The PTC thermistor has recently raised some issues, particularly in the context of carrier energy meters. These problems are mainly focused on the following areas:
First, when manufacturers use a 1.9Un overvoltage transformer, the initial output voltage is relatively low, which leads to insufficient power during carrier transmission. To meet the required instantaneous power, the transformer would need to be larger, increasing the overall cost of the energy meter.
Second, since the transformer can withstand 1.9Un for four hours, many manufacturers do not include a PTC thermistor at the front stage. This results in a higher DC output voltage on the secondary side, requiring electrolytic capacitors with a withstand voltage of up to 35VDC, further increasing the cost.
Third, if the manufacturer doesn’t use a transformer that can resist 1.9Un for four hours and instead adds a PTC thermistor on the primary side, the instantaneous transmission power may improve. However, when a relay meter is used for long periods in transmitting or copying mode, the normal current through the PTC thermistor could match the fault current under 1.9Un conditions. If the ambient temperature is slightly higher during emission or copying, the thermistor might malfunction. Additionally, if the non-operating current increases, the PTC may not activate during the 1.9Un four-hour test, leading to potential transformer damage. As a result, there’s no suitable PTC thermistor available for relay meters, especially when considering self-organizing networks where it's unclear which meter will act as a relay.
The composite PTC thermistor offers a better solution to these challenges.
First, its protection process is independent of the transformer parameters, allowing us to set the non-operating current as high as possible. For example, a current of 30–40 mA at 70°C can meet the high-current requirements of all energy meters during transmission and copying.
Second, under 1.9Un overvoltage conditions, the composite PTC thermistor helps limit the primary voltage of the transformer to below 300V. This allows the use of thicker enameled wire on both the primary and secondary sides, enabling the same-sized transformer to deliver more power—meeting transmission needs while reducing costs.
Third, after using the composite PTC thermistor, the secondary output remains below 20V DC, allowing the electrolytic capacitor’s withstand voltage to drop from 35VDC to 25VDC. This also reduces the required capacitance from 3300µF to 2200µF or even lower, without compromising performance.
In conclusion, using a composite PTC thermistor for overvoltage protection in a carrier energy meter is an excellent choice. It effectively addresses the limitations of traditional PTC thermistors, improves system reliability, and lowers production costs.
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