Semiconductor devices cathode ray tube. Working principle of cathode ray tube and its application

After the deflecting system, the electrons enter the CRT screen. The screen is a thin layer of phosphor deposited on the inner surface of the end part of the balloon and capable of glowing intensely when bombarded with electrons.

In some cases, a conductive thin aluminum layer is deposited over the phosphor layer. Screen properties are determined by its

characteristics and settings. The main screen options are: first And second critical screen potentials, glow brightness, light output, afterglow duration.

screen potential. When the screen is bombarded by a stream of electrons from its surface, secondary electron emission occurs. To remove secondary electrons, the walls of the balloon tube near the screen are covered with a conductive graphite layer, which is connected to the second anode. If this is not done, then the secondary electrons, returning to the screen, together with the primary ones, will lower its potential. In this case, a decelerating electric field is created in the space between the screen and the second anode, which will reflect the electrons of the beam. Thus, to eliminate the decelerating field from the surface of a non-conductive screen, it is necessary to remove the electric charge carried by the electron beam. Almost the only way to compensate for the charge is to use secondary emission. When electrons fall on the screen, their kinetic energy is converted into the energy of the screen glow, goes to heat it and causes secondary emission. The value of the secondary emission coefficient o determines the potential of the screen. The coefficient of secondary electron emission a \u003d / in // l (/ „ is the current of secondary electrons, / l is the current of the beam, or the current of primary electrons) from the screen surface in a wide range of changes in the energy of primary electrons exceeds one (Fig. 12.8, about < 1 на участке O A curve at V < С/ кр1 и при 15 > C/cr2).

At And < (У кр1 число уходящих-от экрана вторичных электронов меньше числа первичных, что приводит к накоплению отрицательного заряда на экране, формированию тормозящего поля для электронов луча в пространстве между вторым анодом и экраном и их отражению; свечение экрана отсутствует. Потенциал and l2\u003d Г / kr corresponding to point A in fig. 12.8, called the first critical potential.

At C/a2 = £/cr1, the screen potential is close to zero.

If the beam energy becomes greater than e£/cr1, then about > 1 and the screen starts charging half-

Rice. 12.8

relative to the last anode of the spotlight. The process continues until the screen potential becomes approximately equal to the potential of the second anode. This means that the number of electrons leaving the screen is equal to the number of incident ones. In the range of beam energy variation from e£/cr1 to C/cr2 c > 1 and the screen potential is quite close to the projector anode potential. At and &2> N cr2 coefficient of secondary emission a< 1. Потенциал экрана вновь снижается, и у экрана начинает формироваться тормозящее для электронов луча поле. Потенциал And kr2 (corresponds to the point IN in fig. 12.8) are called second critical potential or ultimate potential.

At energies of the electron beam above e11 kr2 The brightness of the screen does not increase. For various screens G/ kr1 = = 300...500 V, and cr2= 5...40 kV.

If it is necessary to obtain high brightness, the screen potential is forcibly maintained equal to the potential of the last spotlight electrode using a conductive coating. The conductive coating is electrically connected to this electrode.

Light output. This is a parameter that determines the ratio of light intensity J cv, emitted by the phosphor normally to the screen surface, to the power of the electron beam P el incident on the screen:

Light output ts determines the efficiency of the phosphor. Not all of the kinetic energy of primary electrons is converted into the energy of visible radiation, part of it goes to heating the screen, secondary emission of electrons and radiation in the infrared and ultraviolet ranges of the spectrum. Light output is measured in candelas per watt: for various screens, it varies between 0.1 ... 15 cd / W. At low electron velocities, luminescence occurs in the surface layer and part of the light is absorbed by the phosphor. As the energy of the electrons increases, the light output increases. However, at very high speeds, many electrons penetrate the phosphor layer without producing excitation, and the light output decreases.

Glow brightness. This is a parameter that is determined by the intensity of light emitted in the direction of the observer by one square meter of a uniformly luminous surface. Luminance is measured in cd/m 2 . It depends on the properties of the phosphor (characterized by the coefficient A), the current density of the electron beam y, the potential difference between the cathode and the screen II and minimum screen potential 11 0 , at which screen luminescence is still observed. The brightness of the glow obeys the law

Exponent values p y potential £/ 0 for different phosphors vary within 1...2.5, respectively, and

30 ... 300 V. In practice, the linear nature of the dependence of brightness on current density y remains approximately up to 100 μA / cm 2. At high current densities, the phosphor begins to heat up and burn out. The main way to increase brightness is to increase And.

Resolution. This important parameter is defined as the property of a CRT to reproduce image details. The resolution is estimated by the number of separately distinguishable luminous dots or lines (lines) corresponding to 1 cm 2 of the surface or 1 cm of the screen height, or to the entire height of the screen working surface, respectively. Consequently, to increase the resolution, it is necessary to reduce the beam diameter, i.e., a well-focused thin beam with a diameter of tenths of a mm is required. The resolution is the higher, the lower the beam current and the higher the accelerating voltage. In this case, the best focusing is realized. Resolution also depends on the quality of the phosphor (large phosphor grains scatter light) and the presence of halos due to total internal reflection in the glass part of the screen.

Afterglow duration. The time during which the brightness of the glow decreases to 1% of the maximum value is called the screen persistence time. All screens are divided into screens with very short (less than 10 5 s), short (10" 5 ... 10" 2 s), medium (10 2 ...10 1 s), long (10 H.Lb s) and very long (more than 16 s) afterglow. Tubes with short and very short afterglow are widely used in oscillography, and with medium afterglow - in television. Radar indicators typically use tubes with a long afterglow.

In radar tubes, long-lasting screens with a two-layer coating are often used. The first layer of the phosphor - with a short blue afterglow - is excited by an electron beam, and the second - with a yellow glow and a long afterglow - is excited by the light of the first layer. In such screens, it is possible to obtain an afterglow of up to several minutes.

Screen types. The color of the glow of the phosphor is very important. In oscillographic technology, when visually observing the screen, a CRT with a green glow is used, which is the least tiring for the eye. Zinc orthosilicate activated with manganese (willemite) has this luminescence color. For photography, screens with a blue glow characteristic of calcium tungstate are preferred. In receiving television tubes with a black and white image, they try to get a white color, for which phosphors from two components are used: blue and yellow.

The following phosphors are also widely used for the manufacture of screen coatings: zinc and cadmium sulfides, zinc and magnesium silicates, oxides and oxysulfides of rare earth elements. Phosphors based on rare earth elements have a number of advantages: they are more resistant to various influences than sulfide ones, they are quite effective, they have a narrower spectral emission band, which is especially important in the production of color picture tubes, where high color purity is required, etc. An example is the relatively widely used phosphor based on yttrium oxide activated with europium Y 2 0 3: Eu. This phosphor has a narrow emission band in the red region of the spectrum. A phosphor consisting of yttrium oxysulfide with an admixture of europium Y 2 0 3 8: Eu also has good characteristics, which has a maximum radiation intensity in the red-orange region of the visible spectrum and better chemical resistance than the Y 2 0 3: Eu phosphor.

Aluminum is chemically inert when interacting with screen phosphors, is easily applied to the surface by evaporation in a vacuum, and reflects light well. The disadvantages of aluminized screens include the fact that the aluminum film absorbs and scatters electrons with energies less than 6 keV, therefore, in these cases, the light output drops sharply. For example, the light output of an aluminized screen at an electron energy of 10 keV is about 60% greater than at 5 keV. Tube screens are rectangular or round.

Cathode-ray tube(CRT) - an electronic device in the form of a tube, elongated (often with a conical extension) in the direction of the axis of the electron beam, which is formed in the CRT. A CRT consists of an electron-optical system, a deflection system, and a fluorescent screen or target. TV repair in Butovo, please contact us for help.

CRT classification

The classification of CRTs is extremely difficult, due to their extreme

about wide application in science and technology and the possibility of modifying the design in order to obtain the technical parameters that are necessary for the implementation of a specific technical idea.

Dependences on the CRT electron beam control method are divided into:

electrostatic (with an electrostatic beam deflection system);

electromagnetic (with electromagnetic beam deflection system).

Depending on the purpose of the CRT are divided into:

electron-graphic tubes (receiving, television, oscilloscope, indicator, television signs, coding, etc.)

optical-electronic converting tubes (transmitting television tubes, electron-optical converters, etc.)

cathode beam switches (commutators);

other CRTs.

Electronic graphic CRT

Electronic graphic CRT - a group of cathode ray tubes used in various fields of technology to convert electrical signals into optical ones (signal-to-light conversion).

Electronic graphic CRTs are subdivided:

Depending on the application:

television reception (kinescopes, CRT with ultra-high resolution for special television systems, etc.)

receiving oscilloscope (low-frequency, high-frequency, superhigh-frequency, pulse high-voltage, etc.)

reception indicator;

remembering;

badges;

coding;

other CRTs.

The structure and operation of a CRT with an electrostatic beam deflection system

The cathode ray tube consists of a cathode (1), an anode (2), a leveling cylinder (3), a screen (4), plane (5) and height (6) adjusters.

Under the action of photo or thermal emission, electrons are knocked out of the cathode metal (thin conductor spiral). Since a voltage (potential difference) of several kilovolts is maintained between the anode and cathode, these electrons, aligning themselves with a cylinder, move in the direction of the anode (hollow cylinder). Flying through the anode, the electrons get to the plane regulators. Each regulator is two metal plates, oppositely charged. If the left plate is charged negatively and the right plate positively, then the electrons passing through them will deviate to the right, and vice versa. The height controls work the same way. If an alternating current is applied to these plates, then it will be possible to control the flow of electrons both in the horizontal and vertical planes. At the end of its path, the electron stream hits the screen, where it can cause images.

The cathode ray tube (CRT) is one thermionic device that does not seem to be going out of use in the near future. The CRT is used in an oscilloscope to observe electrical signals and, of course, as a kinescope in a television receiver and a monitor in a computer and radar.

A CRT consists of three main elements: an electron gun, which is the source of the electron beam, a beam deflection system, which can be electrostatic or magnetic, and a fluorescent screen that emits visible light at the point where the electron beam hits. All the essential features of a CRT with an electrostatic deflection are shown in fig. 3.14.

The cathode emits electrons, and they fly towards the first anode A v which is supplied with a positive voltage of several thousand volts relative to the cathode. The flow of electrons is regulated by a grid, the negative voltage on which is determined by the required brightness. The electron beam passes through the hole in the center of the first anode and also through the second anode, which has a slightly higher positive voltage than the first anode.

Rice. 3.14. CRT with electrostatic deflection. A simplified diagram connected to a CRT shows the brightness and focus controls.

The purpose of the two anodes is to create an electric field between them, with lines of force curved so that all the electrons in the beam converge at the same spot on the screen. Potential difference between anodes A 1 And L 2 is selected using the focus control in such a way as to obtain a clearly focused spot on the screen. This design of two anodes can be considered as an electronic lens. Similarly, a magnetic lens can be created by applying a magnetic field; in some CRTs, focusing is done in this way. This principle is also used to great effect in the electron microscope, where a combination of electron lenses can be used to provide very high magnification with a resolution a thousand times better than that of an optical microscope.

After the anodes, the electron beam in the CRT passes between deflecting plates, to which voltages can be applied to deflect the beam in the vertical direction in the case of plates Y and horizontally in the case of plates X. After the deflecting system, the beam hits the luminescent screen, that is, the surface phosphor.

At first glance, the electrons have nowhere to go after they hit the screen, and you might think that the negative charge on it will grow. In reality, this does not happen, since the energy of the electrons in the beam is sufficient to cause "splashes" of secondary electrons from the screen. These secondary electrons are then collected by a conductive coating on the walls of the tube. In fact, so much charge usually leaves the screen that a positive potential of several volts with respect to the second anode appears on it.

Electrostatic deflection is standard on most oscilloscopes, but this is inconvenient for large TV CRTs. In these tubes with their huge screens (up to 900 mm diagonally), to ensure the desired brightness, it is necessary to accelerate the electrons in the beam to high energies (typical voltage of a high-voltage

Rice. 3.15. The principle of operation of the magnetic deflection system used in television tubes.

source 25 kV). If such tubes, with their very large deflection angle (110°), were to use an electrostatic deflection system, excessively large deflection voltages would be required. For such applications, magnetic deflection is the standard. On fig. 3.15 shows a typical design of a magnetic deflection system, where pairs of coils are used to create a deflecting field. Please note that the axes of the coils perpendicular the direction in which the deflection occurs, as opposed to the centerlines of the plates in an electrostatic deflection system, which are parallel deflection direction. This difference emphasizes that electrons behave differently in electric and magnetic fields.

Perhaps there is no such person who would not have encountered devices in his life, the design of which includes a cathode ray tube (or CRT). Now such solutions are being actively replaced by their more modern counterparts based on liquid crystal screens (LCD). However, there are a number of areas in which the cathode ray tube is still indispensable. For example, LCDs cannot be used in high-precision oscilloscopes. However, one thing is clear - the progress of information display devices will eventually lead to the complete abandonment of the CRT. It is the matter of time.

History of appearance

The discoverer can be considered J. Plücker, who in 1859, studying the behavior of metals under various external influences, discovered the phenomenon of radiation (emission) of elementary particles - electrons. The generated particle beams are called cathode rays. He also drew attention to the appearance of a visible glow of certain substances (phosphor) when electron beams hit them. The modern cathode ray tube is able to create an image thanks to these two discoveries.

After 20 years, it was experimentally established that the direction of movement of the emitted electrons can be controlled by the action of an external magnetic field. This is easy to explain if we remember that moving negative charge carriers are characterized by magnetic and electric fields.

In 1895, K. F. Brown improved the control system in the tube and thereby managed to change the direction vector of the particle flow not only by the field, but also by a special mirror capable of rotating, which opened up completely new prospects for using the invention. In 1903, Wenelt placed a cathode-electrode in the form of a cylinder inside the tube, which made it possible to control the intensity of the radiated flux.

In 1905, Einstein formulated the equations for calculating the photoelectric effect and after 6 years a working device for transmitting images over distances was demonstrated. The beam was controlled and the capacitor was responsible for the brightness value.

When the first CRT models were launched, the industry was not ready to create screens with a large diagonal, so magnifying lenses were used as a compromise.

Cathode Ray Tube Device

Since then, the device has been improved, but the changes are evolutionary in nature, since nothing fundamentally new has been added to the course of work.

The glass body begins with a tube with a cone-shaped extension forming a screen. In color imaging devices, the inner surface with a certain pitch is covered with three types of phosphor, which give their glow color when an electron beam hits it. Accordingly, there are three cathodes (guns). In order to filter out the defocused electrons and ensure that the desired beam hits the desired point on the screen accurately, a steel grating - a mask - is placed between the cathode system and the phosphor layer. It can be compared to a stencil that cuts off everything superfluous.

Electron emission begins from the surface of the heated cathodes. They rush towards the anode (electrode, with a positive charge) connected to the conical part of the tube. Next, the beams are focused by a special coil and enter the field of the deflecting system. Passing through the lattice, they fall on the desired points of the screen, causing their transformation into a glow.

Computer Engineering

CRT monitors are widely used in computer systems. Simplicity of design, high reliability, accurate color reproduction and the absence of delays (those very milliseconds of matrix response in an LCD) are their main advantages. However, in recent years, as already mentioned, the CRT is being replaced by more economical and ergonomic LCD monitors.

Application of cathode ray tube

Cathode ray tubes are used in oscilloscopes to measure voltage and phase angles, analyze the shape of a current or voltage waveform, etc. These tubes are used in television and radar installations.

cathode ray tubes are of different types. According to the method of obtaining an electron beam, they are divided into tubes with a cold and heated cathode. Cold cathode tubes are used relatively rarely, since their operation requires very high voltages (30-70 kV). Hot cathode tubes are widely used. These tubes are also divided into two types according to the method of controlling the electron beam: electrostatic and magnetic. In electrostatic tubes, the electron beam is controlled by an electric field, and in magnetic tubes, by a magnetic field.

Electrostatically controlled cathode ray tubes are used in oscilloscopes and are extremely diverse in design. It is enough for students to familiarize themselves with the principle of the device of such a tube containing the main typical elements. These goals are met by a 13LOZ7 tube, which is presented in the table with some simplifications.

A cathode ray tube is a well evacuated glass container with electrodes inside. The wide end of the tube - the screen - is covered with a fluorescent substance from the inside. The screen material glows when electrons strike. The source of electrons is an indirectly heated cathode. The cathode consists of a filament 7 inserted into a thin porcelain tube (insulator), on which a cylinder 6 with an oxide coating of the end (cathode) is put on, due to which electrons are emitted in only one direction. The electrons emitted from the cathode rush to the anodes 4 and 3, which have a rather high potential relative to the cathode (several hundred volts). To shape the electron beam into a beam and focus it on a screen, the beam passes through a series of electrodes. However, students should pay attention to only three electrodes: the modulator (control cylinder) 5, the first anode 4 and the second anode 3. The modulator is a tubular electrode, which is supplied with a negative potential relative to the cathode. Due to this, the electron beam passing through the modulator will be contracted into a narrow beam (beam) and directed by the electric field through the hole in the anode towards the screen. By raising or lowering the potential of the control electrode, you can adjust the number of electrons in the beam, i.e., the intensity (brightness) of the screen glow. With the help of anodes, not only an accelerating field is created (electrons are accelerated), but by changing the potential of one of them, it is possible to more accurately focus the electron beam on the screen and obtain a greater sharpness of the luminous point. Usually, focusing is carried out by changing the potential of the first anode, which is called focusing.

The electron beam, leaving the hole in the anode, passes between two pairs of deflecting plates 1,2 and hits the screen, causing it to glow.

By applying voltage to the deflecting plates, the beam can be deflected and the luminous spot shifted from the center of the screen. The amount and direction of the bias depends on the voltage applied to the plates and the polarity of the plates. The table shows the case when the voltage is applied only to the vertical plates 2. With the indicated polarity of the plates, the displacement of the electron beam under the action of the forces of the electric field occurs to the right. If voltage is applied to the horizontal plates 1, then the beam will shift in the vertical direction.

The lower part of the table shows the way to control the beam using a magnetic field created by two mutually perpendicular coils (each coil is divided into two sections), the axes of which have vertical and horizontal directions. The table shows the case when there is no current in the horizontal coil and the vertical coil provides beam displacement only in the horizontal direction.

The magnetic field of the horizontal coil causes the beam to shift in the vertical direction. The combined action of the magnetic fields of the two coils ensures the movement of the beam across the entire screen.

Magnetic tubes are used in televisions.