1. B ASIC ELECTRONICS G AS - FILLED T UBES

2. INTRODUCTION When some inert gas (e.g. argon, neon, helium etc.) at low pressure is purposely introduced in a vacuum tube, the resulting device is known as a gas- filled tube. A gas- filled tube differs form a vacuum tube in one very important respect.

3. Whereas the control grid in a vacuum tube exercises a fine control over the flow of electrons from cathode to anode and thus permit it to be used as an amplifier, a gas-filled tube does not exhibit such as a control.

4. It is because the flow of electrons from cathode to anode in a gas-filled tube causes ionisation of the gas in the tube. The ionised molecules interfere with the control and make the device useless as an amplifier.

5. INTRODUCTION CONT. In a gas takes place. Once the ionisation of the gas filled tubes can be profitably employed for efficient handling and turning on and off of heavy currents.

6. C LASSIFICATION OF G AS - FILLED T UBES  The construction of gas-filled tubes is similar to the vacuum tubes, except that cathodes, grids and anodes are usually larger in order to carry heavy currents. Gas-filled tubes are generally classified according to the type of emission used viz. (I) Cold-cathode type (II) Hot-cathode type In the cold-cathode type, the cathode is not heated as in a vacuum tube. The ionization of the gas is caused by the energy available from natural sources such bas cosmic rays, radio active particles in air etc. Most of Cold-cathode tubes are used as diodes. In the Hot-cathode type, the cathode is heated just as in an ordinary vacuum tube. The electrons flowing from cathode to plate cause ionization of the gads molecules. Such types are used as diodes, triodes and tetrodes.

7. C ONDUCTION PHENOMENON IN A GAS The operation of gas-filled tube depends upon the conduction process in a gas. Atb low anode-cathode voltage, the ionization of the gas does not occur and the place current is about the same as for a vacuum tube at the same anode voltage. However, at some anode-cathode voltage, called ionization voltage, the ionization of the gas takes place.

8. . The plate current increases dramatically to a large value due to the neutralization of space charge by the positive gas ions. The ionization voltage depends upon the type and pressure of the gas used.

9. D IFFERENCE BETWEEN G AS - FILLED AND VACUUM TUBES. The important differences between gas-filled tubes and vacuum tubes are listed below: (i) The cathode, grid and anode in a gas-filled tube are larger than those of a vacuum tube. (ii) A gas-filled tube can conduct more current than the equivalent vacuum tube. This mainly due to the fact that space charge in the gas-filled tube is neutralized by the positive gas ions.

10. . (iii) A gas-filled tube has far less control on the electrons in the tube than that of vacuum tube. Once the ionization starts, the control of gas- filled tube is tremendously reduced. (iv) The PIV rating of a gas-filled diode is far less than that of vacuum diode.

11. THYATRON A hot- cathode gas triode is known by the trade name thyatron. It consists of three electrodes, namely: cathode, anode, and control grid enclosed in a glass envelope containing some inert gas at low pressure.

12. By adjusting the negative on the control grid, the ionisation the gas in the tube can be at any anode-cathode potential. Once the ionisation starts, the control grid loses all control. It is because the positive ions created due to ionization neutralize the negative charge on the grid, thereby preventing the grid from exerting any further control on the plate current of the tube.

13. G Therefore, the function of control grid is only to start the conduction of anode current. Once the conduction starts, the tube acts as a gas diode. In order to stop conduction in a thyatron, the plate voltage should be reduced to zero for a period large enough for de-ionisation of the gas in the tube.

14. Important Terms. It is helpful to discuss some important terms much used in the analysis of gas-filled tubes. (i) Ionisation potential. The plate voltage at which ionisation starts in a gas-filled known as ionisation potential, striking voltage or firing point. (ii) De-ionisation time. The time taken by the ions to recombine with free electrons in a gas-filled tube is known as de-ionisation time. If de-ionisation time is very small, arc-back may occur.

15. . (iii) Extinction Potential. It is the minimum plate potential below which ionization of gas in gas filled tube cannot be maintained. This is also called de-ionizing potential.

16. S HORT NUMERICAL / TROUBLE SHOOTING PROBLEMS 1. Fig. 5-1 shows the circuit for determining the characteristics of a thermionic gas diode. What does dot in the symbol of diode indicate? 2. What is the function of resistor R in the circuit? 3. Suppose the current increases suddenly to a high value when voltmeter reads 15V in Fig. 5.1. What is the ionizing potential.

17. . I b (mA) mA Eb Ebb 400 200 A Eb (VOLTS ) 0 5 10 15 l b R

18. . 4. Suppose the tube in Fig. 5.1 fires when voltmeter reads 15 V. (i) Will the value of voltmeter change if the potentiometer arm is moved higher towards maximum supply voltage? (ii) Will there be any change in voltmeter reading if resistor R is now removed?

19. 5. The supply voltage in Fig. 5.1 is 300 V. what should be the value of R if maximum current rating of tube is 10A? 6. Fig. 5.2 shows the characteristics of a thermionic gas diode. What is the value of ionizing potential.

20. . 7. What does grid control do in a thyration ? 8. The ionization potential of gas-filled tube is 15 V. What does it mean?