Construction Details of CRT

1)  Electron gun:

It is very simple device which mainly produces thin electron beam. It is the combination of Heater, Cathode and the Grids. All this components are situated at the base of the tube. The heater heats the cathode and then the cathode emits a lot of electrons which make an electron cloud. The electron gun is analogous to the pen. In different parts of the electron gun different types of voltages are required. These voltages vary within the range 200-600 Volts. The electron gun receives voltages from the receiver which is connected with the base pins of the CRT.

2) Heater:

The heater is mainly used to heat the cathode. It is made of metals and twisted in shape. It heats the cathode indirectly. It needs 6-24 Volts dc or ac supply voltage.

3) Cathode:

                   The cathode has plenty of electrons. It is a special type of material which has a very low work function. It is generally a Nickel (Ni) cylinder, coated with thoriated Tungsten (W) or Barium (Ba) and Strontium (Sr) oxides. It is cylindrical in shape such that it can produce radially symmetrical field.

4) Grids:

              Actually the grids act as a help hand for the electrons to be reached on the screen. These electrons finally make the electron beam. The grids are located in the base of the CRT. There are four types of grids.

Such as: a) Control grid

         b) Screen grid/ Accelerator grid

         c) Suppressor grid

         d) Focus grid

  For Black and White Television the first three types of grids are needed and an extra Focus grid is necessary for the Color TV.

        

a)  Control grid: This grid is used to control the flow of electrons. It is generally cylindrical in shape. It has a central hole on itself through which the electrons pass away. That is, it is used to confine the electrons emitted by the cathode. It maintains the negative voltage with respect to the cathode.

b)  Screen grid/Accelerator grid: It is same as the control grid. It mainly accelerates the electrons controlled by the control grid. It is maintained different positive (+ve) voltages with respect to the cathode that varies from 200-600 Volts.

c)  Suppressor grid: The suppressor grid mainly suppresses / stops the overflow of electrons. That is it maintains a constant flow of electrons. It is analogous to the water pipe where there are some holes which help to maintain a constant flow of water through the pipe by passing the water through the holes it selves.       

d)  Focus grid: The focus grid is specially used in the color Television. For better fine focus the focus grid is set at a lower potential such as 0 to 400 V. 

            The focus grid voltage=one fifth of the anode voltage.  

Anode/High voltage side:

             

               The anode of the Television set has some special properties. It operates at a very high voltage. It is not a single electrode. The specialty is that it is coated with phosphor at inside i.e.; here the coating conductor is used. The anode needs a very high voltage (about 33 KV). The high voltage is supplied from a special type of source called Fly back Transformer. Actually the anode attracts the electrons emitted by the cathode (which forms the electron cloud). These electrons hit the screen with a very high speed.  The very high voltage applied to the anode depends upon the diagonal length of the CRT. 

For black and white television:

Voltage applied to the node = 1.2*the diagonal length  KV.

For color television:       

Voltage applied to the node = 1.3*the diagonal length  KV.