If we imagine a basic lowpass filter, the AC components from the non perfect DC from VIN passes the capacitor and down to ground, meanwhile "pure" DC flows to VIN.

If we take a basic closed DC circuit with a battery capacitor, once the capacitor reaches the voltage level of the battery, no current will flow, and therefore "DC is blocked".

So I assume that the capacitor in a lowpass filter act the same. Once its charged and equals the voltage of VIN, no DC will pass.

But what about the AC components/ripple, they pass, do they affect that capacitors charge/discharge. How does the capacitor ever charge/discharge in a lowpass filter like this? Where VIN is DC with AC components.

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    Feb 11 at 19:57
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1 Answer 1


You seem to understand that AC currents flow through a capacitor.

And current is moving charges.

Any time there is a current through a capacitor, the current either charges the capacitor by adding charge, or discharges it by removing charge, and by definition, the voltage of a capacitor depends on the charge stored in it, and capacitance is the relation how much voltage changes when charge changes.

  • \$\begingroup\$ Yeah found my answer online, the ripple current is not the same as AC though, we do not invert and get a negative current like a sine wave, therefore we do not change direction in our charge/discharge. Other than that you are right, charge with rising ripple current and discharge with falling ripple is how it gets flattened into proper DC \$\endgroup\$
    – Larius
    Feb 12 at 8:49
  • \$\begingroup\$ @Larius They are the same thing. I think what you mean is that the capacitor only discharges and charges a bit when you talk about ripple, and AC when it charges and discharges so much it fully discharges and inverts polarity. Discharging is negative current. \$\endgroup\$
    – Justme
    Feb 12 at 9:06

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