The Internet of Things sensor is a new product of modern communication, mainly used in homes, enterprises, logistics companies, express delivery companies, postal and telecommunications systems management. The IoT sensor is connected to the converter, computer, and Internet. A front-end communication product belonging to the modern network. The communication distance is far and near, and the communication method is mostly RS485 mode. The voltage of this communication mode is lower, the current is smaller, and the line length can reach 2KM. Such a line is easily damaged by lightning. .
In practical applications, the RS485 interface circuit is easily damaged. The IoT sensor system itself is a complex system. The IoT sensor products are generally produced by professional manufacturers. The various accessories are designed and produced by different manufacturers. Electrical connection and matching are the main problems of design engineers. Due to the complexity of the system, the electromagnetic environment generated is also very complicated. If there is a slight carelessness, the wiring power cable and data cable will be connected incorrectly, the power cable will be reversed, and the data cable will be short-circuited. . These problems end up in the burnout of the RS485 data interface circuit, or the entire DSP system of the elevator controller is damaged. At the same time, due to the complex electromagnetic environment, the circuit often senses the RS485 circuit or DSP system of the controller that damages the space, such as high voltage, ESD high voltage. There are also long connecting cables that are susceptible to lightning intrusion or damage to the RS485 interface circuit and controller DSP system due to transient high voltages. To deal with the above problems, we must design a reliable protection circuit for the RS485 interface, including overcurrent protection and overvoltage protection. The use of PPTC (Positive Temperature Coefficient Thermistor) device is a cost-effective protection scheme. In the short-circuit fault state, the PPTC device can quickly move from a low-resistance state to a high-impedance state, thereby limiting the current protection RS485 interface and equipment. For overvoltage protection, we can use TVS tube (silicon-based ESD protection device). TVS tube has low clamping voltage, high ESD resistance and small package size. It is ideal for electrostatic protection and overvoltage protection of RS485 interface. The following figure is a typical application using PPTC devices and TVS device protection on the RS485 port.
In the above figure, PPTC1=PPTC2, optional: TG250-120; TG60-010, TG1812-010, etc., regardless of polarity. TVS tube D1, D2, D3 can be used: P6KE6.8AC, regardless of polarity.
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