Current transformer detection circuit

- Aug 16, 2019-

The commonly used current transformer detection circuit is shown in Figure 1(a).

Figure 1(b) shows the equivalent circuit when the current has a current pulse. The current transformer is simplified to the ideal transformer and the excitation Lm model, and Rs is the sample resistance.

When the duty ratio D < 0.5, after the current pulse of the primary current of the current transformer disappears, the magnetic core generates a negative volt-second value by the excitation current flowing through the sampling resistor Rs to realize self-reset [Fig. 1 (d1) ~ (i1) )), because the sampling resistor Rs is small, the negative reset voltage is small; when the current pulse duty cycle is large (D > 0.5), the reset time is short, there is not enough reset volt-second value, so that the magnetic The DC component Id in the core increases, which may cause the magnetic core to gradually become positively biased (as shown in Fig. 1(d2) to (i2)), and the detection is lost. Therefore, the self-reset can only be applied to the current pulse duty. When the ratio is D < 0.5.


It can be seen that this circuit has many disadvantages for detecting a single-polar straight-flow pulse. In the excitation current sense im, there is a direct current component Id, which is easy to cause the magnetic core to saturate. The output voltage signal uR is bipolar and is not convenient for the post-circuit processing.