Doesn't exist old chap.
Excuse egg sucking exercise!
DC has a + and - supply, a line of connectors of lets say + terminations would be called a + rail or a +bus. In DC parlance + and - are also called Positive and Negative.
AC has a live and neutral, quite why as half the time it's one way and the other half the other way <dizzy>
So, if the stay-alive is on the DCC side of the circuit it will be an AC supply (highly unlikely as circuit boards generally don't like AC) if it's on the rectified side (highly likely as circuit boards like to get into bed with DC) then it will be DC.
If the stay-alive is AC (DCC) then you can only use a normal capacitor, if the stay-alive is DC then you can use a dielectric, the advantage of the dielectric is that for it's capacitance value it is significantly smaller in size than a normal capacitor. The bigger the capacitor the longer the stay-alive circuit will work for when supply is removed, as Tim notes above the bigger the capacitor the better, that's capacitive value not physical.
If you had a normal capacitor of say 30uF it may be the size of a baked bean time, the equivalent in dielectric would be AA battery sized, now! If you had space for a baked bean tin, image the power you'd get from a capacitor that was dielectric!
Our cranes use dielectric capacitors on the DC bus to smooth the rectifier ripple, size wise they are about two baked bean tins stacked end to end, touch the terminals on those and basically you'd be lucky to be alive. The ends are designed to blow out on fault, I've seen an end go through 3mm perspex. Dielectrics are superb power houses, just have to remember they have a little bit of a bite if you mishandle them
and they will store their energy for weeks, even months, which is handy to know if you rummage around in a draw with a load of them in there!.
