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Destroyer OS. 19th June 2011 09:51 PM

Adding capacitance to epoxy?
 
Anyone got any ideas how to add capacitance to epoxy/glue? Any liquids that harden? It needs to be able to be colored as well. The level of capacitance is very little, about as much as human flesh holds.

john dozier 19th June 2011 10:00 PM

There are some silver-bearing expoxies. Don't know how much capacitance they would add though.

cbdb 19th June 2011 10:11 PM

It needs to conduct, and have 2 layers with insulation in between. Good luck!

benb 19th June 2011 11:30 PM

I get the feeling there's missing info in this question.

dchisholm 20th June 2011 01:38 AM

Capacitance is directly proportional to a material characteristic called the "dielectric constant" of the insulating material between the conducting elements of a capacitance.

As best I recall some information from a class about 2 decades ago . . .

Common epoxy resins have a dielectric constant on the order of 2.5. When the resin is filled with other material (fiberglass, mica, cellulose, etc) to make printed wiring board substrates, the dielectric constant can be increased up to around 5 or so, depending on the filler material, proportions of resin to filler, etc.

Common window glass probably has the highest dielectric constant (about 8, or a bit less) of practical materials that are readily available to mere mortals (that is, those of us who don't routinely write multi-thousand dollar purchase orders to specialty chemical companies). Interestingly, tempered and high-strength glass has lower dielectric constants than the cheap stuff. I don't know whether the dyes and stains used for artistic glass work have significant effect on the electrical properties of the glass.

Dale

Destroyer OS. 20th June 2011 02:58 PM

dchisholm what is the dielectric constant of human skin?

Perhaps searching for dielectric constant properties will help me stumble upon something that will work.

This is going to act as a skin replacement for a certain application for people no longer able to use theirs. Those are all the details I think I can provide.

benb 20th June 2011 06:36 PM

If it's going to be on a human, such as replacing skin on fingers, then it needs to have a slight electrical conductivity to operate some types of "touch sensitive" electronics. Also, It needs to be electrically connected to actual skin (of a whole person) for a touch panel to detect a touch and the capacitance of a human body.

The human body is a large thing (like maybe a file cabinet) and has, as I recall, something like 100pF to 150pF as measured between it and ground. Many (some?) electronic touch panels use the increase in capacitance when someone touches it with their finger to detect when it is being touched. Others use the increase in capacitance between two adjacent plates caused by a conductor (a finger, with blood just under the skin that conducts) put in close proximity.

A pencil erasor won't activate capacitive-sensitive panels. A piece of tin foil or aluminum foil over it (with or without a thin insulating sheet over it) might. The piece of foil connected to a wire that the other bare end touches your skin will (or I think should) activate it. That should give you something to experiment with to see what will work and what won't.

So it seems to me the dielectric constant of skin is a red herring. If you're working on artificial skin to go on a robotic hand to be worn by a person so they can operate iPods and iPhones, you need a conductive foil or conductive something that goes to a wire that connects to the person's skin. It's the whole body that has the measured capacitance, not just the skin.

Hope this helps, have a nice day, et cetera.

Destroyer OS. 20th June 2011 08:33 PM

That does help greatly. My understanding is now that unless the specific spot can hold a charge of 100pF (which is a capacitor that is pretty good sized on it's own) it will not work.

I have heard of a paint that works, but that is not too practical.

If you grab metal and rub it on one it does not seem to conduct the capacitance. Could it be any form of metal too big is essentially stealing the capacitance instead of allowing it to be conducted? Indeed this does sound like a red herring but if solved the importance of it is not to be underestimated.

Your help is much appreciated.

DF96 26th June 2011 07:32 PM

Quote:

. . specific spot can hold a charge of 100pF . .

If you grab metal and rub it on one it does not seem to conduct the capacitance. Could it be any form of metal too big is essentially stealing the capacitance instead of allowing it to be conducted? Indeed this does sound like a red herring but if solved the importance of it is not to be underestimated.
Charge is measured in coulombs, not farads. You seem to be confusing charge and capacitance. Hence your statement does not actually mean anything. Would you like to try again?


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