Specifications

28
RF SOLUTION-CITEL
Gas Discharge Tubes
These components are made of two or three electrodes in an
enclosure filled with a (non-radioactive) rare gas at a controlled
pressure.
The enclosure is a ceramic tube with its ends closed off by
metal caps that also serve as electrodes.
Their main use is to protect telecommunications lines, but other
uses are possible.
Operation
The gas discharge tube may be regarded as a sort of very
fast switch having conductance properties that change very
rapidly, when breakdown occurs, from open-circuit to quasi-
short circuit (arc voltage about 20V).
There are accordingly four operating domains in the behavior
of a gas discharge tube:
Non-operating domain, characterized by practically infinite
insulation resistance;
Glow domain : At breakdown, the conductance increases
suddenly; if the current drained off by the gas tube is less
than about 0.5A (this is a rough value that differs according
to the type of component), the glow voltage across the
terminals will be in the 80-100V range;
Arc regime :
as the current increases, the gas discharge tube
shifts from the glow voltage to the arc voltage (20V). It is in this
domain that the gas discharge tube is most effective, because
the current discharged can reach several thousand amperes
without the arc voltage across its terminals increasing.
Extinction : At a bias voltage roughly equal to the glow
voltage, the gas tube recovers its initial insulating properties.
Electrical characteristics
The main electrical characteristics defining a gas discharge tube are:
󳀏DC sparkover voltage (Volts)
󳀏Impulse sparkover voltage (Volts)
󳀏Discharge current capacity (kA)
󳀏Insulation resistance (Gohms)
󳀏Capacitance (pF).
DC sparkover voltage
This is the main characteristic defining the gas discharge tube.
It is the voltage at which breakdown will occur between the
electrodes when a slowly increasing voltage (dV/dt = 100
V/s) is applied to the component; it depends on the electrode
spacing, the pressure, and the properties of the gas mixture
and of the emissive substance.
Range of DC sparkover voltages available:
󳀏minimum 75V
󳀏average 230V
󳀏high voltage 500V
󳀏very high voltage 1000 to 3000V
The tolerance on the breakdown voltage is generally ± 20%.