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BASIC ELECTRONICS
GAS-FILLED TUBES
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.
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.
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.
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.
CLASSIFICATION OF GAS-FILLED TUBES

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.
CONDUCTION 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.
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.
DIFFERENCE BETWEEN GAS-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.
(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 gasfilled tube is tremendously reduced.
(iv) The PIV rating of a gas-filled diode is far
less than that of vacuum diode.
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.
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.

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.

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.
(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.
SHORT NUMERICAL/TROUBLE SHOOTING PROBLEMS
1.
2.
3.
Fig. 5-1 shows the circuit for determining the
characteristics of a thermionic gas diode. What does
dot in the symbol of diode indicate?
What is the function of resistor R in the circuit?
Suppose the current increases suddenly to a high
value when voltmeter reads 15V in
Fig. 5.1.
What is the ionizing potential.
Ib (mA)
lb
R
mA
400
Eb
Ebb
200
A
Eb
(VOLTS)
0 5 10 15
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?
5.
6.
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?
Fig. 5.2 shows the characteristics
of a thermionic gas diode. What is
the value of ionizing potential.
.
6.
7.
8.
Fig. 5.2 shows the characteristics of a
thermionic gas diode. What is the value of
ionizing potential.
What does grid control do in a thyration ?
The ionization potential of gas-filled tube is 15
V. What does it mean?