Question

Draw a labelled diagram of cathode ray tube.

Hint:

The combination of the Heater, Cathode, Control Grid, and the Accelerating/Focusing Anodes is collectively known as the Electron Gun assembly. Its primary function is to generate a highly focused, accelerated beam of electrons.

Answer 1

Step 1: Core Operating Concept of the Cathode Ray Tube (CRT):
The Cathode Ray Tube is a specialized vacuum tube that converts electrical video signals into a visible image. It operates by generating, focusing, and accelerating a stream of electrons, then deflecting them onto a phosphor-coated screen.

Step 2: Detailed Architectural Breakdown of the CRT Schematic:
Looking at the internal physical and electrical layout of a standard CRT from left to right, we find the following key components:

• Base Pins: Located at the far left neck of the tube. These are conductive metal contacts sealed within a plastic socket that supply power and bias voltages to all internal electrodes.
• Heater (Filament): A high-resistance tungsten wire coil placed inside the cathode. When energized by a low AC/DC voltage ($6.3\text{ V}$), it heats the cathode cylinder to approximately $800\text{--}1000\text{ K}$.
• Indirectly Heated Cathode ($K$): A small nickel cylinder coated with barium and strontium oxides. When heated, it emits a dense, continuous cloud of free electrons via thermionic emission.
• Control Grid ($G_1$): A metal cup with a tiny central aperture surrounding the cathode. It is kept at a negative potential relative to the cathode. Adjusting this negative bias controls the number of electrons passing through the aperture, regulating the beam current and controlling screen brightness (luminance).
• Pre-accelerating and Accelerating Anodes ($G_2$ & $G_4$): High positive potential cylinders ($1\text{ to }5\text{ kV}$) that attract and accelerate the electron stream along the central axis.
• Focusing Anode ($G_3$): Located between the accelerating anodes. Operating at an adjustable intermediate potential, it creates an electrostatic lens that converges the diverging electron stream into a razor-thin beam.
• Aquadag Coating: A conductive graphite paste coated on the inner surface of the glass neck and funnel. It collects secondary electrons emitted from the screen and helps maintain a uniform electrostatic field.
• Deflection Yoke: Mounted externally around the neck of the tube. It consists of:
  • Horizontal Deflection Coils: Generate an alternating magnetic field to sweep the beam horizontally from left to right (line scanning).
  • Vertical Deflection Coils: Generate an alternating magnetic field to sweep the beam vertically downwards (field scanning).
• Phosphor Screen: The wide, flat front face of the tube, coated internally with a thin layer of zinc-sulfide phosphors. When hit by the high-velocity, focused electron beam, the phosphors convert the kinetic energy of the electrons into visible light (producing the raster/image).