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.
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.
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.
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.
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