INTRODUCTION OF GAS-FILLED TUBE
Gas Tube : In the vacuum tubes, the electrons pass from the cathode to anode in a vacuum. In that type of tubes, extreme care is taken to generate as perfect a vacuum as possible to prevent ionization of gases and the resulting large uncontrolled currents. It may be mentioned here that the logic of the triode is the fine control of free electrons within the valve through the electrostatic fields of the grid and anode. If gas is present even in a little amount, the electrons pass from cathode to anode will be the reason for ionization of the gas. The ionized molecules would interact with the fine control and it makes the device useless amplifier. In some applications, fine control of electrons within the valve is of less importance than the efficient handling and turning HIGH LOW OR on-off of huge currents. In such a condition, some inert gases (e.g argon, neon, helium) at small pressures are purposely introduced into the valve envelope. Such tubes are called as gas-filled tubes. The gas-filled tubes are able of doing many jobs that vacuum tubes never perform and which are especially useful in industrial systems and control circuits. In this chapter, we shall focus our attention on the most important types of gas-filled tubes having special reference to their characteristic properties.
A gas-filled tube is essentially a vacuum tube with a small amount of inert gas at low pressure. The gas pressure in a gas-filled tube normally ranges from 10 mm of Hg to 50 mm. The development of gas-filled tubes is similar to that of vacuum tubes, but one thing is different which is the cathodes, grids, and anodes are normally larger in order to carry huge current. However, the characteristic properties of both are markedly different. Firstly, a gas-filled tube can conduct or pass much * more current than the equivalent vacuum tube. It is due to the electrons passing from the cathode to anode collide having gas molecules and ionize them i.e. knock out electrons from them. The additional electrons pass to the anode together with the original electrons, the resulting plate current will increase. Secondly, a gas-filled tube has very small control over the electrons in the tube as compared to that of a vacuum tube. Once the ionization initiates, the control of the gas-filled tube is continuously reduced.
Gas-filled tubes are normally classified according to the type of electron emission employed. On that basis, they may be classified into two main types namely; cold-cathode type and hot-cathode type.
In this type of gas-filled tubes, the cathode is will never be heated as in a vacuum tube. The ionization of the gas is due to the energy available from natural sources like, cosmic rays, sun rays, or radioactive particles in the air. These natural sources are the underlying reasons for the initiate of flow current in cold-cathode gas tubes. majority of cold-cathode tubes are used as diodes.
Fig. 3.1 shows the schematic symbol of a cold-cathode gas diode, called a glow tube. The dot within the circle shows the presence of gas. Fig. 3.2 shows the schematic symbol of cold-cathode gas triode called a grid glow tube. Hot-cathode type. In this type of gas-filled tubes, the cathode is heated just as in a normal vacuum tube do. The electrons passing from cathode to plate cause ionization of the gas molecules. that types of tubes are used as diodes, triodes, and tetrodes.
Fig. 3.3 shows the schematic symbol of a hot-cathode gas diode, called as phanotron whereas
Fig. 3.4 shows the symbol of hot-cathode gas triode, called as thyratron.
Conduction in a Gas
A gas under normal pressure is a perfect insulator and never conducts current. However, if the gas pressure is small, it is possible to generate a huge number of free electrons in the gas by the process of ionization and thus the gas to become a conductor. This is exactly what happens in gas-filled tubes. The current passes in gas at very low pressure can be Awesomely illustrated by referring to the hot-cathode gas diode shown in Fig. 3.5. The space between cathode and anode of the tube has gas molecules. When the cathode is heated, it emits a huge number of electrons. These electrons make a cloud of electrons near the cathode, known as space charges. If the anode is made positive with respect to the cathode, the electrons (magenta dots) from the space charge speed towards the anode and collide with gas molecules (cyan circles) in the tube. ions slowly drift or moves towards the cathode and neutralize the space charge. Consequently, the tube resistance decreases, resulting in a huge plate current. Hence, it is because of the neutralization of space charge by the positive gas ions that plate current in a gas tube is too much increased.
The following points may be noted regarding the passing current in a gas at low pressure :
(i) At small anode-cathode voltage, the ionization of the gas did not happen and the plate current is about the same as for a vacuum tube at the same anode voltage.
(ii) At some anode-cathode voltage, known as ionization voltage, ionization of the gas takes place. The plate current increases dramatically to a huge value due to the neutralization of space charge by the positive gas ions. The ionization voltage depends upon the type of gas and pressure of the gas in the tube.
(iii) Once ionization has initiated, it is maintained at an anode-cathode voltage much smaller than ionization voltage. However, minimum anode-cathode voltage, known as deionizing voltage, exists below which ionization can’t be maintained. Under such a situation, the positive gas ions combine with electrons to make neutral gas molecules and current passes stop. due to switching action, a gas-filled tube can be used as an electronic switch.
Cold-Cathode Gas Diode
Fig. 3.6 shows the cut-away view of the cold-cathode gas diode. It essentially has two electrodes, anode, and cathode mounted fairly close to each other in an envelope filled with few inert gases at small pressure. The anode is in the shape of a thin wire whereas cathode is of the cylindrical metallic surface having oxide coating. The anode is always a positive potential with respect to the cathode. Operation. Fig. 3.7 shows a circuit that normally used to investigate the operation of the cold-cathode gas diode. Current passes through the tube three successive discharge phases viz. Townsend discharge, the glow discharge, and the arc discharge.
(i) Townsend discharge. At low anode-cathode voltage, the tube current passes an extremely small nearly (1mA). It is due to the cathode is cold and as such no source of electrons is available. However, natural sources (e.g. cosmic rays, etc.) cause some ionization of the gas, creating a small free electron. These free electrons move towards the anode to constitute a little current. This stage of conduction is called *Townsend discharge. In this phase of the situation, no visible light is associated.
(ii) Glow discharge. As the anode-cathode voltage is increased, the electrons get more and more energy. At some voltage, called ionization voltage, ionization of the gas initiate, and the tube current increases to a huge value. The voltage across the tube drops to a small value, which always remains constant regardless of the plate current. At the same time, the glow is seen in the gas and on a portion of the cathode. This phase of flowing current is called glow discharge. The fact that the glow tube maintains a constant voltage across it in the glow discharge portion requires some explanation. In this side, any increase in voltage supply causes huge current to flow; the drop across series resistance R increases but the voltage Eb across the tube always remains constant. As the flow of charges increases, the degree of ionization increases, and the glow covers a larger part of the cathode surface, and hence the ionized gas path between cathode and anode has a large area of cross-section. As resistance is inversely proportional to the area of cross-section, therefore, the tube resistance decreases. Hence, the voltage across the tube always remains constant. The reverse is also true should the voltage supply decrease. Thus in the glow discharge region, the tube resistance changes so as to maintain a constant voltage across it.
(iii) Arc discharge. Once the cathode glow covers the entire surface of the cathode, the xsectional area of gas path cannot increase further. This region is known as abnormal glow. If the current density is further increased, the discharge becomes an arc.
Characteristics of Cold-Cathode Diode
The volt-ampere properties of a cold-cathode diode are shown in Fig. 3.8. At low anode-cathode
voltage, the tube current is very little (1mA) and because of the ionization of gas molecules by the
natural sources. This stage of conduction up to voltage B is called as Townsend discharge and is not self-maintained discharge cause it needs an external source to cause ionization. A few critical voltages such as B, the tube fires, and the voltage across the tube drops (from B to C) and always remains constant according to the plate current. This is the start of the second conduction and is called as glow discharge. In this region (C to D), the voltage across the tube remains constant even if the plate current increases.
After the glow discharge, the voltage across the tube no longer remains constant. Now, if the supply voltage is increased, not only will the circuit current increase but the voltage across the tube will start to increase again. This stage of conduction (D to E) is known as the abnormal glow.
Applications of Glow Tubes
The outstanding characteristic and properties of a cold-cathode gas diode (or glow tube) to maintain a constant voltage across it in the glow discharge region renders it suitable for many control and industrial applications. A few of such applications are discussed below :
(i) As a voltage regulating tube. A glow tube keeps a constant voltage across it in the glow discharge region. This property permits it to be used as a voltage regulating tube. Fig. 3.9 shows a very basic circuit commonly used to maintain a constant voltage across a load. The glow tube (VR tube) is connected in parallel with the RL load across which constant voltage is required. So long as the tube works in the glow discharge region, it will keep a constant voltage (= 150V) across the load. The extra voltage is dropped across the series resistance R.
(ii) As a polarity indicator. As the cathode is surrounded by a property glow, therefore, it can be used to indicate the polarity of a d.c source.
(iii) As an electronic switch, which closes at ionization potential, permitting a huge current to pass, and opens at the deionizing voltage, blocking the current flow.
(iv) As a radiofrequency field detector. A very strong radio-frequency field is capable of ionizing the gas without a direct connection to the tube. Therefore, the tube can show the presence of a radiofrequency field.
Hot-Cathode Gas Diode
A hot-cathode gas diode is mostly used as a rectifier for moderate voltages because of large efficiency and good regulation. A hot-cathode gas diode having an oxide-coated cathode and a metallic anode enclosed in a glass envelope containing few inert gases under low pressure. For proper operation of the tube, the anode is always held at a positive potential with respect to the cathode.
Fig. 3.10 shows a circuit that can be used to investigate the working of a hot cathode gas diode. When cathode is heated, a huge number of electrons are emitted or transmitted. At small anode-cathode voltage, the tube passes a very small amount of current. Under such situations, the gas is not ionized and the tube behaves similarly to a vacuum diode — the voltage across the tube increases with plate current. This happens continuously until anode-cathode voltage becomes equal to the ionization potential of the gas. Once this potential is reached the point, the gas starts to ionize, generating free electrons and positive gas ions. The positive gas ions flow towards the cathode and tend to neutralize the space charge, thus reducing the internal resistance of the tube. If now the plate voltage is increased, the plate current also increases because of the increased degree of ionization. This further decreases the tube resistance. As a result, increase in plate current is offset by the reduction in tube resistance and the voltage across the tube remains constant. Therefore, in a hot-cathode gas diode, not only the internal drop within the tube is small but also it keeps constant. For this reason, a gas diode has better regulation and efficiency than for a vacuum diode when used as a rectifier.
Fig. 3.10 shows the circuit that can be used to find the volt-ampere (Eb/Ib) characteristics of a hot-cathode gas diode. The R series resistance is used to limit the current flow to reach a dangerously large value. Fig. 3.11 shows the plate characteristic of the hot-cathode diode. It is clear that at first, plate current increases slowly with an increase in anode-cathode voltage. However, at some voltage, called ionization voltage (point A), the plate current increases sharply, and the voltage drop across the tube keeps constant. The extra voltage is dropped across the series resistance R. Any attempt to increase the anode-cathode voltage above the ionizing value is fruitless. Increasing the voltage Eb above point A results in huge plate current (Ib) and high drop across R but the voltage Eb across the tube remains constant.
A hot-cathode gas triode is called by the trade name thyratron. As discussed before, gas diode fires at a fixed plate potential, depending upon which type of gas used and what’s gas pressure. Very often it is a must to control the plate potential at which the tube is to fire. Such control is obviously not possible with a gas diode. However, if a third electrode, called a control grid is introduced in a gas diode, this control is not impossible. The tube is then called as hot-cathode gas triode or thyratron. By controlling the negative potential on the control grid portion, the tube may be fired at any plate potential.
Construction of thyratron
Figs. 3.12 (i) and 3.12 (ii) respectively represent the cut-away view and schematic symbol of a thyratron. It has three electrodes, namely; cathode, anode and control grid enclosed in a glass envelope having few inert gases at very low pressure. The cathode and anode are nearly planar. The control grid of thyratron has a special structure some different from that of a vacuum tube. It consists of a metal cylinder surrounding the cathode with one or more perforated discs called grid baffles center the near.
Operation of thyratron
When the cathode is heated, it transmits plenty of electrons by thermionic emission. If the control grid is made sufficiently negative, the electrons do not have the required energy to ionize the gas and the plate current is substantial 0. As the negative grid voltage is decreased, the electrons acquire high speed and high energy. At some grid voltage, called critical grid voltage, ionization of the gas occurs and the plate current increases to a large value. The negative grid voltage, for a given plate potential, at which ionization of the gas starts is called a critical grid voltage. At critical grid voltage, the gas ionizes, generating free electrons, and positive gas ions. The positive ions tend to neutralize the space charge, resulting in a huge plate current. In addition, these positive ions are attracted by the negative grid and neutralize the normal negative field of the grid, thereby stoping the grid from exerting any more control on the plate current of the tube. The grid now doesn’t have control and the tube act as a diode. Therefore, the function of the control grid is only to initiate the passing of anode current. Once the conduction is initiated, the tube behaves as a gas diode. It is most important to realize the usefulness of the control grid. We have seen that the ionization does not begin at low values of plate current. In a gas diode, it needs the plate potential should be increased until sufficient plate current is passing to cause ionization. However, by adjusting the negative voltage on the grid, the required plate current can be obtained to cause ionization. It may be mentioned here that once the thyratron fires, the only way to stop flowing current is to reduce plate voltage to 0 for a period *long enough for deionization of the gas in the tube.
Applications of Thyratron
As the grid voltage has zero control over the magnitude of plate current after the thyratron fires, therefore, it never is used as an amplifier like a vacuum triode. However, due to its triggering action, it is useful in switching and relays applications. Thyratrons are also worked as controlled rectifiers for controlling the amount of direct current power fed to the load. They are also used in the motor control devices.
Gas Filled Tubes | Conduction in a Gas ( Gas Filled Tubes | Cold Cathode Gas Diode | Thyratron | Hot Cathode Gas Diode )
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MCQ’s| Electrons|Atomic | Voltage |Thevenin’s ( Gas Filled Tubes | Cold Cathode Gas Diode | Thyratron | Hot Cathode Gas Diode )
Maximum Power Transfer Theorem |Applications( Gas Filled Tubes | Cold Cathode Gas Diode | Thyratron | Hot Cathode Gas Diode )
Thevenin’s Theorem | Properties | Problems ( Gas Filled Tubes | Cold Cathode Gas Diode | Thyratron | Hot Cathode Gas Diode )
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Reference: Principles Of Electronics By V K Mehta And Rohit Mehta