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Re: Large ferrite cores for SSTC work

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).


 - Jan

 high voltage at http://www.hut.fi/~jwagner/tesla