A very newbie caps question

OK,
I'll get laughed out of the Drawing Board, but here it goes. There has been something really confusing me about caps. When you look at a typical power supply, after a rectifier, there are a few caps to smooth things out. My understanding is that as the cap charges from the ripply "DC" that's coming out of a rectifier, as the crest of the ripple goes down, the charge releases slower and is filled again by the next wave. This is the smoothing effect of the caps and reduces the ripple. Ripple to me is "AC" In this sense to me, caps are changing AC into DC and letting DC pass.

But then the oxymoron to me is that then later in the circuit, caps are used to block DC and let AC pass (decoupling cap, for example). How can a cap change AC into DC in one case, and block DC and let AC pass in another case?

I assume that there must be some fundamental thing I'm not getting (probably something to do with the uF of the cap) so please feel free to rip me a new one and school me with some tough love...

Joel
:guinness:

The rectifiers are converting AC into PDC (pulsed DC). The caps are smoothing out the pulses. The difference between AC and PDC is that even though the amplitude of PDC changes over time, it does not reverse polarity.

Caps have low impedance to alternating current and high impedance to direct current. AC passes easily from one plate of the capacitor to the other, while DC does not pass (not counting leakage in real-world caps). Compare how a filter cap is hooked up and how a coupling cap is hooked up; see where the AC goes when it passes through the cap in each case. You'll realize there's nothing enigmatic about it.

A filter cap in a power supply holds the DC supply at AC ground, in addition to smoothing PDC into steady DC through charge/discharge action. This is important because if signal voltage were riding on the supply, it would couple from one audio circuit into another and result in all sorts of problems from crosstalk to intermodulation to outright oscillation.

This is a really simplified (possibly oversimplified) explanation but it should help to give a mental picture of what's going on. Then you can go on to grasp the finer details more easily when you get to them.