Electron Gun Assembly in CRT
The electron gun assembly of CRT consists of an indirectly heated cathode, a control grid surrounding the cathode, a focusing anode and an accelerating anode.
The sole function of the electron gun assembly is to provide a focused electron beam which is accelerated towards the phosphor screen. The cathode is a nickel cylinder coated with an oxide coating of barium and strontium and emits plenty of electrons, when heated.
The emitting surface of the cathode should be as small as possible, theoretically a point. Rate of emission of electrons or say the intensity of electron beam depends on the magnitude of cathode current, which can be controlled by the control grid in a manner similar to a conventional vacuum tube. The typical values of current and voltage required by an indirectly heated cathode are 600 mA at 6.3 V( ac or dc). The special low power designs use 140 mA at 1.5 V.
The control grid is usually a metal cylinder covered at one end but with a small hole in the cover. This is usually a metal cup of low permeability steel , about 15 mm in diameter and 15 mm long. An aperture of approximately 0.25 mm is drilled in the grid cap for the passage of electrons through it. The grid is kept at negative potential (variable) with respect with cathode and its function is to vary the electron mission and so the brilliancy of the spot on the phosphor screen. The hole in the grid is provided to allow passage for electrons through it and concentrate the beam of electrons along the axis of tube. Electron beam comes out from the control grid through a small hole in it and enters a pre-accelerating anode, which is a hollow cylinder in shape and is at a potential of few hundred volts more positive than the cathode so as to accelerate the electron in the electric field. This accelerated beam would be scattered now because of variations in energy and would produce a broad ill-defined spot on the screen. This electron beam is focused on the screen by an electrostatic lens consisting of two more cylindrical anodes called the focusing anode and accelerating anode apart from the pre-accelerating anode. The focusing and accelerating anodes may be open or close at both ends and if covered, holes must be provided in the anode cover for the passage of electrons. The function of these anodes is to concentrate and focus the beam on the screen and also to accelerate the speed of electrons.
The pre-accelerating anode and the accelerating anode are connected to a common positive high voltage of about 1,500 V. The focusing anode is connected to a lower voltage of 500 V.