Degaussing circuit with delayed generation of a vertical deflection

Abstract: An apparatus for effectively degaussing a CRT incorporating a switching mechanism designed to protect the degaussing coil's magnetic field from interference from the magnetic field of the CRT's vertical deflection yoke (2). Upon powerup a switching means (SW1) supplies the degaussing coil (4) with a current, and the magnetic field generated from the degaussing coil (4) is used to degauss the CRT over a predetermined time. After this degaussing effect is sufficiently achieved, a second switching means (SW2) is activated to supply the vertical deflection yoke (2) with a current. Since no current is flowed in the vertical deflection yoke (2) during the period for degaussing by the degaussing coil (4), no magnetic field is generated therein, and thus mutual interference is prevented and a sufficiently large degaussing effect can be achieved.

Description: FIELD OF INVENTION The present invention relates to cathode ray tube (CRT) displays and more specifically, to the degaussing circuit, within such displays, which is designed to neutralize the earth's magnetic effect. BACKGROUND OF THE INVENTION Degaussing circuits are presently used within standard CRT displays to neutralize picture distortion caused by magnetic flux. FIGURE 4 shows such a conventional CRT apparatus whose function is to project an image on a phosphor screen. This figure includes a vertical deflection circuit 1, a vertical deflection yoke 2 which is driven by a vertical deflection circuit 1, a power circuit 3 for the CRT apparatus, a degaussing coil 4, a thermal resistance element (posistor) 5 connected in series with the degaussing coil 4, a power switch SW3 and an AC power supply AC. The power circuit 3 is powered by the AC power supply AC and supplies drive power directly or indirectly to the vertical deflection circuit 1. The degaussing coil 4 and the thermal resistance element 5 are connected through the power switch SW3 to the AC power supply AC. The resistance of thermal resistance element 5 increases as its temperature increases. FIGURE 5 further describes the operation of this conventional apparatus. When the power switch, SW3 is turned on at time T (0) , a current I (DGC) , as shown in 5(a), flows into the degaussing coil 4 and the thermal resistance element 5, thereby degaussing the CRT. As the temperature of thermal resistance element 5 increases, its resistance value also increases and thereby gradually decreases the amplitude of the current I (DGC) . When the circuit reaches its steady state, no current flows within it. Conversely, as shown in 5(b), the current I LV flowing into vertical deflection yoke 2 is gradually increased after the power supply is activated and eventually reaches a constant amplitude in its steady state. The effect of the degaussing coil as described is undesirable due to interference from the field of the vertical deflection yoke. Typically, efforts to reduce this interference include 1) providing a time constant for the rising edge of the drive signal to the vertical deflection circuit and 2) providing a time constant for the rising edge of the voltage to the vertical deflection circuit. These implementations have not achieved satisfactory results because they do not utilize a time constant sufficient in length to produce the desired results. From EP-A2-0 424 982 a cathode ray tube apparatus according to the preamble of claim 1 is known. However, the second switch for supplying power to the vertical deflection yoke is activated in dependance on the switching on operation of a main switch and is connected between the power supply circuit for the vertical deflection yoke and the vertical deflection yoke. Accordingly, it is an object of the invention to provide a CRT apparatus capable of satisfactorily degaussing the CRT when the power supply is activated, while avoiding interference with the operation of the vertical deflection yoke. SUMMARY OF THE INVENTION This and other objects are attained according to the invention by providing a cathode ray tube apparatus according to claim 1. According to the present invention, when the power supply is turned on, current flows into the degaussing coil by the first switch means so that the CRT is degaussed by the field from the degaussing coil. A predetermined time afterwards, the vertical deflection yoke is supplied with current by the second switch. Accordingly, mutual interference between the degaussing coil and the vertical deflection yoke is prevented and a satisfactory degauss is achieved BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood from the following detailed description, taken in conjunction with the accompanying drawings, in which: FIG. 1 is a block diagram of the construction of one embodiment of the invention. FIG. 2 is a diagram showing the waveform of current at each portion in FIG. 1; FIG. 3 is a diagram of another example of a cathode ray tube apparatus; FIG. 4 is a diagram of a conventional CRT according to the prior art; FIG. 5 is a diagram of the waveform of the current at each portion of FIG. 4. DETAILED DESCRIPTION OF THE INVENTION While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail one specific embodiment, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and is not intended to limit the invention to the embodiment illustrated. In the embodiment shown in FIG. 1, SW1 represents a switch connected in series between the AC power supply AC and a series circuit of the degaussing coil 4 and the thermal resistance element 5. SW2 is a switch connected in series with the power supply line between the AC power supply AC and the power circuit 3. The other portions are constructed in the same way as the conventional CRT apparatus depicted in FIG. 4. The operation of the invention is illustrated by FIG. 2. When the power supply is turned on and the switch SW1 is turned on at time t [0] , the current I [DGC] as shown in FIG. 2(a) flows in the degaussing coil 4 and the thermal resistance element 5 by the AC power supply AC. Then, the amplitude of the current I [DGC] gradually decreases as described above, and finally approaches zero in the steady state. After a time interval T, measured as the elapsed time from t [0] , the time at which switch SW1 was turned on, a sufficiently large degaussing effect on the CRT is achieved and the switch SW2 is turned on. Thus, the vertical deflection circuit 1 is then powered by the power circuit 3, so that the current I [LV] starts flowing in the vertical deflection yoke 2 and gradually increases as shown in Fig. 2(b). Therefore, the vertical deflection magnetic field generated by the current I [LV] from the vertical deflection yoke 2 does not interfere with the degaussing field generated by the current I [DGC] from the degaussing coil 4. As a result, a sufficiently large degaussing effect is achieved by the current I [DGC] in the degaussing coil 4, and thus the effect of the earth magnetism is neutralized. Fig. 3 shows another example of a cathode ray tube apparatus. In this example, the switch SW2 is connected in series with the power supply line between the power circuit 3 and the vertical deflection circuit 1. The other portions are constructed in the same way as in Fig. 1. In this circuit arrangement, the switches SW1 and SW2 are also activated in accordance with the timing shown in Fig. 2. Either arrangement is suitable for use in, for example, a large flat television set in which a large current flows in the degaussing coil.

Claims: A cathode ray tube apparatus, comprising: an AC power supply; a degaussing circuit connected to said AC power supply, said degaussing circuit comprising: a degaussing coil (4) for degaussing a cathode ray tube, a thermal resistance element (5) connected in series with said degaussing coil (4), and first switch means (SW1) connected in series with said degaussing coil (4) and said thermal resistance element (5), wherein the resistance of said thermal resistance element (5) gradually increases after said first switch means (SW1) have been switchend on, thereby decreasing the amplitude of the current flowing through said degaussing coil (4) so that in its steady state substantially no current flows; a vertical deflection yoke (2) being connected to said AC power supply via a power supply circuit (3) for supplying current to said vertical deflection yoke (2); and second switch means (SW2) for delaying the supply of current to said vertical deflection yoke (2) after the time at which said first switch means (SW1) have been switched on, wherein said delay time (T) is long enough for achieving a sufficient large degaussing effect by said degaussing coil (4), characterized in that said second switch means (SW2) are switched on in dependence on the switching on operation of said first switch means (SW1) upon the lapse of said delay time (T) after said first switching means (SW1) have been switched on, and said second switch means (SW2) are connected in series between said AC power supply and said power supply circuit (3) for said vertical deflection yoke (2). A cathode ray tube apparatus according to claim 1, characterized in that said first switch means (SW1), after having been switched on, remain switched on.

Inventors: OGURA TOSHIYUKI, TSUMORI KOKI, OGURA, TOSHIYUKI, TSUMORI, KOKI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: H04N9
IPC codes: H04N9