HF Litz wire as a primary coil

[TMaxElectronics]

I was recently thinking what to do with my little bridge I made for my "small" coil, that kept blowing up drivers/IGBTs (https://highvoltageforum.net/index.php?topic=1058.0).
And since the IGBTs are very fast, I think I'll try my luck with qcw.

Now the question however:
I would like to use some HF Litz wire I got from an old induction cooker plate as the primary, because I have it around and it might fit quite well given the relatively high frequency required for a qcw coil.
But I can't really think of a way of tuning it properly after the coil is assembled, has anybody tried this before and got some tips for me?

[davekni]

One option is to tune by adjusting MMC capacitance.  That's what I use.  Also, the higher coupling coefficient you can get, the less critical tuning is.

[Da_Stier]

Is each strand on this kind of litz wire coated in some isolator or is it more like a normal wire with blank copper strands?
Maybe a brass clamp with some penetrating pins might be usable to move the feedpoint for tuning?

[Steve Ward]

Litz wire is often made from "solderable" magnet wire, which with a little abrasion while applying the iron will often succeed at wetting into the whole bundle of wires producing tap points which should only have a minor effect on the litz performance.

However, I'd agree with others that tuning the primary cap is a very practical way to avoid primary taps if you're careful about knowing your secondary parameters.  JavaTC is my go-to tool for designs. 

Ive built several QCWs with litz wire, and i will say it can make a significant difference in wire temperature (provided the litz is really spec'd for the high freq, your cooktop stuff might only be sized for 20-50khz, so its not as effective as the super fine stuff i used rated for 300-500khz), but generally the primary coil loss is relatively small compared to total energy, so you probably wont see any real boost in spark performance. 

[johnf]

As Da and Steve have said each strand is insulated.
Be warned some of these use polyimide coatings which are not solderable (easily).
Using Litz would tend to increase your "Q" by quite a lot (skin effect) which may also work against what you are trying to achieve. Circuit "Q" is almost always affected most by the quality of the inductor not the capacitor so high "Q" circuits tend to maximise C instead of L
High "Q" circuits tend to test drive circuit stability which in turn tends to make sacrifices to the gods of high dV/dt and dI/dt failures.

[TMaxElectronics]

One option is to tune by adjusting MMC capacitance

Do you mean by adding or removing strings in a bank? I'm kind of hoping to find another big single cap that does everything I need it to do

Be warned some of these use polyimide coatings which are not solderable (easily)

Yeah I have been using W210 copper wire for all of my tesla coils and it is a bit of a pain to connect.
And from lacquer color and the fact that it will potentially have a very hot frying pan on top of it I think that the wire I have is the high temperature stuff too.

...if you're careful about knowing your secondary parameters

For my experience (which I admit is not a lot ) JavaTC can be quite a bit off if the turns count isn't spot on. But since I have everything I need to make one I think I'll just make the secondary, measure it's parameters and then design the primary accordingly.

One thing that I thought about was to use some sort of variable inductor. Basically another small coil made from wire that can easily be connected to for tuning that would drop the frequency just the last little bit. But I'm not quite sure how much useful adjustment rang I could get out of one.

High "Q" circuits tend to test drive circuit stability

How exactly is that? Just because the current rises much more sharply that in a low Q string or is there another mechanism that I don't know about?

[davekni]

"Do you mean by adding or removing strings in a bank? I'm kind of hoping to find another big single cap that does everything I need it to do "

Yes.  MMC arrays generally perform better (lower loss, higher current and voltage capability) than single large capacitors.  Even if you use one large capacitor, adding a few small ones in parallel would work for adjustment.

Adding an inductor in series or parallel will reduce effective coupling factor, the same as excess primary lead length does.  Unless you want less coupling, I wouldn't pursue that option.

My experience with resonant circuits such as DRSSTC is that high-Q improves stability.  I don't understand John's concern.  All my coils use litz-wire primaries.

[johnf]

David
 my concerns were when you operate outside of resonance. resonance is the holy grail and gives max power transfer
lower freq gives a capacitive type complex load and above resonance and inductive complex load.
add feedback without controlling phase and usually the circuit becomes unstable as phase goes towards 180 with circuit gain. Lots of solid state circuits do not have enough phase margin and with the lower freq end being capacitive coupled with more device gain due to the lower freq then the little gremins come out to play
Out of control loops like this are a silicon suppliers dream as you order more sacrifices from them as future offerings.
You and I know how to first simulate and blow up virtual devices before tempting the gods with real silicon and for my part every so often a silicon part tends toward permanent conduction and or oblivion for some unknown reason

all for now

[davekni]

John,

Certainly high-Q resonances are a problem when making broad-band (ie. DC up) feedback loops around them, such as voltage regulation in an SMPS.  Or, specific to my work experience, closed-loop position control around a motor/belt drive.  The belt (spring) and motor and load masses made a relatively-high-Q resonance.  That made high-bandwidth position control very difficult without instability.

On the other hand, for a resonant circuit being driven at resonance, high-Q usually helps keep the circuit locked to the desired frequency.  UD2.7 or similar controller will lock reliably to the highest-Q resonance.