Overview of Ceramic Gas Discharge Tubes

Gas discharge tube, full name ceramic gas discharge tube (GDT), is a type of gap surge protection device. Widely used in lightning protection of communication systems. Gas discharge tubes are usually used as the first or second stage in multi-level protection circuits. Their function is to release the instantaneous overcurrent caused by lightning strikes and limit overvoltage. Due to their high insulation resistance and extremely low parasitic capacitance between electrodes, gas discharge tubes have significant advantages in lightning protection of high-frequency signal lines.

The main disadvantages of the protective characteristics of gas discharge tubes are the large discharge delay and unsatisfactory response sensitivity, resulting in poor effectiveness in suppressing lightning surges with steep wavefronts. In addition, lightning protection in power systems also has the problem of freewheeling.

The classification of ceramic gas discharge tubes

According to the number of electrodes, commonly used gas discharge tubes are classified into two-electrode discharge tubes and three-electrode discharge tubes, with the outer casing material often being ceramic, hence they are called ceramic discharge tubes.

Based on the outer dimensions, commonly used gas discharge tubes are categorized into surface mount discharge tubes and through-hole discharge tubes.

According to the product's pulse voltage, commonly used gas discharge tubes are divided into standard pulse discharge tubes and low pulse discharge tubes.

Working Principle of Ceramic Gas Discharge Tube

Ceramic gas discharge tubes use ceramic sealed package, the interior consists of two or more metal electrodes with gaps, filled with inert gases argon, neon. Generally connected in parallel on the circuit, the device does not act, the resistance value is very high, the equivalent capacitance is very low, can be regarded as open circuit, almost no effect on the circuit. When an abnormal pulse occurs, after reaching the value of the action voltage, the internal resistance drops instantly, releasing the current. When the abnormal high voltage disappears, it will automatically return to the high resistance state and the circuit works normally.

Main Characteristic Parameters of Ceramic Gas Discharge Tubes

  • DC breakdown voltage Vsdc:
    The breakdown voltage value when a DC voltage of 100V/s is applied to the discharge tube. This is the nominal voltage of the discharge tube, commonly used 70V, 90V, 150V, 230V, 300V, 350V, 470V, 600V, 800V and 1000V, as well as high-pressure up to 3600V and so on. The error range: generally ± 20%, some ± 15%, and individual ± 10% or ± 5%.
  • Pulse (impulse) breakdown voltage Vsi:
    The breakdown voltage value when a pulse voltage of 1kV/μs is applied to the discharge tube. Because of the slower reaction speed, the pulse breakdown voltage is much higher than the DC breakdown voltage.
  • Inrush discharge current Idi:
    There are two types of inrush discharge current: 8/20μs wave (short wave) and 10/1000μs wave (long wave). Generally, 8/20μs wave is used more. The inrush discharge current is divided into single inrush discharge current (8/20μs wave impact 1 time) and nominal inrush discharge current (8/20μs wave impact 10 times), generally the latter is about half of the former, 2.5kA, 5kA, 10kA, 20kA …… There are 2.5kA, 5kA, 10kA, 20kA and other specifications.
  • Resistant to AC (I.F.) current discharges: The gas discharge tubes are capable of withstanding AC (I.F.) current discharges of 1 second per discharge and 10 discharges of the rated current.

Selection principle of ceramic gas discharge tube

  • 1. ceramic gas discharge tube to join the prerequisite is the lower limit of the DC breakdown voltage of the ceramic gas discharge tube must be higher than the maximum normal operating voltage in the circuit, in order not to affect the normal operation of the circuit.
  • 2. ceramic gas discharge tube over-holding voltage as high as possible to ensure that the circuit working voltage does not cause continuous conduction phenomenon. When the overvoltage in the circuit disappears, to ensure that the ceramic gas discharge tube in time to extinguish, otherwise it will affect the normal operation of the circuit.
  • 3. to ensure that the impact breakdown voltage value of the ceramic gas discharge tube must be lower than the highest instantaneous voltage value that can be withstood in the circuit.
  • 4. according to the line may run into the intensity of the impact current, to determine the selection of the discharge tube must achieve the impact current resistance.
  • 5. if necessary, ceramic gas discharge tube with appropriate short-circuit device, FS device, also known as failure protection device.

Advantages of Ceramic Gas Discharge Tubes

  • Before the breakdown (conduction) is equivalent to an open circuit, the resistance is very large, there is no leakage current or leakage current is very small;
  • After breakdown (conduction), it is equivalent to a short circuit, which can pass a large current with a small voltage drop;
  • Pulse current capacity (peak current) is very large; 2.5kA ~ 100kA;
  • With bidirectional symmetrical characteristics.
  • The capacitance value is very small, less than 3pF.

Disadvantages of Ceramic Gas Discharge Tubes

  • Lies in the discharge time delay (i.e. response time) is large, the action sensitivity is not ideal, some models will be renewed current phenomenon, a long time renewed current will lead to failure, for the head of the rise of the steeper lightning wave is difficult to effectively inhibit.
  • Due to gas ionization takes some time, so the response speed is slow, the response time is generally 0.2 ~ 0.3μs (200 ~ 300ns), the fastest there is 0.1μs (100ns) or so, before it does not conduct, there will be a larger amplitude of the sharp pulse leakage past, and can not play a protective role.
  • The breakdown voltage consistency is poor, the dispersion is large, generally ± 20%.
  • The breakdown voltage is only a few specific values.

Related Information
Semiware offers a full line of ceramic gas discharge tubes. For more product information, please visit: https://semiware.com/products/general-gas-tube/

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