The interface is the most common component of embedded devices. It is the channel for data transmission. It plays the role of data transmission and isolation protection circuit. Today we discuss a common solution for interface protection design.
The gas discharge tube is a ceramic or glass packaged, short-circuit type protection device filled with low-pressure inert gas (argon or helium), mainly using gas discharge between the two metal plates of the discharge tube to protect, the principle of the gas discharge tube The figure symbol is shown in Figure 1 below.
Figure 1 Common symbols of gas discharge tubes
The working principle of the gas discharge tube is gas discharge. When the voltage between the two poles is sufficiently large, the gas gap between the poles will be broken down, and the original insulation state will be converted into a conductive state, similar to a short circuit. The voltage maintained between the two poles in the conductive state is very low, generally 20 to 50V, so it can protect the circuit of the latter stage. The main indicators of the gas discharge tube are: response time, DC breakdown voltage, impact breakdown voltage, through-current capacity, insulation resistance, inter-electrode capacitance, and freewheeling interruption time.
Figure 2 Working principle of gas discharge tube
In the design of lightning protection circuit, attention should be paid to the selection of parameter values ​​such as DC breakdown voltage, impact breakdown voltage and flow capacity of the gas discharge tube. A discharge tube used with other protection devices on a common AC line is required to be inoperable within the normal operating voltage of the line and its allowable fluctuation range, and its DC discharge voltage should satisfy: min(Ufdc) ≥ 1.8UP. Where Ufdc is the DC breakdown voltage, min(Ufdc) is the minimum value of the DC breakdown voltage, and UP is the peak value of the line normal operating voltage.
Figure 3 lightning protection circuit design
The gas discharge tube can be mainly applied to the phase protection of the AC power supply phase line, the neutral line, the protection between the working place of the DC power supply and the protection ground line; the protection of the signal line line to the ground, the RF signal feeder core line to the shielding layer protection.
The failure mode of the gas discharge tube is an open circuit in most cases, and the failure mode of the short circuit may also be caused when the discharge tube is short-circuited for a long time due to circuit design reasons or other factors. The life of the gas discharge tube is relatively short, and the performance will decrease after multiple impacts. Therefore, the lightning arrester composed of the gas discharge tube has problems of maintenance and replacement after being used for a long time.
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