MXPA96006641A - Depantation circuit for amp range alternate current - Google Patents

Abstract

The present invention relates to a demagnetization circuit for a cathode ray tube, comprising: a source of alternating voltage potential, switching means coupled to said source, a first temperature sensitive device coupled to the switching means, a second temperature sensitive device coupled to the first device, and a demagnetizing coil coupled to the second temperature sensitive device, wherein a demagnetizing current flowing through the first and second temperature sensitive devices is equal to the demagnetization current that flows through the demagnetization coil

Description

DEFLECTION CIRCUIT FOR ALTERNATE CURRENT OF LARGE RANGE This invention relates in general to the field of demagnetization circuits for video display devices, and, in particular, to demagnetization circuits that terminate a demagnetization current after a predetermined period of time. A color cathode ray tube (CRT) requires periodic demagnetization or demagnetization to compensate for the effects that magnetic fields in the environment may have on the metal components of cathode ray tubes. By way of example, the shadow mask of the cathode ray tube can be magnetized due to the magnetic field of the earth or by a magnetic field generated by the operation of a neighboring motor, apparatus or other electrical device. The shadow mask, permanently magnetized in this way, produces localized magnetic fields, which can affect the trajectories of the electron rays of the cathode ray tube and, therefore, the location at which the electron beams strike the phosphor screen of the cathode ray tube. A deviation in the trajectories of the electron beams of the cathode ray tube can result in a lateral displacement of a video image displayed visually by the cathode ray tube. In addition, the purity of the color of the video image displayed by the cathode ray tube can be significantly deteriorated. Color television receivers, computer monitors and other video display devices typically include an automatic demagnetization circuit in order to compensate for the presence of environmental magnetic fields. Such a demagnetization circuit allows the cathode ray tube to be de-skimmed each time energy is applied to the video display apparatus. A typical automatic demagnetization circuit produces an alternative magnetic field that decays toward zero. An implementation of this circuit may comprise a temperature sensitive device, for example a thermal resistance with positive temperature coefficient (PTC), connected in series between a dedicated demagnetisation relay and a demagnetization coil. The demagnetisation relay is energized when energy is applied to the video display apparatus, and is de-energized at a predetermined time previously, for example between 1 and 2 seconds, after the relay has been energized initially. Initially, when the demagnetisation relay is de-energized, no current flows in the demagnetization circuit, so that the thermal resistance with positive temperature coefficient is thermally cold and, consequently, has a low resistance. When energy is applied to the video display apparatus, the demagnetisation relay is energized and the energy from a distribution network source, for example from 120 V ^ g to 60 Hz, is applied to the thermal resistance with positive temperature coefficient. Because the resistance of the thermal resistance with positive temperature coefficient is initially low, a large demagnetization current begins to flow in the demagnetization circuit. The thermal resistance with positive temperature coefficient is self-heating and thus increases its resistance. As a result, the successive peaks of the demagnetizing alternating current decrease monotonically in magnitude. In this way, the automatic demagnetization circuit produces a decaying alternating current, which is shown in Figures 2 (a) and 3 (a), and thus a decaying alternating magnetic field, which demagnetizes all the metallic components that are inside the tube. of cathode rays and that are adjacent to the cathode ray tube within the video display apparatus. The demagnetisation circuit of the single thermal resistance with positive temperature coefficient works well at low voltages of the distribution network as of 120 VRMS »p ro is not effective at high voltages of the distribution network, for example 240 V ^ g. At these high voltages of the distribution network, the demagnetization current may decay too rapidly if the thermal resistance with positive temperature coefficient has a desirably low nominal resistance combined with a small thermal mass. For example, manufacturers of cathode ray tubes typically specify that, after five cycles of demagnetization current, a peak to peak demagnetization current should not decay below 50 percent of the peak to peak demagnetization current. initial If, after five cycles of demagnetization current, the peak-to-peak value of the demagnetization current decays below 50 percent of its initial peak-to-peak value, as illustrated in Figure 4 (a), a significant residual magnetization in the metallic components inside and adjacent to the cathode ray tube, for example in the shadow mask of the cathode ray tube. The term "residual" is used herein to refer to an amount at a time when the demagnetisation relay changes from an energized, or closed, state to a de-energized, or open state. The demagnetizer circuit of the single thermal resistance with positive temperature coefficient can be modified for use with a higher voltage distribution network source, such as 240 V ^ g at 60 Hz. Specifically, for the same demagnetizing coil, the single thermal resistance with positive temperature coefficient can be replaced by a parallel combination of two thermal resistors with positive temperature coefficient having an equivalent nominal value equal to twice the nominal value of the thermal resistance unique with positive temperature coefficient. This dual thermal resistance demagnetizing circuit with positive temperature coefficient operates in a manner similar to the thermal resistance demagnetizing circuit with positive temperature coefficient only to produce the demagnetizing current shown in Figure 4 (b). The two thermal resistances with a positive temperature coefficient are thermally coupled so that they self-heat at the same time as the demagnetization current flows through the demagnetization circuit. The dual thermal resistance demagnetization circuit with positive temperature coefficient works well at high distribution network voltages like 240 V ^ g, but, unfortunately, the dual thermal resistance demagnetizer circuit with positive temperature coefficient is not effective at low distribution network voltages, such as 120 VR S- At that low distribution network voltage, the higher nominal resistance provided by the parallel combination of the thermal resistors reduces the flow of the demagnetizing current, and therefore, the self-heating of the two thermistor resistances. The effective resistance of the two thermal resistors with positive temperature coefficient does not, therefore, become as tied as it would be with a higher distribution network voltage of, for example 240"VRMS- This may result in a current too large of residual demagnetization, as illustrated in Figure 3 (b), and therefore a significant residual magnetization in the metallic components inside and adjacent to the cathode ray tube, at the moment when the demagnetisation relay is de-energized. The intensity of this residual magnetization is proportional to the product of the demagnetization current flowing through the demagnetization circuit at the instant in which the demagnetisation relay is de-energized and the number of turns in the demagnetization coil. cathode rays typically assign each of their cathode ray tubes a specification limit, expressed in units of e Amps-turns, which should not be exceeded when the relay is de-energized to ensure that no significant residual magnetization is induced in the metal components inside and adjacent to the cathode ray tube. The exact point of the waveform of the demagnetization current at which the demagnetisation relay will be de-energized is not known., so that it is desirable to minimize the peak-to-peak amplitude of the demagnetization current just before the moment when the relay is de-energized.

A significant residual magnetization is not desirable, either caused by the demagnetization current decaying too quickly or by being too large a residual demagnetization current because it can induce the same problems that the demagnetization circuit is designed to avoid. In the case of the shadow mask of 1 cathode ray tube, a significant residual magnetization can cause a lateral shift in the video image displayed by the cathode ray tube. The deterioration of the color purity can also result from the residual magnetization of the shadow mask of the cathode ray tube. It would be useful to have a single demagnetization circuit that could be used over a wide range of distribution network sources without generating significant residual magnetization. Neither of the two demagnetization circuits described above can be used over a wide range of distribution network sources without generating a significant residual magnetization. The single thermal resistance demagnetization circuit with positive temperature coefficient can generate a significant residual magnetization at high distribution network voltages, for example of 240 V ^ g, because its demagnetization current can decay too quickly if the thermal resistance with coefficient of positive temperature has a desirably low nominal strength combined with a small thermal mass. Moreover, the thermal resistance demagnetization circuit with dual positive temperature coefficient can generate a significant residual magnetization at low voltages of the distribution network, for example 120 V ^ g, because the demagnetization current flowing in the circuit in the The moment when the demagnetisation relay is de-energized may be too large. A demagnetization circuit, according to an inventive arrangement shown herein, provides an automatic demagnetization circuit for a video display apparatus over a wide range of AC distribution network sources. This demagnetisation circuit for cathode ray tubes comprises: a switching element coupled with an alternating voltage potential source, a first and a second temperature sensitive device coupled in series with the permutation element, one of the devices having a coefficient of positive temperature and the other of the devices having a negative temperature coefficient; and a demagnetization coil having a first terminal coupled to one of the devices and a second terminal coupled to the alternate voltage potential source. A maximum peak-to-peak value of a demagnetization current flowing through the demagnetization coil during a first cycle of the demagnetization current can not be presented. The switching element may comprise a relay, and the first and second temperature sensitive devices may comprise resistors. The foregoing, and other features and advantages of the present invention will become apparent from the following description read together with the accompanying drawings, in which like reference numerals designate the same elements. Figure 1 is a schematic diagram of a demagnetization circuit according to an inventive arrangement described herein. Figure 2 shows demagnetizing current waveforms for particular demagnetization circuits with a distribution network voltage of 120 V ^ g. Figure 3 shows the demagnetizing current waveforms of Figure 2 just before a demagnetisation relay is de-energized. Figure 4 shows demagnetizing current waveforms for particular demagnetization circuits with a distribution network voltage of 240 V ^ g. An automatic demagnetization circuit 10, shown in FIG. 1, can be used to demagnetize metal components in and adjacent to a cathode ray tube inside a video display apparatus (not shown). The video display apparatus may comprise, for example, a color television receiver, a computer monitor or other video display apparatus that includes a cathode ray tube. A source 1 of alternating voltage potential typically comprises an alternating current distribution network voltage. The source 1 typically provides a sine-wave voltage having a value between 90 V ^ g and 270 V ^ and a frequency that is equal to about 50 Hz or 60 Hz. The relay 2 is dedicated to the automatic demagnetization circuit 10; it does not provide power to any other portion of the video display apparatus. The relay 2 is energized when power is applied to the video display apparatus, and is de-energized at a predetermined time, for example between about 1.5 and 2 seconds, after the power is applied to the video display apparatus. The relay 2 can be de-energized at any predetermined time after it has been energized and before the video display apparatus becomes fully operational. Nominal resistance values are selected for thermal resistances 3 and 4 with reference to the characteristics of the particular coil being used, keeping in mind that a cathode ray tube has a specification limit that should not be exceeded when the relay 2 it is de-energized and that, after five cycles of demagnetizing current, the peak-to-peak value of the demagnetizing current shall be lowered by no more than 50 percent of the initial peak-to-peak demagnetizing current. In a preferred embodiment herein, each of the thermal resistors 3 and 4 has a nominal resistance of about 5 O and the nominal coiling resistance of the demagnetizing coil 5 is equal to approximately 10 O. Therefore, when the relay 2 is energized, the source 1 initially observes a direct current impedance that is equal to approximately 20 O and a corresponding demagnetizing current begins to flow through the automatic demagnetizing circuit 10. The thermal resistances 3 and 4 are self-heating according to the demagnetizing current flows. As the thermal resistance with positive temperature coefficient 3 is self-heating, its resistance increases slightly until the thermal resistance with positive temperature coefficient 3 increases markedly. For example, the resistance of the thermal resistance with positive temperature coefficient 3 can be increased from a nominal value of about 5 O to a maximum value of 100 kO before the relay 2 is de-energized. As the thermal resistance with negative temperature coefficient 4 is self-heating, its resistance decreases exponentially. For example, the resistance of the thermal resistance with negative temperature coefficient 4 can decrease from its nominal value of about 5 O to a minimum value of about 0.7 O during the time that the relay 2 is energized. The thermal resistances 3 and 4 are not in thermal contact. Referring to Figure 4 (c), a demagnetizing current waveform 6 of the automatic demagnetization circuit 10, with source 1 providing a voltage of approximately 240 V ^ g, does not decay too quickly, thereby avoiding the problem of magnetization associated with a rapid excessive decay of the demagnetization current. Referring now to Figures 1 and 4 (c), when relay 2 is energized, source 1 initially observes a direct current impedance of 20 O and a demagnetizing current begins to flow. The thermal resistances 3 and 4 they self-warm and their resistance begins to change accordingly. The resistance of the thermal resistance with negative temperature coefficient 4 decreases rapidly, while the resistance of the thermal resistance with positive temperature coefficient 3 increases only slightly. The net result is that the demagnetization current begins to increase because the direct current impedance of the automatic demagnetization circuit 10 decreases-below 20 O. As the demagnetizing current waveform 6 reaches a peak peak amplitude a peak, the thermal resistance with negative temperature coefficient 4 continues to decrease in resistance, but the Curie temperature of the thermal resistance is reached with positive temperature coefficient 3. The resistance of the thermal resistance with positive temperature coefficient 3 then increases rapidly, as it is seen by the monotonically decreasing peak-to-peak amplitudes of the demagnetizing current waveform 6 which follows its maximum peak-to-peak amplitude. A comparison of Figure 4 (c) with Figure 4 (a), showing a demagnetizing current waveform 7 for the single thermal resistance demagnetization circuit with positive temperature coefficient for a distribution network voltage of about 240 V ^ g of the prior art, reveals that the automatic demagnetization circuit 10 delays the setting of the maximum peak-to-peak amplitude of the demagnetization current. This delayed setting of the maximum peak-to-peak amplitude of the demagnetization current is also seen for a distribution network voltage of approximately 120 VR S »by comparing Figures 2 (c) and 2 (a).

For example, with a distribution network voltage of approximately 240 V ^ g, the maximum peak-to-peak amplitude of the demagnetization current waveform 6, which is associated with the automatic demagnetization circuit 10 and is shown in the Figure 4 (c), it does not occur until the second cycle of the waveform of the demagnetization current 6, while the maximum peak-to-peak amplitude of the demagnetization current waveform 67, which is associated with the demagnetization circuit. demagnetization of single thermal resistance with positive temperature coefficient and shown in Figure 4 (a), occurs within the first cycle of the demagnetizing current waveform 7. Thus the demagnetization circuit 10 does not generate significant residual magnetization in a distribution network voltage of approximately 240 V ^ g, unlike the prior art, the single thermal resistance demagnetizer circuit with positive temperature coefficient, due to the The demagnetizing current waveform 6 does not decay too quickly. Referring now to Figures 2 (c) and 3 (c), a demagnetizing current waveform 8 of the automatic demagnetization circuit 10, with the source l providing a voltage of approximately 120 V ^ g, decays from a value residual approximately 160 mA peak-to-peak just before the time relay 2 is de-energized.

Referring now to Figures 1, 2 (c) and 3 (c), when relay 2 is energized, source 1 again initially sees a direct current impedance of 20 O. Again, as a demagnetization current begins to flow, the resistance of the thermal resistance with positive temperature coefficient 3 increases only slightly, while the resistance of the thermal resistance with negative temperature coefficient 4 decreases more rapidly. The net result is that the direct current impedance of the automatic demagnetizing circuit 10 decreases below 20 O and the demagnetizing current increases. As the demagnetization waveform 8 reaches its maximum peak-to-peak amplitude, the thermal resistance with negative temperature coefficient 4 continues to decrease in resistance, but the Curie temperature of the thermal resistance with positive temperature coefficient is reached. the thermal resistance with positive temperature coefficient 3 then increases rapidly, as seen in Figure 2 (c) by the monotonically decreasing peak-to-peak amplitudes of the demagnetizing current waveform 8 following its maximum peak-to-peak amplitude. A demagnetizing current waveform 9, which is shown in Figures 2 (b) and 3 (b), is associated with the dual thermal resistance demagnetizing circuit with positive temperature coefficient of the prior art for a voltage of distribution network of approximately 120 VRMS- A comparison of Figure 3 (c) with Figure 3 (b), showing a peak-to-peak residual amplitude of waveform 9, reveals that, for a distribution network voltage of approximately 120 Vj ^ g, the peak-to-peak amplitude of the demagnetization current is lower in the automatic demagnetization circuit 10 than it is in the demagnetization circuit of the dual thermal resistance with positive temperature coefficient. For example, the peak-to-peak residual amplitude of the demagnetizing current waveform 8, shown in Figure 3 (c), is equal to about 160 mA, while the peak-to-peak residual amplitude of the waveform of demagnetization current 9, shown in Figure 3 (), is equal to approximately 400 mA. Thus, the automatic demagnetizing circuit 10 does not generate a significant residual magnetization at a low distribution network voltage as approximately 120 V ^ g, unlike the prior art, the dual thermal resistance demagnetizing circuit with positive temperature coefficient, because the peak-to-peak residual amplitude of the waveform of the demagnetizing current 8, when the demagnetizing relay 2 is de-energized, is not too large. The automatic demagnetisation circuit 10 has been shown to combine the advantages of both the single thermal resistance demagnetization circuit with positive and dual temperature coefficient with positive temperature coefficient, at the same time, overcoming the limitations associated with the thermal resistance demagnetization circuit. single with positive temperature coefficient in high distribution network voltages and with the dual thermal resistance demagnetization circuit with positive temperature coefficient at low distribution network voltages. As the thermal resistances 3 and 4 are self-heating, the resistance of the thermal resistance with positive temperature coefficient 3 eventually decreases the resistance of the thermal resistance with negative temperature coefficient 4, so that the automatic demagnetizing circuit 10 begins to approach the single thermal resistance demagnetizing circuit with positive temperature coefficient in which the demagnetizing circuit 10 starts to look like a thermal resistance with unique positive temperature coefficient in series with the demagnetizing coil 5. At the same time, the automatic demagnetizing circuit 10 can accommodate a voltage level of 240 V ^ ga from source 1 without causing other magnetization problems because the demagnetizing circuit 10 is initially seen as the dual thermal resistance demagnetizing circuit with positive temperature coefficient because source 1 observes an impedance of corrie Direct connection of 20 O when relay 2 closes.

Claims (6)

1. A demagnetization circuit for a cathode ray tube, comprising: a switching element (2) coupled with a source (1) of alternating voltage potential; a first temperature sensitive device (3) coupled in series with the switching element; and, a demagnetizing coil (5) having first and second terminals, said second terminal coupled to the alternating voltage potential source, - characterized by: a second temperature sensitive device (4) coupled from the first device temperature sensitive to the first terminal of the demagnetizing coil, the second temperature sensitive device responding to changes in temperature in a manner opposite to the first temperature sensitive device.

2. The demagnetizing circuit of claim 1, characterized in that a maximum peak-to-peak value of a demagnetization current (6) flowing through the demagnetization coil (5) during a first cycle of the current of demagnetization

3. The demagnetizing circuit of claim 1, wherein the switching element (2) is characterized by a relay.

4. The demagnetization circuit of claim 1, wherein the first (3) and the second (4) temperature sensitive devices are characterized by resistors.

5. The demagnetizing circuit of claim 4, wherein the first (3) temperature sensitive device is characterized by a positive temperature coefficient. The demagnetization circuit of claim 5, wherein the second (4) temperature sensitive device is characterized by a negative temperature coefficient. REST-fMTÜTJ An automatic demagnetization circuit (10) provides a video display device with demagnetizing capability over a wide range of AC distribution network sources. An alternating current distribution network source (1) is coupled to a demagnetization coil (5) by a series interconnection of a first (3) and a second (4) resistors sensitive to the temperature. The temperature-sensitive resistors have temperature coefficients of opposite tendencies, so that a total direct current impedance of the automatic demagnetization circuit initially decreases after energy is applied to the circuit. * * * * *