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Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
LutzH -
If you really intend to build a power supply utilizing a big ferrite-core HV transformer with a machined Teflon bobbin, here is another option. (It might not be viable, depending on where you live, and if you have access to a lathe, etc and plan to do your own machining on the bobbin.)
I have some "excess" high-density polyethylene rod stock, 2" diameter. Design your bobbin, make a prototype out of HDPE, and test it. If it "survives", then you're done, at a fraction of the cost of a Teflon bobbin. If it fails because of design or construction problems, then you can modify your design and make a second prototype bobbin out of HDPE, and re-test. Finally, when you have a design that survives with a HDPE bobbin, you can go ahead and make a Teflon bobbin if you feel its absolutely necessary.
Here in the US, 2" diameter "Mechanical Grade" PTFE rod is about US$38/foot; 2" diameter "Electrical Grade" PTFE is about US$100/foot. I'll sell the 2" HDPE rod for US$6/foot.
In my opinion, an HDPE bobbin should offer 95% of the performance of a Teflon bobbin, at around 10% of the cost. The only real weaknesses of HDPE vs Teflon are high-temperature operation, and exposure to gamma rays & neutrons.
I'll post an entry in the "Sale and Trade" forum, with photos of the rod stock. I have 2" diameter material in deep blue, lime green, and black. This stuff machines like butter (like Teflon does), and is great for HV insulators, feedthroughts, standoffs, coil forms, low-friction bearings and bushings, etc.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Vacuum encapsulation isn't the same as operation in a vacuum chamber. The dielectric is the potting compound, and that has a maximum electric field it can withstand before breaking down. Hence grading rings might still be needed.
I'm sure HDPE would work just fine, unless this is one of these paralysis-by-analysis projects that you have no intention of ever actually starting.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
For that matter there is also such a thing as vacuum breakdown...so there are always limits, no matter what the "dielectric" is.
In this case he would be vacuum encapsulating to displace air and eliminate/minimize corona. If it works for 50 kv ignition coils it should work for his 25 kv coils.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
jpsmith123 wrote ... ...Speaking of that, one thing I don't understand about this author's design is, if you are going to go through the trouble of using layer insulation in your segmented windings, and then vacuum encapsulate the coil, as he did, then why not just go with a straight multiple layer type coil in the first place? IOW, I wonder what advantage did the author get from the added hassle of segmented windings in this case?
It might be about capacitance and self-resonant frequency. Regardless of HV insulation issues, you want each turn to be physically far away from turns that are far away in voltage.
Registered Member #1721
Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136
"My Brain Hurts" To much rational and completly logical advice, and worst of all, it makes perfect sense. Ok I may explore the HDPE option, I am assuming that it will do fine in mineral oil, correct if it fails then it can be re-done out of PTFE. On this same subject what about Nylon then? Its a little harder and it can be glued, so for the cheap test tranny its HPPE vs Nylon in oil?
On another note I understand that segmented windings are better from a capacitance perspective than layered windings. -But- I have read that if you design the HF driver for ZVS switching then the capacitance issue goes away? So on this project I now find myself back at the starting line having walked a full circle.
Soooo what to do, well I have to do, something so I will move towards the cheap approach and try to do a segmented secondary, on a Nylon, or HDPE Bobbin. This way I am not forced to go ZVS on the driver like with layered windings. Thank You :)
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Yes I agree with you Klugesmith, the capacitance may have been the driving issue behind the bobbin design.
At first I thought that 40 khz was not so high that a carefully done layer winding wouldn't work, e.g., using same-sense windings in each layer and some extra layer insulation...but I simply don't have enough experience winding coils yet to have a very meaningful opinion on it.
In any case, in looking at the picture again, maybe there wasn't enough room to build the coil out radially far enough for a low capacitance multi-layer winding.
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