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Ideal Bobbin Material for HFHV Transformer Project

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Proud Mary

Fri Jul 17 2009, 07:59AM

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

Steve McConner wrote ...

Harry wrote ...

PTFE = "Tufnol"

No, PTFE is Teflon. Tufnol is a fibre reinforced phenolic similar to what Americans call G-10 or Garolite.

Oh dear! Sorry for that!

My own rough and ready solution to this kind of problem is to use stock Nylon tube on which Nylon washers be glued, all the which readily available from ebay. Judging from the diagrams above, it might just suit you too. PTFE tube and washers would be even better, but I have no idea how to glue it inexpensively.

I have used this technique to make RF chokes, and found it very good.

Sulaiman

Sat Jul 18 2009, 07:11AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Don't forget, it's not just winding-to-winding insulation, winding-to-core is also critical.
Electricity tracks along surfaces much better than across airgaps (oil/potting compound etc.)

rp181

Sat Jul 18 2009, 07:38AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Garolite is better for meachanical stress. Use teflon. mcmaster-carr has a teflon glue, and MWS wire also has wire that you bake, so a outer layer melts and adhears together.

jpsmith123

Sat Jul 18 2009, 06:51PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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You're right about the surface tracking concern, Sulaiman.

With that in mind, in the transformer of Fothergill et al., I think the reason for the grooves cut in the bobbin (to the outside of the corona rings) is to increase the path length.

It's too bad they don't give the exact dimensions of the bobbin, or show a picture of the complete transformer, but that bobbin looks massive...it must be at least 4 or 5 inches in diameter and a foot long or so.

jpsmith123

Fri Jul 24 2009, 06:45PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Well I did some more EM simulations (both electrostatic approximation and fullwave @20khz) of a single layer, ferrite cored, HFHV transformer, and it looks like the main corona problem will be due to high Efield at the sharp corners of the core, and not at the ends of the secondary windings.

For my proposed design (10kv-0-10kv), the ends of the secondary apparently don't need any kind of grading rings, as the fields there are below the assumed corona threshold of 30kv/cm, but the field enhancement at the sharp corners of the core looks problematic.

Given the high value of the field, and the limited spacing between the legs of the "U", it seems to me that the only practical solutions will be either some kind of potting or oil immersion, or some kind of shielding.

1248461068 1321 FT72874 Core Field Enhancement

jpsmith123

Sat Aug 22 2009, 03:32AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Today I received the 1.5" ID x 2" OD UHMW PE tubing I ordered from McMaster, and it's nice stuff, IMO.

The ID was a little undersize and the OD a little oversize.

I turned the OD down a little and faced off the ends, making it about 6.5" long.

I'm a newbie at using a lathe, but I was able to get a nice smooth finish on the piece turning it at about 1000 rpm using an indexable tool with a HSS insert (at first I mistakenly used a C2 carbide insert, and the results were terrible).

As far as shielding the core is concerned, I've decided to use a copper foil tape/polypropylene film sandwich; the plastic film can be wrapped around a few extra turns if need be, to take up any space between the core and the bobbin.

Anyway, hopefully tomorrow I'll be able to make the other bobbin.

Herr Zapp

Sat Aug 22 2009, 05:30AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

LutzH -

Relative to your thoughts on epoxy-coating your magnet wire to create a self-supporting coil, are you familiar with "bondable" magnet wire? This is just standard magnet wire that is already coated with a solvent or heat activated adhesive. For the solvent-activated wire, during winding it is just drawn through an alcohol-saturated pad, so by the time the wire reaches the coil being wound, the adhesive is already a bit "tacky" and bonds each turn to the adjacent turns. This holds the coil together well enough to make it self-supporting when it is removed from the winding form. Typically, the coil is post-baked to fully fuse the adhesive and make a rock-hard coil. Magnet wire with the heat-activated adhesive is typically "baked" while it's still on the coil form.

Regards,
Herr Zapp

jpsmith123

Fri Oct 16 2009, 04:02AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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I was able to put 400 turns of 30 gauge wire onto the UHMW PE tube in a defect-free layer, using a lathe turning at a low rpm, while putting some tension on the wire and using a toothbrush as a tool to keep the turns together when necessary.

As it turns out, there is one commercially available epoxy coating product that will supposedly bond to PE and PP,
but it is extremely expensive stuff, too expensive, IMO (1.7 oz for $71 from McMaster).

So for right now the winding is being held in place by tape, while I wait for the "Q-Dope" (GC Electronics) and some clear polyolefin heat shrink tubing I ordered.

As an experiment, I plan on using the "Q-Dope" to temporarily bond the coil ends until the heat-shrink tubing can contract enough to (hopefully) create a permanent mechanical fix.

I wonder if Lutz has anything to report on his efforts yet?

LutzH

Sat Oct 17 2009, 06:28AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
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Posts: 136

Wowww I am Impressed !!!!

I have been busy with a problem teenage daughter so I had to take a break from ALL hobby activities so this is my first chance to visit the forum in a while. This thread has been very productive with some truly great thinking taking place.

I am still determined to take this project to fruition its just going to take a little longer than I thought. A big recent motivator for me was picking up a segmented ceramic / metal particle accelerator tube on ebay for 19.95 USD!!! No kidding, no one bid on it because it was listed as a "Diffusion Pump Part" I figure it should be able to handle 600kV easy so I will defiantly need this transformer to drive the FWCW multiplier. Also I now have a complete IGBT 6 pack module, rated at 1200V at 450amps. The best part is that I have the commercial controller PCB's which go with it, and they work, so I need only a low power 15V control signal to run it. I plan to use 4 of the on board IGBT's in a full bridge configuration to build a high power 20KHz inverter. Then the remaining 2 IGBT's can be used to control the input AC portion of the circuit to regulate the input voltage to the inverter.

Now where to start..... The computer simulations are awesome, and yes I had planned on dropping it into oil, or even Galden fluorocarbon insulating fluid if I can find some more, I need to save enough to cool the ion source which is at the full potential of the CW multiplier at one end of the accelerator tube. Please let me know if you see a cheap Duoplasmatron ion source somewhere? The normal RF and PIG ion sources can only produce beam currents in the uA range, and I am looking for up to 5ma.

Also I want to thank down to earth folks like Harry, who excels at getting me to look at simple solutions to problems, when my head is in the clouds :) The wound coil picture by jpsmith123 is equally impressive in its elegance, as are the suggestions by Herr Zapp and Sulaiman. For now I need some time to "digest" all of these great ideas and thoughts.

Two themes which I cannot get away from are:

1. The idea of vacuum potting self supporting coils around a core, this may be better however on a smaller HV transformer.

and

2. All the bobbin suggestions for which I am leaning in the direction of trying a combination of two ideas: First the coil winding /design concepts of jpsmith123, in combination with the idea of Herr Zapp of using adhesive coated wire. This would at a glance seem like an ideal marriage. This way you have a coil which can only move as a unit so no more slipped winding failure risks etc. Now using PTFE as a bobbin material should not be a problem anymore :) The only wrinkle here is the magnet wire insulation, I am firm in my desire to use polyimide insulated magnet wire due to its inertness to oils and fluids as much as for its high temp properties, so the search is on for 28g "bondable" polyimide insulated wire, and a lathe to borrow some time on, to turn the bobbin.

One last idea to share for now reference "quick and dirty HFHV transformer testing". It is to use a existing commercial switch mode power supply as a ready and easy source of HF, for testing HF transformers. I have a Kaiser 1500v / 1kw laser capacitor charging power supply, which has a medium size ferrite core transformer with a FWB rectifier to obtain the 1500v output. My idea is to "tap" into the primary feed of the power supplies ferrite core transformer, while running it with a 100watt dummy load to keep the regulating circuit happy. I could use a inductor or a capacitor to limit the power that I am tapping off, or a combo of the two? The PS runs at about 20-30KHz, has anyone tried this before? If yes any ideas on the current limiting aspect? Thanks again everyone..........Lutz : )

jpsmith123

Sat Oct 17 2009, 04:28PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Hello Lutz,

Well first let me express my jealousy (: over your HV accelerator tube...I've always wanted one of those. I'm guessing the manufacturer might be NEC?

Anyway, I just got the heat-shrink tubing I ordered, so sometime early next week I'll try to shrink it over the PE tube and see what happens.

In the mean time, I got some CPVC pipe to play with and I'm going to try to make a multilayer bobbin with it. It has a relatively high dissipation factor, yes, but other than that, it has a lot going for it. It seems easy to machine, you can readily buy adhesives, paint, etc., that will stick to it, it's able to stand some reasonably high temperature (200 degrees F, IIRC), and it's cheap and readily available.

The dissipation factor shouldn't be a problem if the shield is inside the bobbin, which shouldn't be too much of a problem for a multilayer arrangement I would think.

I'm presently waiting for some CPVC sheet stock to arrive from McMaster, so I can make the end walls of the bobbin.

BTW, have you considered buying a small lathe from Harbor Freight or on Ebay? I was originally going to go to a local machine shop and try to get a bobbin made, but then I thought, I'm probably going to want to make changes and try different materials, etc., so it may be cheaper and less of a hassle in the long run to just buy my own lathe and learn how to use it myself. Moreover, plastics like CPVC are probably good stock to use when first learning.

Lastly, what kind of ions do you want to accelerate? Are you after hydrogen ions (i.e. protons or deuterons)?

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