Homebrew Capacitors

[Bobby Ironsights]

Has anyone here made their own capacitors? I've been looking online, and there are only one or two posts on them, almost all of them people who are making high voltage caps for homebrew lasers.

I'm interested in making some very large capacity, low voltage, high amperage caps for a homebuilt spot welder that can handle thicker gauge metal than the Microwave oven transformer designs that are more common. I've had a chance to go over the capacitance chapters on the electronics club website, and it seems simple enough.

Is there any reason why I couldn't or shouldn't get some kraft or butcher paper, and some industrial sized rolls of aluminum foil from the bulk store and start wrapping up dozens of cap's and connect them all into a bank in a vat of mineral oil?

I understand that I wouldn't get the efficiency, or small size, or sealed convenience as those nice super capacitors from an actual manufacturer, but I have a fair bit of space in my shop, but not alot of money right now. I'd rather keep the expense under 150-200 dollars if I can, or I'll have to put this project on hold.

4 or 5 volts average, but a couple thousand or more of amp's for a second or two would be great.

The reason I think it would be nice to use capacitors instead of a rack of car batteries is that batteries have a limited number of charge and discharge cycles and doesn't take being fully discharged very well, which would be pretty unhelpful for a spot welder.

Thank you for your time and input,
Bobby

 

[Sceadwian]

Before you can even guess you need to determine the capacitance you actually need and the actual voltage you'll need to get the current for the time period you want, unless you come up with that first you're just guessing when you make these things.

For testing you could try air wrapping a few. I wouldn't use waxed paper though, try poly stretch film like the stuff used to wrap pallets.I've been meaning to try to home roll caps for just that purpose myself. Just don't have any space to do it.

The problem with rolled capacitors like that though is they're well... rolled, they have high inductance, which smooths the output. You'd really have to experiment with a roll of foil and poly film to see. You'll need to keep the foil and the poly wrap tensioned and wrapped very tightly with as few air gaps as possible. Cooling shouldn't be an issue if you're just spot welding, though you'll have to see what the heat build up is like with the foil you're using. For low voltages even the thinnest poly film you can find should work well. Just avoid the stuff that has adhesive on it (generally it's pink I think)

 

[caster.cp]

First of all: it's not the best of the ideas to build a high-voltage capacitor, even if you really know what you're doing. It's a lamer idea to construct one you have looked up on the internet. Capacitors may use things that may explode (if it heats up and enough pressure builds up in its interior).

That said, look for tesla-coil caps. I understand that you need high current, and not voltage, and tesla coilers work with HV. But there's a lot of info out there about building caps to make coils. I've heard one or two stories about caps (made of bottles, stay clear of those) that have exploded and did serious damage to some people. Don't know if they're true or not, but they all seemed plausible to me.

I just don't see how you intend to get thousands of amps out of a cap working at 4 or 5 volts.

i = C* dV/dt, right?

So, to achieve that sort of current, you should build your cap with thousands of farads (good luck with that, I haven't heard about any that get close to that number) or you would only be able to maintain high amps for microseconds, and not 'one or two seconds' (that seems a fairly small amount of time, to us).

My advice: I don't know if you really know your electronics or not, couldn't tell from your post. If you don't really know what you're doing, don't mess with high-voltage, high-current stuff. It's really, really dangerous. It could easily get you killed. It's not worth the hundred bucks you may save (and only may, because, generally, with some few exceptions, industrialized stuff are cheaper than home made projects).

Castilho

 

[tcmtech]

For $150 - $200 you can buy a good used commercial spot welder that will top any capacitive discharge system!
My Miller 2.5 KVA commercial spot welder can do anything from foil to two 3/32" mild steel sheets back to back and I paid $75 for it and another $100 for all new tongs and tips!
Welding anything thicker than 1/16" is regular MIG or TIG welder territory anyway.

 

[Sceadwian]

caster, it's generally a good idea to read the post before responding to it. The OP specifically stated that these were low voltage capacitors, only 4-5 volts. I didn't do the math but apparently you did, you're absolutly right in that you're never going to get that kind of capacitance out of anything home made, the amount of actual physical material you'd need would probably cost more than a used welder like tcm said.

tcmtech, on a side not but related, where would you look for a used but functional welder like that?

 

[gary350]

I have made several capacitors. A flat plate capacitor will store twice the power or a rolled capacitor of the exact same uf rating.

I have worked in industry for 40 years the factory spot welders don't have large capacitor banks in them. They work on a transformer that has 1 turn on the secondary coil. 480 volts AC on the primary and 1 volt or less on the secondary. One time I checked a spot welder with a volt meter just to see what the output voltage was and my meter was reading 1/2 volt.

They use a high current mercury tube as the discharge switch it can handle 60,000. amps at low voltage. The tube does not discharge until the part is completely clamped and locked in place.

Here is a pic of 3 oil filled rolled capacitors that I built. They are .01 uf 40,000. volts each. You can see then at the bottom of this pic. They are inside the 6" diameter white PVC pipes.

**broken link removed**

 

[flat5]

That is an impressive Tesla Coil !

 

[Sceadwian]

A flat plate cap will store twice the energy as a rolled of the same UF rating? Could you please substantiate that with some information because that makes no sense at all.

 

[smanches]

He must mean the same size, as the farad rating is the capacity of the cap.

 

[tcmtech]

tcmtech, on a side not but related, where would you look for a used but functional welder like that?

Watch your local HVAC and other larger local service businesses and fabrication shops "clean out" auctions. Many larger service businesses have a clean out their junk sale every few years or most often bring loads of their old stuff to the local auction houses every now and then.

Around here we have the Northern Auction business which is a sort of community consignment auction place. They run vehicles sales about every other weekend and general junk most alternate weekends and a few farm and industrial equipment auctions during the summer.

If you have a consignment auction place locally bring a $100 and a big pickup because you will never go home empty! (at least I never do) and I guarantee that if you eBay you will make your $100 back with interest and still have too much stuff left over that was just too good to resell!

If you know your "junk" and who wants what it can be excellent source of cash based side income.

 

[gary350]

A flat plate cap will store twice the energy as a rolled of the same UF rating? Could you please substantiate that with some information because that makes no sense at all.

If you make 2 capacitors both .01 uf each the flat plate capacitor will have twice the metal plate surface area as the rolled capacitor so it will store a larger charge of electrons.

If your making a low voltage flat plate cap it is very easy to do with wax paper and aluminum foil. Cut the foil 10" x 10 1/2" square. Cut the wax paper 10" x 10" square. Place a sheet of wax paper on the work bench, place a sheet of aluminum foil on top. Leave a space of 1/4" around 3 edges. The 4th edge will over lap the paper on the right side. Put another layer of wax paper and aluminum foil on top only difference this time put the over lap on the left side. Use a clothes iron to iron it flat the heat will melt the wax and it will all stick to gether. Put on more layers and iron it flat. You may need to add some extra wax to make it all stick together like glue. Over lap on each side is for your wire connection. Connect all the over laps on the left together and connect all the over laps on the right together.

 

[caster.cp]

A flat plate cap will store twice the energy as a rolled of the same UF rating? Could you please substantiate that with some information because that makes no sense at all.

I couldn't understand that either.

The definition of capacitance isn't Q/V? So, if the capacitance is equal, the device MUST store the exact same amount of charge when the same V is applied to its terminals, don't matter if it's rolled, planar, oval, elliptical...

Or am I missing something really obvious here?

caster, it's generally a good idea to read the post before responding to it. The OP specifically stated that these were low voltage capacitors, only 4-5 volts.

I have read it, indeed. Why the comment?

 

[Sceadwian]

If you make 2 capacitors both .01 uf each the flat plate capacitor will have twice the metal plate surface area as the rolled capacitor so it will store a larger charge of electrons.

That makes absoluty no sense logically or electrically. a .01u cap will hold the same charge no matter if it's rolled or flat, if you say otherwise you've gone off your rocker. Capacitance determines charge, period; interacting surface area determines capacitance, if the capacitance is the same the interacting surface area HAS to be the same, so what you're talking about does not make sense as a rolled capacitor will actually have more interacting surface area from the mutual capacitance of the closely rolled plates. Layering the flat plates is the same as rolling them it however a voids the inductance problems, it does not inherantly change the capacitance.

Please explain your claim logically or electrically.

 

[Sceadwian]

caster.cp You gave the poster several warnings against working with high voltages including exploding glass bottle caps even though you knew he was working with low voltages. It's like telling a teetotaller not to drive drunk =)

 

[Bobby Ironsights]

This is all very interesting, if a little above my head, but I had another question, and didn't want to clutter up the board with more started posts.

I'm curious though, because a flat plate capacitor seems alot of work relative to the capacitance compared to a rolled cap; if an accordion folded capacitor wouldn't have less inductance than a rolled capacitor? It seems like a folded capacitor might be a happy medium of work v.s. yield, while still a low inductance design?

thanks for your time,
Bobby