Apparatus in cathode ray tubes for reducing the magnetic field s

Details Apparatus in cathode ray tubes for reducing the magnetic field s Apparatus in cathode ray tubes for reducing the magnetic field s

1987-11-25

1989-07-25

Griffin, Robert L.

Electric lamp and discharge devices: systems

Cathode ray tube circuits

Pulse storing

315 85, 335213, H01J 2906

Patent

active

048517372

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The invention relates to an apparatus in cathode ray tubes (CRT's) for reducing the magnetic field strength in the environment of the CRT, the CRT having a deflecting coil generating a magnetic deflecting field in the transverse direction of the electron beam and a magnetic leakage field in the CRT environment, as well as a screening casing of magnetic material surrounding the deflecting coil.


BACKGROUND ART

Magnetic leakage fields occur in CRT's with magnetic deflection of the electron beam. These fields extend outside the deflection zone and can reach a person in the vicinity of the CRT. The magnetic leakage fields are considered to cause injuries by reason of the electric currents induced in the body cells. The current strength is proportional to the time change in the magnetic field, and relatively large currents are obtained in the cells, e.g. from the return pulse of the scanning line sweep in the CRT. In a known solution for reducing the magnetic field in front of the CRT, a flat short-circuited loop has been placed horisontally above the CRT so that the leakage field is deflected obliquely upwards. This measure is simple, but has a limited field of use, since the field does not decrease but is only given another direction. It has also been proposed to screen the CRT with a casing of magnetic material. The casing cannot cover the display surface of the CRT and gives no reduction of the leakage field in front of it.


SUMMARY

The above problem is solved in accordance with the invention by using electrical loops connected to the deflecting coils for generating magnetic compensation fields, which are counterdirected to the leakage field and reduce the field strength in front of the CRT.
In accordance with the present invention, there is provided apparatus in a cathode ray tube (CRT) for reducing a magnetic field strength in an environment of the CRT, the CRT having a deflecting coil generating a magnetic deflecting field in a transverse direction of the CRT's electron beam and a magnetic leakage field in the CRT environment, the CRT also having a screening casing of magnetic material surrounding the deflecting coil, characterized in that the apparatus includes a first compensation loop which extends outside the CRT in an area at said screening casing and is substantially symmetrical about a horizontal plane at right angles to the direction of the magnetic deflecting field and containing a longitudinal symmetrical axis of the CRT and a first vertical plane which contains said symmetrical axis and is at right angles to the horizontal plane and in that the first compensation loop is electrically connected to the deflecting coil, a projected area of the first compensation loop in said horizontal plane has a size, and a current direction of the first compensation loop is arranged such that a first magnetic compensation field is generated, said first magnetic compensation field being substantially counterdirected to said magnetic leakage field within an area in front of a display surface of the CRT for reducing a magnetic field strength in this area.
In another embodiment of the invention, there is provided an apparatus in a CRT wherein the deflecting coil has forward electrical conductors which partially surround the CRT, characterized in that a second compensation loop, with an upper half and a lower half is situated outside the CRT in an area at the forward electrical conductors of the deflecting coil and extends substantially parallel to a second vertical plane, which is at right angles to the longitudinal symmetrical axis, said second compensation loop being electrically connected to the deflecting coil such that both halves of the second compensation loop generate mutually opposing magnetic fields, a current direction in the second compensation loop being arranged such that the loop generates a second magnetic compensation field counterdirected to said magnetic leakage field within an area around the CRT for reducing a magnetic field strength in this area.


BRIEF DESCRIPTION OF DR

REFERENCES:
patent: 2227029 (1940-12-01), Schlesinger
patent: 3153171 (1961-06-01), Poole et al.
patent: 3735193 (1973-05-01), Ikeuchi
patent: 3784868 (1974-01-01), Shinkai et al.
patent: 3879633 (1975-04-01), Stark, Jr.

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