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PTC's lightning protection scheme in RS485 communication port

Release time:2020-08-14Browse times number :0

1. Basic background 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), high signal-to-noise ratio, convenient control, low cost, multiple nodes can be realized on a single bus, and the variety of transceivers that can be used are more and more affirmed by users.

 

      But with the increase in frequency of use, the problems it encounters are also increasing. As RS485 communication transmission lines are usually exposed outdoors, lightning and electrostatic interference in daily life have become common problems encountered in actual engineering of RS485 communication buses. The working voltage of RS485 transceivers is low, only 5V, and the components themselves have relatively high withstand voltage. Low, usually only -7V ~ +12V, so the overvoltage introduced by 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.

 

      Therefore, how to effectively protect the RS485 chip and the transmission bus is a problem facing every user. Here, we will do a more detailed discussion on the lightning protection and solutions in RS485 communication, as well as the selection of components.

 

      2. RS485 communication protection scheme (Related reading: How long is the storage period of polymer PTC thermistors?)

 

      A. Schematic

 

fuse

 

      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, and the entire circuit presents a larger current due to the conduction of GDT. , Causing 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, thus realizing 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: ××××

      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. (Recommended reading: Is it possible to apply for free samples)

 

      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 enable back-end communication The chip is reliably protected and has a large current capacity.

 

      Four, 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 PPTC with the best current-limiting effect, namely: a recoverable fuse, which can be a recoverable fuse with a factory resistance of about 1.35~6.5 ohms and a non-operating current of 100~200mA. PPTC is determined by its special process. When a normal working current flows through the loop, it presents a very low resistance value and 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 rapidly jump and show a resistance value of tens of K ohms or more due to the heat accumulated in the loop, thereby limiting the current of the entire loop to one In a very small range, effective protection of the load is realized. 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. (Recommended reading: Protection application of self-recovery fuse in no-smoking device)

 

      5. Basis of overvoltage protection standards

 

      IEC61000-4-5, ITU-TK20/K21 and national standard GB9043 all have standards for lightning surge immunity testing. The highest test standard of its communication line is 10/700us, 4KV.


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