Capacitors

[LouisStraub]

Hello everyone, I'm new to this forum and electronics. I am teaching myself from the Electronics Fundamentals text by Floyd. It's a great book, although it occasionally leaves a question unanswered. I am also using MultiSim 10.1 to build and test circurts. My question is as follows:

If a capacitor an an AC circut alternately stores enegry and returns energy to the source it seems to me that the capacitor is only useful 50% of the time. I say this in reference to a DC capicator which in theory holds a charge until it's used. So if you remove the voltage source in an AC circut the capacitor may or may have a charge. I'm sure there is a good reason, could some one please explain.

Thanks in advance for any help in this matter......

 

[MikeMl]

You have the right idea, but I take issue with the "50% useful" statement. Since the goal of putting a capacitor in an AC circuit is to provide a path for an AC current, it is "useful" 100% of the time, regardless of the instantaneous state of charge at any given instant.

As to leaving a capacitor charged if it is suddenly disconnected from an AC circuit, yes, that can leave the capacitor charged with either polarity. It is common practice to put a high-value resistor across a capacitor in an AC Power circuit to discharge it (bleeder resistor) to minimize shock hazard...

 

[BrownOut]

Even an uncharged capacitor is useful. It lies in wait to accept excess charge that is undesirable in the curcuits 'downstream.' It is ready to pass a signal while blocking and DC that might be present. Or it might be cooperating with other elements in the circtuit to produce a signal, which just happens to be at the zero crossing level. Leave the caps in the circuit. They are always useful