The key here is the parameters of resistance and capacitance. That is, during the period when the power supply voltage is lower than the capacitor voltage (stopping charging), the voltage drops through resistance discharge. If the resistance is large, the discharge can be ignored, and the capacitor voltage is equal to the peak value of alternating current. There is no AC (it should be called ripple coefficient). On the contrary, if the discharge current is large (the resistance value is small), a half-wave rectified waveform can be obtained on the capacitor. Alternating current (pulsed DC) can be detected.