equivalent circuit

Hi Folks
I have posted this question earlier, but havent received convincing replies on it yet, so thought of putting it up again.

I have a parallel plate capacitor consisting of two metal plates (M), each coated with a thin layer of insulator (I). The coated plates are then placed at some distance from each other. Through the separation between them, a conducting fluid (S, saline with dielectric constant 80) is allowed to flow through.
So, effectively I have the schematic as:

M | I | S | I | M

and a sine voltage (100 mV p-p, 1 KHz) is applied between the two plates.
What would be the equivalent circuit model for such a system.
I am assuming it will be


-----| |--------| |--------| |------
| |
|--vvvvvv--|

which is three capacitors with a resistance in parallel with the middle capacitor.
The first one is the capacitance due to the coating on plate 1. Second capacitor is due to saline, in parallel with conductance of solution and third is again capacitor due to coating on plate 2.

 

What sort of insulation coating is on the metalic plate? Is it some sort of paint, or an anodic film?

 

Its a very thin layer of spin-on-glass (which can be spun on the substrates). SO, it should be like a layer of paint. It has good dielectric properties.

 

Then I think you have two capacitors - each one being composed of a plate, the insulating film as dielectric, and a conductive solution which is effectively the other plate.

An electrolytic capacitor is made the same way, ie. an aluminium plate as an anode, an anodic film as dielectric, and an aqueous solution as the cathode. In contact with the aqueous solution is an aluminium plate, often refered to as the cathode, but actually put there to give a low resistance conductor for the aqueous solution.

So I think your equivalent circuit is two capacitors and a resistor, all in series.

 

It could have been considered as an electrolytic capacitor if the second plate was coated and exposed to the conducting media. however, what i have is both the plates coated with an insulating layer and conducting liquid flowing between them. Therefore, no electrode is in contact with the conducting solution.

Please post your comments.

 

The equivalent circuit you posted looks correct to me.

 

I was not saying you had an electrolytic capacitor. I was giving the electrolytic as an example of how a conducting fluid can act as a plate of a capacitor.

You originally said ".....The coated plates are then placed at some distance from each other.....". From that I assumed that 'some distance' meant far enough apart so that the dielectric films had no effect on the other, ie. the capacitors could be treated as separate entities.

That being so, you have a capacitor made up of plate/dielectric/plate-of-conducting-fluid, connected via a conducting fluid (resistive), to another capacitor made up of plate-of-conducting-fluid/dielectric/plate.

So effectively, you have two capacitors with a series resistance connected between them.

 

I probably misinterpreted the information provided by Pebe. Thanks for all your inputs so far folks.

 

It seems to me that, since the fluid has a very high dielectric constant (80), there may be significant capacitance in parallel with the resistance of the fluid. If this is true, shreyas's original equivalent circuit is correct.

 

This thread is quite interesting, and gives food for thought.

You may well be right. Shreyus says the saline solution has a dielectric constant of 80, but gives no details of size or separation for the plates, so it's not possible to be specific. But can we assume, for the sake of discussion, a max capacitance of around 100pF? If so, its reactance at 1KHz will be 1.59Mohms. Meanwhile the resistance of the saline solution will possibly be of the order of tens or hundreds of ohms.

I never considered a conducting fluid as being a dielectric. So thinking more about what constitutes a capacitor, I Googled for 'capacitance definition'. There were various results – all in similar vein - but the first hit put it in a nutshell. It said:

"The property of a circuit element that allows it to store an electrical charge.
www.crucial.com/library/glossary.asp"

I could possibly accept that a rubber balloon containing a saline solution and sandwiched between two metal plates may constitute a capacitor. But if the plates are in contact with the saline solution, then there is a leakage path and (at 1KHz) the rate of discharge may well exceed the rate of charge. Thinking more about it - if the device cannot accept a charge, it cannot be a capacitor.

A dielectric should be a good insulator in order to hold a charge, and the concept of a conductive fluid as a dielectric is (to me) quite unique. So I am trying to put this into a comparable context. I am thinking of two submarines submerged in a (saline) ocean. Does a capacitance exist between them? If so, how could it be proved by measurement?

 

Hello Pebe

I am amazed at the quality of information offered in this discussion. Thank you all for that. To be more specific, the "thickness" of the conducting liquid layer (with diel. constant 80) in between the insulation plates is 0.5 mm. This is in contrast to the 0.01 mm of dielectric coating (diel constant = 6) on each plate.

It can be inferred from Gauss' Law that majority of the electric field (resulting in capacitance) should be due to concentrated in the non-conducting coating layers and little in the conducting fluid region. However, the solution should definitely contribute in a small way to the effective capacitance of the system.

Please post your comments. Your visualization of the rubber balloon was truly remarkable.

 

Hi Shreyas,
Well, there’s very little I can add, really. As I said, capacity may exist but I cannot visualise a capacitor that uses a conductive fluid as a dielectric – even less can I see how one would measure it.

From a theoretical point of view I suppose you could consider the plates of the capacitor to be the surface of the solution in contact with your glass dielectrics, and the rest of the solution in between as the dielectric.

If you wanted to do some calculations, you could calculate the capacitance of a given size pair of plates with the 0.5mm separation and a dielectric constant of 80. Then calculate its reactance at 1KHz.

From the specific resistance of the saline solution you could calculate the resistance between the plates, so you would then have the equivalent parallel circuit.

I suspect though, that the resistance would be several orders of magnitude lower than the reactance of the capacitor and the combined impedance would be almost entirely resistive.