Smart meter system has been widely used in the field of industry and life. Automatic meter reading technology is used to read data through communication port, and remote reading mode is used in most cases. For meter application, it is safe and saves time and money. The key to realize this technology is to ensure the safety and reliability of the communication link. Because RS-485 standard has long-distance transmission (more than 1200 meters), the maximum transmission rate can reach 10 Mbps, and high signal noise. At the same time, RS-485 circuit has the advantages of convenient control and low cost, which makes multi-point connection possible. Therefore, RS-485 becomes the standard communication interface of smart meter. But RS-485 transmission line is usually exposed to the outdoor, so it is easy to introduce overvoltage due to lightning and other reasons. The working voltage of RS-485 transceiver is low (about 5V), and its withstand voltage is very low (- 7V ~ + 12V). Once the overvoltage is introduced, it will be broken down. When there is a strong surge energy, you can even see the transceiver burst and the circuit board burnt. Therefore, lightning protection has become a necessary consideration in RS-485 interface design.

  Generally, as shown in Figure 1, PPTC and TVs are used as lightning protection for RS-485.

Design of lightning protection circuit of RS-485 in multi-function watt hour meter

When lightning strikes, the induced overvoltage is introduced by a / b line, passes through PPTC, and then GDT is used as primary common mode protection. Generally, GDT can withstand 10kA (8x20us) surge. After that, the residual voltage has been greatly reduced to less than 1kV, then TVs is used as secondary protection for common mode / differential mode protection, and the voltage to the transceiver is clamped below 12V. At the same time, the pull-up voltage on a / b line can ensure that the voltage on a / b line remains at high level. And realize the surge protection of transceiver. Generally, for over-voltage below 4KV, the primary protection GDT can be omitted. Only TVs can meet the requirement of surge protection. When RS-485 bus is short circuited with power line (such as 220 VAC). PPTC on a / b line can provide short circuit protection.

  But this traditional way has some problems to consider

1: GDT surge breakdown voltage is higher, which means that the resistance value behind is larger. This may affect the transmission distance reduction. 2: The leakage current of TVs is relatively high, about 800ua for smbj6.0ca. This will affect the reliability of point-to-point communication. 3: The response speed of PPTC is slow, so when the power tower is connected, TVs may be broken down by AC

TVs is a semiconductor protection device, which has the advantages of fast response and high reliability. But it’s the clapping protection mode. Its residual voltage will be relatively high, and our sidator as a semiconductor device also has the advantages of fast response and high reliability. But it’s crowbar protected mode. It can keep low voltage after conduction, and has strong anti surge ability and anti lap ability.

  Therefore, refer to figure 3 for the protection scheme using the RS-485 of sidator.

Design of lightning protection circuit of RS-485 in multi-function watt hour meter

When lightning strikes, the sidacto p0080 is used as common mode / differential mode protection. Generally, the sidactor can withstand 800A (8x20us) surge. The voltage to the transceiver is kept at about 4V when the RS-485 bus is short circuited to the power line (such as 220VAC). PPTC on a / b line can provide short circuit protection.

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