On 08/01/2016 04:08 AM, Dale H. Cook wrote: > 1) When there is no constant DC offset to the applied voltage - if > the voltage applied to an electrolytic capacitor reverses the > capacitor can be damaged. When there is no constant DC offset the > second reason can come into play: > > 2) When a large value non-polarized electrolytic can, as previously > noted, cost less than a large film, oil-filled, or other non-polar > capacitor. > > Some of us still deal daily with analog circuitry at work.
There's one other aspect that I believe is germane to this particular application: Linearity. Aluminum electrolytic capacitors are very curious beasts--they're not all capacitance, but rather operate as several different "pure" properties hooked in series. Most people here know about ESR, but there's another one--wet aluminum dielectrics also function as if they had a leaky diode in series with the series resistance and capacitance. This is a result of the way they're constructed--basically two aluminum foil strips, separated by a membrane (often paper) saturated with an electrolyte (commonly borax). A DC current is applied to "form" the capacitor and establish its polarity. Probably nobody here is old enough to remember liquid rectifiers, but my own father related them to me (he grew up in a poor family during the teens and 20s). The idea was that you took a plate of aluminum and a plate of tin, lead or other metal and immersed them in a solution of sodium bicarbonate, borax or some other electrolyte (dad used ammonium sulfate, a waste product of the nearby steel mills' coke ovens, and ran an AC current through them. Eventually, the device polarized and formed a hydroxide/oxide layer on the aluminum and developed a preference to current direction. The sparkbangbuzz website has some interesting observations on this animal. For use in audio circuits, this "series diode" effect can lead to distortion, hence the use of a non-polar device. Take an aluminum electrolytic and put it on your curve tracer and the effect is quite visible. For what it's worth, I don't know if tantalum caps exhibit the same behavior, but I suspect that they do. --Chuck
