What are the downsides of DIY foil capacitors? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: March 12, 2025, 05:04:54 am
content: So some of you probobly made DIY paper capacitors or whatever a long time ago for HV experiments.But lets say you put in a modern attempt at rolling your own with modern materials, and you are not particularly concerned with size efficiency or appearance (i.e. the end terminations can be made with silver epoxy or every layer can get a tab that sticks out of the roll and be soldered together)What are the bad properties of such capacitors, i.e. if you try to characterize them on measurement equipment, when compared to a normal commercial foil cap?There is some unique structures you can make, like a center tap capacitor (using three materials rolled together), or weird shapes (Imagine rolling a triangle up, inspired by the conical inductor)Long ago my attempts at making DIY cardboard tube from kraft paper were actually kind of OK...Things that come to mind1) possibly increased microphonics2) some kind of drift because tension might not be controlled too wellAnd, what would be the best materials for this. Does anyone sell capacitor grade metalized foils? Or are we left with using a separate dielectric and conductor? Is adhesive backed material something to think about for this purpose?

author: paul@yahrprobert.com
author_rank: Regular Contributor
timestamp: March 12, 2025, 08:58:29 pm
content: I once tried to make my own capacitors in a lab setup involving pretty high power RF, using copper foil and polypropylene film. I learned a pretty good lesson in corona induced breakdown of the dielectric. After the breakdown I unrolled my capacitors and found a very pretty fractal pattern in the polypropylene where the arcs tracked across the surface. You can't let air exist in your gaps. Either vacuum impregnate with something like RTV, or put in a layer of oil/paper and wind them tight.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 12, 2025, 09:22:06 pm
content: The main problem is air pockets that can cause a local partial plasma formation. Usually capacitors are much easier to get than the foil to make one.The air issue is less of an issue at DC - still usually not worth it. If at all very small air gap ones.

author: coppercone2
author_rank: Super Contributor
timestamp: March 12, 2025, 09:33:35 pm
content: Ok I was not expecting that one.But what if its low voltage stuff, and signals. Like low noise supply etc. Or filters, or differential filters, etcI also wonder for air gap what would happen if you put soap water on it and rolled it up then vacuum dried it, could that get rid of that problem. Or, rolled it under water. 100% of the stupid bullshit air problems with dry films goes away if you assemble it underwater.Actually, I wonder if using a laminator to laminate copper foil would get rid of 1/2 of the problem you have, if you can't get factory bonded metal/dielectric film.I am way more interested in the signal or low power side right nowIt sounds like a high voltage capacitor rolling machine would need to have a micrometer adjustment on the spool to ensure it does not skew.I wonder how they align it. I would use a indexed base with two microscopes on it to check for total paralelism, ideally, but if your filling that area with glue it does not matter too much

author: TimFox
author_rank: Super Contributor
timestamp: March 12, 2025, 10:18:26 pm
content: In general, the weak point in high-voltage constructions with parallel metal-air-dielectric construction is the path along the interface between gas and solid.Note that in the normal direction (perpendicular to the surfaces), theEfield in a thin air gap is higher than that in the dielectric layer by approximately the dielectric constant of the insulating dielectric.(NormalDis conserved across a discontinuity.)

author: Perrold
author_rank: Contributor
timestamp: March 14, 2025, 04:35:46 am
content: Rolling your own high voltage capacitors is usually not worth it; rolling your own low voltage or low signal capacitors is absolutely not worth it.Purposely flooding HV capacitors or transmission lines with a dilute copper sulfate solution is one somewhat counterintuitive approach to dealing with air gaps and field gradient concentrations. Sure the leakage goes way up but when these components see a few kA or more for a few tens of ns or less the leakage current doesn't matter too much.