What exactly does a filter circuit do?

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What exactly does a filter circuit do?

Posted Date: 2024-02-06

What exactly does the filter circuit do?

1. Utilize the charging and discharging effects of capacitors

In fact, the shaded part of the rectified waveform can be filled by the charge and discharge of the capacitor.

Next, as shown in Figure (a), the capacitor is connected in parallel to the circuit to take advantage of its charging and discharging effects.

Figure (b) shows the relationship between the charging and discharging effects of the capacitor and the rectified waveform. Capacitor C is charged in each stage of ①-⑧, and the charging voltage of capacitor C decreases in each stage of ②-③, ④-⑤, ⑥-⑦, ⑧-⑨, and C releases the charged charge instead.

The colored part of the figure represents the above situation. So the entire waveform is very close to DC.

As mentioned before, the discharge characteristics of the capacitor are related to the time constant = CR determined by the load resistance R and capacitance C, so there are output waveforms shown in ①~④ (see the figure). As a filter circuit, it is desired to have output characteristics that can connect the peaks of the rectified waveform as shown in ④, but since this is a discharge phenomenon, the output will gradually decrease.

Due to the above reasons, a capacitor with a larger capacitance of 10 to 100 microfarads must be used. This allows the capacitor to continue to discharge before being charged by the next rectified waveform.

Although the marked intervals in the figure are large, the half-cycle time of AC is actually very short, 1/50s for 50Hz and 1/60s for 60Hz. Through the charge and discharge of the capacitor, a waveform very close to DC can be obtained.

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