A LCD Monitor
Introduction
LCD or
flat panel computer displays are the latest and greatest offerings in the
desktop computer industry. They have been used for years in the portable and
notebook computing markets, but recent developments have increase performance
and size while reducing costs making them viable in the desktop environment. LCD
displays are lightweight, extremely thin and use much less power than CRT based
monitors.
What
is Liquid Crystal and How It Works?
Liquid crystal was discovered by the
Austrian botanist named Fredreich Rheinizer in 1888. Liquid crystal is an
unusual organic material and it is neither solid nor liquid. That means although
it is liquid in form and appearance, Liquid Crystal exhibits a crystalline
molecular structure that resembles a solid. Liquid crystals are rod-shaped
molecules whose molecules can be aligned precisely when subjected to electrical
fields. As a liquid they are able to flow over and around small grooves and can
change their position depending on applied voltage. When properly aligned, the
liquid crystals allow light to pass through makes the desired images appear.
Molecules are
When coming Molecules line arranged in a into contact with up parallel along
loosely ordered a finely grooved grooves. fashion with surface their long axes
(alignment layer). parallel.
Crystal Molecules in Natural state
An LCD
monitor consists of six layers: a backlight, a sheet of polarized glass
(polarizer), TFT glass, a layer of liquid crystal solution, colour filter/glass
and a second polarized sheet of glass.
Photo source (courtesy of Samsung Electronics)
A fluorescent light source, known as
the backlight, makes up the rearmost slice of bread. Light is shined from behind
the panels. This light passes through the first of two polarizing filters. The
polarized light then passes through a layer that contains thousands of liquid
crystal blobs arrayed in tiny containers called cells. The cells are, in turn,
arrayed in rows across the screen; one or more cells make up one pixel (the
smallest discernible dot on a display). Electric leads around the edge of the
LCD create an electric field that twists the crystal molecule, which lines the
light up with the second polarizing filter and allows it to pass through. Each
crystal either allows light to pass through or blocks the light. The
configuration of the crystals forms the image.
Types of LCD’s
There are two basic kinds of LCD colour displays:
passive-matrix and active-matrix.
In a
colour LCD panel, each pixel is made up of three liquid crystal cells.
Pixels do not actually generate the colours that you see. It is the white light
(backlight) passing through each pixel, which filtered to form the intended
colour. The front glass is coated with colour filter material in front of each
red, green and blue dot (cell). Light passing through the filtered cells creates
the colours you see on the LCD.
Each cell or subpixel, can be
individually addressed with a control voltage. This means, for example, that a
15” LCD Monitor screen that have the resolution of 1024 x768 contains 2,359296
subpixels (1024 x 768 x 3). Occasionally the mechanism that sends the electrical
current to one or more pixels fails; in those instances you'll see a completely
dark cell (bad cell) or a "bad" pixel. Read more information about bad pixel in
the chapter of “Stuck and Dead Pixel in LCD Monitors”.
Passive Matrix LCD
Passive-matrix LCD Monitors use a simple grid to supply
the voltage to a particular pixel on the display. Creating the grid is quite a
process! It starts with two glass layers called substrates. One substrate is
given columns and the other is given rows made from a transparent conductive
material. The rows or columns are connected to integrated circuits that control
when a charge is sent down a particular column or row. The liquid crystal (LC)
material is sandwiched between the two glass substrates, and a polarizing film
is added to the outer side of each substrate. To turn on a pixel, the integrated
circuit sends a charge down the correct column of one substrate and a ground
activated on the correct row of the other. The row and column intersect at the
designated pixel, and that delivers the voltage to untwist the liquid crystals
at that pixel.
For example, if the dot at row 0, column 0 is supposed
to be red, the green and blue dots turn “On” at that point to block white light
through all but the red filter. White light travels through the red filter on
the front glass where it emerges as red. When the red, green and blue dots are
all on, all light is blocked and the pixel appears black. If all three dots are
off, all light passes through and the pixel appears white.
There are disadvantages although the simplicity of the
passive-matrix system is beautiful. First, the response time is slow. Response
time refers to the LCD's ability to refresh the image displayed. The easiest way
to observe slow response time in a passive-matrix LCD is to move the mouse
pointer quickly from one side of the screen to the other. You will notice a
series of "ghosts" following the pointer. Such slow update times make passive
displays poor choices for fast graphic operations (like games), animation and
motion video. Second, their contrast ratio is poor which generally results in
washed out or hazy pictures. Third, the viewing angles for colour passive matrix
LCD’s also are poor at around 45 degrees. That means your clearest view of the
display will be to look at it straight on.
Active-matrix or TFT (thin film transistor) technology
TFT
stand for thin film transistor (or active-matrix) produces colour images that
are as sharp as traditional CRT displays. Basically, TFTs are tiny switching
transistors and capacitors. The three elements provide the red, green and blue
light source for each pixel that your eye perceives. They are arranged in a
matrix on a glass substrate. To address a particular pixel, the proper row is
switched on, and then a charge is sent down the correct column. Since all of the
other rows that the column intersects are turned off, only the capacitor at the
designated pixel receives a charge. The capacitor is able to hold the charge
until the next refresh cycle. And if we carefully control the amount of voltage
supplied to a crystal, we can make it untwist only enough to allow some light
through. This means that the switching occurs right at the cell turning the
white light on or off and the result is faster response times, and less
crosstalk between cells.
When the red, green and blue elements are all off, white
light shines through the three elements, and the pixel appears white. If the
red, green and blue elements are all on, all light is blocked, and the pixel
appears black.
Active-matrix LCD’S response time is very
fast-approximately 16ms and better. Such fast response time provides excellent
performance for graphics or animation applications. The active matrix screen
also provides a comfortable viewing angle of 90 degrees and above. Additionally,
higher drive signals can be used which creates much brighter and higher contrast
images. The disadvantage of active matrix LCD’S is that the price is still high
due to the high cost of building TFT factories and expensive technology used to
fabricate all the tiny transistors (FET) onto the glass plate.
Nearly all modern colour LCDs--both in notebooks and for
desktop monitors is using the active matrix LCD (TFT).
Screen Size
When you purchase a 17-inch CRT monitor, you usually get
16.1 inches or a bit more of actual viewing area, depending on the brand and
manufacturer of a specific CRT. The difference between the "monitor size" and
the "view area" is due to the large bulky frame of a CRT. Unlike CRT monitors,
LCD displays are marketed by the actual screen dimensions. That means if
you purchase a 17" LCD monitor, you actually get a full 17" viewable area, or
very close to a 17". This is the measurement of the displayable area of the
screen from the lower corner to the opposite upper corner of the display. Below
is the rough guide for the screen size:
17” CRT = 15” TFT 19” CRT = 17”-18.4” TFT 21” CRT =
19”-20” TFT
Obviously these are not always exact, but it is a good
rough guide to the sizes. For instance a 21” CRT may offer a viewable area of
more like 20”. Nowadays, 15” and 17” LCD Monitors are fairly rare in the market
because manufacturers are focusing in making 19” model and above and they also
has shifted to producing Widescreen format monitors too.
A diagonal view (screen size) of a widescreen LCD
Monitor
What is Response Time?
Response Time is the specification which many people,
especially gamers, have come to regard as the most important. It translates to
the amount of time it takes for a liquid crystal cell (pixel) to go from active
(black) to inactive (white) and back to active (black) again. In practical
terms, it refers to the speed of the pixels and how fast they can change from
one colour to another, and therefore how fast the picture can be redrawn. The
faster this transition can change the better. This helps reduce the effects of
ghosting/ blurring in games and movies which can result if the response time is
too slow.
The
response time is measured in milliseconds or (ms). Lower numbers mean faster is
the transitions time (e.g. 16 ms is faster than 25 ms.). If you visit any
computer dealers and get the brochure from them you could see a small word (ms)
printed besides the LCD Monitor price list. This is to tell you that the
particular LCD Monitor is running on what milliseconds. Generally the lower the
milliseconds (response time) the more expensive is the LCD Monitor price.
Native Resolutions
The
physical structure of some types of displays, including LCD Monitors/TVs and
plasma panels, defines how many pixels can be displayed at once. The display
produces the sharpest picture when used at its so-called native resolution. This
is the physically number of horizontal and vertical pixels that make up the LCD
matrix of the display.
Setting a computer display to a
resolution lower than this resolution will either cause the monitor to use a
reduced visible area of the screen or it will have to do extrapolation. This
extrapolation attempts to blend multiple pixels together to produce a similar
image to what you would see if the monitor were to display it at the given
resolution but it can result in fuzzy images.
Below are some of the common native resolutions found in
LCD monitors:
14-15": 1024x768 (XGA) 17-19": 1280x1024 (SXGA) 20"+:
1600x1200 (UXGA) 19” (Widescreen): 1440x900 (WXGA+) 20” (Widescreen): 1680x1050
(WSXGA+) 24” (Widescreen): 1920x1200 (WUXGA) 30” (Widescreen): 2560x1600
Contrast Ratio
Contrast ratio is a big marketing tool by the
manufacturers and one that is not easy for consumers to grasp. Contrast ratio
relates to the display's comparative difference between its brightest white
values and its darkest black values. As a rule of thumb, the higher the contrast
ratio, the better. A higher contrast ratio will have truer colours with less
"wash out." The standard offering for lower end models is commonly 700:1. Many
experts recommend a contrast ratio of 1000:1 or better.
Be wary
of quoted specs however, as sometimes they can be exaggerated. Some technologies
boast the ability to dynamically control contrast and offer contrast ratios of
3000:1 and above!
Brightness
Brightness is a measure of the brightest white the LCD
Monitor can display. Typically LCD Monitors are far too bright for comfortable
use, and the On Screen Display (OSD) is used to adjust the brightness setting
down. Higher brightness is good as it leads to a better contrast ratio and can
be useful for dark scenes in games / movies where it might be difficult to
distinguish between shades of grey.
Viewing Angles
A CRT
monitor can be viewed from almost any angle, but with an LCD this is often a
problem. The viewing angle is an especially important consideration if you plan
to have multiple people viewing the LCD monitor at any given time. When you use
an LCD, your view changes as you move different angles and distances away from
the monitor. At some odd angles, you may notice the picture fade (wash out), and
possibly look as if it will disappear from view. The reason for this is because
LCD's produce their image by having a film that when a current runs through the
pixel, it turns on that shade of colour. The problem with the LCD film is that
this colour can only be accurately represented when viewed straight on.
The LCD
monitors are generally rated for their visible viewing angle for both horizontal
and vertical which refers to the degree you can stray from dead centre before
the picture starts to wash out. A theoretical viewing angle of 180 degrees would
mean that it is fully visible from any angle in front of the screen. Many
recommend a viewing angle of at least 140 degrees horizontal and 120 degrees
vertical. The wider the viewing angles, the better. High contrast levels usually
go hand-in-hand with wider viewing angles.
Digital and analogue connections
LCD Monitors are digital devices and thus have to
convert analogue (VGA) signals before they can be displayed. A graphics card
with a digital video interface (DVI) can send the signal straight to the display
in digital format and no conversion required. Many LCD Monitors come with
an analogue input (featuring a D-shaped connector that has 15 pins arranged in
three rows, sometimes labelled D-Sub), some come with both, and only a very few
come with just a digital input.
This is a digital interface that is supposed to allow
for a cleaner and brighter picture compared to standard VGA connectors.
Nevertheless, at this point, many LCD Monitors do such a good job of signal
conversion that digital connections are not as important as they used to
be.
Digital input
Portrait/Landscape modes
Some LCD Monitors pivot so that the longer
edge can go horizontal (Landscape mode) or vertical (Portrait mode). This
feature can be very useful for desktop publishing, Web surfing, and viewing
large spreadsheets, but don't pay extra for it if you won't use it.
Portrait mode
LCD Monitor Life span
Life span, this is typically the time
taken (viewing hours) for the average backlight to dim to 50% of their original
brightness. Generally, LCD monitors last longer than CRTs. A typical LCD
lifespan is 50,000 hours of use compared to 15,000 to 25,000 for a CRT. A longer
monitor lifespan can provide a better return on investment.
LCD application
LCD panels are used in
various applications ranging from smaller portable electronic equipment to
larger fixed location units. Applications such as the display device for digital
watches, portable calculators, LCD Monitor and TV, laptop and notebook, arcade
game machines, automobile navigation systems, industrial machine, video and
digital cameras.
Overview of LCD Monitor
Circuits
Most
LCD Monitors can be broken down into 6 major circuits. Each circuit have its own
function and in this page I will just briefly explain to you the overview of LCD
Monitor and more throughout explanation on each circuit function will be clearly
explain in the following chapters.
Power Supply Circuit
As it
name suggests, the role of the power supply is to provide power to the rest of
the circuits in the LCD Monitor. Normally the output voltages are 12V and 5
Volts and the 5 Volts were brought it down again to 3.3 V and 2.5 Volts through
voltage regulator. However in some LCD Monitor designs, the output voltages may
not be the values I’ve mentioned above. You have to test it with your digital
multimeter
Inverter Circuit
Provide high voltages and current
required by the backlight (lamps). Inverter generates from 600 up to 1000 plus
VAC from one, two or even four high voltage transformer depending on how many
backlights were used.
Internal view of LCD Monitor
Backlight (lamps)
Generate a consistent, uniform light source. The light
generated from the backlight focused through the LCD.
Main board/AD board
Convert the RGB analogue signal into digital signal and
channel it to the LCD driver/controller board.
LCD Driver/Controller board
Accepts additional display information from the Main
Board and drive the transistors in the LCD panel.
LCD Panel
Controls light throughout using the liquid crystal
material.
LCD Monitor Block Diagram
Understanding Power Supply
Board
The
switch mode power supply used to power up LCD Monitor can be either the external
or internal type. The function of the power supply is to convert the main supply
AC 230 volts into DC output voltages to supply to the necessary boards in LCD
Monitor.
The internal type power supply
230
Volts AC supply enters the power supply and to the bridge rectifier ac pins
(normally is the 2nd and the 3rd leg). The
AC supply is then converted into DC output voltage (about 300 VDC-in USA about
155 VDC) where the big filter capacitor filtered off the ripple so that the
power supply will have a nice constant of DC voltage. This high voltage DC
supply is then given to a switching power FET Transistor. This switching FET
transistor circuit is switched on and off at a very high speed by a control
circuit (power IC) which generates very high frequency square wave pulses.
The power FET and power IC
(UC3842B) are separated
The
switching FET transistor circuit switches the given high voltage DC, on and off
at the same high frequency and gives square wave pulses as the output. These
square wave pulses are then given to the primary winding of Switch Mode Power
Transformer. These pulses induce a voltage at the primary winding of the
transformer which will generate voltage at the secondary winding. This voltage
at the secondary winding is then rectified and filtered to produce the required
output.
The
build in power supply have output of usually 12 volts and 5 volts where the 12
volts enters the inverter IC and also audio power amp IC.
The 5 volts will go through one or two voltage
regulators to get the 3.3 and 2.5 volts to power the Scalar IC, MCU, EEprom and
even the LCD driver/controller board.
The power FET transistor already
integrated into the power IC
Please
take note that many latest designs of LCD Monitor power supply designs have the
switching power FET transistor already integrated into the power IC thus you
will not find the power FET in the power supply board.
The External type power supply
Internal view of external power supply
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