Application of gas discharge tube (GDT)

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Gas discharge tube refers to a type of lightning protection tube or antenna switch tube for overvoltage protection. There are two or more electrodes in the tube, which are filled with a certain amount of inert gas. The gas discharge tube is a gap type lightning protection element, which is used in the lightning protection of the communication system.

Introduction
The gas discharge tube is a gap type lightning protection component. It has been widely used in lightning protection of communication systems. Discharge tubes are often used in the first or first two levels of multi-level protection circuits, which play the role of discharging lightning instantaneous overcurrent and limiting overvoltage. Due to the large inter-electrode insulation resistance of the discharge tube, the parasitic capacitance is small The lightning protection of signal lines has clear advantages. The main disadvantage of the discharge tube protection characteristics is that the discharge delay is large, and the operation sensitivity is not ideal. It is difficult to effectively suppress the lightning wave with a large rise in the wave head. There is a problem of continuous current in the lightning protection of the power system.

Commonly used gas discharge tubes are two-electrode discharge tubes and three-electrode discharge tubes, and the materials of their packaging shells are mostly ceramics, so they are called ceramic discharge tubes.

Principle
The working principle of the discharge tube is the gas gap discharge. When a certain voltage is applied between the two poles of the discharge tube, an uneven electric field is generated between the poles: under the action of this electric field, the gas in the tube begins to dissipate. When the applied voltage increases to make the gap between the poles When the field strength exceeds the insulation strength of the gas, the gap between the two electrodes will break down the discharge, changing from the original insulation state to the conductive state. After conduction, the voltage between the two electrodes of the discharge tube is maintained at the residual voltage level determined by the discharge arc This residual voltage is generally very low, so that the electronic equipment connected in parallel with the discharge tube is protected from overvoltage damage.

The working principle of ceramic gas discharge tube can be simply summarized as gas discharge. When enough electricity is generated between the two stages, it will cause the gap between the electrodes to be broken down by discharge. At this time, it will change from the insulated state to the conductive state. This phenomenon is more similar to a short circuit. When in the conductive state, the voltage between the two poles will be lower, generally between 20 and 50V, so it can play a very good role in protecting the subsequent circuit.

The ceramic gas discharge tube adopts a ceramic sealed package, and the interior is composed of two or several metal electrodes with gaps, filled with an inert gas (argon or neon). The basic appearance is shown in Figure 1. When the voltage applied to the two electrode terminals reaches the breakdown of the gas in the ceramic gas discharge tube, the ceramic gas discharge tube begins to discharge, and changes from high resistance to low resistance, so that the voltage across the electrode does not exceed the breakdown voltage.

Structure
Some gas discharge tubes use glass as the tube shell. Some also use ceramics as the package shell, and the discharge tube is filled with inert gas with stable electrical properties (such as argon and neon, etc.). The discharge electrodes of common discharge tubes are generally two or three, and the electrodes are separated by inert gas . According to the setting of the number of electrodes, the discharge tube can be divided into two-pole and three-pole discharge tubes.

The ceramic diode discharge tube is composed of pure iron electrode, nickel-chromium-cobalt alloy cap, silver-copper welding cap and ceramic tube body and other main components. The discharge electrode in the tube is coated with radioactive oxide, and the inner wall of the tube is also coated with radioactive elements to improve the discharge characteristics. The discharge electrode mainly has two rod-shaped and cup-shaped structures. In the discharge tube of the rod-shaped electrode, a cylindrical heat shield is also installed between the electrode and the wall of the tube body. The heat shield can make the ceramic tube body tend to be heated Uniform, so as not to cause local overheating and cause pipe breakage. Radioactive oxides are also coated in the heat shield to further reduce discharge dispersion. In the discharge tube of the cup-shaped electrode, a molybdenum mesh is installed at the mouth of the cup, and the cesium element is installed in the cup, which also reduces the discharge dispersion.

The triode discharge tube is also composed of pure iron electrodes, nickel-chromium-cobalt alloy caps, silver-copper welding caps, and ceramic tube bodies. Unlike the two-electrode discharge tube, a nickel-chromium-cobalt alloy cylinder is added to the three-electrode discharge tube as the third electrode, that is, the ground electrode.

Technical Parameters
(1) DC discharge voltage
Under the effect of a voltage with a rising steeper than 100V / s, the average voltage value at which the discharge tube starts to discharge is called its DC discharge voltage. Due to the dispersion of the discharge, it is necessary to give the upper and lower allowable deviations around this average value at the same time.

(2) Impulse discharge voltage
Under the effect of a transient voltage pulse with a prescribed rising steepness, the voltage value at which the discharge tube starts to discharge is called its impulse discharge voltage. Since the response time or action delay of the discharge tube is related to the rising steepness of the voltage pulse, the impulse discharge voltage of the discharge tube is different for different rising steepness.

(3) Power frequency withstand current
The discharge tube passes the power frequency current 5 times, so that the maximum current of the tube’s DC discharge voltage and insulation resistance does not change significantly is called its power frequency withstand current. When applied to some AC power supply lines or communication lines susceptible to power line induction, attention should be paid to the power frequency tolerance of the discharge tube. Experience shows that the induction power frequency current is small, generally not more than 5A, but its duration is very long; the overcurrent on the power supply line is very large, which can be up to hundreds of amperes, but due to the action of the relay protection device, its duration It is very short, generally no more than 5s.

(4) Impulse withstand current
The maximum current peak value of the discharge tube through the specified waveform and the specified number of pulse currents so that its DC discharge voltage and insulation resistance will not change significantly is called the tube’s impulse withstand current. This parameter is always given under a certain waveform and a certain number of times of flow. Manufacturers often give the impact withstand current of 10 times under the 8 / 20μs waveform, and also give the flow under the 10 / 100μs waveform. 300 times of impact withstand current.

(5) Insulation resistance and inter-electrode capacitance
The insulation resistance of the discharge tube is very large. The value of this parameter given by the manufacturer is generally the initial value of the insulation resistance, which is about several gigaohms. In the continuous use of the discharge tube, the insulation resistance value will decrease. The reduction in resistance will cause an increase in leakage current in the tube during normal operation of the protected system, and may also cause noise interference.

The parasitic capacitance between the discharge tubes is very small. The inter-electrode capacitance of the two-pole discharge tube is generally in the range of 1 to 5pF. The inter-electrode capacitance value can be kept approximately unchanged in a wide frequency range, and the poles of the same type of discharge tube Capacitance value dispersion is very small.

Selection principle
(1) The DC discharge voltage of the gas discharge tube must be higher than the maximum voltage during normal operation of the circuit to avoid affecting the normal operation of the circuit.

(2) The pulse discharge voltage of the gas discharge tube must be lower than the highest instantaneous voltage value that the line can withstand, in order to ensure that the gas discharge tube can respond faster than the line during instantaneous overvoltage, and limit the overvoltage to a safe value in advance .

(3) The holding voltage of the gas discharge tube should be as high as possible. Once the overvoltage disappears, the gas discharge tube can be extinguished in time without affecting the normal operation of the circuit.

(4) The ground wire should be as short as possible and thick enough to facilitate the discharge of large transient currents.

(5) If the overvoltage duration is too long, the gas discharge tube will generate a lot of heat. To prevent damage to the protected equipment due to overheating, the gas discharge tube should be equipped with a fail-safe card device. Today, some new products of gas discharge tubes are equipped with fail-safe cards.

Vacuum brazing
In the application of electric vacuum, the connection joint of high-purity alumina and oxygen-free copper is required to have high strength and air tightness. The high-purity alumina ceramic and oxygen-free copper are directly brazed using Ag-Cu-Ti active solder The effect of brazing temperature and holding time on the joint composition, interface reaction and joint shear strength, the effect of copper matrix material on the structure and interface reaction of the brazed joint was studied. The brazing temperature is 850 ～ 900 ℃ and the holding time is 20 ～ 60 min When the joint shear strength is close to or reaches 90 MPa.

SIMUWU high-temperature vacuum brazing furnace is specially customized for different types of diamond grinding wheels corresponding to a complete process to achieve mass production.

Edited by: Ryan Wu;
Copyright: SIMUWU Vacuum Furnace

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