2-15
electrical characteristics that are advantageous in some circuits. They are capable of carrying much more
current than high-vacuum tubes, and they tend to maintain a constant IR drop across their terminals
within a limited range of currents. The principle of operation of the gas-filled tube involves the process
called ionization.
ELECTRICAL CONDUCTION IN GAS DIODES
An operating gas-filled tube has molecules, ions, and free electrons present within the envelope. In a
gas-filled diode, the electron stream from the hot cathode encounters gas molecules on its way to the
plate. When an electron collides with a gas molecule, the energy transmitted by the collision may cause
the molecule to release an electron. This second electron then may join the original stream of electrons
and is capable of freeing other electrons. This process, which is cumulative, is a form of ionization. The
free electrons, greatly increased in quantity by ionization, continue to the plate of the diode. The molecule
which has lost an electron is called an ion and bears a positive charge. The positive ions drift toward the
negative cathode and during their journey attract additional electrons from the cathode.
The velocity of the electrons traveling toward the plate varies directly with the plate voltage. If the
plate voltage is very low, the gas-filled diode acts almost like an ordinary diode except that the electron
stream is slowed to a certain extent by the gas molecules. These slower-moving electrons do not have
enough energy to cause ionization when they hit the gas atoms. After the plate voltage is raised to the
proper level of conduction, the electrons have enough energy to cause ionization when they hit the gas
molecules. The plate potential at which ionization occurs is known as the IONIZATION POINT, or
FIRING POTENTIAL, of a gas tube. If the plate voltage is reduced after ionization, it can be allowed to
go several volts below the firing potential before ionization (and hence, high-plate current) win cease. The
value of the plate voltage (Ep) at which ionization stops is called the DEIONIZATION POTENTIAL, or
EXTINCTION POTENTIAL. The firing point is always at a higher plate potential than the deionization
point.
GAS TRIODE
The point at which the gas ionizes can be controlled more accurately by inserting a grid into the gas
diode. A negative voltage on the grid can prevent electrons from going to the plate, even when the plate
voltage is above the normal firing point. If the negative-grid voltage is reduced to a point where a few
electrons are allowed through the grid, ionization takes place. The grid immediately loses control, because
the positive ions gather about the grid wires and neutralize the grid’s negative charge. The gas triode then
acts as a diode. If the grid is made much more negative in an effort to control the plate current, the only
effect is that more ions collect about the grid wires—tube continues to conduct as a diode. Only by
removing the plate potential or reducing it to the point where the electrons do not have enough energy to
produce ionization will tube conduction and the production of positive ions stop. Only after the
production of positive ions is stopped will the grid be able to regain control.
Such gas-filled triodes are known as THYRATRONS. Thyratrons are used in circuits where current
flow in the thyratrons output circuit is possible only when a certain amount of voltage is present on the
thyratrons grid. The flow of plate current persists even after the initiating grid voltage is no longer present
at the grid, and it can be stopped only by removing or lowering the plate potential. The symbols for the
gas-filled diode, the voltage regulator, and the thyratron are the same as those for high-vacuum tubes
except that a dot is placed within the envelope circle to signify the presence of gas. Some examples of
gas-filled tube schematic symbols are shown in figure 2-15.
