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In Reply to: RE: We've never really figured out just what parameter Mr. Bench is measuring... posted by Steve O on August 23, 2014 at 13:23:09
"Do you know for sure what he's measuring?"
No, not in the terms you're asking about. I've always assumed this to be an artifact similar to the "memory" that large caps exhibit. It's a little out of my field though, so I just take Steve's work at face value as demonstration of a non-linearity in the caps. Maybe "hysteresis" isn't the best terminology, but it seems to describe the phenomenon being portrayed in the tests.
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Pretty sure "memory" in a cap is more conventionally known as "dielectric absorption" and Bench is definitely not testing for that. Additionally, since DA is important in real world applications, there are well defined, standardized, objective test protocols to characterize it. No need to reinvent the wheel.
As an aside, I believe it's always wise to be a bit skeptical regarding claims for something when all the facts aren't clearly presented...or the claims haven't been verified independently.
It is not very clearly explained in that Steve Bench article in the link. But I suppose the essence of it is that he looks at the equation I= C dV/dt for the current passing through the capacitor, and relates that to the rate of change of the electric displacement D (as per the Maxwell equation curl H -dD/dt ~ J). So he has something proportional to dD/dt on the left-hand side of his equation, and something proportional to dE/dt on the right-hand side (since at constant plate separation, the electric field in the capacitor is proportional to the voltage). His "technical note" at the bottom of the article seems to suggest he is integrating the current to get D, and plotting versus V, which gives E. Thus his plots are supposed to be D versus E, I suppose, which would then be indicative of the constancy, or otherwise, of the dielectric constant as the applied voltage varies. A kind of analogue of the B versus H hysteresis curves in the magnetic case.Another way of putting it would be that by plotting the integral of I against the voltage V, his plot is showing how the capacitance C depends on the applied voltage (straight diagonal line = no dependence on voltage).
Chris
Edits: 08/24/14 08/24/14 08/24/14 08/24/14 08/24/14
"Another way of putting it would be that by plotting the integral of I against the voltage V, his plot is showing how the capacitance C depends on the applied voltage (straight diagonal line = no dependence on voltage)."
So yes, voltage coef of capacitance is what he's measuring (actually just displaying...no numbers involved).
The next question is how audible a large and variable V coef of C could be. Maybe spice has a C=f(V) capability so it could be simulated.
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