Suggestions on PTC thermistors for carrier power meter

Recently, several issues have been identified with the use of PTC thermistors in carrier energy meters. These problems are primarily focused on the functionality and performance of the energy meters under specific conditions. Here's a detailed analysis of the challenges and how a composite PTC thermistor can offer a better solution. Firstly, when manufacturers use a 1.9Un overvoltage transformer, the initial output voltage is relatively low, leading to insufficient power during data transmission. To meet the required instantaneous power, the transformer would need to be larger, which increases production costs significantly. Secondly, since the transformer can withstand 1.9Un for four hours, many manufacturers do not include a PTC thermistor at the front stage. However, this leads to higher DC output voltages from the secondary side, requiring electrolytic capacitors rated at 35VDC. This also adds to the overall cost of the device. Thirdly, if a manufacturer does not use a 1.9Un-resistant transformer but instead adds a PTC thermistor on the primary side, it may improve short-term transmission performance. However, when the energy meter is in a long-term transmitting or copying state, the normal operating current through the PTC thermistor could mimic a fault current under 1.9Un conditions. If the ambient temperature slightly rises during transmission, the thermistor may malfunction. Additionally, if the non-operating current is too high, the PTC might not activate during the 1.9Un test, risking damage to the transformer. This makes it difficult to find a suitable thermistor for relay-based energy meters, especially in self-organized network environments where the role of each meter is uncertain. The composite PTC thermistor offers a more effective solution to these challenges. First, its protection mechanism is independent of the transformer parameters, allowing for optimized non-operating current settings. For example, setting the current to 30–40 mA at 70°C can meet the high-current requirements of all energy meters during data transmission. Secondly, under 1.9Un overvoltage conditions, the composite PTC thermistor helps limit the primary voltage to below 300V. This allows the use of thicker enameled wire in the transformer, increasing power output without increasing size. As a result, the same volume transformer can deliver more power, meeting transmission needs while reducing costs. Thirdly, after using the composite PTC thermistor, the secondary side outputs no more than 20V DC. This reduces the required withstand voltage of the electrolytic capacitor from 35VDC to 25VDC, and the capacitance value can be reduced from 3300uF to 2200uF or even lower, without compromising performance. In conclusion, using a composite PTC thermistor for overvoltage protection in carrier energy meters is an excellent choice that addresses multiple design and cost challenges effectively. It ensures reliable operation, improves efficiency, and provides a flexible solution for various applications.

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