Re: Large ferrite cores for SSTC work

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "rob by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rob-at-pythonemproject-dot-com>

Tesla list wrote:
> 
> Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jwagner-at-cc.hut.fi>
> 
> > Actually, a transformer based feed with one wire going to a magnifier
> > coil has a very important advantage.  If wound so that its self
> > resonance is much higher than that of the coil, the driver only draws
> > large current at coil resonance.
> 
> Umm... wouldn't it be an advantage to have the transformer have
> its self resonant freq at around f_res of the TC secondary? (although, in
> practice, this will be almost impossible to do because f_res is not constant)
> 
> > Whereas the direct primary approach
> > looks like a short circuit everywhere but at resonance, unless you use a
> > huge number of turns,
> 
> I must concur, there - the input impedance of the primary with the
> secondary coil in place is at its lowest point at f_res of the secondary,
> because the secondary has the lowest impedance at f_res.
> 
> I.e. the primary is a low impedance at f_res.  Around f_res, impedance is
> much higher, thus less current draw.
> 
> For example, I've a 300kHz TC secondary coil with an about 20 pri turns
> helical ~4" dia coil. Current draw at 200kHz is close to zero!! whereas,
> current at 300kHz is around 3A rms. (with no primary series cap)
> 
> IF it gets a problem then you can add a series capacitor to the primary
> coil to get a multi resonant system drawing very little current at
> (too-)low frequencies, so no problem here. Tuning isn't a problem either
> if you sense the secondary base current and phase sift vs voltage accross
> the primary coil.
> 
> I've tried the base feed method a number of times, in the range 20kHz to
> 300kHz with different xfrms, but results have always been _very_ wimpy
> compared to the primary coil drive method.
> 
> The main reasons were huge losses due to corona caused by capacitive
> coupling to the ferrite core (NOT by weak insulation!), and also some
> core losses as well as copper losses because all those windings never
> wanted to fit freely in the winding window area, so I had to use thinner
> wire/ribbon than first intended...
> 
> Well, it is very likely that this is due to my lack of skills&experience
> in RF HV xfmr design. Someone with more knowledge could do a much
> better job... :o)
> 
> > in which case the whole scheme is sort of
> > useless.  There won't be much step-up in voltage.
> 
> Well... yes... It probably takes longer to transfer the same amount of
> energy than when compared to the base feed method. But in my opinion the
> base feed is by far less efficient (too much heating losses in
> the transformer and corona on the xfmr surface to core).
> 
> cheers,
> 
>  - Jan
> 
> --
> *************************************************
>  high voltage at http://www.hut.fi/~jwagner/tesla

If the transformer core is heating up and you have corona, then you have
a misdesigned transformer.  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.  Now since we are not at 50ohms, you have to do some
experimentation to find the right match.  It helps to build some
transformers with different amounts of secondary winding.  The one with
the best performance wins.  1:10 ratio is on the high end.   Rob.

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