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Cathode Ray Tube

The cathode ray tube is an electronic vacuum tube used for displaying electrical data in the form of images or videos on the screen. In 1897, German Physicist Karl Ferdinand Braun invented the cathode ray tube. The cathode ray tube is also known as the Braun’s tube. It is used in electronic devices such as televisions (TV), computer monitors, radar, and oscilloscope tubes.

Today the CRT displays were replaced by LCD, LED and plasma displays. Cathode ray tube components of a basic CRT are as follows:

  1. The electron gun
  2. The electron gun is used for generating, controlling and focusing the beam of electrons enclosed in a vacuum tube. The electron gun again internally consists of five components. They are:

    • Heater
    • Cathode
    • Control grid
    • Accelerating anode.
    • Focusing anode
  3. Horizontal deflection plates horizontal deflection plates deflect the electrons horizontally.
  4. Vertical deflection plates vertical deflection plates deflect the electrons vertically. Both the horizontal and vertical deflection plates control the path of electrons beam so that it can be directed towards a specified position on the phosphor-coated screen.
  5. The fluorescent screen

Fluorescent screen is a transparent screen coated on one side with a phosphor that glows brightly when exposed to cathode rays. How cathode ray tube worksThe interior of the tube is a very good vacuum with a pressure of around 0.01 Pa (10-7 atm) or less. At any greater pressure, collisions of electrons with air molecules would scatter the electron beam excessively.

The electron gun is used for generating, controlling and focusing the beam of electrons enclosed in a vacuum tube. The electron gun again internally consists of five components. They are heater, cathode, control grid, accelerating anode, and focusing anode. The heater at the left end in the figure heats the cathode to a high temperature. The cathode is a conductor that emits electrons from its surface when heated to a high temperature through the heating filament. The accelerating anode with a small hole on its center is maintained at a high positive potential of the order of 1 to 20 kV, relative to the cathode. This potential difference gives rise to an electric field directed from right to the left between the accelerating anode and the cathode which accelerates electrons from the cathode to accelerating anode. Electrons passing through the hole in the anode form a narrow beam and travel with constant horizontal velocity from the anode to the fluorescent screen. The area where the electrons strike the screen glows brightly. The control grid controls the flow of electrons between the cathode and the accelerating anode. Hence, it controls the brightness of the spot on the screen.

The focusing anode ensures that electrons leaving the cathode in slightly different directions are focused down to a narrow beam and all arrive at the same spot on the screen. The assembly of heater, cathode, control grid, accelerating anode and focusing anode is called the electron gun. The beam of electrons passes between two pairs of deflection plates: horizontal deflection plates and vertical deflection plates. The electric field between the horizontal deflection plates deflects the electrons horizontally, while the electric field between the vertical deflection plates deflects the electrons vertically. Thus, the electrons travel in a straight line from the hole in the accelerating anode to the center of the fluorescent screen, where they produce a bright spot. The screen consists of a glass which is coated by some fluorescent material like zinc silicate which is semitransparent phosphor substance. The phosphor substance converts the electrical energy into light energy. When the high-velocity electrons strike the phosphorescent screen, the light is emitted from it. The property of phosphor to emit light when its atoms are excited is called fluorescence. The intensity of the glow produced at the screen is determined by the number of electrons striking the screen.

Applications of CRT

  1. In televisions
  2. In cathode ray oscilloscope
  3. In computer monitors
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