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Big home made pulse caps

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pauleddy

Thu Jun 24 2010, 09:52PM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

I have had an idea but donâ€™t know what would be the right way to do it. I would like to be able to make my own capacitors for use in pulse applications, and I donâ€™t want to go and buy my own big caps, mostly because I am broke. I have taken apart a few of those photo caps that I have laying around from an old project and it seams to be made only of two small sheets of metal and some thin paper soaked in something. I was wondering if someone could make really big version of this my taking two rolls of aluminum foil and a few rolls of paper and make roll them to getter to make really big caps. I am not looking for really high voltages I going for like less then 1kv but really high capacitance like 1F.

If anyone has any ideals to add on that will help big time
Thanks

Bjørn

Thu Jun 24 2010, 10:56PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Forget that.

Look here how much work there is to make a nF capacitor

then scale that up by a billion times. Batteries and capacitors can't be made at home cheaper than you can buy them except in special cases. Even if you could manage to make a huge one it is very likely to fail very quickly.

There is a special type of capacitor that can give high capacitance but it is harder to make and it is useless in comparison to a cheap battery in most cases:

pauleddy

Fri Jun 25 2010, 12:21AM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

thanks, but this is more of an idea. for those who like to make things from scrach, maby make one for special needs. i just want to know the posiblities. how whould one go about something like that. i mean how much could you store in somethin like that. im guesing 1f is a little high, maby like >1/2F, and the voltage for those super caps is too, low im looking at a range of 400-500v

Myke

Fri Jun 25 2010, 12:36AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

That's still a HUGE cap. That cap would store 62.5kJ. Like Bjorn said "Forget that."
Look through the forums at the values of the caps people have made by hand. They tend to range from the 100s of pF to a handful of nF. No where close to a uF which is a million times smaller than the capacitance you are asking for.

IntraWinding

Fri Jun 25 2010, 01:07AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

I think the capacitor you dismantled was an electrolytic. They give much higher capacitance per volume than conventional capacitors but the exact chemistry involved is kept secret by manufacturers. However, I have seen examples of people making them successfully (I think Alum was involved, and the right purity of etched aluminium foil) but only as an experiment, and only at low voltage. That doesn't mean you couldn't do it!

Pinky's Brain

Fri Jun 25 2010, 01:59AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Some numbers ...

You can get 20 micron BOPP packing foil for around 30$ for 500 m2, aluminium foil will run you about the same ... so 100$ and a metric ****ton of work will give you up to 3 * 8.85e-12 * 500 / 20e-6 ~= 6.6e-4 F. At a minimal volume of ~0.025 m3 (for instance a block 30 cm on each side).

Getting 1/2 Farad worth of capacitance with a DIY plate capacitor is not realistic.

pauleddy

Fri Jun 25 2010, 03:34AM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

so what whould be any use for somthing like that, making a giant roll plate capcitor. I realize that ya the caps i had are electrolytic so ya that would be imposible same with try to get that high of capacitance. I just enjoy making things my self. I have way too much time and need somthing to fill it.

thanks

radiotech

Fri Jun 25 2010, 08:01AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The chemistry of an electrolytic capacitor has little to do with the electrolyte soaking the paper seperator. The capacitor dielectric is the oxide coating on one side of one of the foils, a few atoms thick, hence very thin, and this produces the great capacity. The electrolyte is the contact/bridge to the other foil.

Experimentation with a lead plate and an aluminum plate in a bath of sal ammoniac, saturated, in a jar about 2 L, will give you two things. Firstly a working electrolytic rectifier and secondly a electrolytic capacitor of about ~ 40 uFd. which will work.

The rectifier can be used to charge car batteries by connecting light bulbs in series directly to 120 volt ac mains.

For those too poor to afford tungar rectifiers...

Marko

Fri Jun 25 2010, 11:29AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys,

I've had several ideas about construction of large homemade capacitors that might be capable of Can Crushing. They all involve low capacitance and extremely high voltage though.
Best of them are -

1. A large amount (talking of 1000 and up) of polystyrene disposable party plates could be stacked with Al foil in between them cut in a circle and preferably doused with some oil to exclude the air between the plates. Alternatively, plastic party cups could be used. They have lower cost per surface area but more difficult geometry. I think people have already explored both cups and plates on this forum and got quite satisfying joules per plate/cup.

2. Alternatively, I thought about using saline solution in between plastic plates as electrodes. This would exclude oil and time consuming foil cutting but would increase internal resistance of the cap. And we would still need strips of foil of some sort to make a contact with each plate.

3. I thought about using paper as a dielectric as well. Paper is by far cheapest dielectric per square meter and is available in cleanly cut formats. Paper would likely need to be doused in oil or wax extensively and used with a lot of thickness clearance to improve the lifetime of the cap. I imagine one could construct a cap with relative ease from a suitable box, couple a thousand A4, or better A3 sheets and kitchen Al foil. This is actually my favorite now and I'll really like to see someone try it.

Any of these processes are probably way too time consuming for me to bother with though, but I'd be happy to see someone of you guys try.
Clearly none of these caps would be suitable for repeated pulsed operation in tesla coils, but could provide a lot of satisfaction in blowing various stuff up and perhaps even crushing cans or coins.

pauleddy

Fri Jun 25 2010, 02:00PM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

Marko wrote ...

3. I thought about using paper as a dielectric as well. Paper is by far cheapest dielectric per square meter and is available in cleanly cut formats. Paper would likely need to be doused in oil or wax extensively and used with a lot of thickness clearance to improve the lifetime of the cap. I imagine one could construct a cap with relative ease from a suitable box, couple a thousand A4, or better A3 sheets and kitchen Al foil.

that is similar to the my first ideal, but insted of sheets, just make a gaint roll. I like Zaping things

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