Cathode Ray Tube Construction and Working

Constructional Details: 

The constructional details of a general purpose CRT are shown in Figure. It consists of the electron gun, deflecting system, fluorescent screen and the housing which is a funnel shaped glass envelope with a base.

Cathode Ray Tube Construction

The electron gun : This assembly consists of an indirectly heated cathode and a grid struc-ure the anodes that deflect and focus the beam. The cathode is surrounded by the grid. The grid s a nickel cylinder. It has a small hole in the centre and is located coaxially with the tube axis. The cathode emits the electrons. The grid is maintained at a negative potential with respect to the cathode. Under the influence of the grid the electrons are prevented from diverging and they pass through the hole in the grid structure. The electrons that are capable of passing through the grid structure's hole travel along the axis of the tube and they contribute the beam current. The magnitude of this beam current is dependent on the grid voltage. The more negative the grid is with respect to the cathode, the less will be the beam current and vice versa. The beam current determines the intensity of the spot on the screen of the CRT. A potentiometer that controls this grid voltage is made available on the front panel of the CRO. This is called the intensity control. Intensity control is a user control. The intensity of the spot or pattern on the screen of the CRO can either be increased or decreased or totally cut off using this control.

The electrons after passing the grid structure will not be having sufficient velocities to reach the screen. Hence they are to be accelerated. It is for that purpose an accelerating electrode called the preaccelerating electrode and another accelerating anode called the second anode are used. As the beam contains electrons that repel each other, the beam tends to diverge. To prevent this and to arrive at a fine beam that would result in a fine spot on the screen of the CRT, a focusing electrode is introduced between the preaccelerating anode and the second anode. Some cathode ray tubes will not be provided with a separate structure for the second anode. Instead of the second anode, a conductive layer will be formed inside the glass envelope which is used as the second anode by applying positive potential. The process of converging the electrons in to a fine form of beam using the anodes is called focusing. Electro static focusing is used in CROs.