Gas Discharge Tubes

Gas discharge tubes are usually formed with a ceramic body filled with a gas mixture containing neon and argon and 2 or more electrodes as shown in Figure 1 . When the voltage across the electrodes exceeds a specified value an arc occurs within the tube, providing a low current path. Gas Discharge Tubes with three or more electrodes can be constructed with a single volume of gas by having holes in the inner electrodes. The result is that an arc formed by a trigger voltage between any two adjacent electrodes will result in a low resistance path between all of the electrodes as the ionized gas fills the entire gas chamber. This can be an important feature for multi line signal ports in which might be sensitive to large imbalances in the voltages between the lines. Gas discharge tubes have the bidirectional crowbar I-V similar to a bidirectional thyristor.

Figure 1 Simple cross section of two and three lead Gas Discharge Tubes

Gas discharge tubes can carry very large amounts of current and have very low capacitance resulting in very little signal loading. The lowest turn on voltage for a GDT is about 75V and the turn on time is relatively long. GDTs are also relatively large and more expensive than other surge protection devices. They excel as primary protection devices in conjunction with other faster turn on and lower voltage secondary protection elements.

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