Introduction to cathode Ray Tube.pptx

About Cathode ray tube

1. CATHODE RAY TUBE Dr. J. Revathi Assistant Professor Department of Biomedical Engineering Dr. N.G.P Institute of Technology

2. Cathode Ray Tube The Cathode R ay T ube (CRT) is a vacuum tube containing an electron gun (a source of electrons or electron emitter) and a fluorescent screen used to view images. It has a mean to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent Electrical waveforms  in oscilloscope, pictures (television, computer monitor), radar targets. CRTs have largely superseded by modern technologies such as, LCD, LED, etc , which offer lower manufacturing and distribution cost.

3. Principle of CRT The Cathode R ay T ube (CRT) is the device that displays images upon a screen using electron beams. It is a vacuum sealed tube to avoid air resistance. The electrons are first created at the back of the CRT by an electron gun. The electrons are shot off the gun at a high velocity and travel through a magnetic field, which steers them in a certain direction

4. The electrons then pass through a filter (shadow mask) and  collide with a phosphor screen, to which they transfer their energy and cause the screen to glow. Through specific direction of these electron beams, images are created and displayed on the screen.

5. CRT

6. Three Main Parts E lectron gun Deflecting plates Fluorsecent screen

7. Electron Gun The electron gun produces the electron beam that travels through the CRT. The electron gun consists of cathode filament accelerating anode

8. Function of Cathode and Anode The filament is insulated coil of tungsten located inside the cathode The current is applied to this coil to heat it. When the cathode is heated, the electrons on the cathode released. The cathode creates an electron cloud (emits electrons) whose electrons are extracted, accelerated and focused into an electron beam. The anode serves only to provide the accelerating voltage for the electrons and not as the electron collector.

9. Deflecting plates The CRT can use either electrostatic or magnetic deflection. so that the beam of electrons can have its direction altered, allowing the light spot to appear anywhere on the face of the tube. The cathode-ray-tube deflection system is made up of two pairs of parallel plates known as the vertical and horizontal deflection plates. The plates are made up of copper or aluminium .

10. Electrostatic Deflection System Due to this, strong electric field is produced between the plates. When electron beam enters in this field, it is deflected towards the plate which is positive. So the path of beam becomes parabolic. Then the beam comes out of this field in straight line and strikes on the screen at a particular point  on the screen .

11. Magnetic deflection system In this system, electron beam is deflected using external magnetic field. To create alternating magnetic field two pairs of deflection coils, known as yoke, are fitted on the neck of CRT. The pair of vertical deflection coils is connected to  y-input  of CRO T he pair of horizontal deflection coils is connected to internal sawtooth wave or external signal at  x-input .

12. CRT screen The CRT screen consists of two main parts: the phosphor screen and the shadow mask. The phosphor screen turns the electron beam into light, and the shadow mask organizes the beam on the screen. It contains millions of tiny red, green, and blue phosphor dots that glow when struck by an electron beam that travels across the screen to create a visible image. The electrons are negative and the screen gives a positive charge so the screen glows.

13. Phosphor persistence  The v arious phosphors are available depending upon the needs of the measurement or display application. The brightness, color, and persistence of the illumination depends upon the type of phosphor used on the CRT screen. Phosphors are available with persistence's ranging from less than one microsecond to several seconds.  For visual observation of brief transient events, a long persistence phosphor may be desirable. For events which are fast and repetitive, or high frequency, a short-persistence phosphor is generally preferable.

14. Advantages of CRT The cathode rayed tube can easily increase the monitor’s brightness by reflecting the light. They produce more colours . The Cathode Ray Tube monitors have lower price rate than the LCD display or Plasma display. The quality of the image displayed on a Cathode Ray Tube is superior to the LCD and Plasma monitors. The contrast features of the cathode ray tube monitor are considered highly excellent.

15. Disadvantages of CRT The electromagnetic fields emitted by CRT monitors constitute a health hazard to the functioning of living cells. CRTs emit a small amount of X-ray band radiation which can result in a health hazard. Constant refreshing of CRT monitors can result in headache. CRTs operate at very high voltage which can overheat system or result in an implosion