What is gas-discharge light sources?
In the normal state gases are good insulators, i.e. they do not carry electric current. However under certain conditions it is possible to break their isolating properties and to force gas to carry elekyotrichesky current.
The set of the phenomena connected with the proyokhozhdeniye of electric current through gas carries the name of gas or electric discharge.
Let's take the vessel, for example the glass tube with the metallic electrodes which are sealed on its ends. Previously we will remove from the tube the atmospheric air and its napolyony what-liboinertnym gas or we will create conditions for formation of vapors of metal. If to put tension to electrodes of the tube, then under the influence of this tension in the tube electric field which, influencing the quantity of free electric charges which is available always in the gas environment, under the known conditions will force to peyoremeshchatsya free and again obrazoyovanny electric charges in the opreyodelenny direction is created. In the tube electric current poyoyavitsya — there is the discharge through gases. Passing of electric current through the tube filled with gas or vapors of metals soproyovozhdatsya by the number of the characteristic light phenomena.
Depending on the type of discharge there is the harakteryony luminescence of gases or vapors of metals which yavlyayotsya by the basis for creation of different type of light sources. The light source in which the discharge through gases phenomenon is used is called the gas-discharge light source. Tension at which process of gazoyovy discharge begins is called ignition tension. This tension defines that its minimum value which has to be attached to the tube that in it there was the discharge. After in the tube there was the discharge, for its maintenance it is necessary to have tension, smaller in size, than tension of peryovonachalny ignition.
This tension buyodt to be called tension of burning of discharge. Depending on the sort of the gas filling the tube, its pressure, distance between electrodes, materiayol of which they are made, their geometrical sizes and some other factors the veliyochina of tension of ignition will change. Let's consider those physical processes which obusyolovlivat the luminescence of gases or vapors of metals at gazoyovy discharge.
Electric current between electrodes of the digit tube is created as a result of movement under deystyoviy electric field of electrons and ions, образующихся from neutral atoms of gas or vapors, находящихся in interelectrode space. In order that process of formation of new svobodyony electric charges in the interelectrode prostranyostvo continued continuously, it is necessary to provide constant replenishment of this space with electrons. This role is carried out by the metallic cathode of the tube. It is known that in metals there is always the big koyolichestvo of free electrons which move haoyotichesk between atoms and molecules of the metal conductor.
However to leave the surface of the metallicheyosky conductor electrons in usual conditions d not moyonut as it is counteracted by forces, deystvuyuyoshchy at its surface. To pull out the electron from the metal poyoverkhnost, he needs to report the dopolnitelyyony energy providing overcoming its uderzhivayuyoshchy forces. Energy which needs to be spent for overcoming by the electron of forces holding it on the poverkhyonost of metal is called electron work function.
Two means of communication to the electron of neobkhodiyomy energy for its exit from the surface of metal are possible. Thermal electron emission when the cathode podogrevayotsya by the electric current passing on it from the poyostoronny power source, or the cold cathode is bombarded by positively ionized atoms, образовавшимися in gas that also leads to its heating.
With the cathode temperature uveliyocheniye the chaotic movement of svoyobodny electrons in the body of the cathode amplifies, and those from them which received sufficient energy for the preyoodoleniye of reacting forces, take off from the metal poverkhyonost. Field emission when near the cold cathode strong electric field due to which electrons escape from the surface of metal is created.
Depending on the kinetic energy store, kotoyory the electron which appeared in interelectrode space as a result of its acceleration the electric field operating between tube electrodes in the moyoment of collision with the neutral atom has bumping or inelastic collision can take place. In the peryovy case the electron possesses the insufficient stock of energy therefore as a result of collision the neutral atom increases the kinetic energy, however the structure of atom does not change.
At the sufficient kinetic energy store of the elekyotron there is inelastic collision of fundamental particles. At the time of collision of the electron with neytralyyony atom it transfers it the part of the energy, and under the influence of this energy one of outer electrons of the neutral atom can pass to the orbit, harakteyorizuyemy with the raised energy level, or the electron can come off absolutely atom, and atom will turn into the positively ionized atom.
Process of transition of the outer electron of the neutral atom into the orbit with the raised energy level is called atom excitation. Each atom has several such energy levels, nazyvayeyomy resonant. Depending on energy, soobshchenyony to atom at collision, the outer electron can peyoreyt on this or that resonant level. Atom does not moyozht long to remain in wild spirits, and через very short period calculated by million fractions of a second the electron from resonant level returns to neutral situation. Upon obyoratny transition of the electron from resonant level to neutral situation there is energy emission in the viyoda of the certain portion of light, or as speak, the light quantum — the photon.
The radiation received as a result of the described process is called the resonant izluyocheniye. The certain pattern of such transition is inherent in each gas or steam of metals. Poluchenyony radiation depending on the sort of gas or steam of meyotall and its pressure has the certain length of the volyona which in turn causes color of this radiation.
With increase in current in the gas-discharge interval of the vozyomozhna of collision of already excited atoms with svobodyony electrons and ions. At the same time still additional energy is reported to such vozbuzhyodenny atoms that leads to transitions of electrons from one rezonansyony levels to others — raised. This process is nayozyvat step excitation. The return transition of electrons with raised power urovyony in neutral situation happens not instantly, and by consecutive transition from levels to povyyoshenny energy on the next level with smaller energy, and then in neutral situation.
The received photon energy at step junction of electrons decreases, and the wavelength of radiation increases. With low pressures of gas and small current densities processes of excitation of atoms mainly play the role in creation of radiation. Light of discharge will be состоять from separate resonance radiations with different lengths of waves therefore the range of radiation of discharge has line character. In process of pressure increment of gas and increase in current density the greatest role is got by stupenyochaty processes of excitation of atoms. The range of radiation of such discharge represents continuous wide strips. The general radiation intensity increases and because upon consecutive transition of electrons from one resonant level to another energy of fotoyon decreases, the wavelength of emitted light respectively increases.
Selecting the sort of gas or steam of metal, their pressure and current density, it is possible to receive discharge radiation with necessary light characteristics. If to put the alternating voltage which periodically changes in size and the direction then to the tube the discharge through gases has some features on which it is necessary to stop.
When in the positive half-cycle tension of piyotayushchy network, increasing, the ignition napryazheyoniya reaches size, in the tube the discharge will be lit. After the discharge zazhiyoganiye tension on the tube will decrease to the veliyochina of tension of burning and during the whole time of the goyoreniye of discharge remains approximately at one level.
Further tension in the power line will decrease настолько that will become less, than it is required for the discharge podderyozhaniye in the tube, and the discharge will stop. The discharge will be lit in the following negative half-cycle again, Yo after achievement by mains voltage of size of the nayopryazheniye of ignition and will stop at its decrease lower than tension of burning. However we almost do not notice process of reignition in the tube as this process proceeds very quickly! Therefore, during one complete alternation of the izyomeneniye of tension attached to the tube in it process of ignition and the termination of discharge repeats twice, and through the tube at the same time there passes alternating current.
Let's consider now features which need to be considered at inclusion of the digit tube in pitayuyoshchy network. If to change the size of the current passing through the digit tube and to change at the same time napryazheyony on its electrodes, then it is possible to establish dependence between this tension and current. This dependence noyosit the name Volt-ampere discharge through gases harakteristiyok.
The bigger current passes through the digit tube, for example, at the electric arc, the more intensively proteyokat process of ionization of neutral gas in mezhelekyotrodny space and electrode voltage of the tube decreases. The current voltage characteristic of the electric arc has the falling character. At such characteristic of the electric arc it is impossible to achieve without application of artificial measures stability or stabilization of discharge.
Really, if, for example, tension on the razryadyony tube for any reasons decreases by neyokotory size, then the circuit current increases. Increase in current, in turn, will cause undervoltage on the digit tube and further increase in the circuit current. If to limit to nothing current size, then it will increase until collapses the chain, any of elements. From this situation it is necessary to draw two conclusions. First, the discharge through gases has no opreyodelenny electrical resistance: it изменяется together with change of the circuit current.
Secondly, for restriction of size of current consistently with the digit tube it is necessary to include the current-limiting soyoprotivleniye which will cause the size of the current which is established in the chain. This tokoogranichivayuyoshchy resistance is called ballast soprotivleyoniy or ballast. Its inclusion in the chain of the razryadyony tube stabilizes discharge. Therefore almost all gas-discharge light sources for the inclusion in the network circuit demand consecutive inclusion of the tokoogranicha with them the howling resistance.
The choice like ballast is defined by the sort of current, proyokhodyashchy by the digit tube, and some other причин. During the work of the digit tube on the direct current as ballast usually apply the ohmic soproyotivleniye (rheostat). On alternating current it is possible to ispolyyozovat the ohmic resistance, inductance or the emyokost. Often ballast resistance during the work of the trubyoka on alternating current represents the combination of these three or two any elements.Top