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Re: Large ferrite cores for SSTC work

Original poster: "Harri Suomalainen by way of Terry Fritz <twftesla-at-qwest-dot-net>" <harri.suomalainen-at-pp.inet.fi>

> Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> > It is there simply to match the FET driver to the input of the
> > magnifier.  If you have < 2:1 SWR into the magnifier, then most of the
> > available power will go to the magnifier and sparks produced.
> By "match", you probably mean a current the driver can deliver at which
> it does not yet blow up? At this (and smaller) output current, the driver
> and coil impedances will always be "matched". I don't know about SWR -
> there shouldn't be any, unless the fets once again decide to blow up or

Conformed. You will be driving the coil at the frequency where it resonates.
At that point it will act almost like a resistive load. (At least all my
did act like that in all other points than the actual spark discharge point.)
No swr problems are there unless you drive it at above or below that point.
Then there will be SWR you'll notice and the coil will not draw much
current either.

Usually you might like to be on a bit high side of the resonant point to
achieve ZVS system. The coil can be used to get there too but the
coil and power lever should be at a suitable level for that. Way too
complex to design at purpose usually.
> > 1:10 ratio is on the high end.   Rob.
> *ouch*! I did built a couple with >1:20 ratio (about 6kVac), although I
> started with 1kV. 10cm was the best streamer length achieved...
> subjectively impressing, objectively not. :o(

The ratio depends a lot on the actual coil. For some coils lower ratio will
be enough for the power wanted. For others you might need a higher
ratio. For example, one coil needed 1:14 for 200W while another
used about 1kW at that ratio. The coil caracteristic impedance is the
key variable here.

This can be predicted very accurately theoretically (say to within 10%).
However, it is usually much simpler to just increase the ratio up to the
point where the power is at a suitable level. You'll be running at lower
power (ie. lower ratio) at prototyping lever anyway untill you tune
it up gradually for more power.