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Ideal Bobbin Material for HFHV Transformer Project

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LutzH

Mon Jul 13 2009, 10:39PM

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

Hello:

I am about to put some time, and effort into determining the cost of having some custom bobbins turned to fit the U126 ferrite core. There seems to be enough interest from other members to make this worthwhile. Given that each person will have a different KV /mA target, this does not seem to present a problem, because this will mostly be a product of the wire size, and the number of windings each person uses. Thus the same generic bobbin could be used by many folks.

This got me thinking about the ideal material to use? My reason for being this "anal" is that often times I have found very expensive, and exotic materials, for a very reasonable cost on the surplus market. My telescope for example has Beryllium vanes hoding the secondary mirror, which I picked up for a song, so to say.

Presently I am aware of about 4 materials which "seem" well suited for the bobbin project, all of which have good dielectric properties, and oil resistance. All of these are also machinable, with no big problems. Here are the materials, that I have come up with so far, grouped by family:

1. Good old Nylon, or Delrin.
2. HDPE, HDPP, and UHMWPE/UHMWPP
3. Teflon/PTFE, FEP and other Fluoropolymers
4. Polyimide based high temp, like Kapton. (Yes it is available in large D rods)
5. Various fiber reinforced versions of the above

Being new to power HF contruction, I would lean towards the materials that I am familiar with from RF construction like Teflon, or a fiber reinforced version of it, to avoid the cold flow issue.

I do recal for example; That adding glass fibers will increase the dielectric constant of the material, which is great if you are building a capacitor, but maybe not for a HF HV Bobbin?

To put it in a nutshell so to speak, besides the HV insulation, and the physical properties of the material, what should I avoid, or look out for in selecting the bobbin material? -If cost is not an issue-

Yes cost is an issue, but while I am at looking, it certainly will not hurt for me to look for the "ideal" material, just in case there is a big rod of it out there somewhere to be had.

If anyone with more HF experience has any thoughts about this, they would be greatly appreciated. My knowledge base is in ionizing radiation, high vacuum, and VM's, but not in HF. Once I select a couple of options on the material, I will post the cost estimates, and options when I get them. Thank you for your time and effort.

Aloha.....Lutz :)

GeordieBoy

Mon Jul 13 2009, 10:52PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Have you looked at the Tufnol family of materials?

Some of the phenolic grades have excellent mechanical stability, are easy to machine, withstand high temperatures and have good electrical properties.

The glass reinforced grades also have excellent mechanical properties but the glass-fibre weave may increase losses in the presence of an intense AC field??? It is also harder to machine!!!

-Richie,

LutzH

Tue Jul 14 2009, 12:11AM

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

Hello:

Thanks for the idea, Tufnol I am familiar with because I still have a 40 x 60 x 1.5cm sheet of it from Tesla, ralated projects.

It is very, very, strong, and machinable (can even cut threads in it), and an excellent insulator when dry. I had considered it, but only the glass reinforced version. I crossed it off the list before because it is somewhat brittle, and because of the moisture absorbtion problem. In Arizona it would be great, but not where I live where it is very humid.

Still thank you for the suggestion, I may still re-visit some other version of this material :)

Proud Mary

Tue Jul 14 2009, 02:49AM

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

PTFE = "Tufnol"

The only problem with this polymer is the great difficulty in getting glue/varnish to stick to it, if that be required.

Nylon stock rods are much cheaper, and do not have the same adhesive problem.

Another possibility is epoxy resin casting, whereon you can make any shape you like.

I am not a polymer expert, and you should my remarks here as a starting point only.

jpsmith123

Tue Jul 14 2009, 03:40AM

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

The core you're using is a U126/91/20?

And do you plan to use a double "U" or a "UI" configuration?

Edit:

Sorry, somehow I missed your statement in your HFHV transformer design thread where you specified the 126/91/20 core with a winding length of 123 mm (which I think implies double a "U" configuration).

Will you be putting one 25 kv secondary coil on each 123 mm side and then grounding the mid-point?

LutzH

Tue Jul 14 2009, 07:43AM

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

Mmmm a cast bobbin, now that's one I did not think of yet regardless of the material. one advantage here is that you could make a bunch of them with one silicon rubber mold. You would need some type of vacuum setup to de-gas the resin. I will definitely kick this idea around, it is worthy of further investigation. Maybe it could be cast from filled HVepoxy, or even the more ridged silicon casting resin used to make powerline insulators. This idea in itself could be a project by itself, if this looks promising I may request a small say 1-4 liter sample of resin :)

This gives me an idea :) How about using thermo setting epoxy coated wire to wind coils onto a form, then baking the wire to allow it to cement together. Then remove the free wire coils from the form, and arrange them on a scaffold which can then potted in insulating resin. In other words no bobbin, just the coils embedded in the potting resin around the core! For a single layer winding I would try this for sure, but for this project it would be kind of complicated.

Reference the core configuration, yes I plan to use 2 U cores. As far as the secondary configuration goes I would like to put 2 secondary bobbins, with 2 secondary windings, onto the double U core, and then put both secondaries in series. One end of each secondary could be grounded, or left floating? Bottom Line: I want to be able to feed a FWVM with up to 50KV, 50mA, at 20-30 KHz. Getting to 50kV to feed a FWVM could be done in two ways as I understand it: One way is to use one transformer with two secondary coils as just described, with a single primary, or a dual primary in series also. The second method would be to use two transformers, each with one end of the secondary grounded, fed in phase. Which is better I am not sure but it just seems like a single transformer is cleaner construction. Thanks everyone for the ideas :)

Steve Conner

Tue Jul 14 2009, 10:08AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Harry wrote ...

PTFE = "Tufnol"

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

jpsmith123

Tue Jul 14 2009, 11:32PM

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

Well I'm about ready to forget about layers and "pies" and associated issues go with a single layer winding and be done with it.

With my core I could add two 100 mm long "I" pieces as extensions giving me a winding length of 163 mm per side; if I leave a margin of, say, 20 mm all around, I should be able to easily get 400 turns of 30 gauge wire or 500 turns of 32 gauge wire on there and maybe a little more.

Even the 32 gauge wire should be able to handle 100 mA I think.

As I see it, if the ungrounded ends of the windings are graded with a few turns of large diameter wire, that may be enough to get the field below the corona threshold of approximately 30kv/cm. (The rest of the winding would be ok as it would be "self-graded").

The down side would be increased reluctance (and therefore decreased primary inductance), a physically bigger transformer and perhaps more complex hardware issues regarding mounting and keeping the assembly together.

LutzH

Wed Jul 15 2009, 10:14AM

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

Yes, I like single layers also, but I just cannot get the turns ratio I want with a single layer, if I went this route I would try the self supporting baked wire coil, potted in HV epoxy, no bobbin this way, just for fun :)

If I was going for 30KV, then I would be done with a single layer. Has anyone done a single layer one with a Faraday Cu foil inter-winding shield? I have seen a commercial single layer design with 2 Cu foil layers: 1. An inter-winding one, and a second insulated one at the full output potential, to which the leads were soldered. The 2 secondaries were in series, feeding a dual VM.

So far it looks like 2 more simple options are best, yes I got a little to anal on this one, sorry folks:) The bobbin material is not that critical.

1. A Nylon, or Teflon bobbin turned on a lathe. Anchoring the wire ends should not be a problem with the Teflon, because I plan to use the split potential rings on the ends, and I can anchor the wire to these, and nylon screw the rings to the bobbin. Both of these materials are cheap on ebay, a big plus!

2. The second option is the cast bobbin idea from Harry, I have a hard time letting this one go, its just such an elegant solution.

The last thing I tried, is to email the big bobbin makers in the US, and China to see if there is anything "off the shelf" out there for this core. Its always worth a shot.

Thanks......Lutz

jpsmith123

Fri Jul 17 2009, 04:43AM

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

I "drew" my proposed transformer in 3D and now I'm having second thoughts about the U100 core. My two hv windings seem to be too close together.

It seems to me that the core you're using would be better suited for a HV transformer than my core.

Here are 3D renderings of mine compared to yours:

1247804904 1321 FT72874 U100 Core With Bobbins

1247804904 1321 FT72874 U126 Core With Bobbins

1247804904 1321 FT72874 U126 Core With Bobbins And Rings

For your core, with cylindrical bobbins that 12.7 mm thick (I didn't include the winding grooves), and with corona rings 10 mm in diameter, the rings are about 27.6 mm apart...so you'll have 50 kv separated by a little over an inch if you do it this way.

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