JPH07212672A - Automatic luminance limit circuit - Google Patents

Abstract

PURPOSE:To provide an automatic beam current limit circuit preventing deterioration and destruction of a CRT by selecting a limit of a beam current flowing to the CRT depending on a screen size applied to, e.g. a television display device. CONSTITUTION:The circuit is made up of a means converting a vertical deflection current into a voltage, a means 106 converting the amplitude of the voltage into a DC voltage, a means 109 detecting a beam current flowing to a CRT 101 and converting the current into a voltage, a means 107 comparing a DC voltage converted from the amplitude of the vertical deflection current with the voltage converted from the beam current flowing to the CRT and providing an output of a discrimination signal, and a means 111 controlling brightness/ contrast of a video signal with the discrimination signal to limit the beam current flowing to the CRT 101.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic brightness limiting circuit in a video display device using a CRT.

[0002]

2. Description of the Related Art In recent years, it has been desired to improve the brightness of a video display device using a CRT, and in particular, a projection CRT is required.
In the image display device, it is required to flow near the beam current which is the limit of CRT.

However, if an excessive beam current flows in the CRT beyond the limit beam current, not only the life of the CRT is shortened but also the temperature of the tube surface is increased due to an increase in the beam current density, which causes damage to the CRT. Circuit to detect the brightness and prevent the brightness from rising above a certain level,
That is, an automatic brightness limiting circuit (hereinafter referred to as an ABL circuit) is introduced.

The operation of the conventional ABL circuit will be described below. FIG. 2 is a block diagram of a conventional ABL circuit. Figure 2
In, the beam current flowing through the CRT 201 is determined by the voltage applied to the cathode and the grid 203, and flows from the high voltage generation circuit 204 through the beam current value detection circuit 205. At this time, the beam current value detection circuit 205 detects the beam current value, and the preset CR
When the maximum beam current value that can be passed to T201 is exceeded, 2
A cathode of the CRT 201 and a grid 20 for controlling the brightness of the video signal by sending a control signal to the video signal control circuit of 06.
The voltage applied to 3 is deepened. For example, the brightness / contrast of the video signal is lowered and the beam current flowing through the CRT 201 is reduced to protect the CRT 201.

FIG. 3 shows an example of the internal configuration of the beam current value detection circuit 205. The beam current flowing through the CRT 201 is represented by the current source 304. In FIG. 3, the resistor 301,
The voltage at the connection point of 302, that is, the detection voltage 305 is the CRT.
Since it is a linear function of the beam current flowing through the CRT 201, that is, the value of the current source 304, the limit beam current value that can be passed through the CRT 201 can be defined. The positive power supply voltage Vcc306 and the diode 303 detect the detection voltage 30 even if the beam current becomes small.
5 works to prevent it from becoming too high.

[0006]

However, in such a conventional configuration, when the deflection size changes, that is, when the aspect ratio changes from 4: 3 to 16: 9, for example, the amount of beam per unit area of the CRT 201. That is, although the beam current density changes, the CR of the beam current value detection circuit
Since the setting of the limit beam current value that can be passed to T201 is uniquely determined, when the screen size is changed, the beam current flows beyond the limit beam current density, the phosphor of the CRT 201 deteriorates, and the life of the CRT 201 is shortened. There was a problem that the increase in the current density would also lead to damage due to the temperature rise of the tube surface.

In order to solve the above-mentioned problems, it is an object of the present invention to provide an ABL circuit in which the set value of the limit beam current value changes when the deflection size changes.

[0008]

In order to achieve this object, the ABL circuit of the present invention has a means for converting a vertical deflection current into a voltage, a means for converting an amplitude of the voltage into a DC voltage, and a CRT. A means for detecting the beam current and converting it into a voltage, a DC voltage obtained by converting the vertical deflection current, and the C
It is provided with means for comparing the voltage obtained by converting the beam current flowing through the RT and outputting a discrimination signal, and means for controlling the brightness / contrast of the video signal by the discrimination signal to limit the beam current flowing through the CRT.

[0009]

According to the present invention, with the above-described structure, a circuit for converting the amplitude of the vertical deflection current into a DC voltage and a circuit for detecting the beam current flowing in the CRT and converting it into a detection voltage are compared with the above voltages to generate a discrimination signal. By providing the output circuit, the size of the screen changes and the beam current value that can be passed to the CRT also changes when the amount of beams per unit area of the CRT, that is, the beam current density changes, and the deterioration and destruction of the CRT are prevented. Is something that can be done.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. FIG. 1 is an AB showing an embodiment of the present invention.
It is a block diagram of an L circuit.

In FIG. 1, 101 is a CRT, 102 is an anode of the CRT, 103 is a deflection yoke, and 104 is CR.
T cathode and grid, 105 vertical deflection circuit, 1
06 is a current-voltage conversion circuit, 107 is an integration circuit, 108
Is a high voltage circuit, 109 is a beam current value detection circuit, 110 is a comparison circuit, and 111 is a video signal control circuit.

The operation of the ABL circuit configured as above will be described. The vertical deflection circuit 105 supplies a vertical deflection current to the deflection yoke 103. The vertical deflection current is converted into a voltage value by the current-voltage conversion circuit 106. This voltage value is integrated in the integration circuit 107, and the amplitude of the vertical deflection current, that is, a DC voltage according to the vertical screen size is output. The beam current flowing through the CRT 101 is determined by the video signal applied to the cathode and the grid 104. CRT1
The beam current value flowing through 01 is the beam current detection circuit 109.
Is detected and the detected voltage is output.

In the comparison circuit 110, the DC voltage corresponding to the vertical screen size is compared with the detection voltage detected by the beam current detection circuit 109, and the determination signal corresponding thereto is output and input to the video signal control circuit 111. Changes the brightness / contrast of the signal. Comparison circuit 1
The operation of the beam current detection circuit 1 will be described below.
If the beam current value from 09 is smaller than the limit beam current of the CRT 101, and the direct-current voltage of the vertical screen size from the integrating circuit 107 is large, that is, if the amplitude of the vertical deflection current is large, the comparison circuit 110 compares and determines the determination signal. Outputs High (defined as a normal state).

Next, when the vertical screen size DC voltage from the integrating circuit 107 becomes small, the discrimination signal outputs Low (defined as an abnormal state), and the video signal control circuit 111 lowers the brightness / contrast of the video signal. C
The value of the beam current flowing through the RT 101 is reduced.

In this way, the vertical deflection size becomes small,
Even if the beam amount per unit area increases, AB
Since the L operation is performed, the deterioration or destruction of the CRT 101 can be prevented in advance.

[0016]

As is apparent from the above embodiments, according to the present invention, there are provided a circuit for converting the amplitude value of the vertical deflection current into a DC voltage and a circuit for detecting the beam current flowing in the CRT and outputting the detected voltage. By providing a comparison circuit that compares them and outputs a determination signal, the size of the screen changes and CR
When the amount of beams per unit area of T, that is, the beam current density changes, the limit beam current value that can be passed through the CRT also changes, and it is possible to provide an ABL circuit that can prevent deterioration and destruction of the CRT.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an ABL circuit showing an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional ABL circuit.

FIG. 3 is a circuit diagram showing an example of a conventional beam current detection circuit.

[Explanation of symbols]

 Reference Signs List 101 CRT 102 CRT anode 103 Deflection yoke 104 CRT cathode and grid 105 Vertical deflection circuit 106 Current-voltage conversion circuit 107 Integration circuit 108 High-voltage circuit 109 Beam current detection circuit 110 Comparison circuit 111 Video signal control circuit

Claims (1)

[Claims] 1. A means for converting a vertical deflection current into a voltage, a means for converting an amplitude of the voltage into a DC voltage, a means for detecting a beam current flowing in a CRT and converting the beam current into a voltage, and a means for converting the vertical deflection current. The present invention comprises means for comparing a DC voltage with a voltage obtained by converting the beam current flowing through the CRT and outputting a discrimination signal, and means for controlling the brightness / contrast of the video signal by the discrimination signal to limit the beam current flowing through the CRT. Automatic brightness limiting circuit.