Question

What is the function of the following in the cathode ray tube:
1) Electron gun
2) Deflection System
3) Fluorescent screen

Answer 1

The features of a CRT can be split into 3 main sections: The electron gun, the deflection system and the fluorescent screen.

Electron Gun
- The role of this section is to produce electrons at a high, fixed, velocity.
- This is done through a process known as thermionic emission.
- A filament in the cathode is heated to the point where its electrons become loose.
- An anode with a high voltage applied to it accelerates the electrons towards the screen due to electrostatic attraction.
- On the way, the electrons pass through a series of control grids which control the brightness of the image produced.
- The more negative the grid, the darker the image and vice versa.

Deflection system
- The role of the deflection system is to control the image produced by controlling the position that the electrons hit the screen.
- It consists of two perpendicular sets of electric/magnetic fields.
- This allows control over both horizontal and vertical axes.
- By controlling the Voltage applied to the fields, it is possible to vary the deflection through Electrostatic force/Motor effect.

Fluorescent screen
- The role of this part is to display where the electrons are hitting the CRT.
- It is a screen coated with a material that emits light when struck by electrons.
- Zinc sulfide or Phosphorus are two commonly used materials.