The name comes from the fact that the tubes run cool to the touch because they are not heated by a filament. A few of such applications are enumerated here.Cold cathode is another name for neon. This unique characteristic renders it suitable for many industrial and control applications. The Cold Cathode Gas Filled Diode or glow tube has an outstanding characteristic of maintaining constant voltage across it in the normal glow discharge region. Applications of Cold Cathode Gas Filled Diode (or Glow Tube): This region of conduction (D to E) is known as abnormal glow discharge region.
With the increase in supply voltage V, not only the circuit current increases but voltage across the tube, E b also starts increasing. After the glow discharge (point D), the voltage across the tube no longer remains constant. In the region extending from C to D, voltage across the tube remains constant though the plate current increases. This is the beginning of the second conduction known as normal glow discharge region. At point B, the tube fires and the voltage across the tube E b drops from point B to point C and remains constant regardless of plate current I b. It is a non-self maintained discharge because it needs an external source to cause ionization. This region, which extends up to point B, is called the Townsend discharge region. and tube current is, therefore, very small. At low anode-cathode voltage, the discharge is because of minor ionization of gas due to external sources such as cosmic rays etc. The volt-ampere characteristic of a Cold Cathode Gas Filled Diode is shown in Fig. Cold Cathode Gas Filled Diode Characteristics: the discharge gets converted into an arc and hence known as arc discharge. If the current density is further increased. This region is known as abnormal glow region.
If the supply voltage V is further increased gradually, a stage arrives when cathode glow covers the entire surface of the cathode and there is no further increase in cross-sectional area of gas path if supply voltage is increased beyond this stage. Hence, the voltage across the tube, E b, in the glow discharge region remains constant. Thus cross section area of ionized gas path between anode and cathode is increased and internal resistance of the tube, being inversely proportional to the area of x-section, decreases. With the increase in tube current, the degree of ionization increases and the glow covers a greater part of the cathode surface. This is only possible if the internal resistance of the tube decreases with the increase in its current and that is exactly what happens in the tube.
Cold cathode series#
In this region, increase in supply voltage V causes increase in plate current I b which increases the voltage drop in the series resistor R but the voltage across the tube E b remains constant. In the glow discharge region, the glow tube maintains a constant voltage across it although there is an appreciable increase in tube current I b. That is why this phase of conduction is known as glow discharge. Simultaneously glow is seen in the gas and on a portion of the cathode. This voltage drop across the tube, E b remains constant regardless of plate current I b.
The voltage drop across the tube E b drops to a low value because of decrease in the internal resistance of tube due to ionization of gas. In this phase of conduction, no visible light is associated and the current is known as dark current.Īs the applied voltage is increased, the electrons acquire more and more energy and stage, known as ionization or striking or breakdown voltage, arrives when the gas starts getting ionized and the tube current rises dramatically to a large value. Townsend who investigated the V-I characteristics of Cold Cathode Gas Filled Diode (glow tube) for the first time in 1901. This stage of conduction is known as Townsend discharge, after the name of J.S.
Cold cathode free#
These free electrons move towards the anode and constitute a small current in the circuit. However, some ionization of gas is caused by natural sources, such as cosmic rays etc., and a few free electrons are produced. When applied voltage (anode-cathode voltage, E b) is low, the tube conducts an extremely small current (say few μA) because the cathode is cold and as such no source of electrons is present. Townsend discharge, the glow discharge and the arc discharge, as illustrated in Fig. On increasing the applied voltage between anode and cathode, electric conduction through the tube passes through three successive discharge phases viz. The positive terminal of variable dc supply is connected to the anode through a current limiting resistor R and negative to the cathode. Circuit arrangement for studying the operation of a Cold Cathode Gas Filled Diode is shown in Fig.
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