Components and Working of CRT (Cathode Ray Tube)
A CRT is just like a vacuum glass bottle. The one end of the tube contains electron gun (i.e. the negatively charged cathode) and the other end of tube is a screen that is coated with pattern of dots (phosphor crystals). The electron gun emits the cathode rays or beam of electrons that strikes on the phosphor coated screen. The phosphor then emits a small spot of light at the point of contact made by the electron beam. These beams of electrons are guided towards the specific position on screen with the help of focusing and deflection system.
The image on the monitor screen is usually made up from at least tens of thousands of such tiny dots or spots glowing together. The light emitted by phosphors fades away very quickly. Therefore in order to maintain picture on screen the electron beam redirected quickly over the same points. In this way the various phosphor crystals keep on glowing continuously. This type of CRT known as refresh type CRT.
Components of CRT
The various components of cathode ray tube are :
- Electron Gun
- Focusing System
- Deflection System
- Phosphor coated screen
1. Electron Gun
The electron gun consists of metal cathode that is cylindrical in shape and a control grid. The control grid is another metallic cylinder that fits over the cathode. In this components of CRT the cylindrical cathode contains a filament (coil of wire) used to supply to heat to cathode by passing current through this it. When heat supplied to cathode via a filament its electrons become loose and get emitted off from cathode surface. These negatively charged electrons then accelerated towards the screen by using a high positive voltage.
There are two ways by which this accelerating voltage can generated:
(i) by using an accelerating anode
(ii) by using positively charged metal coating on the inside of the CRT envelope near the phosphor screen.
Control grid used to control the intensity of electron beam and the brightness on the screen. Done by changing the voltage on the control grid. Also, the brightness of the screen depends upon the number of electrons striking the screen.
When a high negative voltage applied to control grid the electron beam shut off as the various electrons get repelled and stopped from passing through the small hole at the end of the control grid. When a smaller negative voltage applied on control grid, the number of electrons passing through the hole decreased and ultimately the brightness of the screen decreased. A high positive voltage applied, the electron beam is strong as the number of electrons in beam will increase and the brightness of the screen will increase.
2. Focusing System
The purpose of focusing system in CRT is to force the electron beam to converge into a small spot as it strikes the phosphor screen. Focusing system can established either by using electric field or magnetic field. When electrostatic focusing system use the electron beam made to pass through a metallic cylinder that is positively charged (anode). Such a system forms electrostatic lens that focuses the electron beam at the centre of the screen. Magnetic focusing system created by using a coil, mounted around the outside of CRT envelope. This coil generates the magnetic field that creates a magnetic lens.
3. Deflection System
In this components of CRT, the purpose of deflection system is to change the direction of electron beam so that electron beam can make to strike at different locations on the phosphor coated screen. Like focusing system, defection system can also establish either by using electric field or magnetic field. In magnetic deflection system, two pairs of coils mounted on the outside of the working of CRT envelope, with the coils in each pair mounted on opposite sides of the neck of the CRT envelope.
One pair mounted on the top and bottom of the neck, and the other pair mounted on the left and right side of the neck. Horizontal deflection accomplished by the pair of coil mounted on left und right side of neck and vertical deflection accomplished by the pair mounted on top and bottom of neck. The proper deflection amounts are attain by adjusting the current through the coils. In electrostatic deflection system, two pairs of parallel plates are use. One pair of plates mounted horizontally to control the vertical deflection, and the other pair mounted vertically to control horizontal deflection.
4. Phosphor coated screen
The screen is coated with phosphorus crystals called phosphors. These phosphor crystals emit small spots of light when the electron beam strikes on them.
Working of CRT
The electron beam emitted by electron gun possesses the kinetic energy. The spot of light produced by the transfer of this kinetic energy of electron beam to phosphor, atoms coated on the screen. When the electrons in the beam collide with phosphor coating they stop and their kinetic energy absorbed by the phosphor. A part of this energy absorbed by phosphor converted into heat energy because of friction. The major part of the energy absorbed by the electrons in the phosphor atoms.
These electrons, after absorbing energy move up to high quantum energy level (excited state) from their ground state. After a short period, these electrons start dropping back to their stable ground state. They give up their extra energy as small quantum’s of light energy. The frequency or colour of the light produced by phosphor is proportional to the energy difference between the excited quantum state and the ground state. This is the working of CRT.
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