Gas discharge tubes include diodes and triodes, with voltages ranging from 75V to 3500V, over one hundred specifications, and are manufactured, monitored and managed in strict accordance with CITEL standards. The discharge tube is often used in the first stage or the first two stages of the multi-stage protection circuit to discharge and discharge the lightning transient overcurrent and limit the overvoltage.
Gas discharge tube
Gas discharge tube structure introduction Gas discharge tube response time Gas discharge tube voltage limiting circuit Gas discharge tube working principle Gas discharge tube and varistor Gas discharge tube precautions
Introduction to Gas Discharge Tube Structure The working principle of the discharge tube is gas discharge.
When the applied voltage increases beyond the dielectric strength of the gas, the gap between the two poles will discharge and break down, and the original insulation state will be converted into a conductive state. After the conduction, the voltage between the two poles of the discharge tube is maintained at the discharge arc path. Residual pressure level.
The main components of the five-pole discharge tube are basically the same as the two-pole and three-pole discharge tubes, and have good discharge symmetry, which can be applied to the protection of multiple lines. (usually used for protection of communication lines)
Gas Discharge Tube Response Time There is a delay between the moment when the transient overvoltage starts acting on both ends of the discharge tube and the actual discharge time of the tube. This time is called the response time.
The composition of response time: one is the time required for the initial electron-ion to charge particles to randomly generate in the tube, that is, the statistical delay; the second is the time required for the initial charged particles to form an electron collapse, that is, the time delay is formed.
In order to measure the response time of the discharge tube, it is necessary to apply a voltage source with a fixed wave head rising steepness du/dt to the two ends of the discharge tube to measure the response time, and take the average value of the multiple measurements as the response time of the tube.
Comparing the protection performance of the two-pole and three-pole discharge tubes of the gas discharge tube voltage limiting circuit If the AG pole is first discharged, the free electrons generated by the gas liberation inside the tube will quickly cause collision and freeness between the BG poles, so that the BG discharges quickly. When the BG is cut off and discharged, the number of electrons in the tube is greatly reduced due to the combined action of a large number of charged particles (electrons and ions), thereby rapidly suppressing the collision between the other pair of electrodes AG, so that the pair of poles The discharge process is quickly cut off.
In the case of differential mode transient overvoltage protection, both the two-pole discharge tube and the three-pole discharge tube have certain problems, because the electronic device has to withstand the sum of the residual voltage between the two pairs of electrodes, for some fragile electrons. For equipment, the sum of such residual pressures is sometimes unbearable. Additional measures need to be taken, such as connecting a discharge tube between A and B, specifically for suppressing differential mode overvoltage.
The length of the grounding cable has a certain influence on the voltage limiting effect. If the ground connection is long, interference occurs on the line and signal line of the connection itself, or an induced voltage is generated in the surrounding electrical circuit. The commonly used suppression methods are shielding, reducing coupling and filtering. After the discharge tube is turned on, the incident wave is reflected back, so that the subsequent electronic device is protected, but the spatial electromagnetic field generated by the reflected wave current also radiates energy to the surroundings, which needs to be suppressed.
Working principle of gas discharge tube Gas discharge tube and varistor are the main components of lightning arrester. The gas discharge tube is used for a switch type lightning protection device, and the varistor is used for a voltage limiting type lightning protection device.
First, the working principle and characteristics of the gas discharge tube Gas discharge tube generally uses ceramic as the package shell, the discharge tube is filled with inert gas with stable electrical properties, the electrode of the discharge tube generally has two electrodes, three electrodes and five electrodes three structures . When a certain voltage is applied between the poles of the discharge tube, an uneven electric field is generated between the poles. Under the action of the electric field, the gas starts to dissipate. When the applied voltage reaches the interelectrode field strength and exceeds the insulation strength of the inert gas, An arc is generated between the two poles, and the ionized gas produces a "negative resistance characteristic", which immediately changes from an insulated state to a conductive state. That is, when the electric field strength exceeds the breakdown strength of the gas, gap discharge is caused, thereby limiting the interelectrode voltage. That is to say, when there is no surge, it is in an open state, and when the surge arrives, the electrode plates in the discharge tube are closed and turned on. When the surge disappears, the plates return to their original state.
The gas discharge tube is a switch type lightning protection device, generally used for the protection of the first stage or the second stage of the lightning protection engineering; because its inter-pole insulation resistance is large, the parasitic capacitance is small, so it is used for There are obvious advantages to the protection of high frequency electronic circuits. However, the gas discharge tube is difficult to effectively suppress the lightning wave head with a large steep rise due to its long time delay in discharge and the sensitivity of the action. Therefore, the gas discharge tube is generally in the lightning protection project. Most of the applications are combined with the pressure limiting type lightning arrester.
In summary:
The advantage of the gas discharge tube is that the current capacity is large; the parasitic capacitance is small; the residual voltage is low, generally about 900V;
The disadvantages of gas discharge tubes are:
1. The discharge delay is large, the sensitivity of the action is not enough, and the response time is slow, about 80 ns.
2. There is a freewheeling flow, which is not conducive to the protection of AC or more than 20V lines. Therefore, like the spark gap, there is a problem of continuous current interruption.
3. The deterioration indication and the function of fault remote signaling cannot be performed, and the safety factor is not high.
Second, the working principle and characteristics of varistor
The varistor is a non-linear voltage limiting resistor of a metal oxide semiconductor containing zinc oxide as a main component.
The volt-ampere characteristics of the varistor are continuous and incremental, so it does not suffer from the problem of freewheeling.
Its working principle is that the zinc oxide and the additive of the varistor are "sintered" under certain conditions, the resistance is strongly affected by the voltage, and the current rises sharply with the increase of the voltage, and the rising curve is a nonlinearity. index. When at normal operating voltage, the varistor is in a high resistance state. When a surge arrives, it is in a path state, and a powerful current flows through itself to the earth. After the surge, it immediately returned to a high resistance state.
Several important parameters of the varistor:
A: varistor voltage: The varistor voltage is generally considered to be a voltage applied across the resistor when a current of 1 mA flows through the varistor at a temperature of 20 degrees.
The varistor voltage is generally higher in the AC grid than the peak voltage of the grid, which is 0.7 times the peak voltage, while the peak voltage is generally considered to be √2 times the AC grid voltage (the peak voltage at DC is 1.2 times the rated voltage). Expressed as:
VN = VNH ×√2 ÷0.7
The VN in the formula is the varistor voltage; VNH is the rated voltage of the grid.
B: Leakage current: Leakage current refers to the current of the order of magnitude of micro-ampere through the varistor under normal conditions. The smaller the leakage current, the better.
In particular, the leakage current should be emphasized, it must be stable, and it is not allowed to rise automatically during the work. Once the leakage current is found to rise automatically, it should be eliminated immediately, because the instability of the leakage current is to accelerate the aging of the lightning arrester and the explosion of the lightning arrester. The direct cause. Therefore, when selecting the parameter of leakage current, it should not be pursued as small as possible. As long as it is within the allowable range of the power grid, the lightning arrester with a relatively large leakage current value is selected, which is rather stable.
C: Response time: The response time refers to the time required for the voltage applied across the lightning arrester to be equal to the varistor voltage. After this time, the lightning arrester is fully turned on. The response time of the varistor is about 25 ns.
D: Parasitic capacitance: Varistors generally have large parasitic capacitances. Their parasitic capacitance is generally between several hundred picofarads and thousands of picofarads, which is not conducive to the protection of high frequency electronic systems. Because this parasitic capacitance will cause distortion to the transmission of high-frequency signals, which will affect the normal operation of the system. Therefore, for the protection of higher frequency systems, a varistor type lightning arrester with low parasitic capacitance should be selected.
Its advantages:
1. Low residual pressure.
2, the response time is fast, about 25ns.
3. No freewheeling.
4. It can realize the deterioration indication and fault remote notification function. Therefore, its protection effect is safe and reliable. It is currently used in power supply systems, especially in the field of power and telecommunications, and it is even more unique.
Its shortcomings: leakage current; large parasitic capacitance, is not conducive to the protection of high-frequency electronic circuits.
Gas discharge tube and varistor
The ground connection should have the shortest possible length;
The ground connection should have a sufficient cross section to bleed off transient high currents.
The failure mode of the discharge tube.
The discharge tube is subject to mechanical collision, and the over-torque transient over-voltage multiple impacts and internal aging will cause a failure.
There are two modes of failure (ie failure mode):
The first is to present a low discharge voltage and a low insulation resistance state; the second is to exhibit a high discharge voltage state.
Open circuit failure mode is more harmful than short circuit failure mode:
The open circuit failure mode is difficult to detect in time, so no remedial action can be taken.
In today's power SPD products, with failure alarm devices, such as sound, light alarm, color change prompts, etc., these measures are taken to facilitate the timely detection and replacement of the failed SPD.
DC Connector Set, DC Socket Plug, Laptop Charger Adapter
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