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Methods of Winding Large Ferrite Cores

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teravolt

Tue Jun 10 2008, 04:20AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

This is my new coil. it has 30awg and 600 turns. the windings are insulated with 10 mill mylar. I plan to pot it in perifin. do you guys think it will generate 20-30 kv? or do you think it needs more turns.

Dr. Dark Current

Tue Jun 10 2008, 06:10AM

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

teravolt wrote ...

This is my new coil. it has 30awg and 600 turns. the windings are insulated with 10 mill mylar. I plan to pot it in perifin. do you guys think it will generate 20-30 kv? or do you think it needs more turns.

I think it won't go to 20kV, unless you drive it in flyback mode (which means reduced power). For your core I guess it should be safe to run it at 10V/turn so you'll get approx 6kv on the output.

Steve Ward

Tue Jun 10 2008, 06:19PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Nah, that beast of a core can run 40V/turn at 50khz (going from memory here).

I simply doubt you will hold off more than 10kVAC between a few wraps of mylar. Corona is always a problem (unless vacuumed out under oil), and it WILL eat away at the insulation between layers over time. Maybe if you vacuum pot it in parifin you can keep the corona from showing up, but i really doubt you will get every last bit of air out of there, and when you have air and high frequency high voltage, you will have corona.

Dr. Dark Current

Tue Jun 10 2008, 08:03PM

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

Steve Ward wrote ...

Nah, that beast of a core can run 40V/turn at 50khz (going from memory here).

Ok, maybe i got the size measurement wrong from the photo

I thought it is just a few bits bigger than a flyback core

jpsmith123

Wed Jun 11 2008, 12:40AM

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

Hello Teravolt,

I'm wondering, have you measured the self-resonant frequency of your multi-layer 600 turn coil? If not, I suggest you measure it by hooking it up to a signal generator (obviously with some series resistance and with primary absent or open circuited) and find the lowest freq. where it peaks.

I'd really like to know what kind of number you get...depending on what you end up with, you might not be able to drive it at anything close to 50 khz.

BTW, I haven't started my own coil yet, as I'm having trouble finding a good deal on 28 gauge teflon wire. Seems there's plenty of 26 gauge on ebay (which I may end up using) but no 28.

teravolt

Wed Jun 11 2008, 05:20AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Steve ward is right about the carona problem and the mylar is rated for 10kv DC and I don"t know how it will behave. I would like to run it in flyback mode. If it will not hold the voltage off I will use a larger gauge for more current less voltage. Steve I do have acces to vacume and epoxy poring facilities for hv potting. I am not shure that that will help. The plan is to set it up as a bobbin like you did and pot it. I thought your set up is very inavative and I wanted to try it with thease cores. if I pot it in perifin I might be able to repair it but epoxy it will be a throw away if it dies.
I got 2 of theas cores on ebay and I would like to make a full wave cocroft walton multiplier. the demensions are 4*4.5 with a 1"square core probbilty about 3-4kva ea. thanks N.B.

jpsmith123

Fri Jun 13 2008, 01:06AM

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

I guess we can safely say that HV secondary construction techniques for these cores will need further study and experimentation. I'm hoping my idea using teflon wire will work out, but that remains to be seen.

In any case, I guess the fact you didn't answer my question about the SRF of your coil means you didn't measure it and you're not interested in that parameter?

Lastly, may I ask, why do you want to run in flyback mode?

Regards,
jpsmith123

GeordieBoy

Fri Jun 13 2008, 09:41AM

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

Two common advantages of operating low power HV transformers in flyback mode are:

1. Reduced turns-ratio for a given output to input voltage ratio.

2. Reduced reverse voltage on output side rectifiers.

There is also no requirement for an HV rated choke at the output when DC is required with the flyback design. This is a significant cost saving in apps like TVs.

-Richie,

jpsmith123

Fri Jun 13 2008, 03:19PM

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

I can understand "operating [a] low power HV transformer in flyback mode"; I'm not sure why he would want to do that with multikilowatt-capable core and coil he's got.

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