There are five types of single phase induction motor they are
a, Split phase induction motors,
b, Capacitor start induction motors,
c, Capacitor start and run induction motor,
d, Shaded pole induction motor,
e, Repulsion start induction motor.
Of these ,the first three are most commonand so we shall discuss them only in here.
a, Split phase induction motor
We know that normally in all single phase induction motors ,there are two windings namely starting winding(auxillary winding) and running winding(main winding), these two coils are connected in parallel in the split phase induction motor.the centrifugal switch is normally closed and when the motor is started and reaches 80% of its full speed ,the switch opens to disconnect the starting winding which is connected in parallel to the running winding by means of a centrifugal switch. Starting coil has high resistance and low inductive reactance,whereas running coil has low resistance and high inductive reactance.The current on both the starting and running coils are lagging in nature.The phase angles between the starting and running coils is 40-50 degrees.The torque in this motor is low.Its effeciency is around50%.
Applications
Fans,pumps,blowers,etc
b,Capacitor start induction motor
Here the capacitor is aditionally added in between the centrifugal switch and starting winding,because of the addition of capacitor the output power is increased phase angledifference between starting and running coils is approximately 90 degrees power factor is increased due to the addition of capacitor,starting torque is increased and effeciency is also increased.starting current leads the running currrent effeciency is around 55%,starting torque is 350 times of full load torque.
Applications
Compressors,conveyors,air conditioners,etc
c,Capacitor start and run induction motor
Incase of capacitor start motor the capacitance is at the time of starting only and ant the time ofrunning at full speed(rated) it is cut off,but here the capacitor runs till the supply is turned off and so there is an improved effeciency,(i.e) its effeciency is around 65-70%,starting torque is equal to200% full load torque.power factor too increases.
Applications
Washing machines,refrigrators,cooling fans,etc,besides this these motor can be used in all applications of split phase and capacitor start motors.
Wednesday, September 29, 2010
Saturday, September 11, 2010
Applications of LED
The LED's are of different colours such as red,green,blue,yellow,green,etc.They found a wide applications extensively in almost all display boards.the main application is that it is used in seven segment displays,which are used widely in digital indication applications.There are two types of seven segment display's namely
1.Common anode type,and
2.common cathode type.
In the common anode type,the anodes are sort together and a supply of 5v is given and the cathode terminals are connected to the ground,so that the LED's may glow.
And in the common cathode type,the cathode's are sort together and is given to ground,the anode terminals are provided with the supply.
Mostly seven segment display's are used in traffic signal lighting,digital measuring instruments,etc.
Apart from this the LED's are also used in the back-lighting of LCD monitors.
1.Common anode type,and
2.common cathode type.
In the common anode type,the anodes are sort together and a supply of 5v is given and the cathode terminals are connected to the ground,so that the LED's may glow.
And in the common cathode type,the cathode's are sort together and is given to ground,the anode terminals are provided with the supply.
Mostly seven segment display's are used in traffic signal lighting,digital measuring instruments,etc.
Apart from this the LED's are also used in the back-lighting of LCD monitors.
Monday, August 30, 2010
Switches
Toggle Switches
These switches are used in simple applications say two modes and are used mostly in machines were we need a constant switching between two modes.
Selector Switches:
These switches surf between a large number of modes, a simple example of selector switch is a type of ordinary electric fan regulator switch which switches between a wide range.
Drum Switches:
These switches commonly are power supply switches,since they are mostly used to switch ON lathes and drilling machines.They are also used to switch motors to run at forward or in reverse direction.
Rotary Switches:
They perform switching operation throughout 360 degrees example a type of ordinary electric fan regulator switch which rotate 360 degrees.
Limit Switches:
These switches are used to protect a machine part from exceeding its axial distance in case if the part runs off the limit the switch reverses the direction of the part by reversing the direction of rotation of the prime mover mostly a motor.It is actuated by machine part touching it's lever.
Push Button Switches:
These switches are used to control a function either switching 'on' or 'off'.They are mostly used in conventional lathes.
Membrane switches
These switches are of typical use they are used in the contol panel of CNC machines to denote the alpha numeric characters they give a membrane coating to the buttons(switches).
These switches are used in simple applications say two modes and are used mostly in machines were we need a constant switching between two modes.
Selector Switches:
These switches surf between a large number of modes, a simple example of selector switch is a type of ordinary electric fan regulator switch which switches between a wide range.
Drum Switches:
These switches commonly are power supply switches,since they are mostly used to switch ON lathes and drilling machines.They are also used to switch motors to run at forward or in reverse direction.
Rotary Switches:
They perform switching operation throughout 360 degrees example a type of ordinary electric fan regulator switch which rotate 360 degrees.
Limit Switches:
These switches are used to protect a machine part from exceeding its axial distance in case if the part runs off the limit the switch reverses the direction of the part by reversing the direction of rotation of the prime mover mostly a motor.It is actuated by machine part touching it's lever.
Push Button Switches:
These switches are used to control a function either switching 'on' or 'off'.They are mostly used in conventional lathes.
Membrane switches
These switches are of typical use they are used in the contol panel of CNC machines to denote the alpha numeric characters they give a membrane coating to the buttons(switches).
Thursday, August 26, 2010
FET Basics
At this post I'll share some basics about FET (Field Effect Transistor).
Construction:
FETs are traditionally made from silicon, although other semiconductors are used as well. The main body of the FET is doped based upon if the FET is N-type and P-type, while the source and drain are doped in an opposite way. Between them, an insulating oxide is present to which the gate is connected. Applying a voltage to the gate will permit or prohibit electrons from flowing between source and drain.
The most common FET, used in every digital device, is the metal oxide semiconductor field effect transistor, or MOSFET. The MOSFET typically employs a silicon dioxide layer between the gate and body of the FET.
Other types of FETs include the
- FET is a Voltage controlled device (Were BJT is a Current controlled device.)
- FET also have 3 terminals like BJT.
Construction:
FETs are traditionally made from silicon, although other semiconductors are used as well. The main body of the FET is doped based upon if the FET is N-type and P-type, while the source and drain are doped in an opposite way. Between them, an insulating oxide is present to which the gate is connected. Applying a voltage to the gate will permit or prohibit electrons from flowing between source and drain.
The most common FET, used in every digital device, is the metal oxide semiconductor field effect transistor, or MOSFET. The MOSFET typically employs a silicon dioxide layer between the gate and body of the FET.
Other types of FETs include the
- JFET
- MESFET
- MODFET
- FREDFET and quite a number more.
reference: http://www.ehow.com/about_5181217_fet-basics.html
LDR's AND APPLICATIONS
What is LDR?
A photoresistor or light dependent resistor or cadmium sulfide (CdS) cell is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor.
A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.
A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, e.g. silicon. In intrinsic devices the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities, also called dopants, added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (i.e., longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.
Applications
Photoresistors come in many different types. Inexpensive cadmium sulfide cells can be found in many consumer items such as camera light meters, street lights, clock radios, alarms, and outdoor clocks.
They are also used in some dynamic compressors together with a small incandescent lamp or light emitting diode to control gain reduction.
Lead sulfide (PbS) and indium antimonide (InSb) LDRs (light dependent resistor) are used for the mid infrared spectral region. Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.
A photoresistor or light dependent resistor or cadmium sulfide (CdS) cell is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor.
A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.
A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, e.g. silicon. In intrinsic devices the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities, also called dopants, added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (i.e., longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.
Applications
Photoresistors come in many different types. Inexpensive cadmium sulfide cells can be found in many consumer items such as camera light meters, street lights, clock radios, alarms, and outdoor clocks.
They are also used in some dynamic compressors together with a small incandescent lamp or light emitting diode to control gain reduction.
Lead sulfide (PbS) and indium antimonide (InSb) LDRs (light dependent resistor) are used for the mid infrared spectral region. Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.
http://en.wikipedia.org/wiki/Photoresistor
Thursday, August 12, 2010
DC Motor
THE DC MOTOR
The motor acts on the principle that a current carrying conductor in a magnetic field experiences a force.
When a current flows through the coil it has the effect of making the coil rotate about the axis XY.
When the coil is perpendicular to the magnetic field the direction of current must be reversed if the torque is to remain in the same direction.
This is achieved by means of a split ring commutator.
The commutor ensures that the brushes (supplying the current to the coil) are constantly in contact with the side of the coil which maintains the brushes with constant polarity.
The motor acts on the principle that a current carrying conductor in a magnetic field experiences a force.
When a current flows through the coil it has the effect of making the coil rotate about the axis XY.
When the coil is perpendicular to the magnetic field the direction of current must be reversed if the torque is to remain in the same direction.
This is achieved by means of a split ring commutator.
The commutor ensures that the brushes (supplying the current to the coil) are constantly in contact with the side of the coil which maintains the brushes with constant polarity.
Wednesday, August 11, 2010
Liquid Crystal Displays
LCD - short for Liquid Crystal Display devices, have become an important part of our everyday life. Their use range from wristwatches, calculators, and mobile phones, to more demanding high-resolution applications in test instrumentation, computer monitors and high definition LCD Televisions. And the use of LCD panels is growing at an incredible rate. Suffice to say that in the world of HDTV, LCD is already the dominant non-CRT HD display technology; sales of LCD HDTV sets had already exceed 4million units by end 2006 in the US alone.
Further more, a recent study published by iSuppli Corp., a global leader in technology value chain research and consultancy, indicates that despite the present global economic uncertainties, worldwide shipments of LCD TVs are expected to continue to grow - though at a slower rate - reaching 112.6 million units for 2009.
LCD panels are common because they offer some real advantages over other display technologies.
These flat-panel displays are slimmer - LCD Televisions hardly exceeds 3.5 inches in dept, and lighter than an equivalent screen size cathode ray tube (CRT) TV or plasma television. Further more, they draw much less power than alternative flat-panel displays; typically, LCD panels use only about 60% of the power requirements normally associated with plasma displays.
In addition, LCD display panels do not emit harmful electromagnetic waves. To top-up this whole pro list, LCD panels have a half lifetime of around 60,000hrs - meaning that the image brightness of an LCD video panel will fall to half its original value after approximately 60,000 hours of use!
Mind you, liquid crystal displays do not represent the perfect display technology; LCD displays have their drawbacks as well. In particular, viewing angle and display response time may be issues of concern especially with LCD panels from 2nd and 3rd tier display manufactures. And price may also be an issue especially as one moves towards LCD HDTV sets in excess of 50-inch screen size.
In this article, we look at the underlying technology that makes LCD display panels work in order to represent numbers, words, and high-resolution images.
We explain the complexities involved in the physical setup of an LCD display and how this impacts the production of LCD displays. In the process, we discuss the issue of 'bad' pixels. We also explain why manufacturers never guarantee that LCD panels are 100% free of bad, stuck or dead pixels.
LCD Display Technology: Basic Operational Principles:
LCD displays consist primarily of two sheets of polarized glass plates with some liquid crystal solution trapped between them. The type of liquid crystals used in LCD panels have got very specific properties that enable them to serve as effective 'shutters' that close or open to block or otherwise, the passage of light. This blocking takes place in a perpendicular manner to the passage of light on the application of an electric current.
This current through the liquid crystals is controlled by a voltage applied between the glass plates through the use of transparent electrodes that form a grid - with rows on one side of the panel and columns on the other - representing the picture elements or pixels.
But what are Liquid Crystals?
Though the three most common states of matter are solid, liquid, and gaseous, yet some substances can exist in a totally odd state that is a sort of liquid and a sort of solid at the same time.
Equally odd is the behavior of their molecules when substances are in this state, since these tend to maintain their orientation, like the molecules in a solid, but at the same time, they also move around to different positions, like the molecules in a liquid.
This means that liquid crystals are neither a solid nor a liquid, even though from a behavior perspective, these are closer to a liquid state than a solid.
Use of Liquid Crystals in LCD Display Panels
There is a variety of liquid crystals - each with different properties. The liquid crystals used in LCD panels are referred to as Nematic Phase liquid crystals. These have their molecules arranged in a definite pattern.
One type of nematic liquid crystal, called twisted nematic (TN), has its molecular structure naturally twisted.
The orientation of the molecules in the nematic phase is based on the 'director'; this can be anything from a magnetic field - say resulting from the application of an electric current due to an applied voltage across the glass plates holding the liquid crystal solution, to a surface that has microscopic grooves in it.
In the later, the molecules at the various layers of the liquid crystal will gradually align themselves till the molecules at the layer adjacent to the surface will be exactly in line with the direction of the microscopic grooves on the surface.
Microscopic grooves in LCD display panels are applied on the surface of the glass plate that does not have the polarizing film on it, to help align the molecular structure of the liquid crystals as these approach the glass surface in line with the polarization filters on either side of the LCD panel.
Now, the polarization filters on either side of an LCD display are set at 90 degrees to each other (ref. to above diagram). This means that the crystal lineup will go through a 90 degrees twist from one panel surface to the other. When a light shines on the glass surface of the first polarization filter, the molecules in each layer of the liquid crystal solution will guide the light they receive to the next layer. In the process, they will also change the light's plane of vibration to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If the final layer is matched up with the second polarized glass filter, then the light will pass through.
When an electric current is passed through these liquid crystals, they will untwist to varying degrees, depending on the current's voltage. This untwisting effect will change the polarization of the light passing through the LCD panel. As the polarization changes in response to the applied voltage across the glass plates, more or less light is able to pass through the polarized filter on the face of the LCD display.
Backlit versus Reflective
Unlike CRT or plasma displays, LCD displays require an external light source to display the picture. The least expensive LCD displays make use of a reflective process to reflect ambient light over to display the information. However, computer and LCD TV displays are lit with an external light source, which typically takes the form of built-in micro fluorescent tubes - often a few millimeters in diameter - above, besides, and sometimes behind the LCD. A white diffusion panel is used behind the LCD to redirect and scatter the light evenly to ensure a uniform display.
LCD Display Systems - Passive vs. Active Matrix Displays:
There are two main types of LCD displays - passive matrix and active matrix.
Passive Matrix: These are the type of LCD display panels that rely on the display persistence to maintain the state of each display element (pixel) between refresh scans. To a certain extent, the resolution of such displays is limited by the ratio between the time to set a pixel and the time it takes to fade.
To operate, passive-matrix LCDs use a simple grid to supply the charge to a particular pixel on the display. The grid is made up of conductive transparent material - usually indium-tin oxide - over two glass layers (called substrates) housing the liquid crystal solution, with one substrate taking the columns, and the other the rows. The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row. The point of intersection of the row and column represents the designated pixel on the LCD panel to which a voltage is applied to untwist the liquid crystals at that pixel to control the passage of light.
A display can have more than one pixel 'on' at any one time because of the response time of the liquid crystal material. Pixels have a short turn-on time during which the liquid crystal molecules will untwist to control the passage of light. Once the voltage between the respective electrodes addressing a pixel is removed, the pixel behaves similar to a discharging capacitor, slowly turning off as charge dissipates and the molecules return to their twisted orientation.
Because of this response time, a display can scan across the matrix of pixels, turning on the appropriate ones to form an image. As long as the time to scan the entire matrix is shorter than the turn-off time, a multiple pixel image can be displayed.
Passive matrix LCD displays are simple to manufacture, and therefore cheap, but they have a slow response time - in the order of a few hundred milliseconds - and a relatively imprecise voltage control. These characteristics render images that are somewhat fuzzy and lacking in contrast. Passive matrix LCD displays are therefore unsuitable for most of today's high speed, high resolution video applications.
Active Matrix LCD display panels depend on thin film transistors (TFT) to maintain the state of each pixel between scans while improving response times.
TFTs are micro-switching transistors (and associated capacitors) that are arranged in a matrix on a glass substrate to control each picture element (or pixel). Switching on one of the TFTs will activate the associated pixel.
The use of an active switching device embedded onto the display panel itself to control each picture element helps reduce cross-talk between adjacent pixels while drastically improving the display response.
By carefully adjusting the amount of voltage applied in very small increments, it is possible to create a gray-scale effect. While most of today's LCD displays support 256 levels of brightness per pixel, yet some high-end LCD panels used in HDTV LCD televisions support up to 1024 different levels of brightness. This results in improved gray scale performance and therefore improved picture detail in those areas of the image that are primarily all dark or all bright.
Color in LCD Display Panels
For an LCD display to show color, each individual pixel is divided into three sub-pixels with red, green and blue (RGB) color filters to create a color pixel. This is somewhat similar to the way CRT and Plasma use different phosphors to glow red, green, or blue to create color.
With a combination of red, blue and green sub-pixels of various intensities, a pixel can be made to appear any number of different colors. The number of colors that can be made by mixing red, green and blue sub pixels depends on the number of distinct gray scales (intensities) that can be achieved by the display.
If each red, green and blue sub-pixel can display 256 different intensities of their respective color, then each pixel can produce a possible palette of 16.8 million (256x256x256) colors.
Source: http://www.practical-home-theater-guide.com/lcd-display.html
Further more, a recent study published by iSuppli Corp., a global leader in technology value chain research and consultancy, indicates that despite the present global economic uncertainties, worldwide shipments of LCD TVs are expected to continue to grow - though at a slower rate - reaching 112.6 million units for 2009.
LCD panels are common because they offer some real advantages over other display technologies.
These flat-panel displays are slimmer - LCD Televisions hardly exceeds 3.5 inches in dept, and lighter than an equivalent screen size cathode ray tube (CRT) TV or plasma television. Further more, they draw much less power than alternative flat-panel displays; typically, LCD panels use only about 60% of the power requirements normally associated with plasma displays.
In addition, LCD display panels do not emit harmful electromagnetic waves. To top-up this whole pro list, LCD panels have a half lifetime of around 60,000hrs - meaning that the image brightness of an LCD video panel will fall to half its original value after approximately 60,000 hours of use!
Mind you, liquid crystal displays do not represent the perfect display technology; LCD displays have their drawbacks as well. In particular, viewing angle and display response time may be issues of concern especially with LCD panels from 2nd and 3rd tier display manufactures. And price may also be an issue especially as one moves towards LCD HDTV sets in excess of 50-inch screen size.
In this article, we look at the underlying technology that makes LCD display panels work in order to represent numbers, words, and high-resolution images.
We explain the complexities involved in the physical setup of an LCD display and how this impacts the production of LCD displays. In the process, we discuss the issue of 'bad' pixels. We also explain why manufacturers never guarantee that LCD panels are 100% free of bad, stuck or dead pixels.
LCD Display Technology: Basic Operational Principles:
LCD displays consist primarily of two sheets of polarized glass plates with some liquid crystal solution trapped between them. The type of liquid crystals used in LCD panels have got very specific properties that enable them to serve as effective 'shutters' that close or open to block or otherwise, the passage of light. This blocking takes place in a perpendicular manner to the passage of light on the application of an electric current.
This current through the liquid crystals is controlled by a voltage applied between the glass plates through the use of transparent electrodes that form a grid - with rows on one side of the panel and columns on the other - representing the picture elements or pixels.
But what are Liquid Crystals?
Though the three most common states of matter are solid, liquid, and gaseous, yet some substances can exist in a totally odd state that is a sort of liquid and a sort of solid at the same time.
Equally odd is the behavior of their molecules when substances are in this state, since these tend to maintain their orientation, like the molecules in a solid, but at the same time, they also move around to different positions, like the molecules in a liquid.
This means that liquid crystals are neither a solid nor a liquid, even though from a behavior perspective, these are closer to a liquid state than a solid.
Use of Liquid Crystals in LCD Display Panels
There is a variety of liquid crystals - each with different properties. The liquid crystals used in LCD panels are referred to as Nematic Phase liquid crystals. These have their molecules arranged in a definite pattern.
The orientation of the molecules in the nematic phase is based on the 'director'; this can be anything from a magnetic field - say resulting from the application of an electric current due to an applied voltage across the glass plates holding the liquid crystal solution, to a surface that has microscopic grooves in it.
In the later, the molecules at the various layers of the liquid crystal will gradually align themselves till the molecules at the layer adjacent to the surface will be exactly in line with the direction of the microscopic grooves on the surface.
Microscopic grooves in LCD display panels are applied on the surface of the glass plate that does not have the polarizing film on it, to help align the molecular structure of the liquid crystals as these approach the glass surface in line with the polarization filters on either side of the LCD panel.
Now, the polarization filters on either side of an LCD display are set at 90 degrees to each other (ref. to above diagram). This means that the crystal lineup will go through a 90 degrees twist from one panel surface to the other. When a light shines on the glass surface of the first polarization filter, the molecules in each layer of the liquid crystal solution will guide the light they receive to the next layer. In the process, they will also change the light's plane of vibration to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If the final layer is matched up with the second polarized glass filter, then the light will pass through.
When an electric current is passed through these liquid crystals, they will untwist to varying degrees, depending on the current's voltage. This untwisting effect will change the polarization of the light passing through the LCD panel. As the polarization changes in response to the applied voltage across the glass plates, more or less light is able to pass through the polarized filter on the face of the LCD display.
Backlit versus Reflective
Unlike CRT or plasma displays, LCD displays require an external light source to display the picture. The least expensive LCD displays make use of a reflective process to reflect ambient light over to display the information. However, computer and LCD TV displays are lit with an external light source, which typically takes the form of built-in micro fluorescent tubes - often a few millimeters in diameter - above, besides, and sometimes behind the LCD. A white diffusion panel is used behind the LCD to redirect and scatter the light evenly to ensure a uniform display.
LCD Display Systems - Passive vs. Active Matrix Displays:
There are two main types of LCD displays - passive matrix and active matrix.
Passive Matrix: These are the type of LCD display panels that rely on the display persistence to maintain the state of each display element (pixel) between refresh scans. To a certain extent, the resolution of such displays is limited by the ratio between the time to set a pixel and the time it takes to fade.
To operate, passive-matrix LCDs use a simple grid to supply the charge to a particular pixel on the display. The grid is made up of conductive transparent material - usually indium-tin oxide - over two glass layers (called substrates) housing the liquid crystal solution, with one substrate taking the columns, and the other the rows. The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row. The point of intersection of the row and column represents the designated pixel on the LCD panel to which a voltage is applied to untwist the liquid crystals at that pixel to control the passage of light.
A display can have more than one pixel 'on' at any one time because of the response time of the liquid crystal material. Pixels have a short turn-on time during which the liquid crystal molecules will untwist to control the passage of light. Once the voltage between the respective electrodes addressing a pixel is removed, the pixel behaves similar to a discharging capacitor, slowly turning off as charge dissipates and the molecules return to their twisted orientation.
Because of this response time, a display can scan across the matrix of pixels, turning on the appropriate ones to form an image. As long as the time to scan the entire matrix is shorter than the turn-off time, a multiple pixel image can be displayed.
Passive matrix LCD displays are simple to manufacture, and therefore cheap, but they have a slow response time - in the order of a few hundred milliseconds - and a relatively imprecise voltage control. These characteristics render images that are somewhat fuzzy and lacking in contrast. Passive matrix LCD displays are therefore unsuitable for most of today's high speed, high resolution video applications.
Active Matrix LCD display panels depend on thin film transistors (TFT) to maintain the state of each pixel between scans while improving response times.
TFTs are micro-switching transistors (and associated capacitors) that are arranged in a matrix on a glass substrate to control each picture element (or pixel). Switching on one of the TFTs will activate the associated pixel.
The use of an active switching device embedded onto the display panel itself to control each picture element helps reduce cross-talk between adjacent pixels while drastically improving the display response.
By carefully adjusting the amount of voltage applied in very small increments, it is possible to create a gray-scale effect. While most of today's LCD displays support 256 levels of brightness per pixel, yet some high-end LCD panels used in HDTV LCD televisions support up to 1024 different levels of brightness. This results in improved gray scale performance and therefore improved picture detail in those areas of the image that are primarily all dark or all bright.
Color in LCD Display Panels
For an LCD display to show color, each individual pixel is divided into three sub-pixels with red, green and blue (RGB) color filters to create a color pixel. This is somewhat similar to the way CRT and Plasma use different phosphors to glow red, green, or blue to create color.
With a combination of red, blue and green sub-pixels of various intensities, a pixel can be made to appear any number of different colors. The number of colors that can be made by mixing red, green and blue sub pixels depends on the number of distinct gray scales (intensities) that can be achieved by the display.
If each red, green and blue sub-pixel can display 256 different intensities of their respective color, then each pixel can produce a possible palette of 16.8 million (256x256x256) colors.
Source: http://www.practical-home-theater-guide.com/lcd-display.html
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