DIY Capacitors

I'm sure that rolling your own film caps has been discussed before, but I wasn't able to find such a discussion in the archives. I did a little experiment today:

I cut a few 1.25"x12" strips of aluminum from a baking foil roll. Two of these I sintered together at one end, and then I attached leads to my new foil pair and to a third length of foil. I used an iron set to 450C (840F) and determined that the mechanical strength of the joint exceeded the strength of the foil itself, so I would guess that the electrical connection was quite solid.

I then created a stacked-film layer using saran-wrap (vinyl film) as my dielectric. After carefully smoothing the layers to eliminate folds in the film, I attached one end of the stack to a 0.5" wide tongue depressor, and wound the stack around it. To finish the capacitor, I wrapped the roll with electrical tape, which pulled the roll tighter and increased the capacitance.

I estimate the film to be 0.25 mils thick. Based on a figure of 160V per mil, I estimated the voltage rating of this capacitor to be 40V, but empirical testing determined that at least a 50WV rating is reasonable at DC.

Measured value: 22nF
Measured DA: 0.4%
DF: ???
Current handling: ???

I would expect inductance to be reasonably low because of the stacked layers and wide foil. I wonder if the "coaxially wound" Multicap is simply a wound stack like this?

Now, I doubt people will rush out to laboriously wind saran-wrap caps, especially when the resultant capacitance depends heavily on the construction. However, a better dielectric (teflon tape?) could be used, as well as tin or copper film. Suddenly, this seems more attractive.

Any thoughts from the forum?
This is, in fact, how the boys of DIY Tesla Coil and DIY Rail Gun (and the like) make there own capacitors. Of course they use Aluminum Sheeting and Visqueen, and the whole roll is inside of a large PVC tube filled with Mineral Oil. They are usually constructing these because of a need for high voltage capabilities.
It seems to me that the little bit of money for a commercial cap is nothing compared to the labor of DIY caps.
Yeah its fun to say you made your own caps!

Usually too much work, though.

The tesla-coil folks build their own because the price of buying a similiar rated cap from a manufacture would be Mega-expensive.

Try pricing or even finding a 100 farad cap with 100kv ratings, ha-ha. Even, 10 farad caps are in the few hundred dollar range. Car audio guys pay 3-400 for those. 1 farad's are ~$50.

If your going for economy or multiple farad-size, non-critical spec's, make your own. If you don't have time just buy it and save yourself the headache.

Compare the market price with the time required/fun-value to decide. Anything less than a couple of farad, tops, is where I think most would draw the line (a exception given-car audio amps). The caps in the less than a farad range are all reliable, consistent and cheaper, not too mention more time-saving then DIYing.

Giant caps are great for zapping things (e.g. rechargable ni-cd rejuvenator's) too.
They'll just pick up the current in the air and store it too, until you short it on yourself/someone, he-he! Yeooowww!:eek:

I think people are confused about a couple of things.

1) I'm not a DIY capacitor evangelist. I just thought it was remarkably easy and worth considering. Reliability and stability are definitely major issues.

2) Aluminum foil + saran-wrap was just a proof of sense wasting expensive materials when cheap ones are available. The point was if you can do it with that, you can probably do it with better materials as well. I made a teflon cap later that day.

Why use teflon or polypropylene as opposed to mylar? I think that should be obvious: the dielectic has much more favorable properties.
Mark Finnis and I were discussing this in another thread a long time ago. If I recall correctly, we were talking about rolling them, as opposed to stacking them. No, I don't remember which thread, and I no longer have the time or patience to wade through the reams of stuff the search engine coughs up.
Three points in passing:
1) Cleanliness is a necessity. A bit of grit can puncture your plastic film and cause the cap to fail.
2) Kapton and other films are available surplus on the Web. Can't remember where offhand, but I'll try to wake up a brain cell and figure that part out if anyone is interested.
3) Jocko's point is a good one. You might consider dipping the cap in something--wax, perhaps, or some of this vinyl stuff that you use to recoat handles on tools. Another possibility would be to cast the finished cap in resin. I've potted guitar pickups in resin before with excellent results. Incidentally, ordinary tempra paints make perfect dyes for resin; I used black for the pickups, but there's no reason you couldn't use red or blue or whatever suits your fancy.
Go for it.

GRollins, I did roll these caps...I created a stack instead of a two-layer roll because it seemed easier to increase the capacitance that way as opposed to making the "plates" longer.

Jocko, will inconsistent separation of the plates resulting from uneven tension on the film cause bigger problems than inconsistent capacitance? I did make a really terrible 5nF cap from teflon, but I think much of the problem was that I didn't really have the kind of tape I wanted to use.

I would guess that it is static attraction, but if you have a very smooth, clean piece of foil and smooth the teflon against it, it will stick to it slightly, which makes rolling much easier.
Teflon film source


McMaster-Carr sells PTFE film.

for a 6" wide sheet:

0.002" $0.71 a foot
0.005" $1.31 a foot
0.015" $3.26 a foot
0.125" $17.70 a foot

rolls of 1 to 20 feet.

They also sell PET, and polyethylene sheets.

Maybe the 0.002 would work??

Hey anyone tried parchment paper as a dielectric?
<1mil is probably best for low-voltage caps, but I think the 2mil will work fine.

I thought about paper too but was concerned about combusting caps if the dielectric fails its hi pot test. ;) Maybe impregnate the paper with mineral oil?

I think even parchment paper is significantly less compliant than polymer films, which would make achieving a small, consistent gap between the plates harder.
The sound of capacitors

Into the mail this afternoon arrives the new (July 2002) issue of Electronics World with an article, by Cyril Bateman on "Capacitor Sounds". This is, apparently, a two-part series with the initial installment devoted to the design and construction of a very low distortion (<0.001%) Wien Bridge Oscillator for testing caps (of course, AD797's (as well as the ADI SSM2018, a recent freebie from the folks in MA) are used!). I guess the oscillator in my HP339a won't cut the mustard at 0.0018%
Don't underestimate paper

I repaired one that was no more than heavy aluminum foil and waxed paper. I was out in the boondocks with a table saw that wouldn't run because the cap was damaged, and there was nowhere nearby to get a replacement. The waxed paper wasn't much thicker than that used for cooking (freezing?), and was good to at least 200V.

I'm curious if anyone is willing to roll their own electrolytics, or rather stack them. Stacked thay'd have a much lower inductance, but that brings in another slew of problems.

A question that's only slightly relevant here, can anyone direct me to some info on hand rolled torroid transformers with a tape-wound core?


2002-05-17 8:48 pm
Re: Don't underestimate paper

L Daniel Rosa said:
I repaired one that was no more than heavy aluminum foil and waxed paper. I was out in the boondocks with a table saw that wouldn't run because the cap was damaged, and there was nowhere nearby to get a replacement. The waxed paper wasn't much thicker than that used for cooking (freezing?), and was good to at least 200V.

Next time a motor that uses a cap to start won't run, try manually starting it. Works every time! :D