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.
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.
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.
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