DC filter capacitor: connected between the anode and cathode of DC power supply to filter out unwanted AC components in DC power supply and smooth DC power. Originally, electrolytic aluminum capacitors or tantalum capacitors with large capacity were used, but now the trend is to replace them with metallized polypropylene film capacitors.

The output voltage of the rectifier circuit is not pure DC. From the oscilloscope, the output of the rectifier circuit is quite different from DC, and the waveform contains a large ripple component, which is called ripple. In order to obtain an ideal DC voltage, it is necessary to use a filter circuit composed of reactive elements (such as capacitors and inductors) with energy storage function to filter out the pulsating components in the output voltage of the rectifier circuit to obtain DC voltage.

Commonly used filter circuits are passive filter and active filter. The main forms of passive filtering are capacitance filtering, inductance filtering and compound filtering (including inverted L filtering, LC filtering, LCπ filtering and RCπ filtering, etc.). The main form of active filtering is active RC filtering, also known as electronic filters. The magnitude of pulsation component in DC is expressed by pulsation coefficient. The larger this value is, the worse the filtering effect of the filter will be.

Pulse coefficient (S)= the maximum value of the fundamental wave of the AC component of the output voltage/the DC component of the output voltage.

The ripple coefficient of half-wave rectified output voltage is S=1.57, and that of full-wave rectified and bridge rectified output voltage is S≈0.67. The ripple coefficient s = 1/(4) (RLC/t-1) of full-wave and bridge rectifier circuits with C-type filter circuit. (t is the period of the DC pulsating voltage of the rectified output.)

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