Functions of a Cathode Ray Tube and its Applications
Parts of a CRT
- 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.
Applications
Televisions
- Before LCD or Plasma television, the CRT was used to create a moving image.
- It used the same principle as a CRT, and for Black and White televisions, that worked fine.
- B&W TVs were essentially the same thing as a CRT, as all that's needed is the control of the brightness of the beam.
- A CRT TV works by having the electron beam "scan" the screen at an rate faster than our eyes can perceive.
- This means that it shoots across the screen like a machine gun, and the images we see are actually made from many fluorescent dots.
- The fluorescence caused by the beam striking the screen lasts a bit longer so that the next scan can be made without the previous image disappearing.
- It scans twice each time, first filling in the odd "holes" then the even ones.
- Each scan is about 1/50 of a second.
- Colour CRT TVs had 3 electron guns rather than a single one, a shadow mask, and a modified fluorescent screen.
- The 3 electron guns were needed as there were three primary colours (Red, Green and Blue) that could be adjusted in different amounts to create any colour.
- The colours are formed as a result of the shadow mask, which is a layer with holes in it that controls the angle of the incoming electron beams.
- This is because the fluorescent screen is separated into multi-coloured phosphors that are placed adjacent to each other at small intervals.
- Thus it isn't actually a single coloured pixel, but rather 3 very small pixels that join together to form a larger dot.
Cathode Ray Oscilloscopes
- A Cathode Ray Oscilloscope (CRO) is a diagnostic device that allows one to "see" voltage.
- It is essential a Cathode Ray Tube with two perpendicular sets of deflecting electric plates.
- The vertical set is where an input voltage is plugged in for the oscilloscope to display.
- However, the horizontal set is connected to a "sweep generator"
- This is what provides a constant, but adjustable, timebase for the sweeping.
- It essentially creates a "sawtooth voltage."
- This is what causes the image to be animated, and measured with a linear scale.
page revision: 4, last edited: 28 Oct 2011 08:41