JPS5930379A - Beam current limiting device - Google Patents

Abstract

PURPOSE:To prevent the deterioration in doming and focus characteristic, by providing a circuit limiting a beam current flowing to a CRT through a high voltage generating circuit so as to switch the operation of a control circuit in response to the selection of a signal displayed on the CRT. CONSTITUTION:A character and graphic signal of a computer is displayed selectively on a cathode ray tube 1. The high voltage generating circuit 3 comprising a flyback transformer 3a and a diode 3b is connected to the anode of the cathode ray tube 1, and a connecting point 5 with the secondary winding of the transformer 3a is grounded via a resistor 4. Further, a video amplifier circuit 2 is connected to the connecting point 5 and a series circuit comprising a resistor 7 and a switch 6 is connected. The control circuit limiting the beam current is constituted with the series circuit, resistor 4 and the amplifier circuit 2. Further, the operation of the control circuit is switched at a switch 6 in response to the selection of the signal displayed on the cathode ray tube 1 to prevent the deterioration in doming and focus characteristic and a TV receiver is used in common as the display device such as computer.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in beam current limiting devices for color cathode ray tubes and the like.

Conventionally, color cathode ray tubes used as display elements can make the screen brighter by increasing the beam current, but it is known that this can cause problems such as doming or deterioration of focus characteristics due to thermal expansion of the shadow mask. Therefore, it is common practice to provide a beam current control circuit in the cathode ray tube circuit to limit the beam current to an appropriate value. By the way, recently it has been considered that a television receiver can also be used as a display device for a computer, etc. In this case, if the beam current value is set for the television receiver, doming will occur when using it as a computer display device. This caused problems such as:

In other words, since television receivers mainly display moving images, there are few opportunities for the beam current to continuously concentrate on a predetermined portion of the shadow mask.Therefore, the probability of doming occurring is low, and the focus is moderate. However, since display devices such as computers mainly display still images such as characters and figures, doming is likely to occur and a higher level of focus performance is required.

The present invention has been made in view of the above-mentioned points, and is designed to prevent doming and deterioration of focus characteristics by changing the amount of beam current restriction depending on the application.

Hereinafter, the present invention will be explained with reference to the drawings which are one embodiment.

In the figure, (1) is a cathode ray tube, (2) is a video amplification circuit that supplies video signals to the cathode ray tube (1), and (3) is a high voltage generation circuit that supplies high voltage to the cathode ray tube (1). 8a), a diode (8b), etc. Also, (4) is a flyback (8
A resistor is connected between the secondary winding of a) and the ground, and the connection point (5) with the secondary winding is connected to the video gain control circuit etc. in the video amplification circuit (2). (6) is the connection point (5
), (7) is a resistor connected between the switch (6) and the ground, and the video amplification circuit (2),
Together with the resistor (4) and the switch (6), it forms a beam current control circuit. Note that the beam current of the cathode ray tube (1) is equivalent to the high voltage current in the high voltage generation circuit (3), so it is approximately the same as the current flowing from the ground to the resistor (4) (7) and the secondary winding of the flyback transformer (8a). become equal.

With this configuration, a television is connected to the cathode ray tube (1).
゛When displaying a video signal such as a vision broadcast signal,
When the switch (6) is closed and a large beam current flows in this state, the resistance (4) (
The voltage drop due to the combined resistance 7) increases, and by feeding back this voltage to the video amplifier circuit (2), it is possible to prevent an excessive beam current from flowing. Next, when displaying text or graphic signals from a computer, etc.,
When the switch (6) is opened, the resistor (7) is disconnected from the circuit and its resistance value increases.

Therefore, by supplying the same control voltage to the video amplifier circuit (2) while the beam current is relatively small, it is possible to reduce the maximum allowable value of the beam current.

In other words, by selecting the values of resistors (4) and (7), the maximum value of the beam current can be limited to a reasonable value when reproducing television signals and signals from computers, etc., and doming, focusing, etc. It becomes possible to maintain the characteristics in the best condition.

Note that the switch (6) can be operated in conjunction with a switch for selecting the signal reproduction state of a computer or the like to prevent the operation from becoming complicated. Further, the switch (6) can also be configured with a transistor or the like.

As explained above, according to the present invention, it is possible to use a television receiver also as a display device for a computer, etc., and the practical value of the present invention is great.

[Brief explanation of drawings]

The figure is a schematic configuration diagram showing a beam current limiting circuit which is an embodiment of the present invention. In the figure, (1) is a cathode ray tube, (2) is a video amplification circuit, (
3) is a high voltage generation circuit, (4) is a resistor, (6) is a switch, and (7) is a resistor. Agent Shin Kuzuno −

Claims (2)

[Claims] (1) A cathode ray tube that selectively displays video signals such as television broadcast signals and character and graphic signals from computers, etc., a high voltage generation circuit that supplies high voltage to this cathode ray tube, and a beam that flows to the cathode ray tube through this high voltage generation circuit. 1. A beam current limiting device comprising: a beam current control circuit for limiting current, and configured to switch the operation of the beam current control circuit in accordance with selection of a signal to be displayed on the cathode ray tube. (2) The beam current limiting device according to claim 1, wherein the operation switching of the beam current control circuit is linked with a signal selection operation for a cathode ray tube.