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RS485 uses PTC fuse lightning protection

1. Product introduction
RS485 is currently one of the most commonly used serial differential communication methods in the industry. It adopts balanced transmission and differential reception, so it has the ability to suppress common mode interference. Because of its long communication distance (over 1200m) and high transmission rate (10Mbps). The advantages of convenient control, low cost, multi-nodes and a wide variety of transceivers that can be used on a single bus, have been more and more affirmed by users.

However, as the frequency of use increases, the problems it encounters are also increasing. As RS485 communication transmission lines are usually exposed outdoors, lightning and static electricity interference in daily life have become common problems encountered in actual engineering of RS485 communication buses. The working voltage of the RS485 transceiver is low, only 5V, and the withstand voltage of the components themselves is also low, usually only -7V ~ +12V. Therefore, the overvoltage introduced by thunder and lightning can usually instantly damage the RS485 transceiver and cause serious damage to the communication system; In addition, electrostatic electromagnetic interference also seriously affects the data transmission quality of the communication bus.

How to effectively protect the RS485 chip and the transmission bus is a problem facing every developer. Here, we will do a more detailed discussion on the lightning protection and scheme in RS485 communication, and the selection of lightning protection components.

Design polymer PTC fuse for lightning protection

2. RS485 communication protection scheme

A. Schematic

  RS485 lightning protection design drawing
The above is the two-level protection schematic diagram of the RS485 bus. When a lightning strike occurs, the induced overvoltage is introduced from the left end through the transmission line, and the GDT composed of G1~G3 is used for primary protection. At this time, the overvoltage is greatly weakened to about hundreds of volts. The entire loop presents a relatively large current due to the conduction of the GDT, which causes the PPTC to act immediately, limiting the current of the entire loop within the allowable range;
TVS1~TVS2 are used as secondary voltage limiting, so that the voltage to the back-end circuit is clamped at about 8V, so as to realize the protection of the back-end circuit.

B. Protection level
It can pass IEC61000-4-5, level 4 standards:
1.2/50us 4KV
10/700us 6KV
8/20us 2KA
C. Device selection
GDT: ××××
PPTC: WH250-120/145
TVS: ××××

D. Device selection instructions
Gas discharge tube GDT: The DC breakdown voltage is greater than the normal working voltage in the line, and the allowable current of the discharge tube exceeds or equals the maximum current designed to pass.
Self-recovery fuse PPTC: The maximum rated working voltage of PPTC should be greater than the normal maximum working voltage of the circuit. The Ih holding current should be greater than the maximum operating current, and the It operating current should be less than the maximum withstand current of the circuit.
Transient suppression diode TVS: The breakdown voltage VBR of the TVS on the general signal transmission line should be higher than the signal voltage transmitted on the signal line. Under this premise, the VBR should be selected as low as possible. A lower VBR can reliably protect the back-end communication chip and have a larger flow capacity.

Fourth, the selection basis of various devices

The choice of GDT first considers its pressure and flow resistance. The selection of TVS is determined by the working voltage and withstand voltage of the chip, which is generally slightly higher than the highest working voltage of the chip.

The fuse can choose the polymer PPTC with the best current limiting effect, namely:
The restorable fuse can adopt the factory resistance value of about 1.35~6.5 ohm. A recoverable fuse with a non-operating current of 100~200mA. Due to its special process, PPTC has a very low resistance when the normal working current flows in the loop, which can be regarded as a wire. When the current flowing through the loop reaches more than 2 times the normal working current, the resistance value will jump rapidly due to the heat accumulated in the loop, showing a resistance value of tens of K ohms or more. In turn, the current of the entire loop is restricted to an extremely small range, thereby realizing effective protection of the load. When the power is disconnected, the PPTC will automatically return to the factory low resistance value in about 1 minute after the heat is dissipated due to overcurrent.

In order to solve the technical problems of lightning protection in the industry, a special production formula has been developed and a production process developed specifically for the characteristics of lightning protection. Special lightning strike tests have been done on the products to ensure that PPTC will not be cracked or burnt when a lightning strike occurs. It can also effectively limit the large current generated by GDT and TVS after lightning strikes. So as to achieve the purpose of protecting the RS485 chip and the entire communication line.
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