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　　In the power supply circuit, the magnitude of the current varies greatly, ranging from several amperes to tens of thousands of amperes. In order to facilitate the measurement of instruments and instruments, it is necessary to convert to a relatively uniform current. In addition, the voltage on the power supply circuit is relatively high. For example, direct measurement is very dangerous to instruments, instruments and people. The current transformer designed for this purpose plays the role of converter and electrical isolation.

Pointer current display instrument is mostly ampere level, so the secondary current of current transformer is mostly ampere level (such as 5A), while the signal sampled by computer is generally milliampere level (0-5V, 4-20mA, etc.). For this reason, the micro current transformer (the secondary current is milliampere level) is used as the bridge between the current transformer and the computer sampling signal to carry out the secondary conversion of the current.

Current transformer consists of primary coil, secondary coil, core, insulation support and outlet terminal. The core of current transformer is made of silicon steel sheet. Its primary coil is connected in series with the main circuit, and through the measured current I1, it generates alternating flux in the core, so that the secondary coil can induce the corresponding secondary current I2 (its rated current is 5A). If the excitation loss is neglected, I1n1 = I2n2, where N1 and N2 are the turns of primary and secondary coils respectively. Current transformer converter ratio K = I1/I2 = n2/n1. Because the primary coil of current transformer is connected to the main circuit, the insulation material corresponding to the voltage of the primary circuit must be adopted to ensure the safety of the secondary circuit and the person. The secondary circuit consists of the secondary coil of the current transformer, the instrument and the current coil of the relay in series. Current transformers can be roughly divided into two categories: current transformers for measurement and current transformers for protection.