Transformer for DC current magnitude conversion. It uses the non-linearity and asymmetry of the iron core in the iron core coil when it is magnetized by DC and AC currents, and converts the large DC current through the coil into a small DC current inversely proportional to the number of turns through the rectifier circuit. It is mainly used to measure large DC current, and is also used as a current feedback, control and protection component in the rectifier system. Compared with the shunt (see ammeter), it has low power loss and isolation. The circuit of the commonly used DC current transformer is shown in the figure. In the figure, A and B are two identical but independent cores. A large DC current I1 flows through the two primary windings (ie, the feed-through conductor in the figure, which is equivalent to the primary N1 = 1 turn). The secondary windings on both cores are N2 turns, connected in series with opposite polarities, and are exchanged by a sine wave Power supply. As the polarity of the power supply voltage alternates, the two cores will alternate between deep saturation and non-saturation. Under ideal conditions, the difference between the power supply voltage U and the load resistance Rx is borne by the secondary winding on the unsaturated core, and has N1I1 = N2I2 or I1 / I2 = N2 / N1 (I2 is the secondary current), that is I2 and I2 have an inverse relationship with the number of turns. After rectifying I2, it can be read with a DC ammeter or calculated using the voltage drop on the load resistance Rx.