Smart meter systems have been widely used in industrial and living areas. Data is read through the communication port using automatic meter reading technology in the meter, and remote reading is used in most cases. It is safe and saves time and money for meter applications. The key to implementing this technology is to ensure that the communication link is secure and reliable. Since the RS-485 standard has long-distance transmission (1200 m or more), the maximum transmission rate can reach 10 Mbps, and high signal noise is printed. At the same time, the RS-485 circuit has the advantages of convenient control and low cost, making multi-point connection possible. Therefore, RS-485 becomes the standard communication interface for smart meters. However, the RS-485 port transmission line is usually exposed to the outdoors, so it is easy to introduce an overvoltage due to lightning strikes and the like. The RS-485 transceiver has a low operating voltage (about 5V), and its own withstand voltage is very low (-7V~+12V). Once overvoltage is introduced, it will break down and damage. When there is a strong surge of energy, you can even see the phenomenon that the transceiver bursts and the circuit board is burnt. Therefore, lightning protection must be considered as an RS-485 interface design.
Typically, as shown in Figure 1, PPTC and TVS are used as lightning protection for RS-485.
When a lightning strike occurs, the induced overvoltage is introduced by the A/B line, passes through the PPTC, and then the GDT acts as a primary common mode protection. Usually, the GDT can withstand a 10KA (8x20us) surge shock. After that, the residual voltage has been greatly reduced to less than 1KV. Then TVS is used as the secondary protection for common mode/differential mode protection. The voltage to the transceiver is clamped below 12V. At the same time, the pull-up voltage on the A/B line can guarantee A. The voltage on the /B line is held high. And achieve surge protection for the transceiver. In general, for overvoltages below 4KV, primary protection—GDT can be eliminated. Surge protection can be achieved with TVS alone. When the RS-485 bus is shorted to the power line (eg 220VAC). The PPTC on the A/B line provides short circuit protection.
But there are problems with this traditional approach that need to be considered:
1: GDT surge breakdown voltage is higher, which means that the latter resistance value is relatively large. This may affect the transmission distance reduction. 2: TVS has a high leakage current, which is about 800uA in terms of SMBJ6.0CA. This will affect the reliability of peer-to-peer communication. 3: PPTC has a slower response speed, so when the power tower is connected, the TVS may be AC-punched.
TVS is a semiconductor protection device with the advantages of fast response and high reliability. But it is the Clamping protection mode. Its residual voltage will be relatively high and our Sidactor as a semiconductor device also has the advantages of fast response and high reliability. But it is Crowbar protection mode. After the conduction, the voltage is kept low, and at the same time, it has the characteristics of strong anti-surge capability and strong resistance to lap.
So, look at Figure 3, using Sidactor's RS-485 protection scheme.
When a lightning strike occurs, the Sidacto P0080 acts as a common mode/differential mode protection. Typically, the Sidactor can withstand 800A (8x20us) surge surges. The voltage to the transceiver is guaranteed to be around 4V when the RS-485 bus is shorted to the power line (eg 220VAC). The PPTC on the A/B line provides short circuit protection.
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