In circuits that use capacitors, the concepts of lead and lag are often mentioned, but what is the reference reference for lead and lag?
First of all, from the current and voltage relationship of the capacitor, we can see that the current is about the derivative of the voltage, so the current of the capacitor is 90 degrees ahead of the voltage phase, and its vector diagram is as follows:
In practical application, it is generally composed of RC circuits, as shown in the following figure:
We draw a vector diagram of the voltage using the current as a reference reference.
We take Us as the input signal, and the output signal is the voltage Uc at both ends of the capacitor, so we can see from the vector diagram that the output voltage Uc should lag behind the input supply voltage Us.
The following figure shows the simulated waveform of the RC circuit, and it can be seen that when the input signal frequency is about 160Hz, the voltage phase lag of the output capacitor is about 45°, and the phase inconsistency can be clearly seen from the transient waveform.