JPH01282977A - Television receiver - Google Patents

Abstract

PURPOSE:To prevent a power source circuit from going to an over load condition by controlling the cathode bias voltage of a television image tube and limiting a beam current when a current, which flows to a primary side high pressure coil of a fly-back transformer, goes over a prescribed value. CONSTITUTION:When the beam current is rapidly increased, the input current of the primary side high pressure coil of a flyback transformer 2 is increased and the both edges voltages of a resistor R1 in a detecting circuit 11 are increased. When this voltage goes over the prescribed value, a transistor Tr2 is conducted and a transistor Tr3 for control is conducted. Thus, the base voltage of a transistor Tr4 in a bias circuit 7 rises up and the emitter voltages of respective transistors Tr5-Tr7 in a cathode drive circuit 5 rises up. Then, a CRT6 is speedily controlled in a cut-off direction.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a television receiver, and more particularly to a circuit for automatically limiting brightness when the brightness becomes too bright.

<Prior Art> FIG. 2 is a circuit diagram of a conventional ABL (automatic brightness limiting) circuit. When a high voltage current corresponding to the beam current flows through the secondary high voltage winding 3 of the flyback transformer 2,
The voltage at the connection point a between the resistor R2 and the smoothing capacitor Ct decreases, for example, the voltage at the contrast control terminal 1a of the video signal processing circuit 1 decreases, the amplitude of the video signal decreases, and the beam current is limited. It has become so. In FIG. 2, 5 is a cathode drive circuit, 4 is a contrast adjustment circuit for the user to adjust the contrast, 6 is a CRT, and 8 is a horizontal output circuit.

Such a conventional ABL circuit is designed to limit the average beam current, and the beam current is limited from the moment the beam current increases due to the time constants of capacitors C1 and C2 and resistors R2 and R3. at least 6 by
A period of about one field is required.

For this reason, it may not be possible to follow sudden changes in the beam current, and - the beam current may increase over time, overloading the power supply circuit, causing voltage fluctuations and abnormal noises. .

<Object of the Invention> The present invention has been made in view of the above-mentioned points, and an object of the present invention is to make it possible to avoid a temporary overload state of the power supply circuit when the beam current increases rapidly. shall be.

<Configuration of the Invention> In order to achieve the above-mentioned object, the present invention includes a detection circuit that outputs a detection signal when the current flowing in the primary high-voltage winding of the flyback transformer exceeds a predetermined value; and a control circuit that controls the cathode bias voltage of the picture tube in response to the signal to limit the beam current of the picture tube.

<Examples> Examples of the present invention will be described in detail below with reference to the drawings.

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

In the figure, ■ is a video signal processing circuit, and a contrast control terminal Ia of this video signal processing circuit l is connected to a second
Similar to the conventional example shown in the figure, the resistor R2 and smoothing capacitor CI are connected in order to limit the beam current by detecting changes in the high voltage current flowing through the secondary winding 3 of the flyback transformer 2 in response to the beam current. Point a is connected via a resistor R3, and a contrast adjustment circuit 4 for contrast adjustment by the user is also connected.

That is, this embodiment includes a conventional ABL circuit and is designed to limit the average beam current.

The R, G, and B primary color signals from this video signal processing circuit 1 are transmitted to each corresponding transistor T of the cathode drive circuit 5.
These are given to the bases of r5 to Tr7, respectively. The collector output of each transistor Tr5 to Tr7 is CRT6
are given to each cathode respectively.

7 is each transistor Tr5 of the cathode drive circuit 5.
This is a bias circuit for applying a bias voltage to the emitter of Tr7, and the approximately 12V B2 power source is divided by resistors R7 and R8, and a stable bias voltage is supplied by the emitter follower circuit of PNP transistor Tr4. In addition, C4 is a ripple removal capacitor, VRI~
VR3 is a variable resistor for white balance adjustment.

8 is a horizontal output circuit, and horizontal output transistor Tri
, a damper diode DI, a resonant capacitor Or, a horizontal deflection coil DY, a figure-of-eight correction capacitor Cs, and the like.

The above configuration is basically the same as the conventional example.

The television receiver of this embodiment is configured as follows in order to avoid a temporary overload state of the power supply circuit when the beam current increases rapidly.

That is, a detection circuit 11 outputs a detection signal when the input current flowing to the primary high voltage winding IO of the flyback transformer 2 connected to the power supply line exceeds a predetermined value, and a detection signal of this detection circuit 11. and a control circuit 12 that controls the cathode bias voltage of the CRT 6 in response to the beam current.

The detection circuit 11 outputs a detection current when the voltage across the resistor R1 connected in series to the power supply line on the primary side of the flyback transformer 2 exceeds a constant voltage corresponding to the predetermined value. It includes a Zener diode ZD, a transistor Tr2, a resistor R4, and a capacitor C3.

The control circuit 12 includes a control transistor Tr whose base is supplied with the detection current of the detection circuit 11 via a resistor R5.
3 and a resistor R6 connected between the collector of the transistor Tr3 and one end of the capacitor C4, and the emitter of the control transistor Tr3 is connected to the capacitor C4.
It is connected to the other end of 4.

Next, a control operation when there is a sudden increase in beam current in the television receiver having the above configuration will be described.

When the beam current increases rapidly, the input current of the high voltage winding IO on the temporary side of the flyback transformer 2 increases, and the detection circuit ll
The voltage across the resistor R1 increases.

When the voltage across this exceeds a certain voltage corresponding to the Zener voltage of the Zener diode ZD, the transistor Tr2 of the detection circuit II becomes conductive, and a detection current is applied to the base of the control transistor Tr3 via the resistor R5.

As a result, the control transistor Tr3 becomes conductive, the charge in the capacitor C4 of the bias circuit 7 is rapidly discharged via the resistor R6 of the control circuit I2, the base voltage of the transistor Tr4 of the bias circuit 7 increases, and the cathode drive circuit The emitter voltage of each of the transistors Tr 5 to Tr 7 increases, and the CRT 6 is quickly controlled in the cut-off direction, making it possible to instantaneously reduce the beam current.
Temporary overload conditions of the power supply circuit can be avoided.

Further, after the beam current is limited, the detection current from the detection circuit 11 is no longer applied, the control transistor Tr3 of the control circuit 12 is turned off again, and the base voltage of the transistor Tr4 of the bias circuit 4 is changed between the capacitor C4 and the control transistor Tr3. The original state is restored by a time constant determined by resistors R7 and R8. By increasing this time constant, the beam current can be gently returned to a steady state.

In this embodiment, it is possible to cope with sudden fluctuations in beam current in this way, and since the ABL circuit similar to the conventional one is provided, the average beam current is also limited.

In the above embodiment, an example was explained in which the video signal processing circuit l outputs RlG and B primary colors, but R-Y, G-
This can be similarly applied to the case of Y and B-Y color difference output, and in this case, the control circuit that responds to the detection signal can be configured to control the bias voltage, etc. of the luminance signal output circuit. good.

<Effects of the Invention> As described above, according to the present invention, when the current flowing through the primary high-voltage winding of the flyback transformer exceeds a predetermined value,
Since the beam current is limited by controlling the cathode bias voltage of the picture tube, the response is improved compared to the conventional method that limits the average beam current, and it is now possible to follow sudden changes in the beam current. , the power supply circuit will not be overloaded.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIG. 2 is a circuit diagram of a conventional example. 2... Flyback transformer, 11... Detection circuit,
I2...control circuit.

Claims (1)

[Claims] (1) A detection circuit that outputs a detection signal when the current flowing through the primary high-voltage winding of the flyback transformer exceeds a predetermined value, and a detection circuit that controls the cathode bias voltage of the picture tube in response to the detection signal. 1. A television receiver comprising: a control circuit for limiting beam current of a picture tube.