Technologies based on liquid crystals can now be found everywhere. Thanks to the widespread use of LCD displays, we come across them almost every day - they are found in digital watches, calculators, car dashboards, microwave ovens, thermometers, stereo systems, etc. The scope of LCD applications is almost limitless, so it is not surprising that this technology has also affected the most common home device - the TV.

It is thanks to LCD technology that thin, stylish and bright displays were born, which are used everywhere - from ATMs to home living rooms.
But how does it all work?

What makes a crystal “liquid”?

Twist nematic (twisted) crystals are the most common type of liquid crystals today, having a twisted structure. They are used in televisions, monitors, and projectors. A distinctive feature of these crystals is that they predictably respond to electric current, that is, depending on the voltage level, they “unwind” at a certain angle. Hence the name - “liquid”. Of course, the question immediately arises: “How can a solid be liquid?” It's very simple - liquid crystals got their name because of their high flexibility.

How LCD TVs work

Basics: The operating principle of LCD panels is based on adjusting the light intensity. By depositing a liquid crystal (TN) solution between two parallel polarized glass plates, it is possible to control the intensity of light as it passes through the crystal matrix. Depending on the voltage of the electrical discharge, the liquid crystals, spinning at a certain angle, transmit only the required amount of light through the second glass plate. Essentially, LCD displays can switch between a bright state (when the crystals are fully twisted) and a dark state (when the crystals are fully untwisted), or somewhere in between - across the entire gray scale.

Addressing: LCD displays are made up of many tiny segments called “pixels” that form an image. Addressing is the process of “turning on” (no light allowed through) and “off” (light passing through) pixels to form an image on a polarized screen. In the so-called active LCD matrices thin film transistors (TFTs) or small switching transistors and capacitors are used to form an array on a glass plate that conducts electrical charge to the desired pixel. This, in turn, spins the crystals, thereby letting in a predetermined amount of light from the fluorescent lamps located behind them.

Color rendering: The light source in active matrix LCD panels is a fluorescent lamp, which passes a stream of white light through a polarized glass plate located behind the liquid crystal solution. Thus, in theory, the panel screen should glow white - in a position where the crystals are completely twisted and thus able to transmit the full spectrum of light through the polarized panel screen. Since waves of any length are transmitted through the screen, the full spectrum of light can be used to create the desired shades. To do this, each pixel is divided into three sub-elements of the image - red, green and blue, which together create the final shade due to different glow intensities. These subpixels are special filters that cut off waves of a certain length. One such pixel triad can reproduce up to 16.8 million colors.

Advantages of LCD TV

Easier to watch. Flat LCD panels are significantly brighter and have a higher contrast ratio than traditional CRT TVs. This means that they can be used in almost any room, regardless of the light level, since even in very bright sunlight the image will remain crisp and clear, and there will be no glare on the screen under artificial lighting. But perhaps the most important advantage is that you don't have to turn off the lights while watching. It is also important that LCD panels do not damage vision, since their operating principle is based on a technology different from CRT, and the screen does not flicker when displaying images.

The viewing angle of LCD panels is 160°. This means that you can watch TV from almost any position (up to 80° from the center of the screen).

An extremely important indicator affecting the overall image quality is point pitch(or grain pitch), which determines the distance between adjacent subpixels of the same color. The closer these “points” are to each other, the higher the resolution and detail of the image and the greater the viewing angle. Since the dot pitch is measured in millimeters, the following rule applies: the smaller its value, the better and higher quality the image. Therefore, it is worth purchasing LCD TVs with a dot pitch of no more than 0.28 mm (10,000 pixels/inch2).

For a long time, the viewing angle of plasma panels was greater than that of LCD panels. But now, thanks to recent developments, they are practically no different in this parameter. True, this only applies to the most reputable brands. In any case, LCD panel manufacturers have something to strive for.

You can use your LCD TV right out of the box, as it almost always comes with built-in speakers and a tuner. Therefore, given the fact that LCD panels do not require an external tuner to operate, they are perfect for use in small spaces such as bedrooms, living rooms or kitchens.

Sometimes multimedia receivers are included with LCD TVs (although this is quite rare). Before purchasing a specific model, you should familiarize yourself with the additional equipment supplied with the panel, since it is usually not shown in the picture or photograph.

LCD panels project clear, color-rich images, and (most importantly) there are no annoying scan lines on the screen, as each subpixel has its own transistor electrode, and the image is formed point by point. It also allows for a wide variety of colors to be rendered (256 shades of red x 256 shades of green x 256 shades of blue = 16.8 million colors!).

All this requires a huge number of transistors - up to 2.4 million for a resolution of, say, 1024x768 pixels. This means that if even one of them burns out, it will directly affect the subpixel, which will eventually cause the entire pixel to fail. Dead pixels will appear over time. True, they are usually almost invisible and do not interfere with viewing.

Recent developments in the field of LCD technologies have significantly reduced response time display, making television presentations even clearer and more accurate. Response time is the time it takes for a pixel to update, that is, to transition from an active state to a passive (pre-active) state. Response time is measured in milliseconds (ms). For modern LCD panels, this is less than 20ms, as longer response times can result in image "lag" or stuttering (known as "streaking" or "trailing"). Another phenomenon associated with too long a response time is the appearance of residual ghosting in the image ( "ghosting"). It appears when there is a rapid change between dark and bright fragments (or vice versa). In this case, the previous image may remain on the screen.

LCD panel screens can be of several formats - with an aspect ratio of 16:9 (i.e. 16 in width by 9 in length) for viewing HDTV or DVD, or 4:3, typical for television broadcasting. However, if you purchase a widescreen panel (16:9), this does not mean that television programs will be displayed in poor quality due to artificial stretching of the picture. There are several ways to reproduce a 3:4 signal from satellite dishes, VCRs or cable television - in the original ratio (with black bars along the edges of the screen) or in “full screen” mode, where the image is stretched or compressed using special algorithms that reduce the level of distortion. The final image quality depends on how well your TV converts one format to another. However, it is only a matter of time as HDTV is displayed in widescreen format, which will become the basis for television broadcasting in the near future.

LCDs are capable of reproducing both HDTV and TV signals, as well as home video. They can also be used as computer monitors. In fact, LCD panels support any format. They are usually equipped with composite, S-Video and component video inputs, as well as several RGB connectors for connecting to a computer. Their high resolution makes LCD panels ideal for displaying text data and graphics (such as website content).

Some LCD TVs (especially those made by Sharp) are not equipped with RGB connectors. If you want to use the panel as a computer monitor, carefully study the technical characteristics of the model you like.

You can be absolutely sure that your LCD TV will last a long time: the average LCD service life is about 60,000 hours. Even if the TV is turned on 24 hours a day, its resource will last for almost 7 years. If we get closer to reality and consider the case when the TV works, say, 8 hours a day, then the service life will be more than 20 years.

LCD displays last longer than similarly sized plasma TVs. Some manufacturers claim that the service life of their LCD panels can even be 80,000 hours if it is operated properly (i.e. in a room with “standard” lighting and temperature fluctuations of no more than 77°). However, the relevance of these statements in real conditions is questionable.

Also an important factor influencing the service life of the panel is the life of the light source (fluorescent light bulbs). It is extremely important for maintaining the necessary white balance. Over time, this balance can become disrupted, resulting in one of the three primary colors, such as red, being too intense. Therefore, it is better to buy an LCD TV from a well-known brand. Of course, products from manufacturers such as Sharp, Toshiba, JVC or Sony are more expensive than Chinese counterfeits, but at the same time you will be confident that only high-quality lamps are used in your LCD panel, and the precise color balance will be preserved for a long time.

In some cases, the warranty period for light sources may be shorter than for the TV itself. This means that after this period, if the fluorescent lamps malfunction, you will have to buy a new TV. Moreover, some lamps can be replaced, while the rest are built into the device itself. Therefore, before purchasing a panel, again, it is worth carefully studying the technical specifications for warranty and configuration of the backlight system.

DISADVANTAGES OF LCD panels:

1. Lower level of contrast and black color transmission compared to plasma panels.

2. Less-quality “motion correction” system (image artifacts may occur when projecting fast-moving objects).

3. Although LCD panels are not subject to burn-in, there is still a possibility that individual pixels on the screen may burn out. In this case, small black or white dots will appear on the screen. Such defects cannot be corrected. Therefore, if there are too many such dots, then you will most likely have to replace the entire screen.

4. LCD panels are much more expensive than plasma panels of similar size.

Screen size depending on budget and room layout.

Until now, the most popular LCD panels have been small sizes (27" and smaller). One of the reasons for this is that as the panel size increases, the number of pixels increases, and accordingly the number of transistors (three for each pixel). Also large sizes. screens have a detrimental effect on the distribution of light, which, in turn, reduces the quality of color rendering.

Now in Japan and Korea, more and more factories are appearing to produce huge glass plates with built-in transistors, which reduces the cost of producing large-sized LCD panels. Samsung and LG were the first to release such panels. But still, if you want to buy a thin TV with a screen diagonal larger than 37”, it is better to buy a plasma.

Remember not to sit too close to the TV as this will reduce the viewing experience. Ideally, you should try several locations to determine the correct distance between your viewing area and the screen.

Some experts note the fact that if you are too close to the LCD screen, the pixel structure of the image becomes noticeable, which once again emphasizes the importance of choosing the correct viewing distance.

The correct distance between the viewer and the LCD TV depending on the screen size:

• Diagonal from 20 to 27 inches – distance from 1 to 1.5 meters
• Diagonal from 32 to 37 inches - distance from 2 to 2.5 meters
• Diagonal from 42 to 46 inches - distance from 3.5 to 4.5 meters
• Diagonal 50 inches – distance from 3.5 to 5 meters

The whole truth about HD compatibility.

If you are the happy owner of cable television that transmits signals in HDTV format, then the LCD panel, compared to other TVs, will provide an increase in image quality by 10-15%. Most LCD panels are equipped with a built-in ATSC tuner, which allows you to receive HD signals using an antenna. In addition to this, manufacturers often equip their LCD models with cable tuners, which makes them optimal for watching HDTV - both local and national.

Installation of LCD panel.

Now manufacturers of LCD devices are increasingly offering customers new installation options. The time when you had to specially equip a room for installing a TV has passed. Now you can place it almost anywhere. There are about 5 main installation methods, which will satisfy even the most demanding tastes.

Wall mounting allows you to save maximum free space. This type of installation is the cheapest and adds only a few centimeters to the overall depth of the device.

Wall mounting at an angle allows you to place the panel above eye level. Thus, the panel is visible from any place, but the interior design is not disturbed. This type of installation is typically used to place a panel over a fireplace in a bedroom and adds about 10-15cm to the overall depth of the unit.

Desktop mounting is one of the most popular ways to install LCD panels. However, you need to understand that different panels can vary greatly in size, so each model has its own stand (most often it comes complete with the panel).

Hinge mounting uses a swivel joint that allows the device to be “stowed away” when not in use. This type of installation allows the panel to be rotated 120° to the right/left and 10° up/down.

Mounting under the ceiling allows you to place the LCD panel almost anywhere, except when the wall is inaccessible. This type of installation also combines all the advantages of tilted installation for more comfortable viewing. The length of the fasteners is usually from 0.5 to 1 meter and varies depending on the needs of the user.

A television receiver is a device for receiving television signals and converting them into visual and audio images.

A television consists of a visual information display device (kinescope, liquid crystal or plasma panel); chassis - a board that contains the main electronic units of the TV (television tuner, decoder with an amplifier of audio and video signals, etc.), a housing with connectors, control buttons and speakers located on it.

Television radio signals received by the antenna are fed to the radio frequency (antenna) input of the TV. Next, they enter the radio frequency module, also called a tuner, where the signal of the exact channel to which the TV is currently tuned is isolated and amplified. The tuner also converts the radio frequency signal into low-frequency video and audio signals.

The video signal, after amplification, is fed to a color module (color TVs only), which contains a color decoder, and then to a visual information display device. The color decoder is designed to decode color signals of a particular system (PAL, SEC AM, NTSC).

The audio component is fed into the audio channel, where the audio signal is isolated and amplified as necessary. After amplification, the audio signal is sent to a loudspeaker (speaker), which converts the electrical signal into audible sound. If the TV is designed to reproduce stereo or multichannel sound, its audio channel contains a corresponding multichannel audio decoder that divides the audio component into channels.

CRTs come in black and white and color, and they differ in design.

The inside of a black-and-white kinescope screen is covered with a continuous layer of phosphor, which has the property of glowing white under the influence of a flow of electrons. A thin electron beam is formed by an electronic spotlight placed in the neck of the kinescope. The electron beam is controlled electromagnetically, as a result of which it sequentially scans the screen line by line during scanning, causing the phosphor to glow. The intensity (brightness) of the phosphor glow during scanning changes in accordance with the electrical signal (video signal) carrying information about the image.

The inside of a color picture kinescope screen is covered with a discrete layer of phosphors (in the form of circles or lines), glowing red, green and blue under the influence of three electron beams generated by three electronic spotlights. All color picture tubes in front of the screen have a color separation shadow mask. It serves to ensure that each of the three electron beams, simultaneously passing through numerous holes in the mask during scanning, precisely hits “its” phosphor (the first - on the phosphor grains that glow in red, the second - on the phosphor grains that glow in green, the third - on the phosphor grains, glowing blue).

Each electron beam is modulated by its own video signal, which corresponds to three components of a color image. Entering the kinescope, video signals control the intensity of the electron beams and, consequently, the brightness of the phosphors (red, green and blue). As a result, 3 single-color images are simultaneously reproduced on the screen of a color kinescope, which together create a color image.

Modern means of displaying visual information include liquid crystal screens, projection systems, and plasma panels.

In LCD (Liquid Crystal Display) televisions, the image is formed by a system of liquid crystals and polarizing filters. From the rear side, the liquid crystal panel is evenly illuminated by a light source. The cells (pixels) of liquid crystals are controlled by a matrix of electrodes to which a control voltage is applied. Under the influence of voltage, liquid crystals unfold, forming an active polarizer. When the degree of polarization of the light flux changes, its brightness changes. If the polarization planes of the liquid crystal pixel and the passive polarizing filter differ by 90°, then no light passes through such a system.

A color image is obtained by using a matrix of color filters that separate three primary colors from the radiation of a white source, the combination of which makes it possible to reproduce any color. LCD TVs are compact, lack geometric distortion, have no harmful electromagnetic radiation, are lightweight and have low power consumption, but at the same time have a small viewing angle.

In projection TVs, the image is obtained as a result of the optical projection of a bright light image created by the projector onto a translucent or reflective TV screen. Projectors used in projection televisions can be built on cathode-ray picture tubes, liquid crystal matrix semiconductor elements, and laser projection tubes.

The main disadvantages of projection TVs are their bulkiness, high power consumption, low clarity of the enlarged image and a narrow area for placing viewers in front of the TV screen.

The operation of a plasma TV is based on the principle of controlling the discharge of an inert gas in an ionized state between two plane-parallel glasses of a cellular structure located at a short distance from each other. The working element (pixel), which forms a separate point in the image, is a group of three pixels, respectively responsible for the three primary colors. Each pixel is a separate microchamber, on the walls of which there is a fluorescent substance of one of the primary colors. The pixels are located at the intersection points of transparent control electrodes, forming a rectangular grid. During a discharge in the thickness of an inert gas, ultraviolet radiation is excited, which, acting on phosphors of primary colors, causes them to glow. The image is displayed sequentially, point by point, in lines and frames, on the screen.

The brightness of each image element on the panel is determined by the time it glows. If on the screen of a conventional kinescope the glow of each phosphor spot continuously pulsates at a frequency of 25 times per second, then on plasma panels the brightest elements glow constantly with an even light, without flickering. Plasma panels are available in 16:9 image format. The thickness of a panel with a screen size of 1 m does not exceed 10-15 cm, which allows them to be used as a wall-mounted option. The reliability of plasma panels exceeds the reliability of traditional picture tubes.

Today we will understand how the TV works and how the video signal is transmitted. Nowadays the most popular TVs are plasma and liquid crystal. But in order to most fully understand the principle of operation of television, it is better to consider televisions made on the basis of a cathode ray tube.

The basic principle of operation of televisions

Formally, the process of image transfer is quite simple:

1. The photosensitive elements of television cameras convert light radiation into a specific electrical signal.
2. The resulting electrical signal is processed and broadcast.
3. There are three electron guns at the back of the TV. As a result of receiving signals from television, they create beams of electrons and direct them to the inside of the TV, which is coated with a special substance - phosphor. When this substance comes into contact with electrons, a glow is formed.
4. The entire picture on the TV screen is created from the glow of red, green and blue light.

Scheme of operation of a 3D TV

As you can see, the operating principle of old TVs is quite simple. But how does a 3D TV work?

In fact, 3D TVs only create the illusion of three dimensions. The whole principle of creating a three-dimensional illusion is quite simple and is based on the fact that our eyes are at a distance from each other. Based on this fact, it can be assumed that if you show each eye the same image, but from a different angle, the brain will combine these two images, and the result will be a three-dimensional image. The film industry uses methods that rely on this factor.

In the first case, two images are depleted, with each image modified using a color filter. In order to watch this video, you will need glasses with two lenses of different colors. Thanks to these glasses, each eye sees the same image, but from a different angle. This method of creating 3D has been known for quite some time and was first used to give three-dimensionality to the image in black and white cinema. The method using a color filter is usually called anaglyph.

Anaglyph is used less and less in modern films. The color filter has been replaced by the so-called polarization filter. The principle of polarization is similar to that used in anaglyph, but instead of transforming color, it changes the waves of light that the viewer's eye notices. When watching such films, you also need glasses that have lenses of different polarization. This method of creating 3D gives a better and more realistic result.

Not long ago, another method appeared, which has already begun to be used in 3D TVs. The idea is simple - all lenses and filters are installed in front of the screen, and the TV software identifies the user's position and provides a smooth 3D picture.

Operating principle of the remote control

Now all we have to do is find out how the TV remote control works.

The process is actually quite simple:

1. When you press any button on the remote control, two tracks are closed.
2. As a result of this short circuit, an impulse is transmitted to the central chip of the remote control.
3. Next, the central chip sends an electrical signal to the photodiode. Information is transmitted using an infrared signal. This signal is invisible to the human eye, but it can be detected using various equipment (for example, you can use a camera).
4. This signal is caught and processed by the receiver of the TV itself. The signal is checked for information about the remote control model, as well as for the desired command.

The history of mankind contains a whole series of remarkable discoveries and inventions. Television - that is, the transmission of sound and image over vast distances - is rightfully included in this list.

What physical processes underlie the transmission and reproduction of television images? To whom do we owe the birth of television?

How television was born

Scientists from different countries have been working on the creation of foresight for many decades. But TV was invented by Russian scientists: B. L. Rosing, V. K. Zvorykin and Grigory Ogloblinsky.

The first steps that brought the world closer to transmitting images over a distance were decomposition of an image into individual elements using the disk of the German engineer Paul Nipkow, as well as the discovery of the photoelectric effect by the German scientist Heinrich Hertz. The first televisions based on the Nipkow disk were mechanical.

In 1895, humanity was enriched by two great inventions - radio and cinema. This was the impetus for searching for a way to transmit images over a distance.

...The era of electronic television began in 1911, when Russian engineer Boris Rosing received a patent for transmitting images over a distance using a cathode ray tube he designed.

The transmitted image was four white stripes on a black background.

In 1925, Rosing’s student Vladimir Zvorykin demonstrates the full-fledged electronic television he created.

But further research and production of television receivers required huge amounts of money. The famous American entrepreneur of Russian origin, David Sornov, was able to appreciate this great invention. He invested the necessary amount to continue the work.

In 1929, together with engineer Grigory Ogloblinsky, Zvorykin created the first transmitting tube - an iconoscope.

And in 1936, in the laboratory of V. Zvorykin, the first electronic television on lamps received a start in life. It was a massive wooden box with a 5-inch (12.7) cm screen. Regular television broadcasting in Russia began in 1939.

Gradually, tube models were replaced by semiconductor ones, and then just one microcircuit began to replace the entire electronic content of the TV

Very briefly about the main stages of television work

In a modern television system, 3 stages can be distinguished, each of which performs its own task:

• converting an image of an object into a series of electrical pulses called a video signal (image signal);
• transmission of a video signal to the place of its reception;
• converting received electrical signals into optical images.

How does a video camera work?

The production of television programs begins with the operation of a transmitting television camera. Let's consider the structure and operating principle of such a device, developed by Vladimir Zvorykin back in 1931.

The main part of the camera (iconoscope) is a photosensitive, mosaic target. It is onto this that the image created by the lens is projected. The target is covered with a mosaic of several million isolated silver grains coated with cesium.

The operating principle of the iconoscope is based on the phenomenon of external photoelectric effect- knocking out electrons from a substance under the influence of incident light. Light falling on the screen knocks out electrons from these grains, the number of which depends on the brightness of the light flux at a given point on the screen. Thus, an electrical image invisible to the eye appears on the screen.

There is also an electron gun in the tube. It creates an electron beam that manages to “run around” the mosaic screen 25 times every second, reading this image and creating a current in the electrical circuit, called an image signal.

In modern cameras, the image is recorded not on a light-sensitive film, but on a digital matrix consisting of millions of light-sensitive cells - pixels. Light hitting the cells produces an electrical signal. Moreover, its value is proportional to the intensity of the light beam.

To obtain a color image, the pixels are covered with red, blue and green filters. As a result, the matrix captures three images - red, blue and green. Their overlay gives us a color image of the photographed object.

How does the video signal reach the TV?

The resulting video signal has a low frequency and cannot travel long distances. That's why high-frequency EM waves are used as a carrier frequency, modulated (changed) by a video signal. They travel through the air at a speed of 300,000 km/sec.

Television operates on meter and decimeter waves, which can only propagate within line of sight, i.e., cannot circle the globe. Therefore, to expand the television broadcasting area use high television towers with transmitting antennas, Thus, the Ostankino TV tower has a height of 540 meters.

With the development of satellite and cable television, the practical importance of television towers is gradually decreasing.

Satellite television is provided by a number of satellites located above the equator. The ground station transmits its signals to a satellite, which relays them to the ground, covering a fairly wide area. A network of such satellites makes it possible to cover the entire territory of the Earth with television broadcasting.

Cable television provides one receiving antenna, from which television signals are transmitted to individual consumers via a special cable.

How TV works

So, in 1936, the first electronic TV with a cathode ray tube (kinescope). Of course, it has undergone many changes since then, but let’s still look at how images are reproduced on a TV with a cathode ray tube.

It is in this glass flask that the transformation of an invisible electronic signal into a visible image occurs. In its narrow part there is an electron gun, and on the opposite side there is a screen, the inner surface of which is coated with a phosphor. The gun fires electrons at this coating. The number of electrons is controlled by the video signal received by the receiving device. Electrons hitting the phosphor cause it to glow. The brightness of the glow depends on the number of electrons that hit a given point. A combination of points of different luminosity creates a picture. The electron beam hits the screen from left to right, line by line, gradually going down, a total of 625 lines. All this happens at great speed. In 1 second, the electron beam manages to draw 25 static pictures, which we perceive as a moving image.

Color television appeared in 1954. To create the entire range of colors, it took 3 guns - red, blue and green. The screen, accordingly, was equipped with three layers of phosphor of the corresponding colors. Shooting a red phosphor from a red cannon creates a red image, from a blue one - a blue one, etc. Their superposition creates a whole variety of colors corresponding to the transmitted image.

Why TVs have lost weight

The described television receivers with an EL tube are our recent past. They were replaced by more elegant, flat liquid crystal and plasma models. In LCD TVs the screen is thin matrix with a huge density of luminous elements (pixels), allowing you to get an image of good clarity.

The pixels of a plasma TV consist of microlamps filled with 3 types of gases. Their glow creates a color picture.

Digital and analogue television

Until recently, the main television format was analogue. However, television has always responded quickly to new technologies. Therefore, in recent years, video technology has switched to digital format. It provides a more stable and high-quality image, as well as clear sound. Appeared the ability to transmit a huge number of TV channels simultaneously.

The complete transition to the new format will be carried out by 2018. In the meantime, you can use special set-top boxes for old TVs and enjoy digital television services.

The television audience is the largest in the world. After all, this is not only a way to entertain yourself, but also an opportunity to enrich your horizons without leaving home. Internet television is of particular importance in this regard, allowing users to choose a package of channels according to their interests and view past television programs.

If this message was useful to you, I would be glad to see you

Ministry of Education and Science of the Russian Federation Federal Agency for Education

Yerevan State University named after Bunina I.A.

Department of Radioelectronics and

computer equipment

Course work Topic: Construction and repair of LCD panels.

Completed by: student of group FS-61 Popov S.A.

Introduction

1 Design and principle of operation. Types of LCD matrices

2 DC-AC inverters. Types, malfunctions of inverters

3 Installation and repair of LCD panels using the example of a SAMSUNG TV

Introduction Liquid crystals were discovered more than 100 years ago in 1888, but for a long time they were not only practically not used for technical purposes, but were also perceived as nothing more than an interesting scientific curiosity. The first serial devices using liquid crystals appeared only in the early seventies of the last century. These were small monochrome segment indicators for digital watches and calculators. The next important step in the development of LCD technology was the transition from segment indicators to discrete matrices, consisting of a set of dots located close to each other.

For the first time, such a display was used by the Sharp corporation in a pocket monochrome TV. The first working liquid crystal display was created by Fergason in 1970. Previously, LCD devices consumed too much power, had a limited service life, and had poor image contrast. The new LCD display was introduced to the public in 1971 and then it received warm approval. Liquid crystals are organic substances that can change the amount of light transmitted under voltage. A liquid crystal monitor consists of two glass or plastic plates with a suspension between them. The crystals in this suspension are arranged parallel to each other, thereby allowing light to penetrate the panel. When an electric current is applied, the arrangement of the crystals changes and they begin to block the passage of light. LCD technology has become widespread in computers and projection equipment. Note that the first liquid crystals were characterized by their instability and were not very suitable for mass production. The real development of LCD technology began with the invention by English scientists of a stable liquid crystal - biphenyl. The first generation of liquid crystal displays can be seen in calculators, electronic games and watches. Time passes, prices fall, and LCD monitors get better and better. Now they provide high-quality contrast, bright, clear images. It is for this reason that users are switching from traditional CRT monitors to LCD monitors. In the past, LCD technology was slower, it wasn't as efficient, and its contrast levels were low. The first matrix technologies, the so-called passive matrices, worked quite well with text information, but when the picture suddenly changed, so-called “ghosts” remained on the screen. Therefore, this type of device was not suitable for watching videos and playing games. Today, most black-and-white laptop computers, pagers and mobile phones operate on passive matrices. Since LCD technology addresses each pixel individually, the resulting text is clearer than a CRT monitor. Note that on CRT monitors, if the beam convergence is poor, the pixels that make up the image are blurred.

1. Design and principle of operation. Types of LCD matrices.

Unlike CRTs and plasma panels, LCD matrices differ in that they do not emit light themselves, but are merely converters of the light flux emitted by an external source (most often a neon backlight lamp). The principle of their operation is based on the polarization effect of light passed through a liquid crystalline substance in an electromagnetic field. A liquid crystal, unlike a regular crystal, does not have an ordered internal structure; the molecules in it are randomly located and can move freely. Light passed through such a crystal does not change its polarization. However, if the molecules of a liquid crystal are exposed to an external electric field, then they line up in an ordered structure, and the light transmitted through such a medium

acquires directional polarization. But the human eye is not able to detect a change in the plane of polarization of the light flux without additional devices, so another polarized layer is usually placed on the outer part of the LCD matrix, which does not transmit light of a polarization of a different direction (different by 90 degrees), but transmits unpolarized light.

Thus, if light is passed through such a structure, then first it, having passed through the first polaroid, is polarized in the plane of the first polaroid. Next, the direction of polarization of the light flux passing through the layer of liquid crystals will rotate until it coincides with the optical plane of the second polaroid. After which the second Polaroid will transmit a large share of the remaining part of the light flux. But as soon as an alternating potential is applied to the electrodes, the molecules will stretch along the lines of force of the electromagnetic field. Passing polarized light will not change the orientation of the electromagnetic and electrostatic induction vectors. Therefore, the second Polaroid will not transmit such a stream of light. Accordingly, in the absence of potential, the LCD cell is “transparent” to transmitted light. And when the control voltage is set, the LCD cell “turns off”, i.e. loses its transparency. And if the direction of the optical plane of the second polaroid coincides with the first, then the cell will work the other way around: in the absence of potential - transparent, in the presence - dark. By changing the control voltage level within an acceptable range, it is possible to modulate the brightness of the light flux passing through the cell. The very first to appear were LCD monitors with the so-called passive matrix, in which the entire surface of the screen is divided into separate points, combined into rectangular grids (matrices), the control voltage to which, in order to reduce the number of matrix contacts, is applied alternately: at each moment of time to One of the vertical and one of the horizontal control electrodes is set to a voltage addressed to the cell, which is located at the intersection point of these electrodes. The term “passive” itself indicated that the electrical capacity of each cell required a certain time to change the voltage, which resulted in all the images being redrawn for quite a long time, literally line by line. To prevent flicker, such matrices use liquid crystals with a long reaction time. The image on the screen of such displays was very pale, and rapidly changing areas of the image left characteristic “tails” behind them. Therefore, passive matrices in their classical form were practically not used, and the first more or less mass-produced ones were monochrome passive matrices using the technology STN(short for Super Twisted Nematic), with the help of which it became possible to increase the angle of “twisting” of the orientation of the crystals inside the LCD cell from 90° to 270°, which made it possible to provide better image contrast in monitors. Further improvement was the technology DSTN(Double STN), in which one double-layer DSTN cell consists of 2 STN cells, the molecules of which rotate in opposite directions during operation. Light passing through such a structure in a “locked” state loses much more of its energy than before. The contrast and resolution of DSTN turned out to be so high that it became possible to produce a color display in which there are three LCD cells and three optical filters per pixel

primary colors. To improve the quality of the dynamic image, it was proposed to increase the number of control electrodes. That is, the entire matrix is ​​divided into several independent submatrices, each of which contains a smaller number of pixels, so managing them one by one takes less time. As a result, the inertia time of the crystals can be reduced. More expensive than in the case of DSTN, but also a higher quality method of display on a liquid crystal monitor is the use of so-called active matrices. In this case, the principle of one electrode - one cell also applies, however, each pixel of the screen is also served by an additional amplifying element, which, firstly, significantly reduces the time during which the voltage changes on the electrode and, secondly, compensates for the mutual influence neighboring cells on top of each other. Thanks to the transistor “attached” to each cell, the matrix “remembers” the state of all elements of the screen, and resets it only when it receives a command to update. As a result, almost all parameters of the screen image are increased - clarity, brightness and speed of redrawing of image elements, and the viewing angle increases. Naturally, memory transistors must be made of transparent materials, which will allow the light beam to pass through them, which means that the transistors can be placed on the back of the display, on a glass panel that contains liquid crystals. For these purposes, plastic films are used, called Thin Film Transistor (or simply TFT), that is, a thin film transistor. A thin film transistor is indeed very thin, its thickness is only 0.1-0.01 microns. However, the effect of polarized light, which underlies all technologies of modern LCD monitors, still does not allow them to get closer to their cathode-ray brothers in a number of important parameters. Among them, the most important are the still unsatisfactory viewing angles of the liquid crystal display and the still too long response time of the LCD matrix elements, which do not allow them to be used in modern dynamic games, or even for watching high-quality video. But both of these areas are priorities in the development of a modern computer, therefore, at present, the improvement of LCD monitor technology is proceeding in three main directions, allowing, if not eradicating, then at least significantly reducing these shortcomings. Next we will look at all these technologies in more detail.

The most common type of digital panel is based on a technology abbreviated as TN TFT or TN+Film TFT (Twisted Nematic + Film), which is based on traditional twisted crystal technology. The term Film refers to an additional outer film coating that allows you to increase the viewing angle from the standard 90 degrees (45 on each side) to approximately 140 degrees. When the transistor is in the off state, that is, does not create an electric field, the liquid crystal molecules are in their normal state and are arranged in such a way as to change the polarization angle of the light flow passing through them by 90 degrees (liquid crystals form a spiral). Since the polarization angle of the second filter is perpendicular to the angle of the first, the light passing through the inactive transistor will go out without loss, forming a bright point, the color of which is set by the light filter. When the transistor generates an electric field, all the liquid crystal molecules line up,

parallel to the polarization angle of the first filter, and thus in no way affect the light flux passing through them. The second polarizing filter absorbs the light completely, creating a black dot in place of one of the three color components.

TN TFT is the first technology to appear on the LCD market, which still feels confident in the budget solutions category, since the creation of such digital panels is currently relatively cheap. But, like many other cheap things, TN TFT LCD monitors are not without their drawbacks. Firstly, the black color, especially in older models of such displays, is more like dark gray (since it is very difficult to turn all the liquid crystals strictly perpendicular to the filter), which leads to low contrast in the picture. Over the years, the process has improved and new TN panels exhibit significantly increased depth of dark shades. Second, if the transistor burns out, it can no longer apply voltage to its three subpixels. This is important because zero voltage across it means a bright spot on the screen. For this reason, dead LCD pixels are very bright and noticeable. But these two main drawbacks do not prevent this technology from occupying a leading position among 15-inch panels, since the main factor for budget solutions is still low cost.

One of the first LCD technologies designed to smooth out the shortcomings of TN+film was Super-TFT or IPS(In-Plane Switching - approximately this can be translated as “plane switching”), developed by the Japanese companies Hitachi and NEC. IPS represents a kind of compromise when, by reducing some characteristics of digital panels, it was possible to improve others: expand the viewing angle to approximately 170 degrees (which is practically comparable to similar indicators of CRT monitors) due to a more precise mechanism for controlling the orientation of liquid crystals, which and was her main achievement. Such an important parameter as contrast remained at the TN TFT level, and the response time even increased slightly. The essence of Super-TFT technology is that multi-polar electrodes are located not in different planes, but in one. In the absence of an electric field, the molecules of liquid crystals are aligned vertically and do not affect the polarization angle of the light passing through them. Since the polarization angles of the filters are perpendicular, the light passing through the turned off transistor is completely absorbed by the second filter. The field created by the electrodes rotates the liquid crystal molecules 90 degrees relative to their resting position, thereby changing the polarization of the light flux, which will pass through the second polarizing filter without interference.

Among the advantages of IPS technology are clear blacks, a wide viewing angle of up to 170 degrees, and the fact that “broken” pixels now look black, and therefore they are quite unnoticeable. The disadvantage is not so obvious, but significant: the electrodes are located on the same plane, a pair per color element, and block part of the transmitted light. As a result, contrast suffers, which has to be compensated for by more powerful backlighting. But this is a small thing compared to the main disadvantage, which is that the creation

The electric field in such a system requires more energy and takes longer, which increases the response time. Further improvement of IPS technology gave rise to a whole family of technologies: S-IPS (Super IPS), SFT (Super Fine TFT), A-SFT (Advanced SFT), SA-SFT (Super A-SFT).

And finally, the most promising technology developed by Fujitsu today is MVA(Multi-Domain Vertical Alignment) is a further development of VA technology, developed back in 1996. Displays created on the basis of this technology are distinguished by a fairly large viewing angle - up to 160 degrees and a short response time to image changes (less than 25 ms). The essence of MVA technology is as follows: to expand the viewing angle, all color elements of the panel are divided into cells (or zones) formed by protrusions on the inner surface of the filters. The purpose of this design is to enable liquid crystals to move independently of their neighbors in the opposite direction. This allows the viewer, regardless of viewing angle, to see the same shade of color - the lack of this ability was a major drawback of previous VA technology. In the off position, the liquid crystal molecules are oriented perpendicular to the second filter (each of its protrusions), which produces a black dot at the output. When the electric field is weak, the molecules rotate slightly, producing a gray half-intensity point at the output. It is worth noting that the intensity of light for the observer does not depend on the viewing angle, since brighter cells within the field of view will be compensated by darker ones nearby. In a full electric field, the molecules will line up so that at different viewing angles a point of maximum intensity is visible at the output.

Using the achievements of MVA technology, some manufacturers have created their own LCD matrix production technologies. Thus, Samsung uses technology in all its latest developments. PVA(Patterned Vertical Alignment - microstructural vertical placement). The operating principle of PVA is to align liquid crystal molecules at right vertical angles with respect to the control electrodes and form a picture due to their small deviations from the specified position, much smaller than in traditional LCD displays. This, as Samsung notes, reduces inertia and provides a wide conical viewing angle (170 degrees), high contrast levels (500:1) and improved color quality. The potential of MVA technology and its clones is significant. One of its main advantages is the reduced response time. In addition, one can also note such an advantage of MVA as a very good black color. However, the complex design of the panel not only seriously increases the cost of the finished LCD display based on it, but also does not allow the manufacturer to fully realize all the capabilities of MVA due to technical difficulties. Time will tell whether this technology will dominate the LCD market or will be replaced by new developments. In the meantime, MVA is the most technically advanced LCD solution. Conclusions In recent years, the image parameters of LCD panels have improved significantly in such indicators as brightness and contrast, almost approaching

results of CRT monitors. In terms of such an important parameter as the number of displayed colors, a big step forward was also made: there was a transition from 16- to 24-bit color even in mass models of LCD monitors, although from a practical point of view this 24-bit color is still quite far from CRT - monitors. But the pixel response time (i.e., at what speed the pixels take on the desired color) to quickly change the image in LCD displays is significantly longer than in CRTs, which greatly affects the quality of dynamic images (videos, games). After all, if the points do not have time to set the color adequately to the dynamic image, then the observer will note that the image has an unsaturated and “dirty” color.

To evaluate this parameter, monitor manufacturers have introduced the term “response time,” which, however, is used with a number of reservations: total response time, typical and maximum response time. So, the full response time is the sum of the on (activation) and off times of an individual pixel (Full Response Time = Time Rise + Time Fall). This characteristic means the speed of the pixel's response to switching to extreme values: white and black. For normal video playback, the response time should not exceed the duration of one frame - 20 (16) ms at a frame frequency of 50 (60) Hz.

In theory, MVA panels should be the fastest, IPS panels should be the slowest, and regular TN panels should be somewhere in the middle. In practice, there is a significant spread in response times provided by different technologies, even to the point of their overlap.

An equally serious problem with modern LCD displays is the problem of ensuring an acceptable viewing angle of the generated image, the contrast and color parameters of which are noticeably distorted when the viewing angle of the observer changes. Only when the observer looks at the image almost perpendicularly does it look most natural.

Although the viewing angles of their products declared by matrix manufacturers look quite satisfactory on paper, in reality this is not always the case. Thus, most manufacturers of TN+Film matrices indicate that their vertical viewing angle is 90 degrees, but they are silent that in fact in this range the user can observe a more than 10-fold change in brightness (and more than 15-fold - for dark tones). Therefore, real viewing angles, at which a high level of work comfort is maintained, for TN+Film monitors are no more than +/- 10 degrees vertically (and even less for dark grayscale), and horizontally these figures can be increased to +/- 30 degrees.

Things are a little better for MVA and IPS technologies, but there are still big gaps in dark gradations, especially for MVA. The dark field will become noticeably brighter as it deviates from the normal, and then will darken again. This explains why the color rendition of the image is noticeably distorted on the MVA panel, since not only does the contrast of the image decrease, but this process itself occurs nonlinearly. In general, the real viewing angles of MVA panels are both vertical and horizontal no more than +/- 20 degrees

(this is especially noticeable for dark grayscale), and for an IPS panel these angles are approximately twice as large.

DC-AC INVERTERS. Types, malfunctions of inverters.

For the operation of an LCD panel, the light source is of paramount importance, the luminous flux of which, passed through the structure of the liquid crystal, forms an image on the monitor screen. To create a luminous flux, cold cathode fluorescent lamps (CCFLs) are used, which are located at the edges of the monitor (usually top and bottom) and, using frosted diffusing glass, evenly illuminate the entire surface of the LCD matrix. The “ignition” of the lamps, as well as their power supply in operating mode, is provided by inverters. The inverter must ensure reliable start-up of lamps with voltages over 1500 V and their stable operation for a long time at operating voltages from 600 to 1000 V. The lamps in LCD panels are connected using a capacitive circuit (see Fig. A1). The operating point of stable glow (PT - on the graph) is located on the line of intersection of the load straight line with the graph of the dependence of the discharge current on the voltage applied to the lamps. The inverter in the monitor creates conditions for a controlled glow discharge, and the operating point of the lamps is on the flat part of the curve, which makes it possible to achieve a constant glow for a long time and ensure effective brightness control. The inverter performs the following functions: converts direct voltage (usually +12 V) into high-voltage alternating voltage; stabilizes the lamp current and, if necessary, regulates it; provides brightness adjustment; matches the output stage of the inverter with the input resistance of the lamps; Provides short circuit and overload protection. No matter how diverse the market for modern inverters is, the principles of their construction and operation are almost the same, which simplifies their repair.

Block diagram of the inverter.

Rice. 1. CCFL stable glow operating point

The unit for standby mode and turning on the inverter is made in this case on keys Q1, Q2. The LCD panel takes some time to turn on, so the inverter also turns on 2...3 s after the panel switches to operating mode. ON/OFF voltage is supplied from the main board and the inverter enters operating mode. The same block ensures that the inverter is turned off when the LCD panel enters one of the energy saving modes. When a positive ON voltage (3...5 V) is supplied to the base of transistor Q1, a voltage of +12 V is supplied to the main circuit of the inverter - the brightness control unit and the PWM regulator. The unit for monitoring and controlling the brightness of lamps and PWM (3 in Fig. 2) is made according to the circuit of an error amplifier (EA) and a PWM pulse shaper.

It receives the dimmer voltage from the main monitor board, after which this voltage is compared with the feedback voltage, and then an error signal is generated that controls the frequency of the PWM pulses. These pulses are used to control the DC/DC converter (1 in Fig. A2) and synchronize the operation of the converter-inverter. The amplitude of the pulses is constant and is determined by the supply voltage (+12 V), and their frequency depends on the brightness voltage and the threshold voltage level. The DC/DC converter (1) provides constant (high) voltage, which is supplied to the autogenerator. This generator is turned on and controlled by PWM pulses from the control unit (3). The level of the inverter's AC output voltage is determined by the parameters of the circuit elements, and its frequency is determined by the brightness control and the characteristics of the backlight lamps. The inverter converter is usually a self-excited generator. Both single-cycle and push-pull circuits can be used. The protection unit (5 and 6) analyzes the level of voltage or current at the inverter output and generates feedback (OS) and overload voltages, which are supplied to the control unit (2) and PWM (3). If the value of one of these voltages (in the event of a short circuit, converter overload, low supply voltage) exceeds the threshold value, the autogenerator stops operating. As a rule, on the screen the control unit, PWM and brightness control unit are combined in one chip. The converter is made on discrete elements with a load in the form of a pulse transformer, the additional winding of which is used to switch the trigger voltage. All main inverter components are housed in SMD component housings. There are a large number of modifications of inverters. The use of one type or another is determined by the type of LCD panel used in a given monitor, so inverters of the same type can be found from different manufacturers. Let's look at the most commonly used types of inverters, as well as their typical faults.

Inverter type PLCD2125207A from EMAKH This inverter is used in LCD panels from Proview, Acer, AOC, BENQ and LG with a screen diagonal of no more than 15 inches. It is built according to a single-channel circuit with

minimum number of elements (Fig. PZ). At an operating voltage of 700 V and a load current of 7 mA using two lamps, the maximum screen brightness is about 250 cd/m2. The starting output voltage of the inverter is 1650 V, the protection response time is from 1 to 1.3 s. At idle, the output voltage is 1350 V. The greatest depth of brightness is achieved by changing the control voltage DIM (pin 4 of CON1 connector) from 0 (maximum brightness) to 5 V (minimum brightness). The inverter from SAMPO is made according to the same scheme.

Description of the circuit diagram

Rice. H. Schematic diagram of an inverter type PLCD2125207A from EMAKH

+12 V voltage is supplied to the pin. 1 connector CON1 and through fuse F1 - to pin. 1-3 assemblies Q3 (source of the field effect transistor). The boost DC/DC converter is assembled using elements Q3-Q5, D1, D2, Q6. In operating mode, the resistance between the source and drain of transistor Q3 does not exceed 40 mOhm, while a current of up to 5 A is passed into the load. The converter is controlled by a brightness and PWM controller, which is made on a U1 chip of the TL5001 type (analogous to FP5001) from Feeling Tech. The main element of the controller is a comparator, in which the voltage of the sawtooth voltage generator (pin 7) is compared with the voltage of the control device, which in turn is determined by the relationship between the reference voltage of 1 V and the total feedback voltage and brightness (pin 4). The frequency of the sawtooth voltage of the internal generator (about 300 kHz) is determined by the value of resistor R6 (connected to pin 7 of U1). PWM pulses are taken from the output of the comparator (pin 1), which are supplied to the DC/DC converter circuit. The controller also provides protection against short circuit and overload. If there is a short circuit at the inverter output, the voltage at the divider R17 R18 increases, it is rectified and supplied to the pin. 4 U1. If the voltage becomes 1.6 V, the controller protection circuit is activated. The protection response threshold is determined by the value of resistor R8. Capacitor C8 provides a “soft” start when starting the inverter or after the end of a short circuit. If the short circuit lasts less than 1 s (the time is determined by the capacitance of capacitor C7), then normal operation of the inverter continues. Otherwise, the inverter operation stops. To reliably start the converter, the protection response time is selected to be 10...15 times longer than the start and “ignition” time of the lamps. When the output stage is overloaded, the voltage at the right terminal of inductor L1 increases, the zener diode D2 begins to pass current, transistor Q6 opens and the response threshold of the protection circuit decreases. The converter is made according to the circuit of a half-bridge generator with self-excitation on transistors Q7, Q8 and transformer PT1. When the power-on voltage is received from the main monitor board ON/OFF (3

B) transistor Q2 opens and power is supplied to controller U1 (+12 V to pin 2). PWM pulses with pin. 1 U1 through transistors Q3, Q4 goes to the gate of Q3, thereby starting the DC/DC converter. In turn, power is supplied from it to the autogenerator. After this, a high-voltage alternating voltage appears on the secondary winding of transformer PT1, which is supplied to the backlight lamps. Winding 1-2 PTT performs the role of feedback of the self-oscillator. While the lamps are not turned on, the output voltage of the inverter rises to the starting voltage (1650 V), and then the inverter goes into operating mode. If the lamps cannot be ignited (due to a break, “exhaustion”), spontaneous generation failure occurs.

Malfunctions of the PLCD2125207A inverter and how to eliminate them

The backlights do not turn on.

Check the +12 V supply voltage at the pin. 2 U1. If it is not there, check fuse F1, transistors Q1, Q2. If fuse F1 is faulty, before replacing it, check transistors Q3, Q4, Q5 for a short circuit. Then check the ENB or ON/OFF signal (pin 3 of CON1 connector) - its absence may be due to a malfunction of the monitor’s main board. This is checked in the following way: a control voltage of 3...5 V is supplied to the ON/OFF input from an independent power source or through a divider from a 12 V source. If the lamps turn on, then the main board is faulty, otherwise the inverter is faulty. If there is supply voltage and a turn-on signal, but the lamps do not light, then carry out an external inspection of the transformer PT1, capacitors SY, C11 and lamp connectors CON2, CON3, and replace the darkened and melted parts. If at the moment of switching on the pin. 11 of transformer PT1, voltage pulses appear for a short time (the oscilloscope probe is connected through a divider in advance, before turning on the monitor), and the lamps do not light, then check the condition of the lamp contacts and the absence of mechanical damage on them. The lamps are removed from their seats, having first unscrewed the screw securing their housing to the matrix body, and, together with the metal housing in which they are installed, are removed evenly and without distortions. In some monitor models (Acer AL1513 and BENQ), the lamps are L-shaped and cover the LCD panel around the perimeter, and careless actions during dismantling can damage them. If the lamps are damaged or darkened (which indicates a loss of their properties), they are replaced. Lamps can only be replaced with ones of similar power and parameters, otherwise either the inverter will not be able to “ignite” them, or an arc discharge will occur, which will quickly damage the lamps.

The lamps turn on for a short time (about 1 second) and then turn off immediately

In this case, protection against short circuit or overload in the secondary circuits of the inverter is most likely triggered. Eliminate the reasons for the protection operation, check the serviceability of the transformer PT1, capacitors SY and C11 and the feedback circuit R17, R18, D3. Check the zener diode D2 and transistor Q6, and

also capacitor C8 and divider R8 R9. If the voltage at the pin. 5 is less than 1 V, then replace the capacitor C7 (preferably with a tantalum one). If all of the above steps do not produce results, replace the U1 chip. Turning off the lamps may also be due to a failure of the converter generation. To diagnose this malfunction, instead of lamps, an equivalent load is connected to connectors CON2, CON3 - a resistor with a nominal value of 100 kOhm and a power of at least 10 W. A 10 ohm measuring resistor is connected in series with it. Instruments are connected to it and the oscillation frequency is measured, which should be in the range from 54 kHz (at maximum brightness) to 46 kHz (at minimum brightness) and the load current from 6.8 to 7.8 mA. To control the output voltage, connect a voltmeter between the pins. 11 of transformer PT1 and the output of the load resistor. If the measured parameters do not correspond to the nominal, control the magnitude and stability of the supply voltage at inductor L1, and also check transistors Q7, Q8, C9. If, when the right (according to the diagram) diode of assembly D3 is disconnected from resistor R5, the screen lights up, then one of the lamps is faulty. Even with one working lamp, the image brightness is enough for the operator to work comfortably.

The screen flickers periodically and the brightness is unstable

Check the stability of the brightness voltage (DIM) on pin. 4 connectors CON1 and after resistor R3, having previously disabled feedback (resistor R5). If the control voltage at the connector is unstable, then the main board of the monitor is faulty (the test is carried out in all available modes of operation of the monitor and across the entire brightness range). If the voltage is unstable at the pin. 4 controller U1, then check its DC mode in accordance with table. P1, while the inverter must be in operating mode. The faulty microcircuit is replaced. They check the stability and amplitude of oscillations of their own sawtooth pulse generator (pin 7), the signal swing should be from 0.7 to 1.3 V, and the frequency should be about 300 kHz. If the voltage is unstable, replace R6 or U1. Instability of the inverter may be due to aging of the lamps or their damage (periodic loss of contact between the supply wires and the lamp terminals). To check this, as in the previous case, connect an equivalent load. If the inverter operates stably, then it is necessary to replace the lamps.

After some time (from several seconds to several minutes) the image disappears

The protection circuit is not working correctly. Check and, if necessary, replace capacitor C7 connected to the pin. 5 controllers, control the DC mode of controller U1 (see previous fault). Check the stability of the lamps by measuring the level of sawtooth pulses at the output of the feedback circuit, on the right anode D3 (swing about 5 V) with the medium setting

brightness (50 units). If voltage surges occur, check the serviceability of the transformer and capacitors C9, C11. Finally, check the stability of the PWM controller circuit U1.

Inverter type DIVTL0144-D21 from SAMPO

The schematic diagram of this inverter is shown in Fig. 4.

It is used to power the backlight lamps of 15-inch matrices from SUNGWUN, SAMSUNG, LG-PHILIPS, HITACHI. Operating voltage - 650 V at a load current of 7.5 mA (at maximum brightness) and 4.5 mA at minimum. The starting voltage (“ignition”) is 1900 V, the frequency of the lamp supply voltage is 55 kHz (at average brightness). The brightness control signal level ranges from 0 (maximum) to 5 V (minimum). The protection response time is 1...4 s. A U201 microcircuit of type BA9741 from ROHM (its analogue TL1451) is used as a controller and PWM. It is a two-channel controller, but in this case only one channel is used. When the monitor is turned on, +12 V is supplied to the pin. 1-3 transistor assembly Q203 (field effect transistor source). When the monitor is turned on, the inverter ON/OFF start signal (+3 V) comes from the main board and opens transistors Q201, Q202. Thus, +12 V voltage is supplied to the pin. 9 controllers U201. After this, the internal sawtooth voltage generator begins to operate, the frequency of which is determined by the ratings of the elements R204 and C208 connected to the pin. 1 and 2 microcircuits. On the pin. 10 of the microcircuit, PWM pulses appear, which are supplied to the gate of Q203 through an amplifier on transistors Q205, Q207. On the pin. 5-8 Q203 a constant voltage is generated, which is supplied to the self-oscillator (on elements Q209, Q210, PT201). A sinusoidal voltage with a swing of 650 V and a frequency of 55 kHz (at the moment the lamps are “ignited” it reaches 1900 V) from the output of the converter through connectors CN201, CN202 is supplied to the backlight lamps. Elements D203, R220, R222 are used to generate a protection signal and a “soft” start. When the lamps are turned on, the energy consumption in the primary circuit of the inverter increases and the voltage at the output of the DC/DC converter (Q203, Q205, Q207) increases, the zener diode D203 begins to conduct current, and part of the voltage from the divider R220 R222 is supplied to the pin. 11 of the controller, thereby increasing the response threshold of the protection circuit during startup. The stability and brightness of the lamps, as well as short-circuit protection, is ensured by a feedback circuit on elements D209, D205, R234, D207, C221. The feedback voltage is supplied to the pin. 14 microcircuits (direct input of the error amplifier), and the brightness voltage from the main monitor board (DIM) - to the inverse input of the control unit (pin 13), determining the frequency of PWM pulses at the controller output, and hence the output voltage level. At minimum brightness (DIM voltage is 5 V) it is 50 kHz, and at maximum (DIM voltage is zero) it is 60 kHz. If the feedback voltage exceeds 1.6 V (pin 14 of the U201 chip), the protection circuit is turned on. If a short circuit in the load lasts less than 2 s (this is the charging time of capacitor C207 from the reference voltage +2.5 V - pin 15

microcircuits), the functionality of the inverter is restored, which ensures reliable starting of the lamps. If there is a long-term short circuit, the inverter turns off.

Malfunctions of the DIVTL0144-D21 inverter and methods for their elimination

Lamps don't light up

Check the presence of +12 V voltage on the pin. 1-3 Q203, serviceability of fuse F1 (installed on the main board of the monitor). If the fuse is faulty, then before installing a new one, check transistors Q201, Q202, as well as capacitors C201.C202, C225 for a short circuit. Check the presence of ON/OFF voltage: when turning on the operating mode, it should be equal to 3 V, and when turning off or switching to standby mode, it should be zero. If there is no control voltage, check the main board (turning on the inverter is controlled by the microcontroller of the LCD panel). If all of the above voltages are normal, and the PWM pulses are on the pin. 10 there is no V201 microcircuit, check zener diodes D203 and D201, transformer RT201 (can be determined by visual inspection by a darkened or melted case), capacitors C215, C216 and transistors Q209, Q210. If there is no short circuit, then check the serviceability and rating of capacitors C205 and C207. If the above elements are in good condition, replace the U201 controller. Note that the absence of illumination of the backlight lamps may be due to their breakage or mechanical failure.

Lamps turn on and off briefly

If the illumination persists for 2 s, then the feedback circuit is faulty. If, when disconnecting elements L201 and D207 from the circuit, pin. 7 of the U201 chip, PWM pulses appear, then either one of the backlight lamps or the feedback circuit is faulty. In this case, check the zener diode D203, diodes D205, D209, D207, capacitors C221, C219, and inductor L202. Monitor the voltage at the pin. 13 and 14 U201. In operating mode, the voltage at these pins should be the same (about 1 V - at average brightness). If the voltage at the pin. 14 is significantly lower than on pin. 13, then check diodes D205, D209 and lamps for open circuits. With a sharp increase in voltage at the pin. 14 microcircuits U201 (above the level of 1.6 V) check elements PT1, L202, C215, C216. If they are working, replace the U201 chip. When replacing it with an analogue (TL1451), check the threshold voltage at the pin. 11 (1.6 V) and, if necessary, select the value of elements C205, R222. By selecting the values ​​of elements R204, C208, the frequency of the sawtooth pulses is set: on the pin. 2 chips should be around 200 kHz.

The backlight turns off after some time (from several seconds to several minutes) after turning on the monitor

First, check capacitor C207 and resistor R207. Then check the serviceability of the contacts of the inverter and backlight lamps, capacitors C215, C216 (by replacement), transformer RT201, transistors Q209, Q210. Control

threshold voltage at pin. 16 V201 (2.5 V), if it is low or missing, replace the chip. If the voltage at the pin. 12 above 1.6 V, check capacitor C208, otherwise also replace U201.

The brightness changes spontaneously throughout the entire range or in individual operating modes of the TV (monitor)

If the malfunction appears only in certain resolution modes and in a certain brightness range, then the malfunction is related to the main board (memory chip or LCD controller). If the brightness changes spontaneously in all modes, then the inverter is faulty. Check the brightness adjustment voltage (at pin 13 U201 - 1.3 V (at average brightness), but not higher than 1.6 V). If the voltage at the DIM contact is stable, and at the pin. 13 - no, replace the U201 chip. If the voltage at the pin. 14 is unstable or too low (less than 0.3 V at minimum brightness), then instead of the lamps, an equivalent load is connected - a resistor with a nominal value of 80 kOhm. If the defect persists, replace the U201 chip. If this replacement does not help, replace the lamps and also check the serviceability of their contacts. Measure the voltage at the pin. 12 of the U201 chip, in operating mode it should be about 1.5 V. If it is below this limit, check elements C209, R208. Note. In inverters from other manufacturers (EMAX, TDK), made according to a similar scheme, but using other components (except for the controller): the SI443 chip is replaced with D9435, and 2SC5706 with 2SD2190. The voltage at the pins of the U201 chip can vary within ±0.3 V.

Inverter from TDK.

This inverter (Fig. 5) is used in 17-inch monitors and TVs with SAMSUNG matrices, and its simplified version (Fig. 6) is used in 15-inch LG monitors with LG-PHILIPS matrix.

The circuit is implemented on the basis of a 2-channel PWM controller from OZ960 O2MICRO with 4 control signal outputs. Transistor assemblies of the type FDS4435 (two field-effect transistors with a p-channel) and FDS4410 (two field-effect transistors with an n-channel) are used as power switches. The circuit allows you to connect 4 lamps, which provides increased brightness of the LCD panel backlight. The inverter has the following characteristics: supply voltage - 12 V; rated current in the load of each channel - 8 mA; operating voltage of the lamps is 850 V, starting voltage is 1300 V;

output voltage frequency - from 30 kHz (at minimum brightness) to 60 kHz (at maximum brightness). The maximum screen brightness with this inverter is 350 cd/m2; protection response time - 1...2 s. When the monitor is turned on, +12 V is supplied to the inverter connector - to power the Q904-Q908 keys and +6 V - to power the U901 controller (in the version for the LG monitor, this voltage is formed from the +12 V voltage, see the diagram in Fig. P6) . In this case, the inverter is in standby mode. The ENV controller turn-on voltage is supplied to the pin. 3 microcircuits from the microcontroller of the main monitor board. The PWM controller has two identical outputs for powering two inverter channels: pin. 11, 12 and pin. 19, 20 (Fig. P5 and P6). The operating frequency of the generator and PWM are determined by the values ​​of resistor R908 and capacitor C912 connected to the pin. 17 and 18 microcircuits (Fig. P5). Resistor divider R908 R909 determines the initial threshold of the sawtooth voltage generator (0.3 V). On capacitor C906 (pin 7 U901) the threshold voltage of the comparator and protection circuit is formed, the response time of which is determined by the rating of capacitor C902 (pin 1). The protection voltage against short circuit and overload (if the backlight lamps break) is supplied to the pin. 2 microcircuits. The U901 controller has built-in soft start circuitry and an internal stabilizer. The start of the soft start circuit is determined by the voltage at the pin. 4 (5 V) controllers. The DC voltage converter into high-voltage lamp supply voltage is made on two pairs of p-type FDS4435 and n-type FDS4410 transistor assemblies and is forcedly triggered by pulses with PWM. A pulsating current flows in the primary winding of the transformer, and the supply voltage for the backlight lamps connected to connectors J904-J906 appears on the secondary windings of T901. To stabilize the inverter output voltages, the feedback voltage is supplied through full-wave rectifiers Q911-Q914 and the integrating circuit R938 C907 C908 and is supplied to the pin in the form of sawtooth pulses. 9 controllers U901. If one of the backlight lamps breaks, the current increases through the divider R930 R932 or R931 R933, and then the rectified voltage is supplied to the pin. 2 controllers exceeding the set threshold. Thus, the formation of PWM pulses on the pin. 11, 12 and 19, 20 U901 is blocked. In the event of a short circuit in the circuits C933 C934 T901 (winding 5-4) and C930 C931 T901 (winding 1-8), “spikes” of voltage occur, which are rectified by Q907-Q910 and also supplied to the pin. 2 controllers - in this case the protection is triggered and the inverter is turned off. If the short circuit time does not exceed the charging time of capacitor C902, then the inverter continues to operate in normal mode. The fundamental difference between the circuits in Fig. P5 and P6 is that in the first case a more complex “soft” start circuit is used (the signal is sent to pin 4 of the microcircuit) on transistors Q902, Q903. In the diagram in Fig. P6 it is implemented on a capacitor SY. It also uses assemblies of field-effect transistors U2, U3 (p- and n-type), which simplifies their power matching and ensures high reliability in circuits with two lamps. In the diagram in Fig. P5 uses field-effect transistors Q904-Q907, connected in a bridge circuit, which increases the output power of the circuit and reliability of operation in starting modes and at high currents.

Inverter malfunctions and ways to eliminate them

Lamps do not turn on

Check the presence of supply voltage +12 and +6 V per pin. Vinv, Vdd of the inverter connector respectively (Fig. A5). If they are absent, check the serviceability of the main monitor board, assemblies Q904, Q905, zener diodes Q903-Q906 and capacitor C901. Check the supply of +5 V inverter switch-on voltage to the pin. Ven when switching the monitor to operating mode. You can check the serviceability of the inverter using an external power source by applying a voltage of 5 V to the pin. 3 U901 chips. If the lamps turn on, then the cause of the malfunction is in the main board. Otherwise, they check the inverter elements and monitor the presence of PWM signals on the pin. 11, 12 and 19, 20 U901 and, in case of their absence, replace this microcircuit. They also check the serviceability of the windings of the T901 transformer for open circuits and short circuits of the turns. If a short circuit is detected in the secondary circuits of the transformer, first of all, check the serviceability of capacitors C931, C930, C933 and C934. If these capacitors are working properly (you can simply unsolder them from the circuit), and a short circuit occurs, open the installation location of the lamps and check their contacts. Burnt contacts are restored.

The backlights flash for a short time and then go out immediately

Check the serviceability of all lamps, as well as their connection circuits with connectors J903-J906. You can check the serviceability of this circuit without disassembling the lamp unit. To do this, turn off the feedback circuit for a short time, sequentially soldering diodes D911, D913. If the second pair of lamps turns on, then one of the lamps of the first pair is faulty. Otherwise, the PWM controller is faulty or all the lamps are damaged. You can also check the performance of the inverter by using an equivalent load instead of lamps - a 100 kOhm resistor connected between pins. 1, 2 connectors J903, J906. If in this case the inverter does not work and there are no PWM pulses on the pin. 19, 20 and 11, 12 U901, then check the voltage level at the pin. 9 and 10 microcircuits (1.24 and 1.33 V, respectively. In the absence of the specified voltages, check elements C907, C908, D901 and R910. Before replacing the controller microcircuit, check the rating and serviceability of capacitors C902, C904 and C906.

The inverter turns off spontaneously after a while (from a few seconds to a few minutes)

Check the voltage at the pin. 1 (about 0 V) ​​and 2 (0.85 V) U901 in operating mode, change capacitor C902 if necessary. If there is a significant difference in voltage at the pin. 2 from the nominal value, check the elements in the short circuit and overload protection circuit (D907-D910, C930-C935, R930-R933) and, if they are working, replace the controller chip. Check the voltage ratio on the pin. 9 and 10 microcircuits: on pin. 9 voltage should be lower. If this is not the case, check the capacitive divider C907 C908 and feedback elements D911-D914, R938. Most often, the cause of such a malfunction is caused by a defect in the capacitor C902.

The inverter is unstable, the backlight lamps are blinking

Check the performance of the inverter in all operating modes of the monitor and in the entire brightness range. If instability is observed only in some modes, then the main board of the monitor (circuit for generating brightness voltage) is faulty. As in the previous case, an equivalent load is connected and a milliammeter is installed in the open circuit. If the current is stable and equal to 7.5 mA (at minimum brightness) and 8.5 mA (at maximum brightness), then the backlight lamps are faulty and must be replaced. They also check the secondary circuit elements: T901, C930-C934. Then check the stability of rectangular pulses (average frequency - 45 kHz) on the pin. 11, 12 and 19, 20 U901 microcircuits. The DC component on them should be 2.7 V at the P-outputs and 2.5 V at the N-outputs). Check the stability of the sawtooth voltage at the pin. 17 microcircuits and, if necessary, replace C912, R908.

Inverter from SAMPO

The schematic diagram of the SAMPO inverter is shown in Fig. 7.

It is used in 17-inch SAMSUNG, AOC panels with SANYO matrices, in “Preview SH 770” and “MAG HD772” monitors. There are several modifications of this scheme. The inverter produces an output voltage of 810 V at rated current through each of the four fluorescent lamps (about 6.8 mA). The starting output voltage of the circuit is 1750 V. The operating frequency of the converter at average brightness is 57 kHz, while the brightness of the monitor screen is achieved up to 300 cd/m2. The response time of the inverter protection circuit is from 0.4 to 1 s. The basis of the inverter is the TL1451AC microcircuit (analogs - TI1451, BA9741). The microcircuit has two control channels, which makes it possible to implement a power supply circuit for four lamps. When the monitor is turned on, +12 V voltage is supplied to the inputs of the +12 V voltage converters (sources of field-effect transistors Q203, Q204). The DIM brightness control voltage is supplied to the pin. 4 and 13 microcircuits (inverse inputs of error amplifiers). When a turn-on voltage of 3 V (ON/OFF pin) is received from the main monitor board, transistors Q201 and Q202 open and pin. 9 (VCC) of the U201 chip, +12 V is supplied. 7 and 10, rectangular PWM pulses appear, which arrive at the bases of transistors Q205, Q207 (Q206, Q208), and from them to Q203 (Q204). As a result, voltages appear on the right-hand terminals of the chokes L201 and L202, the value of which depends on the duty cycle of the PWM signals. These voltages power oscillator circuits made on transistors Q209, Q210 (Q211, Q212). On the primary windings of 2-5 transformers RT201 and RT202, a pulse voltage appears, respectively, the frequency of which is determined by the capacitance of capacitors C213, C214, the inductance of the windings of 2-5 transformers RT201, RT202, as well as the level of the supply voltage. When adjusting the brightness, the voltage at the outputs of the converters changes and, as a result, the frequency of the generators. The amplitude of the inverter output pulses is determined by the supply voltage and load condition.

Autogenerators are made according to a half-bridge circuit, which provides protection against high currents in the load and breakage in the secondary circuit (turning off lamps, breaking capacitors C215-C218). The basis of the protection circuit is located in the U201 controller. In addition, the protection circuit includes elements D203, R220. R222 (D204, R221, R223), as well as the feedback circuit D205 D207 R240 C221 (D206 D208 R241 C222). When the voltage at the output of the converter increases, the zener diode D203 (D204) breaks through and the voltage from the divider R220, R222 (R221, R223) goes to the input of the overload protection circuit of the controller U201 (pins 6 and 11), increasing the protection threshold for the time the lamps are started. Feedback circuits rectify the voltage at the output of the lamps and it goes to the direct inputs of the controller error amplifiers (pin 3, 13), where it is compared with the brightness control voltage. As a result, the frequency of the PWM pulses changes and the brightness of the lamps is maintained at a constant level. If this voltage exceeds 1.6 V, a short circuit protection circuit will be activated, which will operate while capacitor C207 is charging (about 1 s). If the short circuit lasts less than this time, the inverter will continue to operate normally.

Malfunctions of the SAMPO inverter and ways to eliminate them

The inverter does not turn on, the lamps do not light up

Check the presence of +12 V voltages and the active state of the ON/OFF signal. If +12 V is missing, check its presence on the main board, as well as the serviceability of transistors Q201, Q202, Q205, Q207, Q206, Q208) and Q203, Q204. If there is no ONN/OFF inverter turn-on voltage, it is supplied from an external source: +3...5 V through a 1 kOhm resistor to the base of transistor Q201. If the lamps turn on, then the malfunction is associated with the formation of the inverter turn-on voltage on the main board. Otherwise, check the voltage at the pin. 7 and 10 U201. It should be equal to 3.8 V. If the voltage at these pins is 12 V, then the U201 controller is faulty and must be replaced. Check the reference voltage at the pin. 16 U201 (2.5 V). If it is zero, check capacitors C206, C205 and, if they are working, replace controller U201. Check the presence of generation on the pin. 1 (sawtooth voltage with a swing of 1 V) and, in its absence, capacitor C208 and resistor R204.

The lamps come on, but then go out.

Check the serviceability of zener diodes D201, D202 and transistors Q209, Q210 (Q211, Q212). In this case, one of the pairs of transistors may be faulty. Check the overload protection circuit and the serviceability of zener diodes D203, D204, as well as the values ​​of resistors R220, R222 (R221, R223) and capacitors C205, C206. Check the voltage at the pin. 6 (11) controller chips (2.3 V). If it is underestimated or equal to zero, check elements C205, R222 (C206, R223). If there are no PWM signals on the pin. 7 and 10 microcircuits U201 measure the voltage at the pin. 3 (14). It should be 0.1...0.2 V more than the pin. 4 (13), or the same. If this condition is not met, check elements D206, D208, R241. When performing the above measurements, it is better to use an oscilloscope. The inverter shutdown may be due to a break or mechanical damage to one of the lamps. To test this assumption

(so as not to disassemble the lamp assembly) turn off the +12 V voltage of one of the channels. If the monitor screen starts to light up, then the disconnected channel is faulty. They also check the serviceability of transformers RT201, RT202 and capacitors C215-C218.

The lamps turn off spontaneously after some time (from a few seconds to minutes)

As in previous cases, the elements of the protection circuit are checked: capacitors C205, C206, resistors R222, R223, as well as the voltage level at the pin. 6 and 11 U201 chips. In most cases, the cause of the defect is caused by a malfunction of capacitor C207 (which determines the protection response time) or controller U201. Measure the voltage at the chokes L201, L202. If the voltage rises steadily during the operating cycle, check transistors Q209, Q210 (Q211, Q212), capacitors C213, C214 and zener diodes D203, D204.

The screen flickers periodically and the screen backlight brightness is unstable

Check the serviceability of the feedback circuit and the operation of the error amplifier of the U201 controller. Measure the voltage at the pin. 3, 4, 12, 13 microcircuits. If the voltage at these pins is below 0.7 V, and at the pin. 16 below 2.5 V, then replace the controller. Check the serviceability of the elements in the feedback circuit: diodes D205, D207 and D206, D208. Connect load resistors with a nominal value of 120 kOhm to the CON201-CON204 connectors, check the level and stability of the voltages on the pin. 14 (13), 3 (4), 6 (11). If the inverter operates stably with the load resistors connected, replace the backlight lamps.

Installation and repair of LCD panels using the example of a SAMSUNG TV Models: LW17M24C, LW20M21C Chassis: VC17EO, VC20EO

General information

LCD TVs Samsung LW17M24C, LW20M21C are universal television receivers with screen sizes of 37 and 51 cm. The televisions are designed to receive and reproduce image signals and audio from television programs in the meter and decimeter wavelength ranges of broadcast television of PAL, SECAM and NTSC color television systems. M. TVs provide the ability to connect external sources (VCR, DVD player, video set-top box) to play video recordings, record via video frequency, or to work as a personal computer monitor. TVs allow you to process and reproduce teletext information using a decoder with a 10-page memory.

Main technical characteristics of TVs LW17M24C and LW20M21C LCD panel

TFT-LCD panel, 17" diagonal TFT-LCD panel, 20" diagonal

Synchronization frequency range (automatic frequency adjustment) Horizontal frequency 30...80 kHz 28..33 kHz

Frame rate 50...75Hz

Number of colors displayed 16.2 million |

Matrix response time Less than 25ms

Brightness 450cd/m2

Contrast 500:1

Horizontal viewing angle 160 degrees

Vertical viewing angle 160 degrees

Maximum resolution 1280 x 1024 pixels

Monitor Input Options Video signals RGB Analog, 0.7 V±5% swing, positive polarity, input impedance

75 Ohm Clock signal

Separate (H/V), with TTL levels Nutrition

Alternating voltage 100...24О V with frequency 50...60 Hz Power consumption

Television parameters of the TV system

NTSC-M, PAL/ SECAMJ.(Euro multi) Sound

Mono, Stereo (A2/NICAM) Antenna input

75 Ohm coaxial input Beep Options

Exit UMZCH power: 2.5Wx2

Headphone: 10 mW LF input: 80Hz...20kHz Frequency range

TV signal: 80 Hz...15 kHz | LF input:80Hz...20kHz Types of LF input-output connectors

SCART, RCA, S-VHS

Type of connector for connecting to a PC DSUB(15-KOHTaKT0B) |

TV DESIGN

Structural components of televisions.

The names of the parts and their catalog numbers (Part. No.) are given.

Structural components of the TV LW17M24C Number in Fig. 4.1 Name Part.Nfi

1 ASSY COVER ERONT BN96–01255B

2 LCD-PANEL BN07–00115A

4 SCREW TAPTfTE 6005–000259

5 IP BOARD BN44–00111B

5 ASSY BRKJ PANEL BN96–01564A

6 ASSY MAIN BOARD BN94–00559S

COVER-CONNECTOR BN65–01557A

8 SCREW TARTGGK 6005–000259

9 HOLDER-JACK BN61–01570A

10 SCREW TAPTITE 6005–000277

11 ASSYSHIEED-TUNER BN96–01595A

12 SCREW TAPT1JE 6005–000259

14 SCREW TAPTIJE 6005–001525

15 ASSY-STAND BN65–01555A

15 ASSY COVER BACK BN96–01256B

Structural components of the TV LW20M21C Numbers in Figure 4.2 Name Part. No.

1 ASSY COVER FRONT BN96–01158B