Dielectric Absorption

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
KMN
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Re: Dielectric Absorption

Post by KMN » Fri Jul 20, 2018 9:47 pm

good point, and it will be differential noise too. Not to mention the coil would need to be very near to other circuitry it could easily interfere with.

That is definitely on my mind. I'd be looking for a low loop area coil and possibly try and come up with some kind of small, grounded steel shield box to hover it inside of.... And give all the rest of the high impedance sensitive front end it's own shielding.

Since I can make it in my garage I can make something I'd never be able to afford to mass produce.

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mediatechnology
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Re: Dielectric Absorption

Post by mediatechnology » Fri Jul 20, 2018 9:55 pm

The values Joe and I came up with IIRC were around 10µH in the mic preamp but to implement an RIAA pole would be expensive and hum-prone.

KMN
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Re: Dielectric Absorption

Post by KMN » Sat Jul 21, 2018 1:10 am

It'd be somewhere around a mH.

In order to get the best emi immunity, wire gauge should be as small as possible to enable optimal constructions of a smaller loop area. Up to maybe ten ohms or so series coil R is acceptable. Multilayer is fine.

What I've seen, off the shelf looks nowhere close to optimal for my app. I'd be winding it myself for sure. At least it seems like the construction would be as simple as it gets.

Might not be too bad, from one of my sweat shop suppliers, if I ever got the quantities up there. haha

This would be the type of thing it would be easy to kludge in and try but it sure sounds like having a plan B would be wise, in case this didn't work out.

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JR.
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Re: Dielectric Absorption

Post by JR. » Sat Jul 21, 2018 11:29 am

if you are winding it yourself is there any way to make it hum bucking (cancelling), without altering the primary function?

JR

KMN
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Re: Dielectric Absorption

Post by KMN » Sat Jul 21, 2018 10:36 pm

Hmm, or maybe I should say humm... I hadn't thought of that. I don't know how I'd stop myself from trying it now. Great idea!

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mediatechnology
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Re: Dielectric Absorption

Post by mediatechnology » Wed Oct 31, 2018 3:34 pm

As I rebuilt the workbench I kept this capacitor in a "special place."

1.04 Volts @t=220 days.
We started at about 1.5V.
Slow discharge...

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JR.
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Re: Dielectric Absorption

Post by JR. » Fri Nov 02, 2018 4:10 pm

This capacitor that refuses to discharge still bothers me...

it is as if there is a nonlinearity in the leakage path (or error related to voltmeter)?

JR

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mediatechnology
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Re: Dielectric Absorption

Post by mediatechnology » Fri Nov 02, 2018 6:57 pm

I grabbed about a half-dozen other radial leaded capacitors that sit in my workbench caddy.

Some had been used in the last few months others I dunno.
The ones that had been used may have had their leads shorted as they sat in the caddy.
When measured with the same meter, all read in the mV range.
The largest was about 59 mV it was the same brand, value and working voltage as the one retaining charge. (47/63)
Some were 5-10 mV and one read 0V.

I also hooked up the "dielectrically-absorbed" cap to the 'scope (X1 probe 1MΩ input) input and it reads about 1VDC.
I think this cap is special in that it retains charge well but most of the others that I grabbed had been used for experiments and had retained some charge.

EDIT: BTW I only measure it for a second or less because the meter or scope does begin to discharge it. The DVM takes a reading or two to stabilize. On the scope I could see the discharge begin immediately. Curious to see if after "a few days of rest" it crosses back over 1V. This cap sits perched 2-3 inches from a monitor with a CCFL backlight. Could it be energy-harvesting a field to reduce its discharge rate?

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JR.
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Re: Dielectric Absorption

Post by JR. » Sat Nov 03, 2018 2:32 pm

I love me the internets...
wiki sez wrote: Charging a capacitor (due to a voltage between the capacitor plates) causes an electric field to be applied to the dielectric between the electrodes. This field exerts a torque on the molecular dipoles, causing the directions of the dipole moments to align with the field direction. This change in the molecular dipoles is called oriented polarization and also causes heat to be generated, resulting in dielectric losses (see dissipation factor). The orientation of the dipoles does not follow the electric field synchronously, but is delayed by a time constant that depends on the material. This delay corresponds to a hysteresis response of the polarization to the external field.
When the capacitor is discharging, the strength of the electric field is decreasing and the common orientation of the molecular dipoles is returning to an undirected state in a process of relaxation. Due to the hysteresis, at the zero point of the electric field, a material-dependent number of molecular dipoles are still polarized along the field direction without a measurable voltage appearing at the terminals of the capacitor. This is like an electrical remanence. The oriented dipoles will be discharged spontaneously over time and the voltage at the electrodes of the capacitor will decay exponentially.[2] The complete discharge time of all dipoles can be days to weeks depending on the material. This "reloaded" voltage can be retained for months, even in electrolytic capacitors, caused by the high insulation resistance in common modern capacitor dielectrics. The discharge of a capacitor and the subsequent reloading can be repeated several times.
My problem is literally thinking of leakage in a capacitor as a resistance... The dipole orientation is similar to magnetic domains but inside a capacitor they get aligned by a voltage field, and then relax back to no terminal voltage... there is hysteresis surrounding this relaxation, not unlike in magnetic domains, so no strong mechanism causing them to perfectly completely relax, like a real leakage resistance would fully discharge a capacitor given reasonable time.

This is slowly making more sense.... (I love the internets). :D

JR

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