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Incredibly Low Inductance Inductor?

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wolfman29

Thu Sept 22 2011, 05:14PM

Registered Member #4090 Joined: Fri Sept 09 2011, 04:43PM
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Posts: 13

Hey everyone.

A friend of mine and I are working on a project for which we need to make a coil or something of that sort that will provide an incredibly tiny inductance.

The catch? It must have a hole in the center of it at a centimeter in diameter.

And when I say tiny, I mean REALLY tiny. On the order of hundreds of picohenries. Smaller, if possible. The idea is that we are going to make a can crusher with an ultracapacitor, and we figured that, in order to drain the ultracapacitor in a small enough time frame that it can discharge hundreds or thousands of kiloamps in a fraction of a second.

Any ideas? We have thought of paralleling multiple "fractional" turn inductors to surround the object like a flower, but we aren't sure that would work. A triangular inductor with side length of 2cm seems to be close, but that's still on the order of single nanohenries, which is still to large.

Anyone have any thoughts?

Dr. Slack

Thu Sept 22 2011, 05:45PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Several observations

a) It's not physically possible to get that low inductance with a coil with those dimensions
b) An ultracapacitor will have a far higher internal inductance than you are projecting for the coil, so will limit the current itself
c) A can, inductively coupled to the work coil, will reduce the inductance seen by the capacitor bank, so increasing the current

a rethink is called for

wolfman29

Thu Sept 22 2011, 07:49PM

Registered Member #4090 Joined: Fri Sept 09 2011, 04:43PM
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Posts: 13

Yeah - been doing some research. My buddy (who knows more about caps than I do, or so he claims) seems to think that these things could handle the current discharge - the maximum current discharge on these capacitors, however, is around 1.6kA... so there is the issue.

Rethinking time!

I'll let him know tonight and we may come back here for some more assistance!

Dr. Dark Current

Thu Sept 22 2011, 08:51PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The kilo amp figure you stated is around what would be expected from a can crusher running at kilovolts, not volts, so your idea is not going to work.

wolfman29

Thu Sept 22 2011, 10:33PM

Registered Member #4090 Joined: Fri Sept 09 2011, 04:43PM
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Hm. I would have thought that the concept of a can crusher is generally amps, not power? I thought it was a rapidly changing magnet field that had that effect.

Patrick

Thu Sept 22 2011, 11:43PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

work, power, and energy are all different, also the reason for the very high voltage is to overcome impedance/resistance in the leads to and of the work coil (for very high current), while still having a very fast RLC time constant. (The L and R will be lowest for short straight wires, the capacitence is chosen to have enough current to establish the magnetic field in a short time.)

Amps being joules per second isnt enough to do what you want.

Damage being done to the can is work, which requires some (alot) power.

radiotech

Fri Sept 23 2011, 03:44AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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We have thought of paralleling multiple "fractional" turn inductors to surround the object like a flower, but we aren't sure that would work

Look at this Bitter Solonoid

Patrick

Fri Sept 23 2011, 04:00AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i think if they put CCW then CW then CCW then CW coils in series they would still end up with the same inductance as a straight wire of the same length, like 10nH per inch, a far cry from their desired pico-henry need.

Also as previously stated the ESL of the ultra cap will limit their final minimum value of L way higher then the picohenries they need.

wolfman29

Fri Sept 23 2011, 05:03PM

Registered Member #4090 Joined: Fri Sept 09 2011, 04:43PM
Location:
Posts: 13

Hmm. I like the idea of the bitter magnet... yet how does the current flow through the insulating mica disc? Just connected by wire or something? And why is a near full turn used? I would think that, the less turns one has, the stronger the magnetic field...?

Another idea was to essentially make a pentagon or whatever and just have each side be separated by an insulator, and then parallel the leads of each side? Would that work?

Arcstarter

Fri Sept 23 2011, 05:04PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

wolfman29 wrote ...

Hm. I would have thought that the concept of a can crusher is generally amps, not power? I thought it was a rapidly changing magnet field that had that effect.

Amps per turn is what decides how strong an electromagnetic field is. However, it will take a certain amount of voltage to allow a certain amount of current to flow through the impedance of the coil (Ohm's Law). So it is pretty much based on power.

Also, a can crusher requires the current to rise and fall very very fast. Not even a couple of electrolytics i had in parallel for 800v 1950uf could discharge fast enough to do anything to a can, a few volts just won't work. You really need kv's, and the capacitor should be low inductance and rated for a large voltage reversal, IE a purpose-built poly film capacitor. Or, if you can get your hands on some microwave capacitors (probably 20 or so) you can make a bank and overcharge them for a can crusher.

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