How to understand the equivalent of capacitors passing AC

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How to understand the equivalent of capacitors passing AC [Copy link]

When analyzing capacitor AC circuits, the analysis method of charging and discharging is very complicated and not easy to understand, so the equivalent analysis method should be used. This analysis method is very simple and widely used in circuit analysis, and must be firmly grasped.
　　The two plates of capacitor C1 are insulated, and the AC current cannot directly pass through the two plates to form a loop. It is only because the charging direction of the AC current is constantly changing that a continuous AC current flows through the circuit, which is equivalent to C1 being able to allow the AC current to pass.
　　In fact, the AC current does not pass directly between the two plates. For convenience in circuit analysis, the capacitor is regarded as a component that can directly pass the AC current, as shown in Figure 1-28.

　　Figure 1-28 Schematic diagram of capacitor passing AC equivalent understanding
　　The DC blocking and AC passing characteristics of capacitors are the superposition of the DC blocking and AC passing characteristics of capacitors.
　　In a DC circuit, since the direction of the DC voltage remains unchanged, the charging direction of the capacitor remains unchanged. After the capacitor is fully charged, there is no current flow in the circuit, so the capacitor has a DC blocking effect. The
　　DC blocking and AC passing characteristics of capacitors are often linked, that is, capacitors have DC blocking and AC passing characteristics. Figure 1-29 shows a schematic diagram of the DC blocking and AC passing characteristics of capacitors.

　　Figure 1-29 Schematic diagram of capacitor blocking DC and passing AC characteristics
　　The input signal Ui is a signal composed of DC voltage U1 (dashed line in the figure) and AC voltage U2 (solid line in the figure). The input signal Ui waveform is obtained by adding U1 and U2. In the process of circuit analysis, the working principle of the circuit can be easily understood with the help of signal waveforms.
　　The understanding process of adding DC voltage U1 and AC voltage U2 can be divided into the following moments (see the input signal Ui waveform in the figure).
　　At t0, Ui is equal to U1, U2 is 0V, U1+U2=U1, and Ui waveform is U1.
　　At t1, U1 is still U1, U2 is the positive peak value, and Ui waveform is U1 plus U2 (positive peak value). At this time, Ui is the maximum value.
　　At t2, because U2 is 0V, Ui is equal to U1.
　　At t3, U2 is the negative peak value, so at this time, Ui is U1 minus the negative peak value, and Ui is the minimum.
　　At t4, the addition of the two signal voltages is the same as at t0.
　　Important Tips
　　From the waveform decomposition, it can be seen that the signal waveform shown by Ui is composed of a DC voltage U1 and an AC voltage U2, which provides great help for the next circuit analysis.
　　The input signal Ui is added to the circuit, and the analysis is divided into two cases: DC and AC.
　　(1) Analysis of the DC voltage U1 added to the circuit. Due to the DC isolation function of capacitor C1, the DC voltage cannot pass through C1, so there is no DC voltage at the output end. This is the specific embodiment of the DC isolation characteristic of the capacitor in the circuit.
　　(2) Analysis of the AC voltage U2 added to the circuit. Since capacitor C1 has the function of passing AC, the AC voltage in the Ui signal can form a loop through capacitor C1 and resistor R1, generating AC current in the loop. The AC voltage at both ends of R1 of the AC signal current flowing through resistor R1 is the output voltage Uo.
　　Therefore, the output signal Uo only contains the AC signal component U2 in the input signal Ui, and no DC component U1, thus realizing the circuit function of blocking DC and passing AC.