Gas discharge tubes (GDT) or gas tube arrestors (GTA) provide protection against voltage and current surges in electronic and electrical equipment. They include spark gaps; simple devices with two or three conducting electrodes separated by a gap filled with a gas such as air. The gap allows the passage of a spark that is created when the voltage difference between conductors exceeds the gap’s breakdown voltage. This voltage, known as the spark-over voltage, creates a low-impedance path and diverts the surge current.
Types of Gas Discharge Tube Performance Characteristics
The GlobalSpec SpecSearch Database contains the ability to specify important performance characteristics when selecting a GDT.
- Capacitance range is the range of ability a gas discharge tube has to hold an electrical charge. An effective GDT will have a very low capacitance.
- Sparkover voltage is the main characteristic defining a GDT. It is the voltage at which breakdown will occur between the electrodes when a slowly increasing voltage 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.
- Holdover voltage is the voltage for which the gas tube was designed to operate optimally and safely.
- Impulse spark over is the highest impulse voltage in the wave form achievable before the GDT breaks down and conducts.
- Discharge current is what distinguishes a GDT from other protection devices. It is the maximum current the device can withstand repeatedly (usually 10 pulses) without destruction or alteration of its basic specifications.
Optimum performance in many environments requires a GDT to handle very high currents while limiting over-voltage. GDTs with very low capacitance and very high insulation resistance have little, if any, flow effects on the protected system under normal operating conditions.
Voltage and current surges can occur naturally or artificially. Natural surges may be the result of lightning or electrostatic discharge (ESD). Artificial surges often result from induction loads and electric switches, and when devices are switched on and off. When evaluating products for performance, buyers should consider how often these surges occur and the overall demands of the application. For example, spark gaps are often used as lighting arrestors because of their ability to handle high voltage transients.
Package Type and RF Connectors
Board-level gas discharge tubes are available in various integrated circuit (IC) package types. For example, connectorized devices are designed for attachment with coaxial connectors. Through-hole GTAs have either radial leads that extend from the sides or axial leads that extend from the ends. Surge protectors that are designed for use with surface mount technology (SMT) are also available. These SMT arrestors have a stable breakdown throughout their lives and are used for protecting broadband circuits, telecommunications systems, and industrial electronics.
Application-Specific and Fail Safe Devices
High-voltage gas tubes are used to protect transformers and switch-gears. They’re also used in communications networks and with industrial and commercial electronics. Some gas discharge tubes are equipped with fail-safe devices for situations when the GDT produces excess thermal energy. To prevent heat damage to the magazine or the device’s terminal block, the arrester is fitted with a back-up short-circuit fail-safe device.