Capacitor Design Questions

[dmr4]

I am developing a large scale capacitor to determine the effect of electric fields on tissue samples. The capacitor is a formed by two parallel plates (made of stainless steel), using an overlaping method to reduce edge effects. I am using a high permittivity material (foamglas) in between the plates that will increase the total capacitance of the system, and allow us to use smaller applied voltages to obtain electrical field requirements.

What are some considerations I should take into effect on this?

I feel that heat might be as issue, but without a current I don't know how heat would be generated?

Any incite on this would be very helpful.

Thanks!

 

[dpc]

ac or dc?
frequency?
voltage?
capacitance?

 

[dmr4]

Frequency = 60 Hz, we want to maximize the capacitance so that we don't need to apply voltages higher than the power supply can generate. So Voltage must be less than or equal 20V. I'm fairly certain it is AC, the research specified a sinusoidal voltage, and I think that can only be done using AC voltage.

 

[magoo2]

I'm wondering why you're using stainless steel plates and foamglas for your application.

There are standard power capacitors available, but these use aluminum foil for the plates and have a dielectric fluid between the plates for medium voltages. For low voltages, the standard is now a metalized film, where the metal is deposited on the PPE film. It's a fairly compact design to get high amounts of capacitance in a fairly small can.

Why the special design?

 

[geekEE]

You will get some current flowing because you are using AC. However, with your low frequencies, low voltage, and relatively large distances between small plates (I assume), your current will be very very small.

I assume the samples will be going between the plates. If so, then why use the other materials between the plates? Or am I misinterpreting what you're trying to do?

 

[dmr4]

Magoo2:

The design is unique because we are placing petri dishes between the capacitor plates, that must have an air supply and cannot be subject to contaminants. The stainless steel was chosen because the last iteration of the project used stainless steel plates (which I was not on the design team), I believe this is the case because the system must be sterilyzed and the stainless steel wont rust. I chose stainless steel because of it's relatively inexpensive nature and ease of machinability. And I chose foamglas because the dielectric constant was relatively high at the frequency which our system was running, and it is available locally.

geekEE:

I am placing the other materials between the plates so that I can maximize my capacitance. According to the formulas I have come across if I have a large dielectric constant (with all other things being held constant) I should also yield a large capacitance. Essentially I want to get my capacitance as high as I can so that I can use the readily available power supplies that I have.

All:

As you can tell my background is in mechanical engineering, and the knowledge that I have expressed is that of limited research and introductory level courses in circuits. So if I have said anything that is incorrect, or if my thinking is wrong please correct me. Thanks.

 

[dmr4]

Also,

The distance between the plates is going to be around 1inch to 1.75inches. The overall plate area is going to be maximized but is limited to the dimensions of the incubator which the system must fit into. Currently I am designing the system to have plate areas of approximately 144in^2.

 

[dpc]

Is there a design criteria for the electric field strength?

 

[dmr4]

yes, the electric field strength must be 20 mV/cm.

 

[dmr4]

Also, if I have multiple dielectrics between the plates, how do I model their behavior (i.e. resistors in series)?

Thanks.

 

[geekEE]

If your goal is to maximize your electric field, that's a different requirement than trying to maximize the capacitance. If you're putting foamglas between the plates everywhere except where the petri dish is, you will increase the capacitance, but won't significantly affect the electric field in the petri dish. If you are putting the foamglass between the petri dish and the plates, you are decreasing your electric field because of the increase in distance.

I'd say you should put the plates as close as possible. I don't see the need for the foamglass if all you're trying to do is maximize your electric field. It sounds like you'll be able to get many times your required electric field.

 

[MacGyverS2000]

Possibly the foamglass is there to keep the petri dish from slopping around inside the cap?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[dmr4]

The foamglas is to be the dielectric between the parallel plates and the petri dish. I figured that the large constant (90 I beleive) would offset the added distance between the plates. The dishes we are using are quite tall compared to those I have seen in the past (they are about 1 inch tall), so will adding 1/4 inch of foamglass to each side of the dish really cause that large of an effect and decrease my electric field?

Also, if not foamglas dielectric and stainless steel plate electrodes, what materials do you guys recommend?

 

[IRstuff]

20 millivolts or 20 MEGAvolts?

The field is going to be nonuniform where you ripped out the foamglas, so you might as well just metallize the top and bottom of the petri dish and apply the 20 V directly. That's the absolute highest field that you can achieve with your power supply.

A typical Petri dish is only about 0.75" thick, so that would given you a field of 20V/0.75in = 10V/cm. You're never going to do better than the voltage applied divided by the thickness of the air gap in the Petri dish.

Putting your voltage across a 1.75 in gap drops your field down to 4.5 V/cm. I doubt that there's a usable dielectric with a sufficiently high enough dielectric constant to make up for the decrease field strength. Seems like you'd need a dielectric constant on the order of 10 to do anything useful in terms of reducing the effect of the larger gap.


TTFN

FAQ731-376

 

[dmr4]

The I guess my question becomes, will the energy field only be constant across portions of this capacitor that have a uniform dielectric? Like will the electric field strength be constant across the glass portions of the petri dish, but differ once the e-field makes it to the air portion directly below the glass?

 

[IRstuff]

You still didn't answer the question 20 mV/cm or 20 MV/cm?

TTFN

FAQ731-376

 

[dmr4]

It is 20mV/cm