Degaussing circuit

Details Degaussing circuit Degaussing circuit

2002-01-18

2003-11-25

Sircus, Brian (Department: 2836)

Electricity: electrical systems and devices

Control circuits for electromagnetic devices

Systems for magnetizing, demagnetizing, or controlling the...

C315S008000

Reexamination Certificate

active

06654224

ABSTRACT:

BACKGROUND
1. Field of the Invention
This invention relates to a degaussing circuit of the type commonly used in cathode ray tubes (CRT's), color televisions, color monitors and other like type devices. More particularly, this invention relates to an improved degaussing circuit having a degaussing coil and a single thermistor element with a positive temperature coefficient (PTC) or mono-PTC unit for dual voltage applications.
2. Background of the Invention
Rasters in all cathode-ray tubes (CRTs) are created by the horizontal and vertical sweeping movement of three electron beams. The electron beams emitted by red, green, blue (R,G,B) cathodes are controlled by deflection circuits whose magnetic fields are orthogonal to the direction of the electron beams.
The system of magnetic control, which enables tracing of the raster, has drawbacks because the electron beams are susceptible to changes because of earth and stray magnetic fields, which give off interfering beams. Also, terrestrial flux can reach 0.05 mT (0.5 Gs) in the absence of nearby magnetic structures, with a maximum of 0.2 mT (2 Gs) in the vicinity of steel-frame buildings or underground iron, nickel or cobalt ore deposits. Even more dense magnetic fields can be produced by interfering sources, such as unshielded loudspeakers, motors, and transformers located in direct proximity of the CRT. These kinds of interferences result in tainted, or, in more extreme instances, a complete loss of purity of the primary colors in the CRT. In order to maintain proper pixel excitation by the designated electron beams, various demagnetizing circuits have been utilized in all but the smallest picture tubes. A network that employs these thermistor devices, having positive thermal coefficient (also known asa posistor, or PTC) are connected in series with degaussing coils have been around for a number of years, and are still one of the more commonly used circuits for degaussing applications. However, these circuits that utilize PTC thermistors demonstrate poorly defined characteristics because the PTC thermistors are susceptible to variations in line voltage, load current, and thermal drift. The degaussing circuits produce considerable in-rush current, which in turn create strong electromagnetic fields on their own that are often able to disturb adjacent sensitive electronic equipment.
The basic operational principles of the PTC thermistor circuit are well understood, i.e., upon power-up, a large-magnitude, alternating inrush current generates a magnetic field, whose amplitude, during duration of the first few AC cycles, far exceeds the level of the surrounding stray magnetic fields. Then, the current and its associated flux are gradually reduced to almost nill and the process is terminated. The purpose of this procedure is to cycle through B—H magnetic hysteresis loop of the aperture grill/shadow mask and other ferrous alloy materials of the CRT so that the alternating orientation and diminishing magnitude of the magnetic field vectors reduce remnant flux to the negligible value.
The Trinitrons, having their phosphorous pixels fashioned in a form of vertically elongated strip are more susceptible to vertically oriented parasitic magnetic vectors than to horizontal ones. This is because vertical flux causes horizontal deviation of electron beams. For the same magnitude of horizontally and vertically oriented flux, the latter produces more noticeable color impurities. Thus, to minimize horizontal landing offset, predominantly vertical oriented compensating fields should be generated by the degaussing coils. This dictates horizontal placement of the coils directly above and below the CRT. In practice, since mutual orientation of the television set and the terrestrial magnetic field can vary widely depending on TV spatial positioning and its geographical location, the degaussing coils are mounted at the back of the CRT cone, producing magnetic field vectors angled in reference to the aperture grill. Such orientation of the degaussing coils also boost eddy currents induced in the magnetic shields that cover the back conical side of the CRT.
Further, calculations of magnetic flux produced by the degaussing coils are complicated because of the complex, three-dimensional geometrical form of the coils and their spatial orientation in reference to the CRT. Equations presented below illustrate simplified magnetic relations and, consequently, approximated results.
A vector relations of the scalar functions are represented by the following equations: A typical 67-turn degaussing coil that conducts the inrush current of 15 A
pk
produces magnetomotive force
MMF=I* N
=15 A
pk
*67 trn=1,005 A
pk
trn
For a 32-inch CRT, each of the two quasi-rectangular degaussing coils is about 0.76 m wide and 0.28 m high.
Corresponding magnetic field intensity (H
0
) in geometrical center of the coil can be derived from the Biot-Savart's law &dgr;H=I*&dgr;L*sin(r,dL)/4&pgr;r
2
and converted to accommodate the rectangle-shaped inductor
H
0
=2
IN[x
−2
+y
−2
]
½
p
−1
=2*1,005 A
pk
trn*[(0.76 m)
−2
+(0.28 m)
−2
]
½
*p
−1
=2,435 A
pk
trn/m
The magnitude of this field decreases in reversed proportion to the axial distance
1
=0.25 m measured in direction normal to the plane of the coil, from its center to the aperture grill plane. Based on the same law, we have
H={IN
/2
p
[(0.5
x
)
2
+(0.5
y
)
2
+1
2
]
½
}*{x
/[(0.5
y
)
2
+1
2]
½
+y
/[(0.5
x
)
2
+1
2
]
½
}
H
={1,005 Atrn/2
p
[(0.38 m)
2
+(0.14 m)
2
+(0.25 m)
2
]
½
}*{0.76 m/[(0.14 m)
2
+(0.25 m)
2
]
½
++0.28 m/[(0.38 m)
2
+(0.25 m
2
]
½
}>>1,100 A
pk
trn/m
Vector H constitutes a geometric composite of its horizontal and vertical fractions. The vertical element of that vector is reduced by the angled orientation of the coil that rests on the back side of the CRT. For the CRT having a deflection angle of a=110 deg, the inclination angle of the degaussing coil is approximately b=45 deg, thus
H
y
=cos b*
H=cos
45 deg*1,100 A
pk
trn/m=776 A
pk
trn/m
Finally, the vertical fraction of flux induced by the single coil can be found
B
y
=m
0
m
r
H
y
=4
p
Exp(−7)
H/m
*1*776 A
pk
trn/m>>1 mT (10 Gs)
In spite of apparent simplicity of the demagnetization process, there are several challenging issues. The auto-volt or wide-range AC operation requires automatic adaptation of the degaussing circuit to the line voltage that may span from 88 to 288 volts (proportion in excess of 1:3). Since voltage regulation of the demagnetizing circuits is impractical, the only viable alternatives are switched PTC thermistors and/or current-limiting resistors. Networks based on switched PTC thermistors and/or resistor components reduce the current ratio to 1:1.7 for line voltage variations of (88-153)V or (176-288)V. Larger CRTs require stronger magnetic fields to maintain same flux density (number of magnetic lines per unit area) of the screen. For instance, a CRT enlarged from 27-inch to 36-inch CRT almost doubles its raster area. Magnetic flux has to be increased in the same proportion if the flux density is to remain unchanged. This can be accomplished only by increasing current flowing through the coil, the number of turns, or combination a of both. In either case, cost escalates steeply as increased current requires larger diameter wire, while increased number of turns command more length of wire.
For the former option (current), there are limitations of maximum current output from the residential AC outlet. Excessive surge whose I-t product exceeds that of the household circuit breaker poses risk of its activation. Large-magnitude current surges also induce undesirable voltage sags that may lead to malfunction of the associated en

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