JPH09247576A - Automatic brightness limiting circuit - Google Patents

Abstract

It is an object of the present invention to provide an automatic brightness limiting circuit that can guarantee the cathode life of a CRT display more than ever before. SOLUTION: First brightness limiting means 14 for limiting brightness by detecting an anode current of a CRT display 4;
The second brightness limiting means 19 detects the cathode current of the CRT display 4 for each color signal, and also has a second brightness limiting means 19 for limiting the brightness by detecting the cathode current of the RT display 4. By configuring so as to limit the brightness, the cathode current can be suppressed even if the cathode current of a certain color is large, and the cathode life of the CRT display 4 can be guaranteed.

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 CRT display.

[0002]

2. Description of the Related Art In an automatic brightness limiting circuit (ABL), if the brightness becomes abnormally large, a large anode current may flow into the CRT display, which may overload the high voltage circuit and destroy the horizontal output transistor. In order to prevent this, it is provided in a television receiver and various display devices.

An automatic luminance limiting circuit having the contents shown in FIG. 3 is shown in "NHK color television textbook [above], Japan Broadcast Publishing Association". The R, G and B signals input to the video signal input terminal 1 are supplied to the preamplifier 2 and the drive amplifier 3
The R, G and B signals are input to the cathode 5 for each color signal of the CRT display 4 after being amplified via.

A flyback transformer 6 generates an anode voltage applied to the anode 7 of the CRT display 4. The anode current detector 8 detects the anode current flowing into the flyback transformer 6. Operational amplifier 9
The output of the anode current detector 8 is connected to the non-inverting input (+) and compared with the set voltage 10 connected to the inverting input (-). The cathode of the diode 11 is connected to the output of the operational amplifier 9, and the anode thereof is connected to the contrast control input terminal of the preamplifier 2.

A contrast control input terminal 12 receives a contrast control DAC signal. 13 is a resistor,
It is connected between the contrast control input terminal 12 and the anode of the diode 11.

FIG. 4 shows the relationship between the anode current and the gain of the preamplifier 2 corresponding to the block diagram of FIG. When the anode current is smaller than I ₁ , the anode current detector 8
Is larger than the set voltage 10, the output of the operational amplifier 9 becomes "H" level, and the diode 11 is turned off and disconnected.

Therefore, the gain of the preamplifier 2 becomes G ₁ which is determined by the contrast control DAC signal from the contrast control input terminal 12. When the anode current becomes larger than I ₁ , the output of the anode current detector 8 becomes smaller than the set voltage 10 and the output of the operational amplifier 9 drops.
Then, the diode 11 is turned on, the contrast control DAC signal input to the preamplifier 2 through the resistor 13 is lowered, and the gain of the preamplifier 2 is reduced according to the anode current.

In this way, the CRT display 4
Brightness limiting means for limiting brightness by detecting a charging current
14 is provided and the threshold of the anode current for operating the control loop
Value I ₁ By setting the set voltage 10 corresponding to
Threshold I₁ For the above anode current, the preamplifier 2
The control loop works to reduce the gain,
Of the R, G, B signals applied to the cathode 5 of the spray 4
Operates to suppress amplitude and limit brightness.

[0009]

However, in the above configuration, since the anode current detector 8 detects the total cathode current of the R, G, B signals as the anode current, the R, G, B signals are detected. If the cathode current of one of the two colors is large but the cathode current of the other two colors is small, the anode current becomes smaller than the threshold value I ₁ , and the automatic brightness limiting control loop does not operate.

Therefore, for a color having a large cathode current, there is a problem that the cathode current of the CRT display 4 is not limited and the cathode life of the CRT display 4 is shortened.

An object of the present invention is to provide an automatic brightness limiting circuit which can guarantee the life of the cathode of a CRT display more than ever before.

[0012]

The automatic brightness limiting circuit of the present invention comprises a first brightness limiting means for limiting the brightness by detecting the anode current of the CRT display, and a second brightness limiting means for limiting the brightness by detecting the cathode current of the CRT display. It is also characterized in that the second brightness limiting means detects the cathode current of the CRT display for each color signal by the second brightness limiting means so as to limit the brightness. Even if it is large, it suppresses the cathode current.

[0013]

FIG. 1 is a block diagram showing an embodiment of the present invention.
Will be described with reference to FIG. It should be noted that components having the same functions as those of the conventional example shown in FIG.

In FIG. 1, the R, G, B signals input to the video signal input terminal 1 are amplified via the preamplifier 2 and the drive amplifier 3, and the respective color signals of the CRT display 4 via the cathode current detecting section 15. R, G, B signals are input to the cathode 5 of each.

The cathode current detector 15 independently detects the cathode currents of the R, G and B signals, and the detection signal for each color signal detected by the cathode current detector 15 has its level for each color signal. Is determined to control the gain of the preamplifier 2.

In FIG. 1, one color signal is shown, and the detection signal for each color signal detected by the cathode current detection unit 15 is applied to the inverting input (-) of the operational amplifier 16 and the non-operation of the same operational amplifier 16 is omitted. Compared with the set voltage 17 applied to the inverting input (+), the output of the operational amplifier 16 is connected to the gain control line of the preamplifier 2 via the diode 18. The levels of the cathode current detection signals of the remaining two color signals (not shown) are similarly determined and connected to the gain control line of the preamplifier 2.

The gain control line of the preamplifier 2 is composed of a cathode current detector 15, an operational amplifier 16, a diode 18, etc., and detects the cathode current of the CRT display 4 for each color signal to limit the brightness. In addition to the second brightness limiting means 19 of FIG. 3, a first brightness limiting means 14 which is the same as the automatic brightness control circuit of the conventional example shown in FIG. 3 is provided, and the diode 11 and the diode 18 form the gain of the preamplifier 2. Is connected to the control line (OR).

With this configuration, the operation is as follows. FIG. 2 is a diagram showing the relationship between the anode current and the cathode current and the gain of the preamplifier 2 corresponding to the block diagram of FIG.

The characteristic a shown in FIG. 2 is the first brightness limiting means 1
4 shows the characteristics of the control loop according to No. 4, the horizontal axis is the anode current, and the threshold value I ₁ of the anode current is the first set voltage 1
Set by 0.

The characteristic b shown in FIG. 2 is the second brightness limiting means 1
9 shows the characteristics of the control loop of No. 9, the horizontal axis is R, G, B
With the cathode current of the signal alone, the threshold value I ₂ of the cathode current is set by the second set voltage 17.

The anode current is less than or equal to the threshold value I ₁ , and
When each cathode current of the R, G, B signals is greater than or equal to the threshold value I ₂ ,
Since the anode current of the first brightness limiting means 14 is less than or equal to the threshold value I ₁ , the diode 11 is turned off and disconnected, and
The brightness limiting means 14 does not control the gain of the preamplifier 2. However, since the cathode current is greater than or equal to the threshold value I ₂ , the output of the cathode current detection unit 15 becomes larger than the set voltage 17 and the output of the operational amplifier 16 decreases. Then, the diode 18 is turned on, the contrast control DAC signal input to the preamplifier 2 through the resistor 13 is lowered, and the gain of the preamplifier 2 is reduced according to the cathode current.

The second brightness limiting means 19 is prepared for each of the R, G, B signals, and if even one of the cathode currents of the R, G, B signals becomes the threshold value I ₂ or more, the anode of the diode 18 is turned on. Since the OR (logical sum) connection is made, the control loop described above operates.

As described above, by adding the second brightness limiting means 19, to the anode (cathode) current of the threshold value I ₂ or more and I ₁ or less which cannot be controlled by the conventional first brightness limiting means 14. However, the automatic brightness can be limited, the cathode current can be suppressed, and the cathode life of the CRT display 4 can be guaranteed.

[0024]

As described above, according to the present invention, the first brightness limiting means for limiting the brightness by detecting the anode current of the CRT display and the second brightness limiting for limiting the brightness by detecting the cathode current of the CRT display. CRT with the second brightness limiting means
By configuring the display so as to detect the cathode current for each color signal and limit the brightness, the cathode current is suppressed even if the cathode current of a certain color is large, and C
The cathode life of the RT display can be guaranteed.

[Brief description of drawings]

FIG. 1 is a block diagram of an automatic brightness limiting circuit according to an embodiment of the present invention.

FIG. 2 is a gain-anode (cathode) current characteristic diagram of the present invention.

FIG. 3 is a block diagram of a conventional automatic brightness limiting circuit.

FIG. 4 is a conventional gain-anode current characteristic diagram.

[Explanation of symbols]

 1 video signal input terminal 2 preamplifier 3 drive amplifier 4 CRT display 5 cathode 6 flyback transformer 7 anode 8 anode current detector 9 operational amplifier [first operational amplifier] 10 set voltage [first set voltage] 11 diode [first Diode] 14 First Brightness Limiting Means 15 Cathode Current Detection Section 16 Operational Amplifier [Second Operational Amplifier] 17 Set Voltage [Second Setting Voltage] 18 Diode [Second Diode] 19 Second Brightness Limiting Means

Claims (3)

[Claims] 1. A first brightness limiting means for limiting the brightness by detecting the anode current of the CRT display, and a second brightness limiting means for limiting the brightness by detecting the cathode current of the CRT display. Automatic brightness limiting circuit. 2. A first brightness limiting means for limiting the brightness by detecting the anode current of the CRT display, and a second brightness limiting means for detecting the cathode current of the CRT display for each color signal to limit the brightness. An automatic brightness limiting circuit characterized by having. 3. An anode current detecting section for detecting an anode current flowing into a flyback transformer for driving a CRT display, and an output of the anode current detecting section for comparing the output of the first brightness limiting section with a first set voltage. And a first diode whose cathode is connected to the output of the first operational amplifier and whose anode is connected to the contrast control unit of the preamplifier for amplifying the video signal. , A cathode current detector for detecting the cathode current of the CRT display for each color signal, a second operational amplifier for comparing the output for each color signal of the cathode current detector with a second set voltage, and a cathode for the second Connected to the output of the operational amplifier, the anode is the first
3. An automatic brightness limiting circuit according to claim 2, wherein the automatic brightness limiting circuit comprises a second diode connected to the anode of said diode to form a logical sum.