Basic Operation of CRT

Basic Operation of CRT

A beam of electrons (cathode rays), emitted by an electron gun, passes through focusing and deflection systems that direct the beam toward specified positions on the phosphor-coated screen. The phosphor then emits a small spot of light at each position contacted by the electron beam. Because the light emitted by the phosphor fades very rapidly, some method is needed for maintaining the screen picture. One Way to keep the phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points. This type of display is called a refresh CRT.

Components of CRT are as follows:-

1. HEATER ELEMENT AND CATHODE:-Heat is supplied to the cathode by passing current through heater element. Cathode is cylindrical metallic structure which is rich in electrons. On heating the electrons are released from cathode surface.
2. CONTROL GRID:- It is the next element which follows cathode. It almost covers cathode leaving small opening for electrons to come out. Intensity of the electron beam is controlled by setting voltage levels on the control grid. A high negative cottage applied to the control grid will shut off the beam by repelling electrons and stopping them from passing through the small hole at the end of control grid structure. A smaller negative voltage on the control grid will simply decrease the number of electrons passing through. Thus we can control the brightness of a display by varying the voltage on the control grid.
3. ACCELERATING ANODE:-They are positively charged anodes who accelerate the electrons towards phosphor screen.
4. FOCUSING & DEFLECTION COILS:- They are together needed to force the electron beam to converge into a small spot as it strikes the screen otherwise the electrons would repel each other and the beam would spread out as it approaches the screen. Electrostatic focusing is commonly used in television and computer graphics monitor.
5. PHOSPHOR COATING:-When the accelerating electron beam (collides) is incident on the phosphor cooling, a part of Kinetic Energy is converted into light and heat. When the electrons in the beam collide with the phosphor coating they are stopped and their kinetic energy is absorbed by the phosphor.

 Working

Beam passes between two pairs of metal plates, one vertical and other horizontal. A voltage difference is applied to each pair of plates according to the amount that the beam is to be deflected in each direction. As the electron beam passes between each pair of plates, it is bent towards the plate with the higher positive voltage. The beam is first deflected towards one side of the screen. Then, as the beam passes through the horizontal plates, it is deflected towards, the top or bottom of the screen. To get the proper deflection, adjust the current through coils placed around the outside of the CRT loop.

Explain the operation of electron gun in the working of a CRT.

The primary components of an electron gun in a CRT are the heated metal cathode and a control grid. Heat is supplied to the cathode by directing a current through a coil of wire, called the filament, inside the cylindrical cathode structure. This causes electrons to be “boiled off” the hot cathode surface. In the vacuum inside the CRT envelope, the free, negatively charged electrons are then accelerated towards the phosphor coating by a high positive voltage. The accelerating voltage can be generated with a positively charged metal coating on the inside of the CRT envelope, near the phosphor screen, or an accelerating anode can be used. Intensity of the electron beam is controlled by setting voltage levels on the control grid, which is a metal cylinder that fits over the cathode. A high negative voltage applied to the control grid will shut off the beam by repelling electrons and stopping them from passing through the small hole at the end of the control grid structure. A smaller negative voltage on the control grid simply decreases the number of electrons p[assign through. Since the amount of light emitted by the phosphor coating depends on the number of electrons striking the screen, we control the brightness of a display by varying the voltage on the control grid.

Color CRT Monitors

A color CRT monitor displays color picture by using a combination of phosphors that emit different colored light. By combining the emitted light a range of colors can be generated. Two basic methods for producing color displays are:

a)      a) Beam Penetration Method

Random scan monitors use the beam penetration method for displaying color picture. In this, the inside of CRT screen is coated two layers of phosphor namely red and green. A beam of slow electrons excites only the outer red layer, while a beam of fast electrons penetrates red layer and excites the inner green layer. At intermediate beam speeds, combinations of red and green light are emitted to show two additional colors- orange and yellow. The speed of the electrons, and hence the screen color at any point, is controlled by the beam-acceleration voltage. Beam penetration has been an inexpensive way to produce color in random-scan monitors, but only four colors are possible, and the quality of pictures is not as good as with other methods.

b)     b) Shadow Mask Method

Raster scan system are use shadow mask methods to produce a much more range of colors than beam penetration method. In this, CRT has three phosphor color dots. One phosphor dot emits a red light, second emits a green light and third emits a blue light. When the three beams pass through a hole 'in the shadow mask, they activate a dot triangle, which appears as a small color spot on the screen. The phosphor dots in the triangles are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask. In some low-cost systems, the electron beam can only be set to on or off, limiting displays to eight colors. More sophisticated systems can set intermediate intensity levels for the electron beams, allowing several million different colors to be generated. 

Compare and contrast shadow mask and beam penetration method

Both methods are used in color CRT monitors. Beam penetration method is used for random scan monitors. In beam penetration two layers of phosphor red and green are coated inside CRT screen, the display of color depend on how far electron excites outer red layer, then green layer. This method can produce four colors i.e. red, green, orange, and yellow. This is less costly method as compared to shadow mask. But it can produce less color as compared to shadow mask. The quality of picture is also poor as compared to shadow mask. Shadow mask is used for raster scan systems. It can produce wide variety of colors .There is three phosphor color dots at each pixel position. One phosphor dot emits red light, another emit green light, third emit blue light. Three guns one for each color is used. Three beams pass through holes in shadow mask, and a small color spot on screen is appeared. Shadow masks are used as display device for home computers, color TV set etc