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Spiral impulse generator = rolled cap Blumlein induction coil!

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Proud Mary

Wed Apr 23 2008, 11:11AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I have seen references to VIG oscillators, where the lines form part of an LC tuned circuit, so it may be possible to drive them quite hard.

My chief concern is about the 'goodness' of the dielectric, on which the velocity of propagation and voltage efficiency depends, not to mention holding off break downs.

If we could sort out the dielectric, almost anyone could achieve hundreds of kV and maybe more with such a simple device. I've noticed, though, that the seemingly more advanced designs often have more complex segmented winding arrangements.

Dr. Shark

Wed Apr 23 2008, 10:21PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I redid my VIG with a significantly larger diameter, which as Harry pointed out is supposed to significantly increase the voltage efficiency. I also went for a 1:1 turns ratio with 9 turns each. On my 8cm diameter "core", this gives a D/n of nearly one, so the calculated efficiency should be about 50% without a ferrite core. Going for a much larger former would have improved the efficiency more, but what good would that be if it left me with only 3 turns from my 3m long strips, and therefore a maximum step up ratio of three?

I also redesigned the spark gap for super-low inductance, because apparently the inductance of the "fast side" of the generator is dominated by the switch. The two contacts are not separated by air but by 0.2mm of dielectric, so there is essentially zero loop area between them. Of course this does not allow repetitive operation without burning the dielectric.

Testing the thing immediately showed inter-turn arcing, I should have payed more attention winding the thing (It's getting late, and I am loosing interest in this project). As far as I could tell though, the output spark was still quite weak, most energy being burned in the switching spark gap.

Proud Mary

Wed Apr 23 2008, 10:37PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Dr. Shark wrote ...
Testing the thing immediately showed inter-turn arcing, I should have payed more attention winding the thing (It's getting late, and I am loosing interest in this project). As far as I could tell though, the output spark was still quite weak, most energy being burned in the switching spark gap.

Getting the dielectric just right seems to be everything with this device. I'm sure there must be a source of a suitable material that is cheap and easy to get if only we could think what it was.

I've read several papers where the lines are described as metalized tapes, so I know I have that part right. But I have doubts about my plan to use several thicknesses of 100mm wide insulating tape as the dielectric, because of the difficulty of not trapping any air while I'm winding it with five thumbs.

Kraft paper and oil is always an option, I suppose. It has nothing like the dielectric properties of polyethylene or PTFE, but can be laid on thick and cheap!

Firefox

Thu Apr 24 2008, 01:08AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

What about teflon tape? Its not exactly cheap, but it is an excellent insulator, and you can use relatively small amounts of it to insulate the coils.

Proud Mary

Thu Apr 24 2008, 09:13AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Firefox wrote ...

What about teflon tape? Its not exactly cheap, but it is an excellent insulator, and you can use relatively small amounts of it to insulate the coils.

Yes, PTFE would be my first choice, but I have no idea where I might find 100mm wide tape of suitable thickness at a price I could afford. I'm sure it is available from specialist plastic stock suppliers, but having bought PTFE tubes and rods before, I have no doubt it would be expensive.

Dr. Shark

Thu Apr 24 2008, 11:29AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

PTFE would also be advantageous from a loss tangent point of view, apparently the Mylar that I am using is quite lossy, whereas PTFE is one of the best plastic dielectrics. I have no idea where it could be obtained though.
The insulating tape you plan on using is probably going to be PVC, do you know the dielectric properties of that? I think it is not commonly used for capacitors, but I don't know why.

...

Thu Apr 24 2008, 02:30PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Not sure how think you need the dialectic, but you can get 6" wide .005" for only $2/ft, 6" bu .010" for $3/ft, and many other sizes in between.

A little pricey, but not completely unrealistic...

Proud Mary

Thu Apr 24 2008, 04:07PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Dr. Shark wrote ...

PTFE would also be advantageous from a loss tangent point of view, apparently the Mylar that I am using is quite lossy, whereas PTFE is one of the best plastic dielectrics. I have no idea where it could be obtained though.
The insulating tape you plan on using is probably going to be PVC, do you know the dielectric properties of that? I think it is not commonly used for capacitors, but I don't know why.

PVC has a typical dielectric constant of 3, and a dielectric strength between about 550V - 750V/mm.

Mylar has a typical dielectric constant of 3.2, and a dielectric strength of around 7kV/mm, vastly superior.

PTFE has a dielectric constant of around 2.1, a dielectric strength of 500V - 1kV/mm, but up to 5kV/mm in 'modified' PTFE, (almost as good as Mylar) but the dielectric strength per mm declines with increasing thickness for some reason.

I'm don't think my plan to use four thicknesses of PVC inslulating tape is likely work except at low charge voltages, so I shall have to keep an eye out for cheap sources of plastic shim or tape of suitable thickness.

If I were to charge my VIG with 14kV DC (rectified from an old but good 10kV NST) then the dielectric would need to hold off at least 28kV at the far end of the line. Allowing a 50% safety margin, this would mean the dielectric must be good to 42kV - which would be 6mm of Mylar.

I am beginning to understand why people have written about the use of capacitor winding machines in the construction of VIGs, though there are usually ingenious amateur means of getting almost as good results for almost none of the price.

Dr. Dark Current

Thu Apr 24 2008, 04:19PM

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

Harry wrote ...

PVC has a typical dielectric constant of 3, and a dielectric strength between about 550V - 750V/mm.

I really doubt that it has lower strength than air. I assume that mains cables etc. and all the wires with thick rubbery insulation are PVC insulated. Once I tried 40kV DC on a wire with some 1mm insulation and it didn't break down.

Proud Mary

Thu Apr 24 2008, 05:21PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Plasmaddict wrote ...

Harry wrote ...

PVC has a typical dielectric constant of 3, and a dielectric strength between about 550V - 750V/mm.

Forgive me, I am not used to American notation, and misread the American "mil" as being mm in the table I was using. I have now learnt that 1 American mil = 0.0254 millimeters, which seems a very odd figure to use in metrology. The figures I have misquoted above should be read as American 'mils' and my conclusions about them ignored!

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