Cathode ray tube device that reduces magnetic field leakage

Details Cathode ray tube device that reduces magnetic field leakage Cathode ray tube device that reduces magnetic field leakage

2002-04-10

2003-10-07

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

With electromagnetic wave radiation preventing or shielding...

C315S008000, C315S370000, C313S402000, C313S408000, C335S214000, C335S213000

Reexamination Certificate

active

06630791

ABSTRACT:

BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a cathode ray tube (CRT) device provided with a deflection yoke, and particularly relates to a technique for reducing a magnetic field escaping as leakage from the deflection yoke.
(2) Related Art
In recent years, standards have been developed in Northern Europe in response to concerns about a low-frequency magnetic field given off by a CRT device. There is apprehension that such a magnetic field may affect the human body. Especially in Sweden, the standards, such as the MPR II and TCO standards, have been established with the aim of suppressing the magnetic field escaping from a deflection yoke or a horizontal deflection coil in particular. The magnetic field escaping as leakage from the deflection yoke or the horizontal deflection coil is referred to as the “magnetic field leakage” hereinafter. To meet the leakage limits prescribed by the standards, necessary measures should be taken for the CRT device to reduce the magnetic field leakage.
There have been techniques suggested in order to reduce the magnetic field leakage. As one example of such techniques, a magnetic field is generated as a “cancel magnetic field” in the direction opposite to the magnetic field escaping as leakage from the deflection yoke. For doing so, a “cancel coil” is used for generating the cancel magnetic field so as to cancel the magnetic field leakage.
A CRT device using a cancel coil is disclosed in Japanese Laid-Open Patent Application No. 3-165428 (referred to as the first prior art) and No. 6-176714 (referred to as the second prior art).
For the CRT device disclosed in the first prior art, a cancel coil for reducing the magnetic field leakage is set above an upper part of a deflection yoke and a current is supplied to the cancel coil so that a cancel magnetic field is generated.
FIG. 1
shows a schematic circuit diagram of a horizontal deflection coil
27
and a cancel coil
28
of the first prior art.
As shown in
FIG. 1
, the horizontal deflection coil
27
and the cancel coil
28
are connected in series. By the passage of a horizontal deflection current through the cancel coil
28
as well as the horizontal deflection coil
27
, the cancel coil
28
can generate a cancel magnetic field that varies in accordance with the variations in the magnetic field leakage from the horizontal deflection coil
27
. The cancel coil
28
is positioned so that the cancel magnetic field is generated in a proper direction to cancel the magnetic field leakage.
Meanwhile, for the CRT device disclosed in the second prior art, a cancel coil for reducing the magnetic field leakage is made up of a closed-circuit winding and set at each of upper and lower parts of a CRT so as to face a deflection yoke.
FIG. 2
shows a schematic circuit diagram of a horizontal deflection coil
37
and a cancel coil
38
of the second prior art.
As shown in
FIG. 2
, the cancel coil
38
made up of the closed-circuit winding is set facing the horizontal deflection coil
37
. With this construction, an electromotive force is produced inside the cancel coil
38
in accordance with variations in the magnetic field leakage resulting from the generation of the horizontal deflection magnetic field. By means of the electromotive force, the cancel coil
38
generates a cancel magnetic field in a proper direction so as to cancel the magnetic field leakage.
However, the CRT devices employing the techniques stated in the first and second prior arts respectively have the following problems.
As for the first prior art, the deflection current needs to pass through the cancel coil
28
that does not contribute to the horizontal deflection. Thus, power has to be unnecessarily consumed and, in addition to this, the deflection sensitivity may be deteriorated.
As for the second prior art, power does not need to be supplied to the cancel coil
38
and so the problem of the first prior art does not occur. However, the second prior art has another problem. If the magnetic field escaping as leakage from the deflection yoke is harmful to the human body, the magnetic field leakage should be reduced in front of a front panel of the CRT device, where a user is expected to be most times. However, the cancel coils
38
are set at the upper and lower parts of the CRT, facing the deflection yoke, so that the magnetic field leakage cannot be effectively reduced at a significant position where the reduction of leakage is required most. In order to reduce the magnetic field leakage at this position, the number of turns forming the cancel coil
38
may be increased. However, the increased number of turns of the cancel coil
38
may in turn adversely affect the horizontal deflection magnetic field.
Just as with the magnetic field leakage, electric field leakage is also subject to the Swedish MPR II and TCO standards. The electric field leakage is ascribable mainly to that an electric field generated due to a difference in voltage between the facing deflection coils included in the deflection yoke is given off to the outside. A technique for reducing such an electric field leakage is disclosed in, for example, Japanese Laid-Open Patent Application No. 5-207404 (referred to as the third prior art).
For the CRT device disclosed in the third prior art, a reverse voltage supplying unit is provided to supply a voltage having a reversed polarity to the waveform of the deflection voltage applied to a deflection coil. Also, an electrode is set at the top and bottom of the inner wall of the CRT at the front panel side. The reverse voltage supplying unit supplies the reverse voltage to the pair of electrodes. This enables the electrodes to generate an electric field having the reversed polarity to the VLMF (Very Low Magnetic Field) leakage (i.e., unwanted VLMF leakage). The electric field with the reversed polarity can cancel the unwanted VLMF leakage.
Using the technique of the third prior art, however, the reverse voltage supplying unit needs to be further provided. In addition to this, the magnetic field leakage cannot be reduced using this technique.
SUMMARY OF THE INVENTION
Therefore, it is a first object of the present invention to provide a CRT device that can prevent unnecessary power consumption and reduce a magnetic field leakage with a simple construction at low costs.
It is a second object of the present invention to provide a CRT device that can prevent unnecessary power consumption and reduce magnetic and electric field leakages with a simple construction at low costs.
The first object of the present invention can be achieved by a cathode ray tube device made up of: a cathode ray tube that has a front panel and a funnel; an electron gun that is set inside a neck of the funnel and projects electron beams onto an inner surface of the front panel; a deflection yoke that is set on the funnel at the neck and deflects the electron beams projected by the electron gun; and a cancel coil that has at least one closed-loop coil, makes an interlinkage with a magnetic field leakage that escapes from the deflection yoke, and generates a magnetic field in a direction so as to cancel the magnetic field leakage, wherein each closed-loop coil is set at either a first position or a second position, the first position being at a top of the cathode ray tube with a part of the closed-loop coil running along a top edge of an effective display region of the front panel, and the second position being at a bottom of the cathode ray tube with a part of the closed-loop coil running along a bottom edge of the effective display region.
With this construction, the magnetic field leakage from the CRT makes an interlinkage with the closed-loop coil, so that the magnetic field leakage can be canceled. Since the closed-loop coil is arranged along the top or bottom edge of the effective display region, the magnetic field leakage occurring at a significant position where the reduction of leakage is required most can make an interlinkage with the closed-loop coil. Consequently, the effect of canceling the magnetic fi

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