L
Larry Brasfield
- Jan 1, 1970
- 0
Ken Smith said:Ken Smith said:[...]
Methinks that common ceramic capacitors are not
actually piezoelectric.
I think, you think wrongly here. Ceramic capacitors with high K materials
are often enough piezoelectric to make them a bad idea for small signal
work. They make a voltage if you thump on them. This makes your circuit
into a bad microphone.
If you define "piezoelectric" loosely enough, then your "bad
microphone" is relying on piezoelectric effect. By such a
loose definition, even vacuum is piezoelectric.
True but I was not defining it loosely as indicated below.
Ok, I am going to have to take a more nuanced position here.
You and I were addressing two different phenomena.
Your issue is with capacitors which have, in fact, developed a
piezoelectric characteristic. (Not terribly surprising since high
K capacitors are made of materials similar to those use for
piezoelectric tranducers.)
My issue is with incorrectly calling any instance of mechanical
transduction in a capacitor "piezoelectric". As my point about
vacuum should make clear, that usage is sloppy and at odds
with the accepted definition. (Of course, with enough usage,
sloppier definitions become accepted. Such is language.)
[...]What do you think happens when you thump a capacitor
that is not biased?
As stated elsewhere, I got tens of uV of signal from a non-biased
capacitor by dragging my fingernail along the PCB material near it.
I might squirm out here by insisting that your capacitor was in fact
biased, even if you could not measure DCV at its terminals. That
would be a fair description of the condition of a poled PZT, (from
which your capacitor was likely made). But that would not be an
out, because my original position, "[C]ommon ceramic capacitors
are not actually piezoelectric.", is not quite right, as your experience
shows. So I will grant your point without negating the essential
truth of what I was stating.
Your poor microphone, at some time during its life, transitioned
from being a mere capacitor into a piezoelectric device. This
most likely occured when it was held under bias for a long
period of time. (It could occur in a short time at a temperature
higher than most electronics see.) It became poled, probably
not very well since it was inadvertant. You may have seen or
heard of dielectric absorption. High K capacitors are also
subject to it, and until their internal and only slowly available
charge is gone, they will be slightly piezoelectric, as your poor
microphone undoubtedly was.
You may also have heard biased capacitors, when subject to
AC currents having components in the audio range, make a
sound. In all likelyhood, that sound is not due to piezoelectric
effect. (A very small fraction of it could be, but not enough
to reach audibility.) It is due to simple compression of the
dielectric, by opposing charges, which varies in degree as
the AC occurs.
At any rate, under your relentless, pinpoint attack, I must
alter my position: Common ceramic capacitors are not
piezoelectric simply because they are made from materials
used for their ablity to be made piezoelectric. They may
become slightly piezoelectric for that reason, but without
being poled either above their Curie temperature or for a
long time, they cannot exhibit strong piezoelectric effects.
Nevertheless, weak piezoelectric effect is a hazard to be
aware of with all capacitors and especially high K ones.
How about now?
How about not sufficiently nuanced?