Every one must have seen TV/Computer picture tubes. Have you ever thought, How the display on TV screen comes?? Its all because of the CRT ( of any mode) used in them. CRT denotes for Cathode Ray Tube. In any class of CRT, it consists five fundamental parts:
- Electron Gun
- Horizontal deflection plates
- Vertical deflection plates
In a CRT, electron gun emits a very high intensity beam of electrons. As this beam passes through anodes, it is focused and accelerated towards the screen. The anodes carry positive charge. These electrons after being focused and accelerated pass on through anode and strike a screen whose inner surface is coated with Phosphor. The Phosphor glows visibly as it can be seen from the face of CRT.
If we don’t use deflection of the electron beam around the screen of CRT, we will get only a bright spot in the center of the screen. It is the deflection of the beam which makes it possible to display various pictures.
The deflection of beam can be achieved by following methods :
- Magnetic field deflection: In this type of deflection, coils are used in place of deflection plates. When current is passed through these coils, the beam changes its direction at right angles to the magnetic lines of flux. The magnetic field generated will be directly proportional to the current fed in the coils. The greater the current will be, the stronger the magnetic field, and so the farther the beam will be deflected. Two coils are used in this method. One coil is used to achieve horizontal deflection and the other coil to achieve vertical deflection. These coils may be placed on the outer side of the tube neck inspite of being placed inside like deflection plates.
- Electrostatic field deflection: In this method, deflecting plates are used in place of the coils. The electron beam is negatively charge due to electrons. And so, the electron beam is pulled towards the positive deflecting plates and repelled away from negative deflecting plates. In this method, two pairs of deflecting plates are used. One plate is used for horizontal plane and the other plate is used for the vertical plane. As the electrostatic field increases with the increase in voltage and so we can achieve more deflection just by increasing voltage.