What is the difference in an AC capacitor over a DC capacitor?

[Andy-In_over_my_head]

A lot of times I read that the capacitor needs to be DC or AC.
And the rules of the game are that you never have the one you need... so can you use the one you have?   
In this case I have large AC caps but not DC.... and I need DC.....
I play around and experiment a lot however I know my limitations and when to ask...
I don't like things exploding in my face... I'm male.....
Thanks in advance.

[Seekonk]

You can use AC caps on DC. AC caps have a much higher DC rating.  All capacitors have microscopic air bubbles between the foil layers.  Under a changing voltages, corona forms in these voids eventually destroying the insulation.  AC capacitors are designed to deal with this.  X2 capacitors actually have a layer of foil in between, making two capacitors in series and reducing corona effects in half..

[ebclr]

AC capacitor does not have polarity, but the constructions are generally based on films without any kind of electrolyte. The capacitor has a big volume/capacitance and is expensive also.

Dc Capacitors are based on an electrolyte, that makes a very low capacitance/volume but works only on DC voltage because the electrolyte  can't isolate on both direction

[CatalinaWOW]

In addition, AC capacitors have to deal with significant current flow in and out of the capacitor each cycle.  Many DC capacitors do not, although those specified for switched mode power supplies and other similar applications will often have the most challenging specifications in this regard.

[Brumby]

This is the fundamental difference:

AC capacitor does not have polarity ...

Described as "non-polarized", they can handle a voltage of either polarity on the terminals and, thus a an alternating voltage.  DC is just a special case where the polarity of the voltage does not change, so you can use AC capacitors - as is - in a DC application.

NOW comes the big warning .... when it comes to capacitors used in MAINS circuits, you must be very careful to get properly rated capacitors.  A prime example is the X and Y rated capacitors used for EMI suppression.  DO NOT get this wrong.



Just as an extra fun fact ... you can make a non-polarized capacitor by taking two polarized capacitors and connecting them in series - back to back.  Whether that's a good idea or not is another thing.

[David Hess]

In the past when using non-polarized capacitors like ceramic, film, and paper, it was common to use DC rated capacitors in AC applications including X and Y line applications with a suitable derating.