Suggestions on PTC thermistors for carrier power meter

Recently, some problems have been discovered through access to the PTC thermistor. These problems are mainly concentrated on the carrier energy meter, specifically the following:

First of all, when the manufacturer uses the 1.9Un overvoltage transformer, the initial output voltage of the transformer is relatively small, resulting in insufficient instantaneous output power during carrier transmission. If the instantaneous output power is to be met, the transformer volume must be made larger, which will increase. The overall cost of the energy meter product.

Secondly, since the transformer can resist 1.9Un for four hours, the manufacturer does not add a PTC thermistor in the front stage of the transformer. Under the 1.9Un state voltage, the secondary DC output voltage of the transformer is higher, and the withstand voltage of the electrolytic capacitor needs to reach 35VDC, which also increases. The cost of use.

Third, if the manufacturer does not choose a transformer that is resistant to 1.9Un for four hours, but instead adds a PTC thermistor to the transformer primary to deal with 1.9Un for four hours, although the instantaneous transmit power will be improved, but when a certain carrier table When the relay meter is used, the block energy meter is in the transmitting/copying state for a long time, and the normal working current flowing through the PTC thermistor is exactly the fault current of the transformer at 1.9Un, if the block of electric energy meter is just slightly higher When the emission/copying is performed at ambient temperature, the thermistor will inevitably malfunction. If the non-operating current of the thermistor is further increased, the PTC thermistor does not operate during the 1.9Un four-hour test, and the transformer is inevitably damaged. Therefore, as a power meter for relaying, there is no suitable thermistor to meet its use requirements, and under the premise of self-organizing network technology at the user site, we do not know which power meter will be used as a relay table.

The composite PTC thermistor introduced can solve these problems better.

First, since the protection process of the composite PTC thermistor is independent of the transformer parameters, we can choose its non-operating current as much as possible. For example, the current of 30-40 mA at 70 ° C ambient temperature can basically meet the high current requirement of all electric energy meters in the state of launching and copying.

Secondly, under the condition of 1.9Un overvoltage, after the composite PTC thermistor participates in the protection partial voltage, the primary voltage of the transformer will not exceed 300V. Under the premise that the transformer is not resistant to 1.9Un for four hours, its primary and secondary can be used. The thick enameled wire enables the same-volume transformer to output more power, that is, to meet the transmission power requirement, and also to reduce some costs.

Third, since the transformer secondary does not output more than 20V DC after using the composite PTC thermistor, the withstand voltage of the electrolytic capacitor can be reduced from 35VDC to 25VDC, and the large capacity of the electrolytic capacitor is not required to provide instantaneous emission. The power and capacitance values ​​can also be reduced from 3300uF to 2200uF or even lower.

Using a composite PTC thermistor for overvoltage protection in a carrier energy meter is an excellent choice.