JPH0117890Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a protection circuit for a cathode ray tube, and more particularly to a protection circuit for protecting the fluorescent surface from burn-in when horizontal deflection is stopped.

Cathode ray tubes, which are generally used in video projectors, are constructed so that the electron beam is directed directly onto the fluorescent surface without using a shadow mask, and the beam current is extremely high in order to increase brightness. . Therefore, in a video projector using such a cathode ray tube, if the operation of the deflection system is stopped for some reason, the fluorescent surface will be burned out in an instant. For this reason, video projectors are provided with a cathode ray tube protection circuit that instantly stops the operation of the high voltage generation circuit when the horizontal deflection stops.

FIG. 1 is a circuit diagram showing an example of a conventionally commonly used cathode ray tube protection circuit. In the figure, reference numeral 1 denotes a transformer for horizontal deflection current detection, and its primary coil 1a is connected to the horizontal deflection current path, that is, between the horizontal deflection coil and the ground. 1b is a secondary coil of the transformer 1, one end of which is connected to ground, and the other end connected to a diode 2 for peak rectification. 3 is a capacitor that smoothes the rectified output of diode 2, and 4 is a high voltage stop that is activated when the level of the smoothed output from capacitor 3 drops beyond a predetermined reference level to stop the operation of a high voltage generation circuit (not shown). It is a control circuit.

In the cathode ray tube protection circuit configured in this way, when the horizontal deflection current HI flows through the primary coil 1a of the transformer 1, a pulse-like bipolar output is generated in the secondary coil 1b at the sudden change portion. Ru. This output signal is subjected to peak rectification in the diode 2, so that a positive pulse-shaped output signal is generated at the horizontal deflection period. The pulse signal of the horizontal deflection period thus extracted is smoothed by the capacitor 3 and supplied to the high voltage stop control circuit 4 as a constant level DC signal. The high voltage stop control circuit 4 constantly monitors the level of the smoothed output from the capacitor 3. If the horizontal deflection is stopped for some reason, the level of the smoothed output will suddenly decrease.
By detecting this and generating a control signal A, the operation of a high voltage generating circuit (not shown) is abruptly stopped, thereby preventing the fluorescent surface of the cathode ray tube from burning out.

However, in the circuit configured as described above, the operating speed cannot be increased below this horizontal period because the output signal of the diode 2, which is in the form of a pulse with a horizontal deflection period, is smoothed.

In contrast, in recent years, there has been a demand for video projectors that have larger projection surfaces and can produce brighter images, and along with this, the beam current of cathode ray tubes has also increased. It is becoming. Therefore, as in the conventional cathode ray tube protection circuit described above, if the detection operation is delayed for more than the horizontal deflection period, there is a problem in that the cathode ray tube cannot be reliably protected.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a cathode ray tube protection circuit that can detect cessation of horizontal deflection more quickly. Hereinafter, a detailed explanation will be given using the drawings.

FIG. 2 is a circuit diagram showing an embodiment of the cathode ray tube protection circuit according to the present invention, and the same parts as in FIG. 1 are indicated using the same symbols. In the same figure, 1c
is the primary coil of the transformer 1, and in particular, the middle point is connected to ground. A diode 5 is connected between the capacitor 3 and the end of the secondary coil 1c to which the diode 2 is not connected. That is, in the circuit shown in FIG. 2, positive and negative pulses are generated in the secondary coil 1c of the transformer 1, and these pulses are taken out via the diodes 2 and 5.
The positive and negative pulses generated simultaneously from the diode 2.5 are combined in the capacitor 3. In this case, when the positive polarity pulse shown in FIG. 3a and the negative polarity pulse shown in FIG. It is a signal of the state. Therefore, the capacitor 3 only needs to absorb small changes that occur during synthesis, and as a result, the time constant can be significantly reduced, and the detection speed is also faster than before. This will greatly speed up the process. Therefore, the horizontal deflection stopping operation can be quickly detected, and the cathode ray tube can be reliably protected accordingly.

As explained above, the cathode ray tube protection circuit according to the present invention detects the horizontal deflection current as positive and negative polarity pulses through transformer coupling, and combines and smoothes both pulses. , the smoothing efficiency has become extremely high, and as a result, the value of the smoothing capacitor can be made sufficiently smaller than in the past, making it possible to detect the stop of horizontal deflection more quickly and reliably. Has excellent effects.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an example of a conventional cathode ray tube protection circuit, Fig. 2 is a circuit diagram showing an embodiment of the cathode ray tube protection circuit according to the present invention, and Figs. 3 a to c are the circuits shown in Fig. 2. FIG. 1...Transformer, 1a...Primary coil, 1c...
...Secondary coil, 2, 5...Diode, 3...Capacitor, 4...High voltage stop control circuit.

Claims (1)

[Scope of utility model registration request] A transformer that supplies a horizontal deflection current to the primary coil, a pair of diodes that rectify the output generated in the secondary coil of this transformer, take out and combine the positive and negative outputs, and both of them. A small-capacity capacitor that smooths the composite signal on each output side of the diode, and a high-voltage control that detects when the smoothed output of this capacitor has decreased beyond a reference value and cuts off the application of high voltage to the cathode ray tube. A cathode ray tube protection circuit comprising a circuit.