APPARATUS IN CATHODE RAY TUBES FOR REDUCING THE MAGNETIC FIELD STRENGTH IN THE TUBE ENVIRONMENT

Abstract: A cathode ray tube (3) (CRT) has a deflecting coil (1) surrounded by funnel-like casing (4) of magnetic material. The deflecting coil generates a magnetic deflecting field (B) for the electron beam and a magnetic leakage field (BL) in the CRT environment. The leakage field is composed of a dipole field and a quadrupole field. To reduce the magnetic field strength in the CRT environment a magnetic compensation field which is counterdirected to the leakage field is generated. The compensation field is composed of a dipole field which is generated by a first compensation loop (7) and a quadrupole field which is generated by a second compensation loop (9). The first compensation loop (7) is substantially flat and at right angles to the magnetic deflecting field (B). The second compensation loop (9) is flat and at right angles to the longitudinal symmetrical axis (z) of the CRT and has an upper (9a) and a lower (9b) part which generate two mutually opposing dipole fields (DK2, DK3). The centres of gravity (TP1 and TP2) of the compensation loops lie on the symmetrical axis (z) respectively at the forward edge (6) of the funnel-like casing (4) and the forward part of the deflecting coil (1). The compensation loops (7, 9) are connected in series with the deflecting coil (1).

Description: APPARATUS IN CATHODE RAY TUBES FOR REDUCING THE MAGNETIC FIELD STRENGTH IN THE TUBE ENVIRONMENT TECHNICAL FIELD The invention relates to an apparatus in cathode ray tubes (CRPs) 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 direc¬ tion of the electron beam and a magnetic leakage field in the CRT environ- ment, as well as a screening casing of magnetic material surrounding the deflecting coil. BACKGROUND ART Magnetic leakage fields occur in CRPs 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. DISCLOSURE OF INVENTION 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 couπterdirected to the leakage field and reduce the field strength in front of the CRT. BRIEF DESCRIPTION OF DRAWINGS An embodiment of the invention will now be described in detail with reference to a drawing, where Figure 1 is a perspective view of the CRT deflecting coil, Figure 2 schematically illustrates the electrical connections of the deflecting coil, Figure 3 is a cross-section of the CRT, Figure 4a is a perspective view of the deflecting coil, Figure 4b is a plan view from one side of the deflecting coil, Figure 4c is a plan view from behind of the deflecting coil, Figure 5 is a plan view of the CRT from above with a first compensation loop, Figure 6 illustrates the compensation loop in perspective, Figure 7 illustrates the electrical connection of the compensation loop to the CRT deflecting coil, Figure 8a is a plan view from behind of the CRT with the first and a second compensation loop, Figure 8b is a plan view of the CRT from one side with the first and the second compensation loop, Figure 9 illustrates an alternative embodiment of the first compensation loop, Figure 10 is a diagram illustrating the time variations of the magnetic field strength in the environment of the CRT and Figure 11 is a further diagram of the magnetic field strength. BEST MODE FOR CARRYING OUT THE INVENTION Figure 1 is a sketch of a known magnectic deflecting coil 1 in a CRT 3, the display surface 3a of which is indicated in the Figure. The coil has an upper half la and a lower half lb, which are connected in parallel as illustrated in Figure 2. The coil has many turns, but for the sake of simplicity it is illustrated with only one turn. The coil is placed at the rear portion of the CRT exterior to the CRT, and its funnel- like shape follows that of the CRT. At the forward end of the coil 1 facing towards the display surface the coil halves la and lb have forward conductors lc and Id which extend in a half circle outside the CRT 3. Electrical currens I- and I„ in the coil halves, where I, *4 I 2 , generate a vertical magnetic deflection field B in the deflection zone of the CRT. An electron beam 2 through the deflection zone is deflected laterally and impignes απ the display surface 3a. The lateral deflection, the so-called scanning line sweep, takes place at a frequency of 31,7 kHz, while the deflection in height, the image sweep, takes place with a frequency of about 50 Hz and is taken care of with the aid of a coil not illustrated in the Figure. The CRT 3 is illustrated in a first verticl plane through the longitudinal symmetrical axis z thereof in Figure 3. This plane is parallel to the direction of the deflecting field B and in Figure 1 it is denoted by VP1. The rear part 3b of the CRT is surrounded by the deflecting coil 1, as mentioned. In turn, the coil is surrounded by a screening ferrite casing 4 with a funnel-like shape, which shields the deflecting field B against extraneous disturbances. The deflecting coil 1 for the high-frequency line sweep generates a magnetic leakage field BL outside the CRT. The ferrite casing 4 acts on this leakage field so that its field lines 5 substantially depart from the forwardly facing outer edge 6 of the ferrite casing. The leakage field BL is composed of a magnetic dipole field DL and a magnetic quadrupole field KL, as will be explained below with reference to Figures 4a, 4b and 4c. The deflecting coil 1 is illustrated in Figure 4a, and for the sake of clarity the upper half la and the lower half lb have been shown spaced from each other. In Figure 1 there is a horisontal plane HP, which includes the symmertical axis z and is at right angles to the deflecting field B, the coil 1 having a projection in this plane which is illustrated in Figure 4b. The coil is passed through by the currents I, and I« and generates the above- mentioned dipole field DL, which can be characterized with a magnetic dipole DL Also in Figure 1 there is a second, vertical plane VP2 at right angles to the symmetrical axis z and in this plane the deflecting coil 1 has a projection illustrated in Figure 4c. The upper half la of the projected deflecting coil is passed through by the current I, and generates a magnetic dipolde field which can be characterized as a magnetic dipole D2. This dipole is paralllel to the symmetrical axis z and is situated at the forward conductor lc of the upper coil half la. In a corresponding way, the lower half lb of the deflecting coil generates a magnetic dipole field with the current L,, and this field can be characterized as a magnetic dipole D3 situated at the forward conductor Id of the lower coil half lb. Both dipoles D2 and D3 are in mutual counter -direction and together form a magnetic quadrupole Kl, which characterizes the above- mentioned magnetic quadrupole KL. The leakage field BL is considerd, as mentioned hereinbefore, to exercise an injurious action on a person being in the vicinity of the field. To reduce this action, the field strength of this field can be reduced, as will be described below. In accordance with the present invention, two magnetic compensation fields are generated, a dipole field DK and a quadrupole field KK. for counteracting the magnetic leakage field BL. The dipole field DK is here cαunterdirected to the dipole field DL of the deflecting coil, and the quadrupole field KK is cαunterdirected to the quadru¬ pole field KL of the deflecting coil. The CRT 3 is shown from above in Figure 5 with the deflecting coil 1 and the ferrite casing 4. The compensating dipole field DK is generated by a first compensation loop 7 situated substantially in the horisontal plane. The surface in the horisontal plane HP surrounded by the first compensation loop has its centre of gravity TP1 on the symmerical axis z at the forward-facing outer edge 6 of the ferrite casing 4. The loop in the example is made with a rectangular part 7a between the dashed lines in the Figure and two lobes 7b. These lobes extend from the rectangular part 7a slopingly forwards along the rear side of the CRT 3 outwards such as to be flush with the outer edge of the display surface 3a. The loop 7 has a plurality of turns, but for the sake of simplicity it is only shown with one turn in the Figure. The first compensation loop 7 is illustrated in perspective in Figure 6. In the area 7a the turns of the loop are partially separated for surrounding the ferrite casing 4 and the CRT 3. The remaining parts of the loop are in the horisontal plane HP. The loop 7 is electrically connected in series to the deflecting coil 1, as schematically illustrated in Figure 7, and is passed through by the currents L + l * . With the aid of the loop 7 there is generated a magnetic dipole field DK, which extends in an area in front of the CRT display surface 3a. By selecting a suitable current direction in the loop 7 the compensating dipole field DK will be in couπterdirection to the dipole field DL generated by the deflecting coil 1, as illustrated in Figure 5. The field strength of the compensating dipole field DK may be varied by varying the number of turns in the loop 7, and by changing the superficial size of the loop. The compensating dipole field DK is characterized here as a magnetic dipole DK1. This dipole has the same size and position as the above-mentioned dipole Dl for the leakage field DL, and the dipoles DK1 and Dl are mutually cαunterdirected. By adjusting the first compensating loop 7 in this way, the strength of the dipole field DK may be adjusted so that the leakage field DL is counteracted and the resulting field strength heavily reduced. This reduction of the field strength is obtained in a large area in front of the display surface 3a, if the centre of gravity TP1 of the compensation loop is disposed as described above. The CRT 3 is illustrated from behind in Figure 8a with the ferrite casing 4 and the first compensation loop 7. The compensa¬ ting quadrupole field KK is generated by a second compensation loop 9 with an upper half 9a and a lower half 9b. In Figure 8b the CRT is illustrated from one side with bath compensation loops 7 and 9. The second compensation loop is substantially flat and parallel to the second, vertical plane VP2 and surrounds a surface having a centre of gravity TP2 on the longitudinal symmertical axis z at the forward conductors lc and Id of the deflecting coil 1. In the illustrated embodiment the loop 9 is symmertical about both the first vertical plane VP1 and the horisontal plane HP. However, the loop 9 may need to have a somewhat different and asymmertic form to compensate for the irregularities in the leakage field KL, which can be caused by such as an unillustrated metal frame retaining the CRT 3. The second compensation loop is electrically connected in series to the first compensation loop 7 and the deflecting coil 1, as schemati- cally illustrated in Figure 7, and is passed through by the current + I * In the upper half 9a of the second compensation loop 9 there is generated a magnetic field, which is characterized as a magnetic dipole DK 2, and in the lower half 9b there is generated a counter-directed dipole field which is characterized as a magnetic dipole DK3. Both magnetic dipoles DK2 and DK3 constitute together a magnetic quadrupole KK1 which characterizes the above-mentioned compen¬ sating quadrupole field KK. By suitable selection of current direction in the loop 9, loop size and number of turns, the second compensation loop 9 can be adapted so that the generated quadrupole field KK counteracts the quadrupole field KL of the deflecting coil 1 and heavily reduces the magnetic field strength in the environment of the CRT 3. An alternative embodiment of the first compensation loop 7 is illustrated in Figure 9. A compensation loop 8 is put together from two part loops 8a and 8b, which are electrically coupled in series with each other and with the deflecting coil 1. The part loops are flat and lie in the horisontal plane HP. The surfaces surrounded by the part loops have their common centre of gravity TP1 at the same point as the first compensation loop 7 at the front edge 6 of the ferrite casing 4. It should be noted that the compensation loop 7, as different from the compensation loop 8, affects the quadrupole field in the environment of the CRT 3. The compensation loop 7 namely has a loop part 7c according to Figure 6, which is parallel to the second vertical plane VP2. The size and number of turns of the second compensation loop 9 must be adjusted with respect to the implementation of the first compensation loop. In Figure 10 there is illustrated a diagram with an example of how the magnetic field strength in the environment of the CRT is affected by the compensation loop 7. In Figure 11 there is a diagram illustrating the corresponding effect when both compensation loops 7 and 9 are connected. The y-composant of the magnetic field is measured in the horisontal plane HP along a circle of radius 40 cm surrounding the CRT. The centre of the circle is on the longitudinal symmertical axis z in the vicinity of the centres of gravity TP1 and TP2 of the loops, so that the distance between the display surface 3a and the measuring point on the z axis is 30 cm. The numerals along the X-axis in the respective diagrams denote the time variation in mT/s of the magnetic field. The measured values for the CRT without any compensation loop are plotted on a graph 10. The measured values with the first compensation loop 7 connected are plotted on a graph 11. Measured values with both the first 7 and the second 9 compensation loops connected are plotted on a graph 12 in Figure 11. Apparatus have been described above for generating magnetic compensation fields BK, which counteract the magnetic leakage filed BL coming from the deflecting coil 1 for the line sweep. A leakage field coming from a deflecting coil for the image sweep can also be counteracted with the aid of a corresponding apparatus.

Claims: C L A I M S 1. Apparatus in cathode ray tubes (CRPS) 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, characterized in that the apparatus includes a first compensation loop (7,8) which extends outside the CRT (3) in an area at said screening casing (4) and is substantially symmetrical about a first plane (HP) at right angles to the direction of the magnetic deflecting field (B) and containing the longitudinal symmetrical axis (z) of the CRT and a first vertical plane (VP1) which contains said symmetrical axis (z) and is at right angles to the horizontal plane (HP) and in that the first compensation loop (7,8) is electrically connected to the deflecting coil (1), the projected area of the first compensation loop (7,8) in said first plane (HP) has a size, and the current direction (L +I 7 ) of the first compensation loop (7,8) is arranged such that a magnetic compensation field (DK) is generated, said field being substantially counterdirected to said magnetic leakage field (DL,KL) within an area in front of the display surface (3a) of the CRT (3)-for reducing the magnetic field strength in this area. 2. Apparatus as claimed in claim 1, where the deflecting coil has forward electrical conductors which partially surround the CRT, characterized in that a second compensation loop (9), with an upper (9a) and a lower (9b) half is situated outside the CRT (3) in an area at the forward conductors (lc,ld) of the deflecting coil (1) and extends substantially parallel to a second vertical plane (VP2), which is at right angles to the longitudinal symmetrical axis (z), said second compensation loop being electrically connected to the deflecting coil (1) such that both halves (9a,9b) of the loop (9) generate mutually opposing magnetic fields (DK2,DK3), the current direction ( + ,) in the second compen- sation loop (9) being arranged such that the loop generates a magnetic compensation field (KK) counterdirected to said leakage field (DL,KL) within an area round the CRT (3) for reducing the magnetic field strength in this area. 3. Apparatus as claimed in claim 1 or 2 where the screening casing of magnetic material is funnel-shaped, and has a wide end with its edge facing towards the display surface of the CRT, characterized in that the first compensation loop (7,8) substantially extends in said horisontal plane (HP) and in that its projected area in said horisontal plane (HP) has its centre of gravity (TPl) on the longitudinal symmertical axis (z) at the wide end edge (6) of the screening casing (4). 4. Apparatus as claimed in claim 2 or 3, characterized in that the projected area of the second compensation loop (9) onto said second vertical plane (VP2) has its centre of gravity (TP2) on the longitudinal symmertical axis (z) at the above-mentioned forward conductor (lc,ld) of the deflecting coil (1) facing towards the display surface (3a). 5. Apparatus as claimed in any one of claims 1-4, characterized in that the first compensation loop (7,8) is connected in series with the deflecting coil (1). 6. Apparatus as claim in any one of claims 2-5, characterized in that the second compensation loop (9) is connected in series with the deflecting coil (1).

Inventors: JOHANSSON ROLAND THOMAS WILHEL, LAONGH STIG ARNE, MADSEN KNUD, JOHANSSON, ROLAND, THOMAS, WILHELM, LAONGH, STIG, ARNE, MADSEN, KNUD
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1987
Main classification: H01J29
IPC codes: H01J29, H04N9