The interface is the most common component in embedded devices, serving as a channel for data transmission while providing isolation and protection for the circuit. Today, let’s explore a common solution for interface protection design.
A gas discharge tube is a short-circuit protection device encapsulated in ceramic or glass, filled with low-pressure inert gases (such as argon or neon). It primarily utilizes gas discharge between the two metal electrodes to achieve protection. The schematic symbol for a gas discharge tube is shown in Figure 1 below.
Figure 1 Common Symbol for Gas Discharge Tube
The working principle of the gas discharge tube is gas discharge. When the voltage between the electrodes is sufficiently high, the gas gap between them will break down, changing from an insulating state to a conductive state, similar to a short circuit. In the conductive state, the voltage maintained between the electrodes is very low, generally between 20 to 50V, thus providing protection for the subsequent circuit. The main parameters of a gas discharge tube include: response time, DC breakdown voltage, impulse breakdown voltage, current capacity, insulation resistance, inter-electrode capacitance, and continuation interruption time.
Figure 2 Working Principle of Gas Discharge Tube
In the design of lightning protection circuits, attention should be paid to the selection of parameters such as the DC breakdown voltage, impulse breakdown voltage, and current capacity of the gas discharge tube. For discharge tubes used in conjunction with other protective devices on ordinary AC lines, it is required that they do not operate within the normal operating voltage of the circuit and its allowable fluctuation range. Thus, its DC discharge voltage should satisfy: min(Ufdc)≥1.8UP. Where Ufdc is the DC breakdown voltage, min(Ufdc) indicates the minimum value of DC breakdown voltage, and UP is the peak value of the normal operating voltage of the circuit.
Figure 3 Lightning Protection Circuit Design
Gas discharge tubes can primarily be applied in protecting the phase line and neutral line at AC power sources, as well as between the working ground and protective ground of DC power; protection for signal lines against ground, and protection for the core of RF signal feed lines against the shielding layer.
The failure mode of gas discharge tubes is mostly open circuit. If due to circuit design reasons or other factors, the discharge tube remains in a short circuit state for a long time and burns out, it can also lead to a short circuit failure mode. The lifespan of gas discharge tubes is relatively short, and their performance will decline after multiple impacts. Therefore, lightning protectors composed of gas discharge tubes may require maintenance and replacement after prolonged use.
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