the voltage-current characteristic of a gas discharge tube
For the construction of a
sunlamp, a mercury vapour arc
discharge was the most
effective source of ultraviolet
radiation. In a basic
configuration, the sunlamp
consisted of two electrodes
that were located at the outer
ends of a sealed glass tube,
filled with mercury vapour.
When a DC-voltage was
applied over the electrodes,
electrons would be forced to
travel from one electrode to
the other, causing collisions
between the gas atoms and
the moving electrons. Due to
these collisions, photons
would be released with some
dedicated frequencies in the
ultraviolet part of the
In the diagram above, a simplified representation of the voltage-current characteristic
of a gas discharge tube is given. As can be read from the diagram the relation
between voltage and current is not linear and the impedance may even become
negative. For a better understanding the presentation of the various discharge
transitions is drawn disproportionally. Also the transitions at the breakdown points D
and G of the characteristic are idealised and drawn sharper than will be the case in
reality. The diagonal straight lines represent the voltage-current characteristic of the
serial resistor at different values of the supply voltage U. At I=0 the voltage over the
resistor UR will be zero. At UL=0 UR will be equal to the supply voltage and,
according to Ohm's law, the maximum current U/R will flow through the resistor. In any
stable situation, for instance in F, UR is equal to U minus UL and that is on the
crossing point of both voltage-current characteristics.
The physical processes in a gas discharge between two electrodes are very much
depending on the actual current the discharge is stabilised on. Basically there are
three discharge states with a relatively sharp transition from one state to another.
The discharge type between B and C is called a 'dark discharge', that between E and
F is called a 'glow discharge' and a discharge beyond G is called an 'arc discharge'.