JPS639713B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a high voltage limiting circuit for preventing X-ray leakage in a television receiver. In a television picture tube, X-rays are emitted by striking a material with an electron beam at a high acceleration voltage. International safety standards set the allowable amount of X-ray leakage at a distance of 5 cm from each side of the receiver's cabinet to less than 0.5 milliroentgen per hour. The amount of leaked X-rays increases rapidly as the accelerating voltage of the electron beam increases, so in order to ensure safety against X-ray leakage, if the anode voltage of the picture tube exceeds a certain limit, high-voltage restriction must be applied. It is necessary to operate the circuit to limit the anode voltage. The present invention relates to such a high voltage limiting circuit. FIG. 1 is a circuit diagram showing a conventional high voltage limiting circuit. In the figure, a circuit configuration in which a rectifier diode 3 and a smoothing capacitor 4 are connected to a high voltage detection winding 2 wound around a flyback transformer 1 allows an anode voltage V ₁ to be supplied from the flyback transformer 1 to a cathode ray tube, which is a television picture tube. Obtain a high voltage detection voltage V ₂ proportional to . By connecting the midpoint of this voltage V ₂ divided by resistors 5 and 6 to the input terminal 8 of the high voltage limiting circuit control section 12 connected to the horizontal oscillation circuit 7 through a Zener diode 9, the anode voltage V ₁ can be set to a certain value. When this happens, the Zener diode 9 is made conductive, and the high voltage limiting circuit control section 12 is operated to stop the oscillation operation of the horizontal oscillation circuit 7, thereby stopping high voltage generation. However, in such a conventional circuit, the terminal 1
In the case of a failure mode in which the DC voltage (hereinafter referred to as +B voltage) _V3 supplied from 0 to the horizontal deflection circuit via the winding of the flyback transformer 1 becomes high, it operates normally, but the horizontal resonance of the horizontal output circuit Capacity 1
In the case of a failure mode in which transformer 1 opens, the tuning conditions of flyback transformer 1 change and V ₂ /V ₁
voltage ratio is lower than normal. What happens in such a case will be explained next. Figure 2 is a graph showing the cathode ray tube anode voltage on the vertical axis and the cathode ray tube anode current on the horizontal axis, and 14 is an X-ray limit curve, that is, when the anode voltage increases beyond this characteristic curve 14, X-ray leakage increases. The curve 15 indicating exceeding the permissible limit is a characteristic curve of the cathode ray tube anode voltage during normal operation. 1
3 is a curve in which when the cathode ray tube anode voltage increases beyond this curve, the high voltage limiting circuit control section 12 operates to stop the operation of the horizontal oscillation circuit 7 in the circuit shown in FIG. When the failure mode is such that the +B voltage _V3 increases,
Since the control unit 12 operates along the characteristic curve 13, it is possible to achieve the objective of suppressing X-ray leakage below the permissible limit. However, it has been found that in a failure mode in which the horizontal resonant capacitor 11 is open, the control section 12 operates along the characteristic curve 16 for the reasons described above. curve 16
As is clear from the figure, while the curve 13 is set between the X-ray limit curve 14 and the normal cathode ray tube voltage characteristic curve 15, there is a portion that exceeds the X-ray limit curve 14, and this portion exceeds the X-ray limit curve 14. In this case, X-ray leakage cannot be suppressed below the permissible limit. In other words, the conventional high-voltage limiting circuit shown in Figure 1 may not function in accordance with its intended purpose of suppressing X-ray leakage below the permissible limit when the failure mode is that the horizontal resonant capacitor opens. , a big problem arose. This invention was made in order to solve the problems of the conventional high voltage limiting circuit as described above.
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a high voltage limiting circuit for a television receiver that can function reliably in accordance with the intended purpose of preventing leakage of X-rays even when the horizontal resonant capacitor is open. The main point of the configuration of the present invention is that in a conventional high voltage limiting circuit, a voltage proportional to the DC voltage and a voltage proportional to the anode voltage supplied to the cathode ray tube are supplied from a DC voltage supply circuit that supplies DC voltage to the horizontal deflection circuit. These two voltages are normally applied as reverse bias, and when the horizontal resonant capacitor connected to the DC voltage supply circuit is in an open state,
The present invention is characterized in that a switching means is provided which performs switching by changing the above-mentioned voltages to forward bias, and the high voltage limiting circuit is configured to operate by the switching as well. Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 3 is a circuit diagram showing an embodiment of the present invention, and a portion S surrounded by a broken line is a circuit provided according to the present invention. Refer to the same figure. +B voltage V ₃ is divided by resistors 17 and 18, and high voltage detection voltage V ₂ is divided by resistor 1
Divide by 9,20. Then, an NPN transistor 22 is connected to the ground through each divided point and the resistor 21 as a switching element as shown in FIG. Connect to. Generally the anode voltage V ₁ is

[Formula] (C _p is mainly the horizontal resonant capacitance, L is the composite inductance such as the horizontal inductance of the deflection yoke and the primary inductance of the flyback transformer), and V ₁ /V ₃ is

Proportional to [formula]. Further, since V ₂ /V ₃ are almost the same, the following equation holds true. V ₂ /V ₃ (when horizontal resonant capacitor 11 is normal) <
V ₂ /V ₃ (when the horizontal resonant capacitor 11 is open) Here, using this property, in the circuit shown in _FIG. becomes reverse biased and when horizontal resonant capacitor 11 is open, bias V _BE
resistors 17, 18, 1 so that
9 and 20 can be set. As a result, under normal conditions, the high voltage limiting circuit control section 12 does not operate.
When the horizontal resonant capacitor 11 becomes open, the switching transistor 22 becomes conductive, and a voltage is generated at the input terminal 8 of the high voltage limiting circuit control section.
The high voltage limiting circuit control section 12 operates. In this embodiment, the detection voltage of the same winding as in the conventional high voltage limiting circuit is used as the high voltage detection voltage, but the same effect can be achieved even if the voltage of the winding of another flyback transformer is used. Furthermore, the same effect can be obtained by using, in addition to the +B voltage, a voltage obtained by rectifying the AC voltage input to the television receiver as the voltage to be compared with the above voltage. With the above configuration, according to the present invention, even when the horizontal resonant capacitor is open, the high voltage limiting circuit can be operated reliably, and the X This has the advantage that safety against line leakage can be established.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional high voltage limiting circuit;
Fig. 2 is a characteristic diagram showing an X-ray limit curve showing the permissible limit of leakage of X-rays, a characteristic curve of the cathode ray tube anode voltage during normal operation, an operating characteristic curve of the high voltage limiting circuit control section, etc., and Fig. 3 is a characteristic diagram showing the present invention. FIG. 2 is a circuit diagram showing an example of the above. In the figure, 1 is a flyback transformer, 2 is a high voltage detection winding, 3 is a rectifier diode, 4 is a smoothing capacitor, 5 and 6 are respective voltage dividing resistors, 7 is a horizontal oscillation circuit, and 8 is an input terminal for the high voltage limiting circuit control section. , 9
is a Zener diode, 10 is a DC voltage input terminal,
11 is a horizontal resonant capacitor, 12 is a high-voltage limiting circuit control section, 13 and 16 are operating characteristic curves of the high-voltage limiting circuit control section, 14 is an X-ray limit curve, 15 is an anode voltage characteristic curve of a cathode ray tube during normal operation, 17-2
0 indicates a voltage dividing resistor, 21 indicates a high voltage limit detection resistor, and 22 indicates a switching transistor.

Claims (1)

[Claims] 1. A high-voltage limiting circuit for a television receiver that extracts and monitors a first voltage proportional to the anode voltage from a flyback transformer that outputs the anode voltage of the television picture tube, and operates when the anode voltage exceeds a certain voltage. , a second voltage proportional to the DC voltage is obtained from a DC voltage supply circuit that supplies DC voltage to the horizontal deflection circuit, and a third voltage proportional to the anode voltage is obtained, and the second voltage is always connected to the horizontal deflection circuit. and third
When both voltages are applied as a reverse bias and the horizontal resonant capacitor connected to the DC voltage supply circuit is in an open state, switching is performed by changing both the second and third voltages to a forward bias. 1. A high-voltage limiting circuit for a television receiver, characterized in that it is provided with switching means and is operated by the switching.