DEGAUSSING CIRCUIT FOR CRT

Abstract: A degaussing circuit for degaussing a cathode ray tube includes a degaussing coil zmounted on a back of the cathode ray tube and a positive temperature coefficient thermistor connected in series with the degaussing coil. The resonance frequency determined by the degaussing coil and the positive temperature coefficient thermistor is set to be below about 14 kHz or above about 17 kHz.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a degaussing circuit for a CRT (cathode ray tube) used in a color television, a color monitoring device, a color display device, or the like. 2. Description of the Prior Art Generally, although CRTs have shadow masks, there will be still some color shading observed due to undesirable magnetic field, such as the earth's magnetic field, effecting on the CRT. In order to eliminate the unwanted magnetic field, a degaussing circuit is proposed which includes a degaussing coil inserted with a positive temperature coefficient thermistor with respect to the power source. The degaussing coil is usually provided by one loop wound on the back surface of the CRT, or by two loops provided on the upper half and lower half of the back surface of the large size CRT to increase the degaussing effect. Generally, the positive temperature coefficient thermistor made of a material containing barium titanate has some capacitive component C, which is about 4-40 nF when measured during the high impedance stable condition obtained after the completion of degaussing operation as effected immediately after the commencement of the power supply. Since the degaussing coil itself has an inductance of about 10-20 mH, the degaussing circuit defined by a combination of the positive temperature coefficient thermistor and the degaussing coil will have a resonance frequency (fo=1/2π√LC) which sometimes take about 15-16 kHz. Also, since the horizontal scanning frequency of the deflecting yoke provided around the electron gun of the CRT is 15.75 kHz, the signal generated by the deflecting yoke may resonate with the signal through the degaussing coil such that the magnetic flux produced by the deflecting yoke links with the degaussing coil to generate induction current, which may undesirably raise to about 100 mA (normally several mA), in the degaussing coil, resulting in undesirable picture fluctuation even after the completion of the degaussing. Although it is possible to prevent the degaussing coil from being adversely affected by the magnetic field produced by the deflecting yoke by providing a suitable shield, such a shield results in high manufacturing cost. SUMMARY OF THE INVENTION The present invention has been developed with a view to substantially solving the above described disadvantages and has for its essential object to provide an improved degaussing circuit which does not resonate with the horizontal scanning signal, and yet which is stable in operation, and simple in construction. In accomplishing these and other objects, a degaussing circuit according to the present invention is so arranged that the series resonance frequency determined by the degaussing coil and the positive temperature coefficient thermistor is set to be below about 14 kHz or above about 17 kHz. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become apparent from the following description taken in conjunction with a preferred embodiment thereof with reference to the accompanying drawings, throughout which like parts are designated by like reference numerals, and in which: Fig. 1 is a circuit diagram of a degaussing circuit of the present invention; Fig. 2 is a circuit diagram of an equivalent circuit of the circuit shown in Fig. 1; Fig. 3 is a diagrammatic view of the degaussing circuit of Fig. 1 applied to the CRT, when it is viewed from back; Fig. 4 is a diagrammatic view of the degaussing circuit of Fig. 1 applied to the CRT, when it is viewed from side; Fig. 5 is a graph showing a residual current through the circuit of Fig. 1 at the resonance frequency; Fig. 6 is a view similar to Fig. 3, but showing a modification thereof; and Fig. 7 is a view similar to Fig. 1, but showing a modification thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, a degaussing circuit of the present invention is shown, which comprises a positive temperature coefficient (PTC) thermistor 2, a degaussing coil 3 and a main switch SW connected in series between an AC power source 1. As shown in Figs. 3 and 4, degaussing coil 3 is provided on a back conical side of a CRT (cathode ray tube) 5 such that a loop portion 3a is provided above an electron gun 6 and deflecting yoke 7 of CRT 5 and another loop portion 3b is provided below the loop 3a so as to include therein electron gun 6 and deflection yoke 7. Since CRT 5 is covered by two loops 3a and 3b, the degaussing can be done with a high efficiency. In operation, when the main switch SW is turned on, a great amount of electric current flows through degaussing coil 3. Then, after a certain period of time, i.e., when the electric resistance of PTC thermistor 2 becomes high due to the temperature rise of the PTC thermistor itself, the current flowing through degaussing coil 3 will be decreased. By that time, the degaussing operation of various magnetic parts, such as a shadow mask, is completed. Thereafter, very small amount of current I (referred to as stable current), such as below several tens of milliamperes, should flow through degaussing coil 3, as will be described hereinbelow. The resonance frequency fo of the degaussing circuit can be given by the following equation: fo=1/2π√ LC in which C is a capacitive component of PTC thermistor 2 measured while it has gained a high resistance and L is an inductance of degaussing coil 3, as shown in Fig. 2. A relationship between the resonance frequency fo and the stable current I as measured when degaussing coil 3 is mounted on the back of CRT 5 is shown in Fig. 5. It has been observed that when the stable current I is greater than about 50 milliamperes (limit current), there will be an image fluctuation and/or horizontal stripes appear in the picture. Such increase of the stable current I is due to, as has been described above in connection with the prior art, the induction current generated in the degaussing circuit caused by the AC magnetic field (15.75 kHz) which is produced by the horizontal beam scan signal used in deflection yoke 7. In the light of the above fact, according to the present invention, the degaussing circuit is so arranged that the resonance frequency fo thereof is offset from the horizontal scanning frequency (15.75 kHz) of deflecting yoke, preferably about or below 14 kHz, or about or above 17 kHz. By this arrangement, is it possible to maintain the stable current I below 50 milliamperes, i.e., below the limit current, such as to several milliamperes. According to one example, since the inductance L of degaussing coil 3 is about 10-20 mH, the capacitive component C of PTC thermistor 2 is made to be about or below 4.5 nF or, about or above 10 nF by adjusting the structural factors thereof, such as composition, sintering conditions and/or crystal size (particle diameter) of the PTC thermistor 2. According to one embodiment, PTC thermistor 2 has a composition as given below. (Ba 0.815 Sr 0.18 Y 0.005)TiO₃ + 0.05wt% Mn + 1.5wt% SiO₂ It is noted that as the capacitive component C of PTC thermistor 2 becomes great, a high frequency surge current from power source 1 will pass through the PTC thermistor 2, resulting in undesirable magnetization of various elements provided in the CRT 5. Thus, it is preferable to arrange the capacitive component C of PTC thermistor 2 to be below 4.5 nF. It is to be noted that the manner in which degaussing coil 3 is mounted on CRT 5 can be varied such as a double loop, as shown in Fig. 6. Furthermore, as shown in Fig. 7, it is possible to connect another PTC thermistor 6 in parallel to the power source 1 and in a thermally coupled manner with PTC thermistor 2 so as to aid the heating up process of PTC thermistor 2. According to the present invention, since the resonance- frequency fo of the degaussing circuit is arranged to be about or below 14 kHz, or about or above 17 kHz so as to be offset from the frequency of the horizontal scanning signal used in the deflection yoke, the stable current obtained after the degaussing operation can be maintained below the limit current. Thus, there will be no image fluctuation or horizontal stripes appear in the picture. Furthermore, since there is no fixed requirement of the capacitive component of the PTC thermistor, but can be varied about or below 4.5 nF or, about or above 10 nF, it is possible to easily design the degaussing circuit using various types of degaussing coil 3 having the inductance of about 10-20 mH.

Claims: A degaussing circuit for degaussing a cathode ray tube having a degaussing coil mounted on a back of said cathode ray tube and a positive temperature coefficient thermistor connected in series with said degaussing coil, characterized in that: a series resonance frequency determined by said degaussing coil and said positive temperature coefficient thermistor is below about 14 kHz. A degaussing circuit as claimed in Claim 1, wherein said positive temperature coefficient thermistor has a capacitive component of below about 4.5 nF. A degaussing circuit for degaussing a cathode ray tube having a degaussing coil mounted on a back of said cathode ray tube and a positive temperature coefficient thermistor connected in series with said degaussing coil, characterized in that: a series resonance frequency determined by said degaussing coil and said positive temperature coefficient thermistor is above about 17 kHz. A degaussing circuit as claimed in Claim 3, wherein said positive temperature coefficient thermistor has a capacitive component of below about 10 nF.

Inventors: MIHARA, MASAHITO
Applicants: MURATA MANUFACTURING CO., LTD.
Application year: 1988
Main classification: H04N9
IPC codes: H04N9