LCD is a kind ofThin film transistorDrivenActive matrixLiquid crystal display, which mainly uses current to stimulate liquid crystal molecules to generate points, lines and surfaces to form a picture with the back lamp.IPS、TFT、SLCDAll belong to the subcategory of LCD.[1]Its working principle is that under the action of electric field, the arrangement direction of liquid crystal molecules changes to make the external light sourceTransmittanceChange (modulation), complete electric optical conversion, and then use R, G, BTricolorDifferent excitations of signals, through the three primary colors of red, green and blueFilter film, complete time domain andSpatial domainThe color of is redisplayed.[1]
LCDLiquid Crystal Display may not be a new term for many users, but the history of this technology may be far beyond our imagination.As early as the end of the 19th century,AustriabotanistLiquid crystal, that is, liquidcrystalThat is to say, a substance has the fluidity of liquid and some arrangement characteristics similar to crystals at the same time.Under the action of electric field, the arrangement of liquid crystal molecules will change, thus affecting its optical properties. This phenomenon is calledElectro-optic effect。Using the electro-optic effect of liquid crystal, British scientists made the first liquid crystal display, namely LCD, in the last century.[2]
The first liquid crystal display device in the world appeared in the early 1970s, which is called TN-LCD (twisted nematic) liquid crystal display.AlthoughMonochrome display, it is still extended toElectronic watch, calculators, etc.[2]
Polarizing filmalso calledPolarizer(Polarizer）The polarizer is divided into an upper polarizer and a lower polarizer. The polarization functions of the upper and lower polarizers are perpendicular to each other and act like a fence. The light wave components are blocked as required, for example, the light wave components perpendicular to the fence of the polarizer are blocked, and only the light wave components parallel to the fence are allowed to pass through.[3]
Glass substrate(Glass Substrate) in LCD can be divided into upper substrate and lower substrate, whose main function is to clamp the space between the two substratesLiquid crystal materials。The glass substrate is generally made ofMechanical propertiesExcellent, heat resistant and resistantChemical corrosionAlkali freeBorosilicate glass。aboutTFT-For LCD, one layer of glass substrate is distributed with TFT, and the other layer of glass substrate is depositedColor filter。[3]
Black Matrix is used to separate red, green and blue in the color filter with the help of materials with high shading performanceTricolor(to prevent color confusion), prevent light leakage, so as to improve theColor blockContrast of.In addition, in TFT-LCD, the black matrix can also mask the internal electrode wiring orThin film transistor。[3]
Color filter(Color Filter)Color filterFilm, which is used to generate three basic colors of red, green and blue, and realize the full range of LCDColor display。[3]
Orientation film(Alignment Layer), also known as alignment film or directional layer, is used to enable liquid crystal molecules to achieve uniform alignment and orientation on the level of micro size.[3]
Transparent electrode is divided into public electrode and pixel electrode,Input signalThe voltage is loaded between the pixel electrode and the common electrode.Transparent electrodes are usually deposited on glass substratesIndium tin oxide(ITO) material constitutes a transparent conductive layer.[3]
Drive IC is actually a setIntegrated circuit chipThe device is used to adjust and control the phase, peak value, frequency, etc. of the potential signal on the transparent electrode, establish the driving electric field, and finally realize the information display of the liquid crystal.[3]
stayLCD panelMedium,Active matrixLCDIt's at twoGlass substrateIt is composed of twisted nematic (TN) liquid crystal material sealed between.The upper glass substrate close to the display screen is deposited with red, green and blue（RGB）Three color filter (or color filter film), black matrix and public transparent electrode.The lower glass base plate (the base plate far from the display screen) is equipped withThin film transistor(TFT) devicesTransparent pixelsElectrode, storage capacitance, grid lineThe signal lineEtc.An alignment film (or alignment layer) is prepared on the inner side of the two glass substrates to orient the liquid crystal molecules.Liquid crystal material is poured between two glass substrates to disperseGasket(Spacer) to ensureUniformity。The four sides are bonded with the aid of the frame sealing glue to play a sealing role;Make the upper and lower glass substrates common by means of silver dot adhesive processElectrode connection。[3]
Up and downGlass substrateOn the outside of thePolarizationFilm (or polarizing film).When a voltage is applied between the pixel transparent electrode and the public transparent electrode, the arrangement state of liquid crystal molecules will change.At this point,incident lightThe intensity of liquid crystal transmission also changes accordingly.LCD display is based onLiquid crystal materialsOfoptical rotationWith the control of electric field, information display can be realized.[3]
Backlight module
Backlight module
LCD product is a nonActive lightingelectronic device, does not have luminous characteristics, must rely onBacklight moduleCan only be obtained by the emission of light source inDisplay performanceTherefore, the brightness of LCD is determined by its backlight module.It can be seen that the performance of backlight module directly affectsLCD panelDisplay quality of.[3]
Reflector SheetReflectorThe main function is to completely send the light from the light source to the light guide plate, so as to reduce the useless consumption as far as possible.[3]
Light guide plate(Light Guide Plate) is mainly used to guide the light emitted by the side light source to the front of the panel.[3]
Prismatic sheetPrism Film, also known as Brightness Enhancement Film, is mainly used to refract andtotal reflection, focus on a certain angle, and then emit from the backlight to achieve the screen brightening display effect.[3]
Diffuser(Diffuser）The main function is to correct the side light of the backlight module to be uniformArea light sourceTo achieve the effect of optical diffusion.The diffuser can be divided into upper diffuser and lower diffuser.The upper diffuser is located between the prism and the liquid crystal module, which is closer to the display panel.The lower diffuser is located between the light guide plate and the prism, which is closer to the backlight.[3]
working principle
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Liquid crystal display
LCD is a kind of display made of liquid crystal.Liquid crystal is a kind of liquid crystal between solid and liquidOrganic compoundAt room temperature, it shows both liquid fluidity and crystalOptical anisotropyWhen heated, it will become transparent liquid, and when cooled, it will become crystalline turbid solid.[3]
Under the action of electric field, the arrangement of liquid crystal molecules will change, thus affectingincident lightThe beam passes through the liquid crystal to produce a change in intensitylight intensityChanges, further throughPolarizerThe effect of light and shade is reflected in the change of light and shade.According to this, by controlling the electric field of the liquid crystal, the change of light and shade can be achieved, so as to achieve the purpose of information display.Therefore,Liquid crystal materialsIts function is similar to small "light valves".[3]
Because there are control circuits anddrive circuit 。When the electrode in the LCD generates an electric field, the liquid crystal molecule will be twisted, so that the light passing through it will be refracted regularly (the optical rotation of the liquid crystal material), and then filtered by the second layer of polarizer and displayed on the screen.
It is worth pointing out that,Liquid crystal materialsBecause it does not emit light, LCD usually needs to configure additional light sources for the display panel. The main light source system is called“Backlight module”, where,Backlight panelIt is composed of fluorescent materials, which can emit light, and its main function is to provide a uniform backlight.[3]
Monochrome display
LCD technology is to pour liquid crystal between two planes with thin grooves.The slots in these two planes are perpendicular to each other (intersecting at 90 degrees).In other words, if the molecules on one plane are arranged in north-south direction, the molecules on the other plane are arranged in east-west direction, and the molecules between the two planes are forced into a 90 degree torsion state.Since the light travels along the arrangement direction of molecules, the light is also twisted 90 degrees when passing through the liquid crystal.When a voltage is applied to the liquid crystal, the liquid crystal molecule will rotate to change the light transmittance, thus realizingGray scaleDisplay.[3]
LCD usually consists of two mutually perpendicular polarizers.PolarizerIts function is like a fence, blocking light wave components as required.For example, the light wave component perpendicular to the fence of the polarizer is blocked, and only the light wave component parallel to the fence is allowed to pass through.Natural light is randomly scattered in all directions.Two mutually perpendicular polarizers should normally block all natural light trying to penetrate.However, since the two polarizers are full of twisted liquid crystals, the light will be twisted 90 degrees by the liquid crystal molecules after passing through the first polarizer, and finally pass through the second polarizer.[3]
Color display
For laptop or desktop LCD, more complex color displays are required.[3]
As far as color LCD is concerned, it is also required to have colors specially used for color displayFilter layer, which is called "color"filter(Color Filter), also known as "color filter".In the color LCD panel, each pixel is usually composed of three liquid crystalsCellEach cell has a red, green or blue (RGB) three color filter in front of it.In this way, light passing through different cells can display different colors on the screen.[3]
colourfilterBlack matrix and public transparent electrode are generally deposited in front of the display screenGlass substrateOn.Color LCD can create colorful pictures in a high-resolution environment.[3]
Dynamic image display
human beingsVisual organ(Eyes) 'perception of dynamic images“Visual vestige”The phenomenon of high speed movement in the human brainChina CouncilForm a short impression.The principle of "visual vestige" has been applied in early cartoons, films and the latest game programs, so that a series of gradual images can be displayed in front of people quickly and continuously, forming dynamic images.[3]
When the speed of multiple images exceeds 24 frames/s, people's eyes will feel continuous images.This is also the reason why the movie plays at 24 frames per second.If the display speed is lower than this standard, people will obviously feel the pause and discomfort of the picture.According to this indicator, the display time of each screen should be less than 40ms.High definition display of fast moving pictures, general imageMovement speedMore than 60 frames/s.That is to say, the interval time of each frame of the active screen is 16.67ms.[3]
If the response time of the LCD is greater than the interval time of each frame of the picture, people will feel that the picture is somewhat blurred when watching the fast moving image.response timeIt is a special indicator of LCD.The response time of LCD refers to the response speed of each pixel of the display to the input signal, that is, the LCD is composed of“Dark rotationLight or from "light to dark"reaction time。The smaller the value, the better. The response time is fast enough to ensure the continuity of the picture.If the response time is too long, the LCD may displaydynamic image There is a feeling of tail shadow dragging.The response time of LCD is generally 2-5ms.[3]
TFT drive
TFT-LCD structure diagram
TFT meansLCD panelOn glass substratetransistorArray, so that each LCD pixel has its ownSemiconductor switch。Each pixel can control two pieces through dot pulseGlass substrateThe liquid crystal between theActiveSwitch to achieve independent and precise control of each pixel "point-to-point".Therefore, each node of the pixel is relatively independent and canContinuous control。[3]
TFT type LCD is mainly composed ofGlass substrate, grid, drainSource pole, semiconductor active layer (a-Si), etc.[4]
TFT arrays are generally associated with transparent pixel electrodes, storage capacitors, grid linesThe signal lineAnd co deposited on the rear glass substrate of the display screen (the substrate far away from the display screen).The configuration of such a transistor array is helpful to improveLCDOfreaction rate In addition, it can also control the display grayscale to ensure that the LCD image color is more realistic and the picture quality is more pleasing to the eye.Therefore, most LCDs, LCD TVs and some mobile phones are driven by TFT, whether they are small and medium-sized LCDs with narrow viewing angle twisted nematic (TN) mode or parallel arrays with wide viewing angle（IPS)IsomodeLarge scale liquid crystal televisions (LCD-TV), commonly known as“TFT—LCD”。[3]
IPS
IPS screen（In-Plane Switching，Planar transformation）Technology isHitachiLCD panel technology introduced in 2001, commonly known as "Super TFT".IPS screen is a technology based on TFT. Its essence isTFT screen。IPS solves the viewing angle problem by making the molecules have the same apparent length in all directions.[5]
IPS hard screenThe reason why it has clear and ultra stabledynamic display The effect depends on its innovative horizontal transformationMolecular arrangement, changed VASoft screenThe molecules are arranged vertically, so it has a more solid and stable liquid crystal structure.Not in the superficial sense,Hard screenIs inLCD panelAdd a hard protective film to avoidLCD screenSubject to external hard objectsStab wound。[5]
SLCD
S-LCD panel isPVAPanel,SamsungMain PVA modeWide perspectiveS - LCD panel adopts PVA technology, which adopts transparent ITO electrode layer, so its higherOpening ratioBetter thanMVABrightness output;PVA technology also has a high contrast capability of 500:1 and a primary color display capability of up to 70%.[6]
technical parameter
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brightness
The maximum brightness of a liquid crystal display is usually determined byBacklightTo decide that it is technically achievableHigh brightness, but this does not mean that the brightness value increasesGaoyueGood, because the display with too high brightness may cause eye injury to the viewer.LCD is a kind of substance between solid and liquid, which cannot emit light itself. It needs additional light sources.Therefore, the number of lamps is related to the brightness of the LCD.[7]
resolving power
Resolution refers to the number of display pixels per unit area.LCDPhysical resolutionIs fixed forCRT displayAs long as the adjustmentelectron beamOfDeflection voltage, you can change different resolutions.However, it is much more complicated to implement in LCD, and the display effect must be simulated through calculation. The actual resolution is unchanged.Since not all pixels are magnified at the same time, there is a scaling error.When LCD is used in nonStandard resolutionThe text display effect will become worse, and the edge of the text will be blurred.[7]
Chromaticity
LCD is important, of course, colorPerformance。We know that any color in nature is composed of red, green and blueBasic colorComposed of.For example, the LCD panel with a resolution of 1024 × 768 is composed of 1024 × 768 pixels, and each independent pixel color is controlled by three basic colors: red, green, and blue (R, G, B).For LCD monitors produced by most manufacturers, each basic color (R, G, B) reaches 6 bits, that is, 64 expressiveness, so each independent pixel has 64 × 64 × 64=262144 colors.Many manufacturers also use the so-called FRC (Frame Rate Control) technology to display full-color pictures in a simulation way, that is, each basic color (R, G, B) can reach 8 bits, that is, 256 kinds of expressiveness, so each independent pixel has up to 256 × 256 × 256=16777216 colors.[7]
contrast ratio
contrast ratioIt is the ratio that defines the maximum brightness value (all white) divided by the minimum brightness value (all black).Control ICfilterandDirectional membraneAnd other accessories related to the contrast of the panel.[7]
response time
response timeIt refers to the LCD display forInput signalOfreaction rate , that is, the liquid crystal turns from dark to bright or from bright to darkreaction time, usually in milliseconds.Of course, the smaller the value, the better.If the response time is too long, the LCD may displaydynamic image There is a feeling of tail shadow dragging.[7]
viewing angle
LCDviewing angle Symmetric from left to right, but not necessarily from top to bottom.For example, when the backlightincident lightThrough polarizing plate, liquid crystal andOrientation filmAfter that, the output light has a specificDirectional characteristicIn other words, most of the light emitted from the screen has a vertical direction.If we look at an all white picture from a very oblique angle, we may see black or color distortion.Generally speaking, the upper and lower angles should be less than or equal to the left and right angles.If the viewing angle is 80 degrees from left to right, it means thatnormalThe screen image can be clearly seen at 80 degrees.However, due to different vision ranges, if you do not stand in the best visual angle, the color and brightness you see will have errors.[7]
visible area
The size indicated by the LCD is consistent with the actual screen range.For example, a 15.1-inch LCD displayApproximately equal toThe visual range of the 17 inch CRT screen.[7]
Advantages and disadvantages
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advantage
The main principle of the liquid crystal display is to stimulate liquid crystal molecules to generate points, lines and surfaces with the back lamp to form a picture. It has the following four characteristics:[8]
(1) Thin body, space saving, and relatively bulkyCRT displayIn contrast, LCD only needs one third of the space of the former;[8]
(2) It saves electricity and does not generate high temperature. It is a low power consumption product, which can completely avoid scalding compared with CRT displays;[8]
(3) No radiation, good for health, LCD is completely radiation free;[8]
(4) The screen is soft and does not hurt the eyes. Unlike CRT technology, the LCD screen does not flicker, which can reduce the damage to the eyes caused by the display. The eyes are not easily tired.[8]
shortcoming
(1) Small visual deflection angle;
(2) Easy to produce imagesTailing phenomenon(e.gmouse pointer Rapid shaking), which is because most ordinary LCD screens are 60Hz (60 frames per second), but this problem mainly occurs in games when LCD displays were just popular (i.e“Picture tearing”）；
(6) When the resolution is lower than the default resolution of the display, the picture will be very blurred;
(7) When the resolution is greater than the default resolution of the display (it needs to be set by the software), the color at the details will be lost.[10]
Maintenance knowledge
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Safe cleaning
If the displayScreen panelThere is dust on it, and it should be operated under the advice of professional maintenance personnel. Individuals should not look for a piece at randomDuster cloth, or something rough, because it is easy to damage the LCD screen due to improper personal operation. The correct wiping method should select a clean and soft cloth to wipe, so that the display screen panel will not be damaged.Do not put water orCleaning agentSpray directly on the screen, dip a little special detergent on the soft cloth, and gently wipe the screen, which avoids short circuit caused by detergent flowing into the screen.When wiping the screen panel of the display, be careful not to use hard objects to touch and scratch the panel. Do not let any liquid enter the gap at the boundary of the display. There are many examples of damage to the display caused by liquid accidentally entering the gap.[11]
It is recommended that when cleaning, after 20 minutes of physical separation of power and data lines, the display should be placed horizontally (to avoid liquid flowing into the gap due to gravity). After slightly wetting the screen wiping cloth, pay attention to the boundary when gently wiping, and never spray detergent or water directly onto the screen (to avoid liquid flowing into the gap due to gravity).When the liquid evaporates, it can be used later.[11]
Avoid water ingress
Never let anything with moisture enter the LCD.Of course, once this happens, don't panic. If you find that there is only fog on the screen surface before starting up, just wipe it off with a soft cloth. If the moisture has entered the LCD, put the LCD in a ventilated and dry place, and gradually evaporate the moisture inside. If the screen is "flooded" seriously,It is better for ordinary users to call the service provider for help, because serious moisture will damage the components of the LCD, cause corrosion of the LCD electrodePermanentDamage.In addition, you should also try to avoid using LCD monitors in humid environments.[11]
Avoid bumping
The LCD display is fragile, so you should be careful not to be "bruised" by other devices during normal use.When using the cleaner, pay attention not to spray the cleaner directly on the screen. It may flow into the screen and cause a short circuit. The correct way is to use a soft cloth to stick the cleaner on the screen and gently wipe the screen. Remember, the ability of the LCD to resist "impact" is very smallcrystalAnd sensitiveElectrical componentsIt will be damaged when being hit, so please do not hit sharp objects.[11]
Time work
The pixels of the LCD are composed of many liquid crystals. If the LCD is used continuously for a long time, the crystals will age or burn out. Once the damage occurs, it is permanent and irreparable.Generally, do not keep the LCD on for a long time (more than 72 consecutive hours). If not in use, turn off the LCD.[11]
LCD overload mainly includes the following points:
(3) Often with differenttime intervalChange the display content on the LCD screen;
(4) Please turn off the monitor when you are free.[11]
Air requirements
Generally, when the humidity is kept between 30% and 80%, the display can work normally, but once the indoor humidity is higher than 80%, condensation will occur inside the display.Its internalPower transformerIt is also easy to generate electric leakage when other coils are affected with damp, which may even cause short circuit of wiring.Therefore, the LCD display must be protected from moisture. The display that has not been used for a long time can be powered on regularly for a period of time, so that the heat generated by the display during operation can drive out the moisture in the machine.[11]
Faults and maintenance
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White screen
White screen
The white screen is caused by the normal backlight and the screen does not correctly receive the signal sent from the main board, which indicates that the high-voltage part is good, and the white screen is caused by the T-CON board on the upper part of the LCD screen stops working or works abnormally.It may be caused by the relevant fault of the circuit on the drive board that supplies power to the LCD T-CON board (the voltage is generally 5V, and a few LCD screens use 12V or 3.3V power supply) or the main chip andMCUPoor or damaged characteristics.There are chips that have been written into various types of MCU programs on the market. As long as the chip on the original main control board is removed and the new chip with the program printed is replaced, this fault can be solved.[8]
Black screen
During actual maintenanceBlack screenThe black screen can be divided into three types: power light flashing, permanently on or off.[8]
(1) The LCD power light flashes. The power light flashes during maintenanceHigh pressure plateThe rear switch tube is short circuited, and when the LCD drive board sends out a signal to open the high-voltage boardcontrol voltageThen, the boost circuit began to work, but one or a pair of switch tubes were short circuited to the ground, and the main power supply was pulled down, which led to the power supply of the LCD driver board being pulled down. When the power supply of the LCD board was abnormal, the start signal of the high-voltage board was immediately canceled,Power supply boardThe voltage output returns to normal again, so the power light turns on again and againOnce outThe fault can be solved by replacing the corresponding faulty element.[8]
(2) The LCD power light is on for a long time, which can confirm that the power supply is normal, mainly high voltageInverter circuitFinal stage or power supply stage elementCalorific valueLarge, caused by long-term workFaulty solderingCaused by;The most likely fault of this circuit is the booster coil, which is generally due to poor contactKey inspectionThis circuit.[8]
(3) The LCD power light does not light up. If the power light does not light up, the power part should be checked, mainly to check whether the 12V and 5V voltages are normal. If the power light does not light up, the 12V and 5V voltages should not be output. The key point should be to check whether the switch tube isthermal stabilityPoor, orBaseWhether there is faulty soldering and capacitanceswellingPhenomenon, in additionPWMThe pulse width modulation integrated controller used in the single chip switching power supply of SG6841, UC3842UC3843Samefailure rateHigher components.[8]
Backlight fault
In addition to the above fault phenomena, the LCDBacklight Faults are also common, mainly manifested as aging and complete damage, that is, no light.After the backlight is aged, you can see the image yellowing, and adjust the color temperature andwhite balanceNone of them can be solved very well. At the same time, the brightness of the screen is adjusted to the maximum, and it is difficult to meet the use requirements. This reminds you that it is time to change the light.[8]
After the display with single lamp tube structure is damaged, turn it on in daylightDownward squintOn the screen, you can see dim graphic display.New multi lamp display shows image brightness after a lamp is damagedUnevenFor example, the upper part is dimmer, which means that the upper lamp tube is damaged. Touch the position of the display lamp tubeTemperature contrastIt can also be used to judge whether the light tube is damaged, and the normal display screen light tube position temperature should be consistent.But becauseNew displayMost have high pressure balanceprotection circuit As a result, the damage of a light tube is often not caused by uneven brightness, but the display cannot be turned on or the black screen will appear after turning on, which depends on the specific display circuit.[8]
