1 Aug
2016
1 Aug
'16
4:08 a.m.
At 04:03 AM 8/1/2016, drlegendre wrote:
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.
There are two principal situations where non-polar electrolytics are used:
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.
Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html
1 Aug
1 Aug
9:56 a.m.
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
5:48 p.m.
">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.
--Chuck
2845
days inactive
2846
days old
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