Understanding of Cathode Ray Tube – CRT

Computer technology is going to see major advances in sophisticated 3-dimensional modeling and image processing; the users will see desktop computers with the computational power of today’s super-computers. Even graphics capabilities would be available to the average user at a reasonable cost. To make this, ultra-high-resolution monitors will be required. There are different display systems like cathode ray tubes (CRTs), liquid crystal displays (LCDs), electroluminescent displays (ELDs), plasma displays and light-emitting diodes (LEDs) are available in the present technology. Here we are going to discuss the Cathode Ray Tube (CRT).

Working Principles

When the two metal plates are connected to a high voltage source, the negatively charged plate called the cathode emits an invisible ray. The cathode ray is drawn to the positively charged plate, called the anode, where it passes through a hole and continues traveling to the other end of the tube. When the ray strikes the specially coated surface, the cathode ray produces a strong fluorescence or bright light. When an electric field is applied across the cathode ray tube, the cathode ray is attracted by the plate bearing positive charges. Therefore a cathode ray must consist of negatively charged particles. A moving charged body behaves like a tiny magnet, and it can interact with an external magnetic field. The electrons deflected by the magnetic field. And also when the external magnetic field is reversed, the beam of electronics is deflected in the opposite direction.

In a cathode ray tube, the cathode is a heated filament and it placed in a vacuum. The ray is a stream of electrons that naturally pour off a heated cathode into the vacuum. Electrons are negative. The anode is positive, so it attracts the electrons pouring off the cathode. In a TV’s cathode ray tube, the stream of electrons is focused by a focusing anode into a tight beam and then accelerated by an accelerating anode. This tight, high-speed beam of electrons flies through the vacuum in the tube and hits the flat screen at the other end of the tube. This screen is coated with phosphor, which glows when struck by the beam.

Operation of CRT

Cathode Ray Tube (CRT) is a computer display screen, used to display the output in a standard composite video signal. The working of CRT depends on the movement of an electron beam which moves back and forth across the back of the screen. The source of the electron beam is the electron gun; the gun is located in the narrow, cylindrical neck at the extreme rear of a CRT which produces a stream of electrons through thermionic emission. Usually, A CRT has a fluorescent screen to display the output signal. A simple CRT is shown below.

The operation of a CRT monitor is very simple. A cathode-ray tube consists of one or more electron guns, possibly internal electrostatic deflection plates, and a phosphor target. CRT has three electron beams – one for each (Red, Green, and Blue) is clearly shown in the figure. The electron beam produces a tiny, bright visible spot when it strikes the phosphor-coated screen. In every monitor device, the entire front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. An image (raster) is displayed by scanning the electron beam across the screen. The phosphor’s targets are beginning to fade after a short time, the image needs to be refreshed continuously. Thus CRT produces the three color images which are primary colors. Here we used a 50 Hz rate to eliminate the flicker by refreshing the screen.

The main parts of the cathode ray tube are cathode, control grid, deflecting plates and screen.

Cathode

The heater keeps the cathode at a higher temperature and electrons flow from the heated cathode towards the surface of the cathode. The accelerating anode has a small hole at its center and is maintained at a high potential, which is of positive polarity. The order of this voltage is 1 to 20 kV, relative to the cathode. This potential difference creates an electric field directed from right to left in the region between the accelerating anode and the cathode. Electrons pass through the hole in the anode travel with constant horizontal velocity from the anode to the fluorescent screen. The electrons strike the screen area and it glows brightly.

The Control Grid

The control grid regulates the brightness of the spot on the screen. By controlling the number of electrons by the anode and hence 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 whole assembly of cathode, control grid, focusing anode, and accelerating electrode are called the electron gun.

Deflecting Plates

Two pairs of deflecting plates allow the beam of electrons. An electric field between the first pair of plates deflects the electrons horizontally, and an electric field between the second pair deflects them vertically, the electrons travel in a straight line from the hole in the accelerating anode to the center of the screen when no deflecting fields are present, where they produce a bright spot.

Screen

This may be circular or rectangular. The screen is coated with a special type of fluorescent material. Fluorescent material absorbs its energy and re-emits light in the form of photons when the electron beam hits the screen. When it happens some of them bounces back just like bouncing off a cricket ball from a wall. These are called secondary electrons. They must be absorbed and returned to the cathode if it is not so they accumulate near the screen and produce space charge or electrons cloud. To avoid this, aqua day coating is applied on the funnel part of CRT from inside.

Advantages of CRT

1. CRT’s are less expensive than other display technologies.
2. They operate at any resolution, geometry and aspect ratio without decreasing the image quality.
3. CRTs produce the very best color and gray-scale for all professional calibrations.
4. Excellent viewing angle.
5. It maintains good brightness and gives long life service.

Features of CRT

The use of CRT technology has quickly declined since the introduction of LCDs but they are still unbeatable in certain ways. CRT monitors are widely used in several electrical devices such as computer screens, television sets, radar screens, and oscilloscopes used for scientific and medical purposes.

Now you have got a clear idea about the cathode ray tube and if any queries on this topic or the electrical and electronic projects leave the comments below.

Photo Credit:

• Cathode Ray Tube by allaboutcircuits