Cathode Ray Tube Working Principle of a CRT


The cathode ray tube is the main element of a cathode ray oscilloscope. We will discuss the elements of a CTR one by one in this article.

cathode ray tube
Cathode Ray Tube

Cathode of CRT

There is a heater element in the cathode of a cathode ray tube. The cathode is made of nickel. It has a cylindrical cap-like structure. There is a coating of bromine and strontium oxides on the cathode for emitting electrons. The emitting surface is very tiny, just like a point. There is a supply of 600mA at 3.6 volts to the cathode for the electron emission. The emitted electrons create an electron beam in the CTR.

Control Grid of CRT

A one-side open cylinder partially overlaps the cathode. We refer to it as the control grid. The control grid has a small hole at the center to allow electrons to pass through. It has a negative potential. Because of the negativity, it can control the emission of electrons from the cathode. We can adjust the negative potential of the control grid with the help of a potentiometer attached with its circuit. So by turning the potentiometer, we can easily adjust the intensity of the electron emission.

Focusing Anode of CRT

The next is the focusing anode of the cathode ray tube. We apply 500 volts to it. The focusing anode is a hollow cylinder. It contracts the electron beam. Therefore, it increases the sharpness of the image on the CRT screen.

Accelerating Anode of CRT

The next hollow cylinder is the accelerating anode. The accelerating anode has typically 1.5 KV potential. The high positive potential accelerates the electrons in the beam towards the screen.

Pre-accelerating Anode of CRT

In some types of cathode ray tube, there is a pre-accelerating anode in addition to the accelerating anode. If there is a pre-accelerating anode in the CTR. The position of that anode would be here in between the control grid and focusing anode. The function of the pre-accelerating anode is to provide acceleration to the electrons before these enter in the focusing anode. The pre-accelerating anode has commonly the same potential that of accelerating anode. Therefore, there is a connection between the pre-accelerating and accelerating anode.

Horizontal Plates of CRT

The next element is the set of the horizontal plates. The purpose of these two horizontal plates is to provide an electric field on the electron beam. We apply the input voltage signal between the plates through the input circuitry of the CRO. Hence, there is a vertical electric field created between the horizontal plates. According to the variation of the input signal, the intensity of the electric field varies. As a result of this variation, the deflection force acting the electron beam also varies. Therefore, the electron beam moves vertically.

Vertical Plates of CRT

There is another set of vertical plates in the cathode ray tube. The purpose of these two vertical plates is to provide another electric field on the electron beam. We apply a periodic sawtooth input voltage signal between the plates through the concerned circuitry of the CRO. Hence, there is a horizontal electric field created between the vertical plates. Due to the variation of the periodic sawtooth signal, the intensity of the electric field also varies periodically. As a result of this variation, the deflection force acting the electron beam also varies continuously in the horizontal direction. Therefore, the electron beam also moves horizontally from left to right.

Fluorescent Screen of CRT

There is a fluorescent coating on the internal surface of the cathode ray tube screen. When electrons strike on the fluorescent screen, a glow of light appears. So, the electron-beam creates a bright spot on the screen. Due to the combination of the horizontal and vertical motions of the electron beam, the glowing-spot draws an image of the waveform on the screen. The waveform means the waveform of the input signal. The input signal means the signal which we apply across the horizontal plates.

It was the basic working principle of a cathode ray tube (CTR).

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