JPH05344455A - Brightness limiting circuit - Google Patents

Abstract

(57) [Summary] [Purpose] To make the circuit configuration of the brightness limiting circuit relatively simple.
It [Structure] Common base type transistors 111 to 113
The current addition circuit 105 is formed by using this
Cathode current I through path 105_KR, I_KG, I_KBAdd
And load the output load of the current adding circuit 105
Switching is performed by the switching circuit 106. Load switching circuit 1
Switching pulse P supplied to 06₂The vertical back
Low level of the porch period, including other video parts
By setting the signal to be high level at
Output signal S appearing on the load of path 105 _{twenty one}Automatic cutoff
ACO voltage signal S for_21AOr cathode current I_KR, I
_KG, I_KBComposite voltage signal S corresponding to the composite current of_21BCum on
Can be used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a brightness limiting circuit suitable for application to, for example, a projection display device.

[0002]

2. Description of the Related Art In recent years, projection type display devices that combine light emitted from three projection tubes of RGB colors to project an image on a screen with a screen of 35 inches or more have become popular. It is progressing.

In the projection tube used in such a projection display device, in order to magnify the fluorescent screen image, it is necessary to make the image 10 times brighter than a normal direct-viewing tube.

Therefore, in order to prevent burning of the phosphor due to the electron beam impinging on the phosphor, the projection type display device detects the beam current, that is, the cathode current. A brightness limiting circuit configured to automatically limit or adjust the brightness is incorporated.

FIG. 5 shows the configuration of a brightness limiting circuit according to the prior art.

In FIG. 5, reference numerals 1 to 3 respectively indicate
It is a CRT for R, G, and B. Cathode current switch circuits 7 to 9 also serving as cathode drive amplifier circuits are connected to the cathodes 4 to 6 of the CRTs 1 to 3, respectively.

Video signals R ₁ (for red) and G (for green) from the primary color reproduction / luminance correction circuit 10 are input to the input terminals 11 to 13 of the cathode current switch circuits 7 to 9, respectively. Video signal B for G ₁ and B (for blue)
₁ is supplied.

A switching pulse P ₁ is supplied from the terminal 15 to the cathode current switch circuits 7-9. The switching pulse P ₁ is, as shown in FIG. 2C,
Of the video signals R _{1 to} B ₁ , the vertical blanking period V.
This signal is high level only during the vertical back porch period 122 in BLK and is low level during the remaining period.

One of the output voltage signals S _{2 to} S ₄ of the cathode current switch circuits 7 to 9 is supplied to the combined current detection circuit 16 and the single tube maximum current detection circuit 17.

The other output voltage signals S _{5 to} S ₇ of the cathode current switch circuits 7 to 9 are sent to automatic cutoff current detection circuits (hereinafter referred to as ACO (Auto Cut Off) current detection circuits) 21 to 23. Supplied.

[0011] The output signal S ₁₀ of the output signal S ₉ and single tube maximum current detection circuit 17 of the composite current detection circuit 16, adder circuit 2
5 is added and supplied to the primary color reproduction / luminance correction circuit 10 as a luminance detection signal S ₁₇ .

The output signals S _{11 to} S ₁₃ of the ACO current detection circuits 21 to 23 are supplied to the primary color reproduction / luminance correction circuit 10 through terminals 31 to 33, respectively.

A luminance signal Y, a color difference signal (BY) and a color difference signal (RY) are supplied to an input terminal of the primary color reproduction / luminance correction circuit 10 from a color demodulation circuit (not shown) through terminals 35 to 37. ing.

Each of the cathode current switch circuits 7 to 9 has the same circuit connection configuration and has resistors 41 to 45, transistors 46 to 50, and diodes 51 and 52. It should be noted that, of the cathode current switch circuits 7 to 9, the cathode current switch circuits 8 and 9 are not attached with the reference numerals in order to avoid complexity.

In the cathode current switch circuits 7 to 9, the transistors 46 to 50 have withstand voltage, for example, in connection with driving the cathodes 4 to 6 of the CRTs 1 to 3.
A high voltage transistor of 300V (volt) or more is used. + 210V is supplied as a power supply voltage to the cathode current switch circuits 7 to 9.

The ACO current detection circuits 21 to 23 are respectively
The comparison and amplification circuit 53 and the reference voltage E ₁~ E₃To generate
And a reference voltage source 54. To avoid complexity
Among the ACO current detection circuits 21 to 23, the ACO current
The detection circuits 22 and 23 are not labeled with these symbols.

The combined current detection circuit 16 outputs the output voltage signal S
Adder circuit 5 for adding _{2 to} S ₄ and outputting an addition signal S _15.
5, a reference amplification circuit 56, and a reference voltage source 57 that generates a reference voltage E ₄ .

The single tube maximum current detection circuit 17 includes a resistor 54
To 56, capacitors 57 to 59, and transistors 61 to 6
3, a reference amplification circuit 64, and a reference voltage source 65 for generating a reference voltage E ₅ . The output voltage signal appearing on the emitter side of the transistors 61 to 63 is called the maximum voltage signal S ₁₆ .

The brightness limiting circuit according to the prior art is basically constructed as described above, and its operation will be described below.

As described above, the switching pulse S ₁ supplied from the terminal 15 has the vertical blanking period V.BL of the video signals R _{1 to} B ₁ as shown in FIG. 2C.
This signal is high level only during the vertical back porch period 122 within K. The switching pulse P ₁ is applied to the transistor 4 constituting the cathode current switch circuits 7 to 9.
Supplied to base 7. The emitter of the transistor 47 is grounded.

Therefore, the transistor 47 is turned on and the transistor 49 is turned off while the switching pulse P ₁ is at a high level. Also, the transistor 48 is activated.

Therefore, the cathode currents I _KR , I _KG , I _KB
Flows only in the transistor 48. Transistor 48
Output voltage signals S _{5 to} S ₇ having a voltage value obtained by multiplying the current values of the cathode currents I _KR , I _KG , and I _KB by the resistance value of the resistor 45 are obtained on the collector side of the.

The voltage values of these output voltage signals S _{5 to} S ₇ and the reference voltages E _{1 to} E _{3 of the} reference voltage source 54 are respectively compared by the comparison amplifier 53, and the difference between them is amplified to output the output signal S _11. is supplied to the primary color reproduction, brightness correction circuit 10 as a to S _13.

In the primary color / luminance correction circuit 10, the output signals S _{11 to} S ₁₃ generated in the vertical back porch period 122, that is, the cathode currents I _KR and I at the pedestal level are obtained.
CR based on the output signals S _{11 to} S ₁₃ corresponding to _KG and I _KB
On the screens of T1 to T3, so-called automatic cutoff adjustment is automatically performed in which white balance is automatically adjusted in a relatively dark portion of an image.

On the other hand, when the switching pulse P ₁ is at the low level, that is, the effective vertical scanning period 121 (see FIG. 2A).
In the period 150 (see FIG. 2B) including the period during which the image is displayed on the screens of the CRTs 1 to 3 (see FIG. 2B), the transistor 47 is
Is turned off. As a result, the bias current of the bias current supply circuit composed of the resistors 41 to 43, the transistor 50, and the diode 51 is supplied to the base of the transistor 49.
Turns on. In this case, the video signals R _{1 to} B ₁ supplied to the base of the transistor 46 that operates as an emitter follower are supplied to the CRT _{1 to} CRT ₁ through the transistor 49.
3 cathodes 4-6. The transistor 48 is in the off state because the transistor 49 is in the on state.

As a result, a color image having a predetermined brightness is obtained on the screens of the CRTs 1 to 3. In this case, the cathode currents I _KR , I _KG and I _KB are supplied to the collector side of the transistor 46 through the transistor 49 and the transistor 46. Therefore, on the collector side,
Output voltage signals S _{2 to} S _{4 which} are products of the current values of the cathode currents I _KR , I _KG , and I _KB and the resistance value of the resistor 44 appear.

The added voltage of these output voltage signals S _{2 to} S ₄ is obtained as an added signal S _{15 at} the output side of the adding circuit 55 which constitutes the combined current detecting circuit 16. This addition signal S ₁₅
Is compared with the reference voltage E ₄ of the reference voltage source 57, and the difference is amplified by the comparison and amplification circuit 56 to obtain the output signal S _{9, which} is supplied to one input terminal of the adder circuit 25.

On the other hand, the output voltage signals S _{2 to} S ₄ are supplied to the transistors 61 to 63 constituting the single tube maximum current detection circuit 17.
Will be supplied to each base. In this case, the transistor 61
Since the collector side of ~ 63 is connected to the power source + 12V and the emitter side is commonly connected, the output voltage signals S _{2 to} S
Of the _four , only the output voltage signal having the maximum value is supplied to one input terminal of the comparison and amplification circuit 64 as the maximum voltage signal S ₁₆ . The resistors 54-56 and the capacitors 57-
The series circuit of 59 is a circuit for removing high frequency noise.
The maximum voltage signal S ₁₆ is compared with the reference voltage E ₅ of the reference voltage source 65, and the difference is amplified by the comparison and amplification circuit 64 to obtain the output signal S _{10, which} is supplied to the other input terminal of the adder circuit 25. It

Output signal S _{10 of the} single tube maximum current detection circuit 17
And the output signal S ₉ of the combined current detection circuit 16 is added to the addition circuit 2
In step 5, the added signal is supplied to the primary color reproduction / brightness correction circuit 10 as a brightness detection signal S ₁₇ .

Therefore, the primary color reproduction / luminance correction circuit 10
Limits the amplitudes of the video signals R ₁ , G ₁ , and B ₁ according to the value of the brightness detection signal S ₁₇ , so that the brightness on the screen of the CRT 1 to CRT 3 in the effective vertical scanning period 121 is equal to or less than a certain brightness. Can be limited to.

After all, the single tube maximum current detection circuit 17 is
In order to prevent the CRT of any one of T1 to T3 from having excessive brightness, in other words, the cathode current of any one of the cathode currents I _KR , I _KG , and I _KB , to become excessive, the cathode current I _KR is prevented. , I _KG , I _KB , and the combined current detection circuit 16 detects three voltages.
The voltage corresponding to the sum of the cathode currents I _KR , I _KG , I _KB is detected so that the sum of the cathode currents I _KR , I _KG , I _KB of RT1 to 3 does not become excessive.

As described above, even in the above-mentioned conventional brightness limiting circuit,
By detecting the cathode currents I _KR , I _KG , and I _KB of the CRTs 1 to 3, it is possible to prevent burning of the fluorescent screens of the CRTs 1 to 3 in advance.

[0033]

However, in the above-mentioned conventional brightness limiting circuit, the cathode current switch circuits 7 to 9 use the high breakdown voltage transistors 46 to 50, and the number thereof is large, and the material cost is high. There was a problem that was high. Further, there is a problem that the arrangement area of components is relatively large and the mounting area and volume are relatively large. Further, the cathode currents I _KR , I _KG , and I _KB are supplied to the transistor 4 by using the switch transistor 47 and the like.
Since it is switched to the 6 side or the transistor 48 side,
The switching circuit for current switching becomes complicated,
In this respect also, there is a problem that the manufacturing cost becomes high and the mounting area and volume become relatively large.

The present invention has been made in view of the above problems, and an object thereof is to provide a brightness limiting circuit having a relatively simple circuit configuration.

[0035]

The first aspect of the present invention is, for example, as shown in FIG. 1, each CR of a three-tube projection type display device.
In the brightness limiting circuit for detecting the cathode currents I _KR , I _KG , and I _KB of the CRTs 1 to 3 by detecting the collector currents of the transistors 84 to 86 for driving the cathodes of T 1 to T 3, Cathode current I _KR , I _KG ,
The current addition means 105 for adding I _KB and the load switching means 106 for switching the load of the current addition means 105 are provided, and the cathodes of the respective CRTs 1 to 3 according to the output signal S ₂₁ appearing at the load of the current addition means 105. Current I _KR , I _KG , I _KB
Is to limit.

In the second aspect of the present invention, for example, as shown in FIG. 1, the current adding means 105 includes a PNP transistor 111.
Transistors 84 to 8 for driving respective emitters and cathodes of three grounded base type transistor circuits having
And resistors 101 to 103 between the respective collectors of 6 and
Has a circuit inserted therein.

In the third aspect of the present invention, for example, as shown in FIG. 1, by detecting the collector currents of the cathode driving transistors 84 to 86 of the CRTs 1 to 3 of the three-tube projection display device, CRT1 to CRT cathode current I _KR ,
In the brightness limiting circuit that detects I _KG and I _KB , each CRT
Current adding means 105 for adding the cathode currents I _KR , I _KG , I _KB of 1 to 3, load switching means 106 for switching the load of the current adding means 105, and each CRT 1 appearing on the input side of the current adding means 105. 3 cathode current I _KR , I
_The output signal S ₂₁ that appears in the load of the current adding means 105 has a maximum voltage value detecting means 17 for detecting the maximum voltage among the voltages E _KR , E _KG , and E _KB corresponding to _KG and I _KB. The cathode currents I _KR , I _KG and I _KB of the CRTs 1 to 3 are limited according to the output signal S ₁₀ of the maximum voltage value detecting means 17.

According to a fourth aspect of the present invention, for example, as shown in FIG. 4, the cathode current of each CRT of the above three CRTs is detected by detecting the collector current of the transistor for driving the cathode of each CRT. In the brightness limiting circuit for detecting CRT, the current adding means for adding the cathode current of each CRT and the output side of the current adding means are connected to correspond to at least one scanning line on the screen of each CRT. Voltage signal generating means 156 for generating a voltage signal of a predetermined level or higher only when the period becomes a high brightness of a predetermined level or higher.
And the cathode current of each CRT is limited in accordance with the output signal of the voltage signal generating means 156.

In the fifth aspect of the present invention, for example, as shown in FIG. 4, the cathode current of each CRT of the above three CRTs is detected by detecting the collector current of the transistor for driving the cathode of each CRT. In the brightness limiting circuit for detecting the CRT, the current adding means for adding the cathode current of each CRT and the output side of the current adding means are connected, and the period corresponding to all the scanning lines on the screen of each CRT is predetermined. A voltage signal generating means 157 for generating a voltage signal of a predetermined level or more only when the brightness becomes higher than the level, and limits the cathode current of each CRT according to the output signal of the voltage signal generating means 157. It is the one.

[0040]

According to the first aspect of the present invention, the current adding means 10 for adding the cathode currents I _KR , I _KG , and I _KB of the CRTs 1 to 3 is added.
5 and load switching means 106 for switching the load of the current adding means 105, and according to the output signal S ₂₁ appearing in the load of the current adding means 105, the cathode currents I _KR , I _KG , I of the respective CRTs 1 to 3. _{I try} to limit the _KB .

Therefore, the output signal S ₂₁ of the current adding means 105 can also be used as the detection signal S _21A for automatic cutoff, and the combined current I _{K of the} cathode currents I _KR , I _KG and I _KB.
Since it can also be used as the detection signal S _21B , the cathode current switch circuit becomes unnecessary.

According to the second aspect of the present invention, the current adding means 10
5 has a circuit in which resistors 101 to 103 are respectively inserted between the emitters of three grounded base type transistor circuits having PNP transistors 111 to 113 and the collectors of transistors 84 to 86 for driving cathodes. ing.

Therefore, the circuit configuration of the current adding means 105 becomes relatively simple.

According to the third aspect of the present invention, the current adding means 105 for adding the cathode currents I _KR , I _KG , I _KB of the CRTs 1 to 3 and the load switching means 106 for switching the load of the current adding means 105. , Cathode currents I _KR , I _KG , I _{KB of the} CRTs 1 to 3 appearing on the input side of the current adding means 105.
Of the voltages E _KR , E _KG , and E _KB corresponding to the maximum voltage value detecting means 17 for detecting the maximum voltage, and the output signal S ₂₁ and the maximum voltage value appearing in the load of the current adding means 105. Each CRT 1 according to the output signal S ₁₆ of the detection means 17
The cathode currents I _KR , I _KG and I _KB of 3 to 3 are limited.

Therefore, the output signal of the current adding means 105
S_{twenty one}The detection signal S for automatic cut-off_21AUsed as
Wear. Further, the output signal S of the current adding means 105_21BAnd up
Output signal S of large voltage value detecting means 17_TenDepending on each C
Cathode current I of RT1 to 3 _KR, I_KG, I_KBComposite value I
_KCan be restricted and each CRT 1-3
Cathode current I flowing_KR, I_KG, I_KBLimit values individually
You can Furthermore, the cathode current switch circuit
It becomes unnecessary.

According to the fourth aspect of the present invention, the current adding means for adding the cathode current of each CRT and the output side of the current adding means are connected to each other, and at least one on the screen of each CRT is connected.
A voltage signal generating means 156 for generating a voltage signal of a predetermined level or higher only when the period corresponding to one scanning line becomes high brightness of a predetermined level or higher.
The cathode current of each CRT is limited according to the output signal of 56.

Therefore, when the cathode current corresponding to any one scanning line on the screen of the CRT exceeds a predetermined value, the cathode current value can be limited.

According to the fifth aspect of the present invention, the current adding means for adding the cathode current of each CRT and the output side of this current adding means are connected to correspond to all the scanning lines on the screen of each CRT. A voltage signal generating means 157 for generating a voltage signal of a predetermined level or higher only when the period becomes high brightness of a predetermined level or higher, and the cathode current of each of the CRTs is determined according to the output signal of the voltage signal generating means 157. I try to limit it.

Therefore, when the cathode currents corresponding to all the scanning lines on the screen of the CRT exceed a predetermined value, the cathode current value can be limited.

[0050]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the brightness limiting circuit of the present invention will be described below with reference to the drawings. In the drawings referred to below, the same reference numerals are given to those corresponding to those shown in FIG.

FIG. 3 shows the configuration of an example of a CRT projection display device to which an embodiment of the brightness limiting circuit according to the present invention is applied.

FIG. 1 shows a detailed configuration of an embodiment of a brightness limiting circuit in the CRT type projector.

The CRT projection display device in the example of FIG. 1 has an antenna 71. The RF signal received by the antenna 71 is selected by the tuner 72 and converted into an intermediate frequency signal.

The intermediate frequency signal is amplified by the intermediate frequency amplifying circuit 73 and then supplied to the audio receiving circuit 74 and the video detecting circuit 75. The audio receiving circuit 74 reproduces an audio signal from the audio intermediate frequency signal and supplies it to the speaker 76. Sound is reproduced by the speaker 76.

On the other hand, the video detection circuit 75 forms the composite video signal CV. The signal system of the composite video signal CV is separated into the luminance signal Y and the color signal C by the Y / C separation circuit 76 and supplied to the color demodulation circuit 77.

In the color demodulation circuit 77, from the luminance signal Y and the color signal C, the luminance signal Y and the color difference signals (RY), (B-
Y) and these signals are converted into the luminance limiting circuit 80
Is supplied to the primary color reproduction / luminance correction circuit 10 which constitutes
Hereinafter, description will be made with reference to FIG.

The primary color reproduction / luminance correction circuit 10 has a terminal 10
Video signals R ₁ , G ₁ , and B ₁ of three primary colors for R, G, and B are formed from these luminance signal Y and color difference signals (RY) and (BY) supplied to a to 10c. Output amplifier circuit 81
Supply to ~ 83. When forming the video signals R ₁ , G ₁ and B ₁ , the luminance detection signal S ₁₇ and the automatic cutoff voltage signal (hereinafter referred to as ACO (Auto Cut Off) voltage signal) S _21A Those video signals R ₁ , G ₁ , B
The brightness of ₁ is limited and adjusted.

The video signals R ₁ , G ₁ , and B ₁ amplified by the output amplifier circuits 81 to 83 are output transistors 84 to 86.
Supplied to the base of. Each output transistor 84-86
The cathodes 4 to 6 of the CRTs 1 to 3 are driven through the emitters of the.

For each beam current of CRT1 to CRT3,
Cathode current I_KR, I_KG, I _KBBut the output transition
ACO current / composition through the collectors of stars 84-86
In the resistors 101 to 103 forming the current detection circuit 100,
Each is supplied. ACO current / combined current detection circuit 1
00 is the cathode current I_KR~ I_KBCurrent adder to add
Current adding circuit 105 as a stage, and this current adding circuit 1
Load switching as load switching means for switching load 05
Circuit 106 and reference voltage E at one terminal_FourGroups that generate
A quasi-voltage source 57 is connected, and the current adding circuit 10 is connected to the other terminal.
Output signal S of 5 _{twenty one}(Composite voltage signal S_21B) Is supplied
It has a comparison and amplification circuit 56.

The output signal S ₂₁ is the combined voltage signal S
The component of _{21B and} the component of the ACO voltage signal S _21A are signals included in time division (the reason will be described later).

The output signal S ₂₁ and the reference voltage E ₄ are compared by the comparison and amplification circuit 56, and the amplified signal of the difference is supplied to one input terminal of the addition circuit 25 as the output signal S ₉ .

The current adding circuit 105 is configured as a grounded base transistor circuit having PNP type transistors 111 to 113. That is, the transistor 1
The bases of 11 to 113 are commonly connected to the bias voltage source 114 of the voltage E7. The collectors are also connected in common, and the common connection point is the resistor 115 as a load and the resistor 11 forming the load switching circuit 106.
6 are respectively connected to one terminal.

The other terminal of the resistor 115 is grounded. The other terminal of the resistor 116 is connected to the transistor 117.
Grounded through. The switching pulse P ₂ is supplied from the synchronization signal generating circuit 131 (see FIG. 3) to the terminal 118 on the base side of the transistor 117.

FIG. 2B shows the waveform of the switching pulse P ₂ . FIG. 2A shows a composite video signal CV (in FIG. 1,
It shows a waveform of a 1 V (1 vertical) scanning period of (appearing on the output side of the video detection circuit 75).

The 1V scanning period is composed of a vertical blanking period V.BLK which is a non-video period and an effective vertical scanning period 121 which is a video period. Here, the switching pulse P ₂ is a signal which becomes low level in the vertical back porch period 122 of the vertical blanking period V.BLK and becomes high level in the other periods.

The synchronizing signal generating circuit 131 separates the vertical synchronizing signal and the horizontal synchronizing signal from the composite video signal CV, supplies them to the vertical deflecting circuit 132 and the horizontal deflecting circuit 133, respectively, and switches based on the vertical synchronizing signal. Generate pulse P ₂ .

Vertical deflection circuit 132 and horizontal deflection circuit 133
Respectively form a sawtooth current signal for deflection, C
Supply to a deflection coil (not shown) mounted on RT1 to RT3. Further, a high voltage is generated in the high voltage generation circuit 134 using the output signal of the horizontal deflection circuit 133, and CRT1 to
3 anode electrodes and the like.

In this way, images corresponding to the video signals R ₁ , G ₁ and B ₁ are formed on the screens of the CRTs ₁ to 3. The projection lens and the screen arranged in front of the screens of the CRTs 1 to 3 are omitted.

Referring again to FIG. 1, the current adding circuit 105
One end of the resistors 101 to 103 forming the
Current I_KR, I_KG, I_KBVoltage signal E corresponding to_KR，
E_KG, E _KBOccurs. That is, the voltage signal E_KR，
E_KG, E_KBIs the resistance value of the resistors 101-103.
Current I_KR, I_KG, I_KBIt is the value multiplied by the current value. Na
Actually, this voltage signal E_KR, E_KG, E_KBThe tiger
The voltage that appears at the emitters of transistors 111-113 is
Although included as an ass value, single tube maximum current detection circuit 1
7 connected to one input terminal of the comparison and amplification circuit 64
Reference voltage E of the reference voltage source 65_FiveThe base ground
The voltage of the bias voltage source 114 of the transistors 111 to 113 is increased.
By setting the same value as the pressure E7, the bias
The value is removed by the comparison and amplification circuit 64.

The single tube maximum current detection circuits 17 are respectively
It has three transistors 61 to 63 each having a collector and an emitter connected in common. The common collector is connected to the power supply voltage + 12V. Transistors 61-
One terminal of the capacitors 57 to 59 and one terminal of the resistors 54 to 56 are connected to the base of 63. The other terminals of the capacitors 57 to 59 are grounded.

[0071] The other terminal of resistor 54 to 56, the above-mentioned cathode current I _KR, I _KG, voltage signal E _KR corresponding to I _KB, E _KG, is E _KB is supplied. Here, the resistor 54-
Since a series circuit of 56 and capacitors 57 to 59 is provided for removing high frequency noise signals, voltage signals E _KR , E _KG , and E are provided at the bases of the transistors 61 to 63.
_KB will be supplied as is.

Since the transistors 61 to 63 have an emitter follower configuration in which their emitters are commonly connected,
Among the supplied voltage signals E _KR , E _KG and E _KB , the voltage signal having the maximum value appears on the emitter side. The signal that appears on the emitter side is called the maximum voltage signal S ₁₆ . As described above, the maximum voltage signal S ₁₆ and the reference voltage E ₅ are compared by the comparison and amplification circuit 64, and the difference is amplified and supplied as the output signal S _{10 to} one input terminal of the adder circuit 141. The output signal S ₁₀ is the cathode currents I _KR , I _KG ,
Since the maximum voltage signal is selected from the voltage signals E _KR , E _KG , and E _KB corresponding to I _KB , the maximum value of the voltage signal is actually selected. It is called a tube maximum current detection circuit 17.

The brightness adjusting signal S ₂₃ is supplied from the brightness adjusting circuit 142 to the other input terminal of the adding circuit 141. The output signal S ₂₄ of the adder circuit 141 is supplied to the other input terminal 25 of the adder circuit 25.

The output signal of the adder circuit 25 is the brightness detection signal S.
It is supplied to the primary color reproduction / luminance correction circuit 10 as ₁₇ .

Next, the operation of the above embodiment will be described.

The switching pulse P ₂ supplied to the terminal 118 of the load switching circuit 106 is a signal which becomes a low level only in the vertical back porch period 122 as shown in FIG.

In the vertical back porch period 122 corresponding to this low level period, since no image is displayed on the screen of the CRTs 1 to 3, the cathode currents I _KR , I _KG ,
The value of I _KB is a relatively small value.

Therefore, the cathode current I in this period is
_Since a voltage signal obtained by combining _KR , I _KG , and I _KB is obtained with a relatively high voltage amplitude value, only the load resistor 115 serves as the load resistor, so that the base of the transistor 117 is at a low level during the vertical back porch period 122. The signal is given to turn it off to reduce the load.

Therefore, in this case, the voltage signal S ₂₁ obtained during the vertical back porch period 122 is used by the primary color reproduction / luminance correction circuit 10 as the ACO voltage signal S _21A . Actually, the voltage signal S ₂₁ is taken into the primary color reproduction / luminance correction circuit 10 only during the back porch period 122.

In the primary color reproduction / luminance correction circuit 10, this A
Based on the CO voltage signal S _21A , white balance adjustment is automatically performed in a relatively dark portion of the image on the screen of the CRTs 1 to 3.

On the other hand, when the switching pulse P ₂ is at the high level, that is, the effective vertical scanning period 121 (CRT
Period 15 including the period in which images are displayed on the screens 1 to 3)
At 0, the transistor 117 is turned on. As a result, the load of the current adding circuit 105 becomes a parallel combined resistance value of the resistors 115 and 116, which is so-called heavy.

For this reason, the image display period is relatively long.
Cathode current I with a large value_KR, I _KG, I_KBSynthetic current of
I_KIs the product of the value of and the parallel combined resistance value of the output signal
S₂1 (composite voltage signal S_21B) As a comparison and amplification circuit 56
Appears on one of the input terminals. In this case, the reference voltage source 57
Reference voltage E_FourComparison amplification times without changing the value of
Output signal S of path 56₉Synthetic current I_KCorresponding voltage signal
It can be used as an issue.

After all, by providing the current adding circuit 105 and the load switching circuit 106, the ACO current / combined current detection circuit 100 is the cathode current switch circuit described with reference to FIG. 7. Composite current detection circuit 16
And the ACO current detection circuits 21 to 23 are also used, and the circuit configuration is relatively simple.

Referring again to FIG. 1, the cathode currents I _KR , I
Voltages E _KR , E _KG , and E _KB proportional to _KG and I _KB are supplied to the bases of the transistors 61 to 63 that form the single tube maximum current detection circuit 17. In this case, the voltages E _KR , E _KG , E
Of the _KB , only the transistor to which the voltage having the maximum value is supplied is in the active state, the other transistors are in the off state, and the output signal S ₁₆ appears at the emitter of the transistor in the active state.

As described above, only the signal component of the output signal S ₁₆ is output from the comparison and amplification circuit 64 and supplied as the output signal S _{10 to} one input terminal of the adder circuit 141. In the adder circuit 141, the output signal S ₁₀ and the output signal S ₂₃ from the brightness adjustment circuit 142 are added and supplied as an output signal S _{24 to} one input terminal of the adder circuit 25.

In the adder circuit 25, the output signal S ₂₄ and the output signal S ₉ are combined to form a luminance detection signal S _17, which is supplied to the primary color reproduction / luminance correction circuit 10.

After all, the brightness detection signal S ₁₇ is the output signal S output from the ACO current / combined current detection circuit 100.
₉ , the output signal S ₁₀ output from the single tube maximum current detection circuit 17, and the output signal S ₂₃ output from the brightness adjustment circuit 142.
It becomes a synthetic signal of. Therefore, the primary color reproduction / luminance correction circuit 10 can effectively adjust the luminance and limit the luminance based on the luminance detection signal S ₁₇ which is the combined signal.

Thus, according to the above embodiment, each CR
The current addition circuit 105 for adding the cathode currents I _KR , I _KG , and I _KB of T1 to T3 and the load of the current addition circuit 105 are used for the effective vertical scanning period 121 (the period in which an image is displayed on the screen of CRT1 to 3). ) Is included, and the load switching circuit 106 is switched between a vertical back porch period 122 which is the other period.

Therefore, the circuit including the current addition circuit 105 and the load switching circuit 106 is the same as the ACO current detection circuits 21 to 23 (see FIG. 5) and the combined current detection circuit 16 (see FIG. ACO that can also be used as
Since the current / combined current detection circuit 100 can be integrated into one circuit, the overall circuit configuration becomes relatively simple.

That is, the output signal S ₂₁ of the current addition circuit 105 can be used as the ACO voltage signal S _21A as a detection signal for automatic cutoff, and can also be used as the combined voltage signal S _21B. ..

Further, the output transistors 84 to 86
Cathode current I supplied from the _KR, I_KG, I_KB1
The two current adding circuits 105 add the
The load on the output side of the flow addition circuit 105 is connected to the load switching circuit 106.
Since it was designed to be switched with
Cathode current switch that requires a large number of high voltage transistors
The switch circuit 7 (see FIG. 5) becomes unnecessary.

Furthermore, the configuration of the current adding circuit 105 is 3
Since the resistors 101 to 103 are inserted between the emitters of the individual grounded base transistors 111 to 113 and the collectors of the output transistors 84 to 86, the circuit configuration is relatively simple.

As the configuration of the current addition circuit 105, a grounded-gate type field effect transistor can be used instead of the grounded-base type transistor. An operational amplifier IC can also be used.

Furthermore, according to the example of FIG. 1, the voltage signals E _KR , E _KG and E corresponding to the cathode currents I _KR , I _KG and I _KB are obtained.
_{Since KB} is supplied as it is to the bases of the transistors 61 to 63 whose emitters are commonly connected, any one of the CRTs 1 to 3 among the CRTs 1 to 3 is used.
When it is detected that an excessive current flows into the
A cathode current of 3 can be limited.

Furthermore, in the example of FIG. 1, since the circuit structure is relatively simple, the arrangement area of the components for forming the circuit is small, and the mounting area and volume are also small. Manufacturing costs can also be reduced.

FIG. 4 shows the construction of the essential parts of another embodiment of the present invention. In this case, the example of FIG. 4 is the ACO shown in the example of FIG.
Of the current / combined current detection circuit 100, the reference voltage source 57
Shows a portion obtained by replacing the circuit portion of the comparison and amplification circuit 56 to which is connected. It should be noted that components corresponding to those in FIG. 1 are designated by the same reference numerals and detailed description thereof will be omitted.

In the example of FIG. 4, two sets of circuits having the comparison and amplification circuit 56 to which the reference voltage source 57 is connected are arranged in parallel,
A resistor 151 and a capacitor 15 are provided on the input side of one set.
2 is inserted into the first time constant circuit 156, and the second side of the resistor 154 and the capacitor 155 is inserted into the input side of the other set.
The time constant circuit 157 is inserted.

Here, the time constant of the first time constant circuit 156 is selected to be the time constant of two horizontal scanning lines (may be selected to the time constant of one horizontal scanning line), and the second time constant is selected. The time constant of the time constant circuit 157 is selected as a time constant corresponding to horizontal scanning for one screen.

When the time constant is selected in this way, two adjacent signals are displayed on the screens of the CRTs 1 to 3 by the output signal of the comparison amplification circuit 56 connected to the first time constant circuit 156.
It becomes possible to limit the cathode current by detecting a so-called peak value corresponding to one scanning line. Further, the cathode currents corresponding to all the scanning lines on the screens of the CRTs 1 to 3, so-called average values, are detected by the output signal of the comparison and amplification circuit 56 connected to the second time constant circuit 157 to limit the cathode currents. It will be possible. Note that the example of FIG. 4 also has all the effects described in the example of FIG.

Further, the present invention is not limited to the above-mentioned embodiments, and it goes without saying that various configurations can be adopted without departing from the gist of the present invention.

[0101]

As described above, according to the first aspect of the present invention, the current adding means for adding the cathode current of each CRT and the load switching means for switching the load of the current adding means are provided. The cathode current of each CRT is limited according to the output signal appearing in the load of the current adding means.

Therefore, the output signal of the current adding means can be used as a detection signal for automatic cutoff, and each CRT can be used.
Since it can also be used as a detection signal of a combined current of the cathode current of, the effect that the cathode current switch circuit becomes unnecessary can be obtained.

According to the second aspect of the present invention, the current adding means includes resistors between the emitters of the three grounded base type transistor circuits having PNP transistors and the collectors of the cathode driving transistors. It has an inserted circuit.

Therefore, it is possible to obtain the effect that the circuit configuration of the current adding means is relatively simple.

According to the third aspect of the present invention, the current adding means for adding the cathode current of each CRT, the load switching means for switching the load of the current adding means, and each of the CRTs appearing on the input side of the current adding means. A maximum voltage value detecting means for detecting a voltage having a maximum value among the voltages corresponding to the cathode currents of the above, and depending on the output signal appearing in the load of the current adding means and the output signal of the maximum voltage value detecting means. Therefore, the cathode current of each CRT is limited.

Therefore, the output signal of the current addition means can be used as a detection signal for automatic cutoff, and further, according to the output signal of the current addition means and the output signal of the maximum voltage value detection means, each CRT can be used. It is possible to obtain an effect that the combined value of the cathode currents can be limited and the cathode current value flowing in each CRT can be individually limited. Furthermore, the effect that the cathode current switch circuit becomes unnecessary is obtained.

According to the fourth aspect of the present invention, current adding means for adding the cathode current of each CRT and at least one scanning line on the screen of each CRT are connected to the output side of the current adding means. And a voltage signal generating means for generating a voltage signal of a predetermined level or higher only when the period corresponding to the above becomes high brightness of a predetermined level or higher, and the cathode of each CRT according to the output signal of the voltage signal generating means. I try to limit the current.

Therefore, when the cathode current corresponding to any one of the scanning lines on the screen of the CRT exceeds a predetermined value, the cathode current value can be limited.

According to the fifth aspect of the present invention, the current adding means for adding the cathode current of each CRT and the output side of this current adding means are connected to correspond to all the scanning lines on the screen of each CRT. A voltage signal generating means for generating a voltage signal of a predetermined level or higher only when the period becomes high brightness of a predetermined level or higher, and limits the cathode current of each CRT according to the output signal of the voltage signal generating means. I am trying to do it.

Therefore, when the cathode currents corresponding to all the scanning lines on the screen of the CRT exceed a predetermined value, the cathode current value can be limited.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram provided for explaining the operation of the example of FIGS. 1 and 5.

FIG. 3 is a block diagram showing a configuration of a CRT system projection display device to which the luminance limiting circuit of the example of FIG. 1 is applied.

FIG. 4 is a circuit diagram showing a configuration of a main part of another embodiment of the brightness limiting circuit according to the present invention.

FIG. 5 is a circuit diagram showing a configuration of a brightness limiting circuit according to a conventional technique.

[Explanation of symbols]

1-3 CRT 4-6 Cathode 17 Single tube maximum current detection circuit 84-86 Output transistor 100 ACO current / composite current detection circuit 101-103 Resistor 105 Current addition circuit 106 Load switching circuit 111-113 Transistor I _KR , I _KG , I _KB cathode current

Claims (5)

[Claims] 1. A brightness limiting circuit for detecting the cathode current of each CRT by detecting the collector current of a transistor for driving the cathode of each CRT of a three-tube projection display device. And a load switching means for switching the load of the current adding means, and limits the cathode current of each CRT according to the output signal appearing in the load of the current adding means. A brightness limiting circuit characterized by. 2. The current adding means has a circuit in which a resistor is inserted between each emitter of the three grounded base type transistor circuits having PNP transistors and each collector of the cathode driving transistor. The brightness limiting circuit according to claim 1, wherein: 3. A brightness limiting circuit for detecting the cathode current of each of the CRTs of a three-tube projection display device by detecting the collector current of a transistor for driving the cathode of each of the CRTs. Among the voltages corresponding to the cathode currents of the respective CRTs appearing on the input side of the current adding means, the voltage having the maximum value is detected. And a maximum voltage value detecting means for controlling the cathode current of each CRT according to the output signal appearing in the load of the current adding means and the output signal of the maximum voltage value detecting means. Brightness limiting circuit. 4. A brightness limiting circuit for detecting the cathode current of each of the CRTs of a three-tube projection display device by detecting the collector current of a transistor for driving the cathode of each of the CRTs. And a current adding means for adding the
Of the voltage signal generating means for generating a voltage signal of a predetermined level or higher only when the period corresponding to at least one scanning line on the screen becomes high brightness of a predetermined level or higher. Each of the above CRTs according to the output signal
A brightness limiting circuit characterized in that the cathode current of the device is limited. 5. A brightness limiting circuit that detects the cathode current of each CRT by detecting the collector current of the cathode driving transistor of each CRT of the three-tube projection display device. And a current adding means for adding the
A voltage signal generating means for generating a voltage signal of a predetermined level or higher only when the period corresponding to all the scanning lines on the screen becomes high brightness of a predetermined level or higher, and the output signal of the voltage signal generating means is According to the above CRT
A brightness limiting circuit characterized by limiting the cathode current of the.