drlegendre at gmail.com
Mon Aug 1 19:48:16 CDT 2016
">Non-polar caps are used in locations where they must pass AC.
Conventional polarized aluminum and tantalum capacitors are also used in
locations where they must pass AC, such as interstage coupling capacitors
and bypass capacitors, not to mention electrolytics as power supply filter
capacitors. Any capacitor that could not pass AC would not be a capacitor."
That's not true AC in either case. It's fluctuating DC, or perhaps AC
riding on a DC bias equal to at least half the peak value of the waveform.
True AC switches polarity every 180' of the cycle. If you try to pass true
AC with an electrolytic, you will encounter the diode-like / rectifier
behavior that was also previously noted, whenever the voltage changes sign
to oppose the marked polarity of the cap.
On Mon, Aug 1, 2016 at 11:56 AM, Chuck Guzis <cclist at sydex.com> wrote:
> 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.
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