generating means 6 composed of a frame-shaped metal metal plate, which is insulated and disposed so as not to contact with the surface of the CRT 1. Fig. 4 is similar to the frame shape in Fig. 3, except that the vertical side metal plate is not used, while the lower side metal plate is used. That is, the inverse field generating means 6 shown in Fig. 4 comprises a metal plate divided into upper side and lower side, and is insulated so as not to contact with the surface of the CRT 1. The inverse field generating means 6 shown in Fig. 4 is effective on the irradiation in the forward 45-degree direction in consideration of the irradiation pattern to the periphery of the CRT 1. The difference in irradiation pattern due to difference of the inverse field generating means 6 is shown in Fig. 5, in which (a) is an irradiation pattern without inverse field generating means 6, (b) is one by using the frame-shaped metal plate in Fig. 3, and (c) is one using the upper and lower split metal plate in Fig. 4. As known from Fig. 5, as compared with the front direction, the irradiation level is lower in the 45-degree direction. Accordingly, in the case of (b) of the frame shape in Fig. 3, excessive cancellation occurs in the 45-degree direction, and the irradiation level is worse as compared with the front surface. Hence, in the case of (c) of the upper and lower divided shape shown in Fig. 4 by eliminating the metal plate in the vertical side of the frame shape in Fig. 3, effective cancellation is also enabled in the 45-degree direction. Moreover, the inverse field generating means 6 may be also composed of a conductor of other shape, for example, a coil-shaped lead wire, or a demagnetization coil may be also used. Other structural examples of the detector 13 in Fig. 1 are shown in Fig. 6, Fig. 7 and Fig. 8. Fig. 6, disclosing an embodiment not forming part of the present invention, is a front view of the CRT 1, and as shown in Fig. 6, the detector 13 is disposed near the front side shadow mask 4 of the side surface of the CRT 1. The detector 13 is composed of a coupling 16, a shield 17, and an insulation sheet 18. The coupling 16 and shield 17 are composed of conductors, and the coupling 16 is disposed in the periphery of the shadow mask 4 of the CRT 1. The coupling 16 forms a coupling capacity enclosing the glass wall of the CRT 1 and the anode potential band including the shadow mask 4, and its static capacity is, for example, a capacity of scores of picofarads. The anode alternating-current component is divided in C by the coupling capacity and the input capacity of the inverting amplifier 14. This divided output is handled as the detection value. The shield 17 is disposed so as to cover the coupling 16 in order to cut off the effects of undesired electric field from outside. At this time, the coupling 16 and the shield 17are insulated by the insulation sheet 18 so that the parts may not contact with each other. Fig. 7 is a sectional view of essential parts of the present invention disposing the detector 13 in a CRT neck area 19 of Fig. 1. This detector 13 is also composed of coupling 16, shield 17, and insulation sheet 18 same as in Fig. 6. The coupling 16 and shield 17 are composed of conductors, and the coupling 16 is disposed in the CRT neck area 19 around an anode (g4) 20 in the electron gun of the CRT 1, and is coupled with the anode 20 by a capacity of, for example, scores of picofarads, and the anode alternating-current voltage component is detected in C division. The shield 17 is disposed so as to cover the coupling 16 for cutting off the direct effects from outside such as deflection yoke 2. At this time, the coupling 16 and shield 17 are insulated by the insulation sheet 18 so that the parts may not contact with each other. Fig. 8 is a sectional view not forming part of the present invention of parts disposing the detector 13 around the anode cable 3 in Fig. 1. This detector 13 also composed of coupling 16, shield 17, and insulation sheet 18 same as in Fig. 6. The coupling 16 and shield 17 are composed of conductors, and the coupling 16 is disposed so as to surround the anode cable 3, and is coupled with the anode potential band by a capacity of, for example, scores of picofarads, and the anode alternating-current voltage component is detected in C division. The shield 17 is disposed so as to cover the coupling 16 for cutting off the effects of undesired electric field from outside. At this time, the coupling 16 and shield 17 are insulated by the insulation sheet 18 so that the parts may not contact with each other. The detector 13 of the anode alternating-current voltage component may be other means for detecting by capacitive coupling from the anode potential band. As described hereabove, the leakage field decreasing device for CRT display of the invention according to claim 1 is capable of canceling effectively the principal components of deflection component and high voltage ripple component, in the leakage field irradiated in the CRT display front direction, so that the leakage field decreasing device of low frequency can be obtained inexpensively and effectively. According to the experiment, the leakage field irradiated in the CRT front direction could be decreased infinitely to zero by the invention.

Claims: A CRT display device including a CRT display (1) and a leakage field decreasing device comprising: a detector (13) for detecting an alternating-current voltage component by means of capacitively coupling said detector with an anode potential; an inverting amplifying circuit (14) for amplifying and inverting a polarity of said alternating-current voltage component; an inverse field generating means (6) disposed near a CRT front surface for generating an electric field in a CRT front direction; characterised in that said detector (13) is disposed at the surface of a CRT neck area (19) around an anode (20) of a CRT electron gun. The CRT display device according to claim 1 characterised in that said detector (13) comprises a coupling (16) for capacitive coupling with said anode potential, a shield (17) for cutting off any nearby electric field and an insulator (18) isolating said coupling and being disposed between said coupling (16) and said shield (17).

Inventors: KINOSHITA TAKASHI, KOHGA TETSUYA, KINOSHITA, TAKASHI, KOHGA, TETSUYA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD
Application year: 1996
Main classification: H01J29
IPC codes: H01J29, H04N3