In the lines of power generation, transformation, transmission, distribution, and consumption, the currents vary greatly, ranging from a few amperes to tens of thousands of amperes. In order to facilitate measurement, protection and control, it needs to be converted into a relatively uniform current. In addition, the voltage on the line is generally relatively high, such as direct measurement is very dangerous. The current transformer plays the role of current conversion and electrical isolation. For pointer type ammeters, the secondary current of the current transformer is mostly ampere level (such as 5A, etc.). For digital instruments, the sampled signal is generally milliampere (0-5V, 4-20mA, etc.). The secondary current of the miniature current transformer is milliampere, and it mainly acts as a bridge between the large transformer and the sampling.