Anatomy of coupling capcitors.

[RockinRocket]

I dissected a Mustard coupling capacitor to see what was really inside and other than a piece of foil stuck to a clear piece of plastic (dialect?) rolled up there really wasn't anything else to it. How do these block DC? Anyone have a good link on the dissection of caps step by step how it works would be great. Thanks

[Phil_S]

A cap is basically 2 sheets of conductive foil with a wire lead attached to each piece of foil. Other stuff goes between the sheets (goo:dialectic.) Then, roll it up and put a casing on it. Well, maybe more sophisticated, but not that much so. It transmits AC and blocks DC. The DC can't get across the goo.

[RockinRocket]

Am I wrong in thinking that from one lead to the other theres a long rolled up piece of foil that the signal passes from one end to the other. When I opened it up all I saw was a piece of foil with a clear backing on I rolled upt. So how does this block DC?

[Firestorm]

Sometimes the "goo" is actually goo. Sometimes it's just polyester or polyethylene (or other plastics), sometimes paper soaked in oil. Since there's no connection between the foil sheets (or not supposed to be), DC can't pass it. Technically AC can't pass it either, but since AC goes both ways, it charges one side, then the other so the AC shunts around the cap. Google Leyden Jar for and explanation of the earliest, simplest cap. A cap stores charge.

[pdf64]

There's 2 sheets of foil, with a sheet of insulation sandwiched between.
That's then rolled up, and one lead connected to each sheet.
dc can't pass through the insulation.

[vibratoking]

You should look at Youtube for an explanation of how a cap works. The explanation is not all that simple. The reason DC does not pass and AC does requires an understanding of electric charge and how electrons and protons behave. Stating that DC can't pass through the dielectric is not helping IMO. Caps and electric charge are are explained in many different sites and videos much better than you're going to get in the little white box on TAG.

[RockinRocket]

There's 2 sheets of foil, with a sheet of insulation sandwiched between.
That's then rolled up, and one lead connected to each sheet.
dc can't pass through the insulation.

Thanks for the replys everyone. I overlooked the foil when I "examined " it. The foil is on both sides.. And it makes complete sense of how it works.

If a coupling capacitor epoxy shell has a slight crack in it or slightly smushed from shipping in padded envelopes is the integrity of the cap compromised? Are caps robust being fairly simple in design?

[Firestorm]

You should look at Youtube for an explanation of how a cap works. The explanation is not all that simple. The reason DC does not pass and AC does requires an understanding of electric charge and how electrons and protons behave. Stating that DC can't pass through the dielectric is not helping IMO. Caps and electric charge are are explained in many different sites and videos much better than you're going to get in the little white box on TAG.

Oh, oh. Physics. Charge is weird. You need it only to understand how much energy a cap can store (also a function of voltage). Electrons can move around (because they're small and bound only by the EM force. Protons can't go anywhere because they're big and bound by the strong force.) But protons have charge, so the charge is present on the electron depleted side of a cap (or a diode, or a transistor). The opposite charge is present on the other side, so when signals wiggle, the state of the charge varies with the frequency of the wiggle. Unless you want to be a physicist or engineer, that's probably close enough.