(Yeah, but is there a difference between metal foil and metal film?)
I thought it might be a bit difficult without any support between the plates, yes. How about a ring of polypropylene along the outer edges? If the foil is thin enough to crinkle easily, maybe multiple rings, enough to support the plates but thin enough rings that their higher d.e. constant is negligible? Alternatively a spiral of PP.
Also, how much benefit would I get by using rhodium-plated (or gold-plated since I don’t need a perfect lattice surface in a cap…) copper or silver plates, versus something less ideally conductive like aluminium? I obviously don’t want oxide on the plates, well, silver oxide wouldn’t be an issue due to its conductivity unless we’re talking about tarnish…
(unplated silver > gold-plated anything? Dunno.). I guess oxidation is avoided in polymer dielectric based caps since there’s no interface with air anyway.
Uhh, hmm, I don’t have any readily available facilities. Frankly, I don’t even know what type of company exactly I should contact to have this manufactured once I eventually have the idea fully formulated.
So you could advise me on that, who to contact for a custom cap with self-made schematics, provided it’s not going to require aerospace level engineering (surely it won’t)?
I have followed this thread with bemused interest, since the only metal plate capacitors I have ever encountered were the variable capacitors used to tune old radios. It sounds like the OP is going retro with a vengeance!
My interest too - I played with many old radios as a kid
I recall setting up two parallel metal plates in a physics lesson once, and measuring some sort of charge on it, but it was pretty small.
@Arenith - keeping a 1 metre square piece of metal a tenth of a millimeter away from another such plate without using a film of some sort between them seems fairly aerospacey to me, though I may be wrong.
If you are in the USA I would hope there will be some folks could chime in here with contacts?
Hmmmmm.
Well, wow. My intuition has led to the principle roots of antiquity radio capacitors…
It’s good to start from the roots and then from the ground up, right?
What kind of other applications do moderately powerful parallel metal plate caps have other than radios? Might I use them in amplifiers?
well capacitors are general purpose circuit elements that crop up all over the place, radio tuned circuits (the old variable metal plate capacitors being the perfect example), radio frequency amplifiers, audio amplifiers, and others too.
different requirements suggest different max voltage, different capacitance values etc. etc.
air gap capacitors are almost certainly the rarest, as they are the hardest to make stable, and the hardest to make usable values for above a few hundred pF.
you usually see values in audioelectronics of several uF - up to a million times more capacitance than the typical air gap ones in radios.
the ways of moving from pF up to uF being increased plate area, decreased plate gap, both of which suggest a film dielectric that can then be rolled up with two foil plates into a sensible sized package.
i expect wikipedia has lots of info on this, and probably suggests why no one makes air gap capacitors, due to their impracticality in most usage requirements.
i’m genuinely not trying to put you off, i look forward to seeing some photos of nicely engineered bespoke air gap capacitors in an external speaker crossover for example, but do not underestimate the engineering involved
air-gap caps are also limited in voltage handling, because arcing across the plates is highly undesirable, and the insulating ability of dry air is only about 70kV per inch of gap.
So would the minimal dielectric constant difference (and other properties) between air and vacuum give way to much better voltage handling? If vacuum isn’t the best cap gap medium, then what is…?
Well, that’s helpful to hear, to give it up, but NOW I’m better off to learn about vacuum constructions.
Vacuum, vacuum, kindly give me way, I promise I’ll stay, vacuum come my way…
So just for a final clarification, are vacuums and perhaps some gas mixtures other than dry air the only options of keeping a metal plate capacitor stable?
Really there’s not that much engineering for me to do if I want this done, unless I want it in a compact size. A custom-made vacuum tube without a heater and a minimal plate distance, is that it…? I know it’s expensive and superfluously bulky, I just want something as simple as possible.
Do the math. What value of capacitance do you need? Use the formulas above to see what size you will need. You’ll likely find that superfluously bulky doesn’t really describe the size…
Let’s say you need 100uF (just a wild guess.) Say you can somehow keep the plates exactly 0.1mm apart. Then you’d need square plates that are approx 1351 sq yards or 36 yards on a side.
You can’t get that number down to a reasonable size without a better dielectric, titanium dioxide could bring it down to approximately four yards on a side. To get to a reasonable size you’d need a quality dielectric like those used in commercially available capacitors, hmmm, I wonder why that is?
I didn’t mean to sound snarky (if you took it that way.) I, of all people, know what it’s like to swim against the tide of conventional design, but all down this thread we’ve been bringing up that size matters and have given the formula for size multiple times. We’ve also given links to good resources explaining this and other relevant things (like dielectric breakdown.)
I’ve learned a lot by doing thought experiments and then trying to build things. Usually you find out why everyone else does things a certain way. But now and then you learn enough about your subject matter to do something non-conventional that better suits your needs. The key is that it takes a lot of work, not just asking some questions on-line. Seek out information about capacitors and read it (https://en.wikipedia.org/wiki/Capacitor). If you don’t understand something that seems to matter seek out good educational material about electronics (say at the Kahn Academy: https://www.khanacademy.org/ or MIT OpenCourseWare: https://ocw.mit.edu/index.htm, or even your local library or secondary education institutions.)
Thanks Ted, no, didn’t take it that way, haha. I appreciate your time to put this into proper perspective.
I must have done the initial math (seriously) wrong here, square yards, yeah, nope. Understandably even square meters aren’t exactly manageable.
I’ll ask more about capacitors when I’ve learned what a capacitor is, let’s say.
)