The working principle is as follows: when the rectifier circuit outputs pulsating DC voltage, the load current will increase or decrease. When the load current increases, the induced electromotive force will be generated in the inductance coil in the opposite direction to the current, trying to prevent the current from increasing. When the load current decreases, the induced electromotive force in the same direction as the current will be generated in the inductance coil, thus reducing the pulsation degree of the load current and obtaining a smoother DC output voltage on the load. The larger the inductance, the better the filtering effect.

(4) The composite filter is a filter composed of inductance and capacitance or resistance and capacitance. Its working principle is the same as that of capacitance filter and inductance filter, but after filtering for more than two times, the output waveform is smoother and the load is close to the power supply voltage of dry battery. LC-Ⅰ filter and LC-Ⅱ filter are both composite filters.

(1) There are three common filtering circuits: capacitive filtering, inductive filtering and composite filtering. According to different rectifier circuits, filter circuits can be divided into half-wave rectifier capacitor filter circuit, full-wave rectifier capacitor filter circuit, bridge rectifier capacitor filter circuit, full-wave rectifier inductor filter circuit, bridge rectifier inductor filter circuit, LC-I filter circuit, LC-II filter circuit, etc.

(2) Capacitive filtering is to connect a large capacitor in parallel with the load after the rectifier circuit. Due to the charging and discharging of the capacitor and the voltage at both ends of the capacitor, the pulsation degree of the output voltage UL of the rectifier circuit is greatly weakened, and the waveform is almost smooth, which plays a filtering role. The bridge rectifier capacitor filters the output voltage waveform (actually the filtered output waveform). In this capacitor filter circuit, the larger the capacitor capacity or the larger the load resistance, the slower the capacitor discharge and the smoother the output voltage.

(3) Inductive filtering is to connect an inductive coil in series between the output end of the rectifier circuit and the load. It uses the characteristics of small DC resistance and large AC resistance of inductance to filter (that is, cross flow).