JPH053097A - Display device - Google Patents

Abstract

PURPOSE:To realize a simple and low cost circuit for a half-lighting of a cold cathode-ray tube. CONSTITUTION:A transistor Q1 is connected between a drive circuit 2 to light a cold cathode-ray tube 1 and a drive power source. In parallel to the transistor Q1, a Zener diode ZD1 is connected. A transistor Q2 to drive the transistor Q1 is driven directly by a digital signal from a microcomputer 3. When the transistor Q1 is ON, the voltage from the drive power source is all fed to the drive circuit 2, and the cold cathode-ray tube 1 is lighted in a normal condition. When the transistor Q1 is OFF, the voltage fed to the drive circuit 2 is fed by reducing the amount of the Zener voltage owing to the voltage drop by the Zener diode ZD1. As a result, the brightness of the cold cathode-ray tube 1 is reduced as the amount of the voltage reduction, and a half-lighting condition of the cold cathode-ray tube 1 is generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a display device for backlighting a transmissive LCD by semi-lighting a cold cathode tube.

[0002]

2. Description of the Related Art When the backlight of a transmissive LCD is turned off, the display becomes invisible. However, the cold cathode tube used as the backlight has a life of 20,000 hours. Usually, this life is shorter than that of LCD. Therefore, a circuit as shown in FIG. 3 has been used, which is capable of half-lighting the cold-cathode tube for display and extending the life.

In FIG. 3, the cold cathode fluorescent lamp 1 is driven to be lit by a driving circuit 2 composed of an inverter, and the power source of the driving circuit 2 is supplied from a driving power source through a transistor Q ₁ . .. Further, the digital signal from the system control microcomputer 3 is converted into an analog signal by the D / A conversion circuit 4, and the output signal of this D / A conversion circuit 4 controls the transistor Q ₂ to change the transistor Q ₁ to It was on / off controlled.

[0004]

Although the D / A conversion circuit 4 is used for adjusting the voltage of the transistor Q ₂ , there is a problem that the D / A conversion circuit 4 is expensive. The present invention has been made in view of the above points, and provides a display device intended to realize a simple and inexpensive circuit for half-lighting a cold cathode tube.

[0005]

The present invention is a transmissive LCD.
In a display device in which a cold cathode tube is used as a backlight of the device and the cold cathode tube is half-lit, a driving circuit for driving the cold cathode tube to light and a switching element inserted between the driving circuit and a power source. And a voltage drop element connected in parallel with the switching element, and a second switching element that is driven by a digital signal output from a microcomputer for system control to control ON / OFF of the switching element. Is.

[0006]

When the switching element is turned on via the second switching element by the digital signal from the microcomputer, all the power is supplied to the drive circuit and the switching element is turned off. Reduces the voltage supplied to the drive circuit due to the voltage drop by the voltage drop element, lowers the brightness of the cold cathode tube, puts the cold cathode tube in a semi-lit state, and directly outputs the second signal by a digital signal from the microcomputer. By controlling the switching element and eliminating the D / A conversion circuit, an inexpensive and simple circuit is realized.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. As shown in FIG. 1, the feature of the present invention is that the D / A conversion circuit used conventionally is eliminated, and the transistor Q ₂ is directly driven by the digital signal from the microcomputer 3,
An inexpensive Zener diode ZD ₁ is connected as a voltage drop element between the collector and the emitter of the transistor Q ₁ . Other configurations are the same as the conventional one.

Then, the transistor Q ₂ is directly turned on / off by the digital signal from the microcomputer 3,
The transistor Q ₂ controls the on / off of the transistor Q ₁ . When the transistor Q ₁ is on, all the voltage from the driving power supply is supplied to the driving circuit 2, and the cold cathode tube 1 is lit in the normal state. When the transistor Q ₁ is off, the voltage supplied to the drive circuit 2 is reduced by the Zener voltage due to the voltage drop due to the Zener diode ZD ₁ and is supplied by that amount.
, The cold cathode tube 1 is in a semi-lighted state. In this way, by using the Zener diode ZD ₁ , a simple and inexpensive lighting circuit for the cold cathode tube 1 can be realized.

In this embodiment, the Zener diode ZD ₁ is used to stabilize the supply voltage, but the same effect can be obtained by using a resistor instead of the Zener diode ZD ₁ . By the way, the method for turning off the EL backlight is to turn off automatically after a certain time has elapsed, depending on the setting of the dip switch or the like, regardless of the input from the outside. However, due to the nature of the display,
Since it is necessary to display the sequencer information, it is difficult to visually recognize the display when the backlight is off. In such a state, the screen change due to the sequencer state change (for example, an abnormal screen) requires attention of the operator. Will result in a decrease in Therefore, only when there is no external input, the backlight should be turned off after a certain period of time.

Therefore, the backlight of this embodiment is turned off as follows. That is, by setting a certain time with the DIP switch, if there is some input from the outside to the display unit, if the backlight is lit at that point, the elapsed time up to that point is cleared. Then, the set time is measured again from that point.

When there is no input within the fixed time, the extinguishing operation is automatically performed. Further, when the backlight is being output at that time, it is considered that the state has changed due to an input from the outside, and the operation of forcibly turning on the backlight is performed. FIG. 2 shows an explanatory view of this case, in which a count number is entered in the counter buffer in accordance with the setting of the dip switch, and after a certain period of time, the flag is turned on and the backlight is turned off.

Further, when there is an external input, the value of the counter buffer is transferred to the current value buffer, the flag is turned off and the backlight is turned on. Then, the current value buffer is subtracted for each scan, and the current value buffer becomes 0
Then, the flag is turned on and the backlight is turned off.
By doing this, when using the backlight, LC
D is a semi-transmissive type, and since the display content can be confirmed even when the backlight is off, it can be turned off as much as possible when the backlight is not needed, and the life of the backlight itself can be extended. In addition, when the input from the outside or the state of the sequencer changes, the backlight is forcibly turned on.
The effect of preventing the operator from being overlooked can be produced.

[0013]

As described above, the present invention uses a cold cathode tube as a backlight of a transmissive LCD, and in a display device in which the cold cathode tube is half-lit, a drive circuit for lighting the cold cathode tube. A switching element inserted between the drive circuit and the power supply; a voltage drop element connected in parallel with the switching element; and a digital signal output from a system control microcomputer. Second for controlling on / off of switching element
When the switching element is turned on via the second switching element by the digital signal from the microcomputer, all the power is supplied to the drive circuit so that the switching element When it is off, the voltage supplied to the drive circuit is reduced by the voltage drop by the voltage drop element, the brightness of the cold cathode fluorescent lamp is lowered, and the cold cathode fluorescent lamp is in a semi-lighted state. By directly controlling the second switching element with the digital signal of (3), the D / A conversion circuit can be eliminated and an inexpensive and simple circuit can be realized.

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram.

FIG. 3 is a circuit diagram of a conventional example.

[Explanation of symbols]

1 cold cathode tube 2 drive circuit 3 microcomputer Q ₁ switching element Q ₂ second switching element

Front page continuation (72) Inventor Sadamitsu Koike 1048 Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Works Co., Ltd.

Claims (1)

Claim: What is claimed is: 1. A display device in which a cold cathode tube is used as a backlight of a transmissive LCD, and the cold cathode tube is half-lit, and a drive circuit for driving the cold cathode tube to light.
A switching element inserted between the drive circuit and the power source, a voltage drop element connected in parallel with the switching element, and the switching element driven by a digital signal output from a system control microcomputer. And a second switching element for controlling ON / OFF of the display device.