JP2002218489A - Degaussing circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a degaussing circuit that supplies an optimum alternate attenuation current causing no rush current to a degaussing coil. SOLUTION: A solid-state relay SSR that is closed at a prescribed phase of an AC input voltage and a positive temperature coefficient thermistor Rt are connected in series with the degaussing coil. Since the SSR starts its conduction at a prescribed phase angle independently of the phase of the AC input voltage at application of a power switch SW, the transient current flowing through the degaussing coil is suppressed and the highest degaussing effect can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a degaussing circuit used for a color CRT or the like.

[0002]

2. Description of the Related Art Generally, in a color CRT used for a color video monitor device, a color television receiver, and the like, when affected by geomagnetism or an external magnetic field, an improper electron beam reaches the fluorescent screen of the color CRT. , And a position error (mislanding) at the arrival point occurs, causing color shift and color unevenness. On the other hand, a magnetic shield plate and a shadow mask or an aperture grill are incorporated in the color CRT in order to reduce the influence of the external magnetic field, and a magnetic shield member is provided outside the color CRT. If the shielding member is magnetized under the influence of an external magnetic field and remains (magnetized), it also causes color unevenness. Therefore, in order to eliminate the magnetization of the magnetic shield member, a circuit is provided for applying a damped oscillating magnetic field from outside the color CRT to demagnetize it.

A general degaussing circuit is shown in FIG.
A positive characteristic thermistor Rt is connected in series to the degaussing coil, and when the power switch SW is turned on, power is supplied to the degaussing coil via the positive characteristic thermistor Rt. Therefore, when the power switch SW is turned on,
An alternating current is applied to the degaussing coil, but the resistance value of the positive temperature coefficient thermistor Rt gradually increases due to self-heating. As a result, the current supplied to the degaussing coil is gradually attenuated, and the degaussing coil generates an alternating decaying magnetic field.

[0004]

However, in the conventional degaussing circuit as shown in FIG. 9, the waveform of the alternating decaying current flowing through the degaussing coil changes in accordance with the turning-on timing of the switch SW. That is, when the switch SW is at about 90 ° or about 270 ° in the phase of the AC voltage, an inrush current flows and the maximum change ratio of the alternating damping current increases. Here, the “maximum change ratio” refers to, for example, in the alternating decay current waveform shown in FIG. . . . Then, the smallest value of the ratio of attenuation from a certain current peak value to the next peak value, min (| In + 1 | / | In
|). The greater the maximum change ratio, the more the demagnetizing effect is obtained because the alternating decay current is attenuated more slowly.However, the smaller the value of the maximum change ratio, the sharper the attenuation. It will vary.

FIG. 10 shows the result of actual measurement under the following conditions.

[Positive characteristic thermistor] Resistance value: 18Ω Element size: φ12.3 × t2.2mm (disc) Curie point temperature: 50 ° C. [Measurement conditions] Input power supply voltage 240V AC (50Hz) Load series resistance: 18Ω Ambient temperature : 25 ° C In this example, the maximum change ratio is the largest when the input phase of the AC voltage is 0 ° or 180 °, and a good demagnetizing effect is obtained. However, when the power switch SW is turned on at 90 ° and 270 °, , The maximum change ratio is the smallest, and the demagnetizing effect is reduced. As described above, the rush waveform of the alternating decay current flowing through the degaussing coil changes depending on the input timing of the AC voltage, thereby causing variations in the degaussing ability.

In view of the above variation, it is effective to increase the element size of the positive temperature coefficient thermistor as a measure for securing sufficient demagnetizing ability. That is,
By increasing the element size of the positive temperature coefficient thermistor, its heat capacity increases, the temperature rise due to energization becomes slow, and the maximum change ratio can be reduced. However, if the element size of the positive temperature coefficient thermistor is increased, the entire degaussing circuit becomes larger, and if the element size is increased while maintaining an appropriate Curie point temperature,
Since the resistance value in the steady heat generation state is reduced, a current that constantly flows through the degaussing coil increases, and a problem such as a display image of a color CRT fluctuating occurs.

An object of the present invention is to solve the above problems,
An object of the present invention is to provide a degaussing circuit in which a small and optimal alternating decay current is supplied to a degaussing coil.

[0009]

According to the present invention, there is provided a circuit for demagnetizing an object to be degaussed by supplying an alternating decaying current to a degaussing coil adjacent to the object to be degaussed, wherein a predetermined phase of an AC input voltage is provided. And the PTC thermistor is connected in series to the degaussing coil to apply the AC input voltage.

As described above, by starting the current supply to the degaussing coil at a predetermined phase of the AC input voltage, it is possible to suppress the rush current flowing through the degaussing coil and to slow down the alternating damping current. In particular, if the phase of the AC input voltage is 0
By conducting the relay at an angle of 180 ° or 180 °, the inrush current can be sufficiently suppressed, and a high demagnetizing effect can be obtained.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of a degaussing circuit according to a first embodiment of the present invention will be described with reference to FIGS.

FIG. 1 shows two different degaussing circuits. In (A), Rt is a positive characteristic thermistor, and the positive characteristic thermistor Rt and the solid state relay SSR are connected in series to the degaussing coil. This solid-state relay is a zero-voltage switch-type relay, and starts conducting when the AC input voltage becomes almost 0V. The DC power supply circuit in the figure inputs an AC input voltage and supplies a drive voltage to a solid state relay.

FIG. 2 is a diagram showing the configuration of the solid state relay SSR shown in FIG. In FIG. 2, when a DC input signal current flows to the input terminal,
The photothyristor of the photothyristor coupler PC is turned on, and a current flows through D4-PC-D1-R4. At this time, the voltage generated at both ends of the resistor R4 causes the triac BC
A shunt current flows to the gate of R, turning on the BCR.

In FIG. 2, a zero voltage switch circuit is constituted by resistors R3 and R2 and transistor Q1. That is, when Q1 turns on, the photothyristor does not turn on. Therefore, when the voltage is almost zero, Q
1 turns off and the photothyristor PC turns on.
The CR starts conducting at the zero cross timing.

FIG. 3 shows operation waveforms of the degaussing circuit shown in FIG. (A) is an AC input voltage signal, (B),
(D) is the timing of turning on the power switch SW, (C),
(E) shows the waveform of the current supplied to the degaussing coil in that case. As shown in FIG.
When W is applied at the timing of t1, the solid state relay is turned on from the immediately after zero cross timing t2, and the energization of the degaussing coil is started. Further, for example, as shown in FIG.
When W is input, the zero-cross timing t immediately thereafter is input.
In step 4, the solid state relay is turned on to start energizing the degaussing coil.

As described above, since the demagnetizing current is always started to be supplied at the zero-cross timing of the AC input voltage regardless of the turning-on timing of the power switch SW, no inrush current occurs, and the alternating attenuation current gradually decreases. Is

FIG. 4 shows the relationship between the ON timing of the power switch SW and the maximum change ratio. The horizontal axis represents the ON timing of the switch SW by the phase angle.

The measurement conditions are as follows.

[Positive thermistor] Resistance value: 18Ω Element size: φ12.3 × t2.2mm (disc) Curie point temperature: 50 ° C. [Measurement conditions] Input power supply voltage 240V AC (50Hz) Load series resistance: 18Ω Ambient temperature : 25 ° C As described above, the maximum change ratio is high and stable regardless of the ON timing of the power switch SW.

In the example shown in FIG. 1B, another positive characteristic thermistor Rt1 is connected in parallel to a series circuit of the positive characteristic thermistor Rto and the degaussing coil. The two positive temperature coefficient thermistors Rt0 and Rt1 are thermally coupled. Therefore, the manner of attenuating the alternating decay current flowing through the degaussing coil after the power is turned on is substantially the same as that shown in FIG. 1A, but in the steady state after the degaussing operation is performed, the positive characteristic thermistor Rt1 Heats the other positive temperature coefficient thermistor Rto to keep its resistance high. As a result, the current flowing through the degaussing coil in a steady state is suppressed, and the generation of an unnecessary alternating magnetic field during periods other than the degaussing operation can be suppressed.

Next, the degaussing circuit according to the second embodiment is shown in FIG.
This will be described with reference to FIG. In FIG. 5, a zero-crossing detection circuit detects a zero-crossing of an AC input voltage signal and generates a square wave signal. The phase shift circuit shifts this signal by a predetermined phase to generate a trigger signal. The SSR is a solid state relay using a bidirectional semiconductor switch element such as a triac, and is turned on at the timing of a trigger signal.

FIG. 6 shows the above-mentioned zero-cross detection circuit and its operation waveform. G0, G1 and G2 in the figure are inverter gate circuits each formed by a C-MOS, and G0 is a gate G by feeding an inverted output signal back to the input.
1 is supplied with a zero voltage of the AC voltage signal (the threshold voltage of the C-MOS). Since the AC input voltage signal is input to the gate circuit G1 via the capacitor, the output voltage is inverted every time the AC input voltage exceeds approximately 0V.

FIG. 7 is a diagram showing a configuration of the solid state relay SSR in FIG. In FIG. 7, when a trigger signal is input to the input terminal and the input terminal, the phototriac of the phototriac coupler PC is turned on, and R2
-A current flows through PC-R3. At this time, a voltage generated at both ends of the resistor R3 generates a shunt current to the gate of the triac BCR, and the BCR is turned on.

The phase shift circuit shown in FIG. 5 outputs a trigger signal to the SSR with a predetermined time delay from the rising or falling timing of the zero-cross detection signal. Further, the phase shift circuit includes a time constant circuit including a variable resistor, for example, and the amount of phase shift can be adjusted by the value of the variable resistor. This makes it possible to set so as to start energizing the degaussing coil at an optimal timing. Normally, as shown in the first embodiment, it is optimal to start energizing the degaussing coil at the timing of the zero crossing of the AC input voltage signal. Since the AC voltage is applied to the RL series circuit including the resistor R including the component and the inductance L of the degaussing coil, the magnitude of the transient term of the current flowing through the degaussing coil differs depending on the phase when the AC voltage is applied. That is, the delay φ of the current phase in the steady state when the AC voltage is applied to the RL circuit is tan ⁻¹ ωL / R, and the transient term when the phase θ when the AC voltage is applied is θ = φ Becomes 0. Incidentally, when θ = φ + π / 2, the transient term becomes the maximum, and a large transient current flows. Therefore, the phase shift circuit may be determined so as to output the trigger signal with a delay of a phase angle of tan ^-1 ωL / R from the zero cross timing of the AC input voltage.

[0025]

According to the present invention, the inrush current flowing through the degaussing coil can be suppressed, and the attenuation of the alternating attenuation current can be reduced. In particular, by conducting the relay when the phase of the AC input voltage is 0 ° or 180 °, the rush current can be sufficiently suppressed, and a high demagnetizing effect can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing two examples of a degaussing circuit according to a first embodiment.

FIG. 2 is a diagram showing a configuration example of a solid state relay in the same circuit.

FIG. 3 is a waveform chart showing the operation of each part of the degaussing circuit.

FIG. 4 is a diagram showing a relationship between a power-on timing and a maximum change ratio by the degaussing circuit and a conventional degaussing circuit.

FIG. 5 is a diagram showing a configuration of a degaussing circuit according to a second embodiment.

FIG. 6 is a diagram showing a configuration example of a zero-cross detection circuit in the degaussing circuit.

FIG. 7 is a diagram showing a configuration example of a solid state relay in the degaussing circuit.

FIG. 8 is an explanatory diagram of an alternating decay current flowing through a degaussing coil and a maximum change ratio thereof.

FIG. 9 is a diagram showing an example of a conventional degaussing circuit.

FIG. 10 is a diagram showing the relationship between the AC voltage input timing and the maximum change ratio by the conventional degaussing circuit.

[Explanation of symbols]

 Rt-Positive Thermistor SSR-Solid State Relay

Claims (2)

[Claims] 1. A circuit for demagnetizing an object to be degaussed by supplying an alternating decay current to a degaussing coil adjacent to the object to be degaussed, comprising: a relay that conducts at a predetermined phase of an AC input voltage; And a degaussing circuit connected in series to the degaussing coil so as to apply the AC input voltage. 2. The degaussing circuit according to claim 1, wherein the phase is set to approximately 0 ° or 180 °.