It has been more than ten years since the primary of the electric meter transformer began to use PTC thermistor for overvoltage protection. Due to the use of the thermistor, the parameter matching requirements of the transformer are quite strict, and the ambient temperature has a relatively large influence on the protection speed. Instrument manufacturers are eager to have a universal PTC thermistor, regardless of the size of the power supply load, regardless of the difference between the transformer manufacturers, without considering the impact of the ambient temperature on the reaction speed, to meet all single-phase or three-phase meter transformer primary The overvoltage requirement can properly reduce the influence of surge voltage and group pulse on the accuracy of the meter. When an overvoltage is applied, it responds immediately in a short time, so that the primary voltage of the transformer is clamped at about 220V, and the meter works normally. After the overvoltage is lowered, it returns to the original working state.
This composite PTC thermistor. It is used to solve these problems that are plagued by power meter companies. Packing a PTC thermistor and a varistor (the varistor voltage varies according to the rated voltage of the transformer), using the current and temperature generated by the varistor overvoltage to make the thermistor respond quickly, using thermal The effect of rising resistance on the voltage and current in turn protects the varistor. Due to the comprehensive protection effect of current and voltage between the thermistor and the varistor, we do not care about the varistor damage, in the design of this composite PTC thermistor, we have the varistor The varistor voltage and the rated operating current of the thermistor have been boldly designed: they can meet the primary overvoltage protection of all types of transformers, and can also speed up the response of the thermistor during overvoltage protection at any ambient temperature. Group pulse, the influence of high residual voltage on the accuracy of the meter during the surge process, after the protection process is completed, the transformer primary can maintain the normal working voltage of 220±20% due to the influence of the heat dissipation of the varistor.
After the composite PTC thermistor does not exist a 1.9Un overvoltage, we only need to require the transformer to work at 300VAC for a long time. This is the same volume for a transformer that requires a large instantaneous power output. Power.
In order to verify the reliability of the long-term operation of the composite PTC thermistor. On the high-pressure aging life test bench, 450V high-voltage power-on for 1 minute, power-off for 5 minutes and 1000 hours; in the high-temperature aging room, the 70°C environment continues for high-voltage 450V for 1000 hours. The thermistor and varistor were intact in 200 samples.
Application circuit:
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