Gas Discharge Tubes - GDTs

The Gas Discharge Tube (GDT) is among the oldest of high-power switches. It was used in early radars for microwave power switching and is still used in electric power systems and communication systems for protection against high-voltage surges. It works a bit like the safety valve in the pressure cooker. When the steam pressure inside the cooker exceeds a certain limit, the safety valve opens and discharges steam till the pressure inside drops. The GDT works similarly, only it discharges electricity. Gases are insulators. Sufficiently high voltages applied across two nearby points will break gas molecules into positive and negative ions. The ionized gas readily conducts electricity, which is how a lightning discharge occurs. Once the high-voltage disappears, the ions recombine to become molecules. The gas again becomes an insulator. This principle is used in protecting valuable equipment against transient high voltages on power and communication lines. Actual GDTs consist of two electrodes in a sealed tube filled with an inert gas. The distance between the electrodes, the nature of the gas, and the pressure inside are all controlled during manufacture to permit a discharge above a desired voltage. This tube is connected electrically across the line or the equipment to be protected. As soon as the voltage rises above the designed safety level, the tube conducts by passing the high voltage to ground. The equipment remains protected.