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Methods of Winding Large Ferrite Cores

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jpsmith123

Sun Jun 01 2008, 01:43AM

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

I think it may be possible to wind a multilayer coil (for, say, a 10 kv peak output) while keeping the e-field in the coil below the corona threshold...this by using the right type and size of wire, the right winding details, e.g., insulation thickness between layers, etc., thereby avoiding vacuum potting.

My main concern has now become the issue of stray capacitance, and once again it all seems to get back to inevitable trade-offs. If I want to minimize core loss and maximize magnetizing inductance (Lm), I need more turns, but this will increase stray capacitance.

At this point, I'm seriously thinking about using two Us and two Is for the core, giving me a single-layer winding length of about six inches.

teravolt

Sun Jun 01 2008, 07:54AM

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

hello jpsmith123 I have 2 of those cores and I want to make a full wave cockroft muliplier like Steve Ward and pot the windings in perifin wax wich seems to work well. I have tried this with a large flyback and got 3.5" sparks continuously and 4" at flyback death using the mazelii circuit. I losely figure it has 300 turns. With about 40kv out definatly turns ratio has nothing to do with output voltage. p.s. what is the web site adress and part number fo your bobbins. thanks Nathan B.

jpsmith123

Tue Jun 03 2008, 03:21AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Teravolt I sent you a PM regarding bobbins for this core.

BTW I'm still not sure what approach I'm going to use to drive this transformer...thus I'm still researching it. I'm wondering, what kind of driver circuit are you considering?

teravolt

Tue Jun 03 2008, 05:15AM

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

this is my last victom befor I potted it with perifin. it worked good ontill I got 4 inch sparks. with a little tuning I got the mazzilli circuit to work well.

jpsmith123

Fri Jun 06 2008, 01:48AM

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

Here's a home-built 1 kw, 1.5 kv, hard-switching half-bridge regulated power supply that uses the U100/57/25 core:

Steve Ward

Fri Jun 06 2008, 02:23AM

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

I abandoned the multi-layer windings for a few reasons. Firstly, as you pointed out, the stray C made it too difficult to play nicely in the SLR setup. Secondly, even my attempts with vacuum potting in epoxy resulted in corona and internal breakdowns when getting anywhere above 10kV peak output voltage. Using transformer oil is an option, but i typically dont like to use oil because im always left with some sort of mess. A single layer winding insulated with hot-melt glue is my preferred method now, but i only use this for moderately high voltages up to 12kVDC (for guess what? charging a capacitor! :P), and thats *after* a voltage doubler.

Earlier experiments with 2 secondary windings (on 2 separate legs of the core) did provide plenty of voltage, up to 20kV peak, but eventually failed from flash-over problems.

I would be curious to try out a split coil setup with some diodes in-between each section. That should help the corona issue too, as the pulsing DC E-field is a lot less corona-forming than AC at this frequency!

When i need really high voltages, i just use my large C-W multiplier, which is quite effective when driven at 50khz or more.

uzzors2k

Mon Jun 09 2008, 02:19PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

How did you vacuum pot your transformers Steve? I take it the one in your CW stack is still alive? I tried making an HV transformer this weekend by wrapping each layer in paper towel (!) and soaking the whole thing in oil. Despite my effort it failed after just a few arcs. Internal arc-over I assume, and oil starts to pour out of it for some reason. I have no idea where the extra pressure could be coming from.

And my SLR inverter suffers from capacitive coupling to the core! Winding many hundred turns only to have them fail is really demotivating... jpsmith123, be sure to allow for enough space between the core and bobbin.

Dr. Dark Current

Mon Jun 09 2008, 02:30PM

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

Uzzors, I use the PP transparent sheets that are used for some book covers I think. I put one layer of it, if I want to be extra sure I put two layers. I made a ~15 layer transformer that did put out some 20kV on a flyback core.

If you want acceptable winding capacitance then I would wind your coils in ~4 layers, and put 4 layers of the PP transparency between each layer (

uzzors2k

Mon Jun 09 2008, 02:41PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Really, and it hasn't died yet? I've made capacitors with that kind of plastic before, so I have some around here somewhere. I'll try making a new secondary then.

Dr. Dark Current

Mon Jun 09 2008, 02:48PM

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

Uzzors wrote ...

Really, and it hasn't died yet? I've made capacitors with that kind of plastic before, so I have some around here somewhere. I'll try making a new secondary then.

Yes, the secondary itself is fine, it has just arced to the core from the outermost layer... (I was also abusing it under cooking oil outputting ~30kv but it still wanted to arc to the core, making bubbles there..)

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