Primary current feedback on the DRSSTC-3

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Yesterday i managed to solve all of the problems that i was facing
with my DRSSTC-3 and using primary current feedback.  Today im happy
to claim longer sparks :-)

For anyone who is basing a DRSSTC design off of my designs, you might
want to check out the updated schematic for my DRSSTC-3:

http://www.stevehv.4hv.org/DRSSTC3/DRSSTC-OCDsch.JPG

OCD stands for over current detection... not obsesive compulsive disorder ;-).

Anyway, I borrowed some things from our very own DRSSTC newbie, Terry
Fritz ;-)  I liked the idea of clamping the output of the CT with
zeners and using some schottkeys to take care of the slow recovery
time.  I modified the flip flop circuit so that it doesnt have any
issues with needing noise in the feedback input to start oscillating.
You can also see the OCD in that schematic, it works beautifully.  I
can set it anywhere from around 200A to 600A, and i do hope to
eventually be using near 600A ;-).

I found something interesting with the tuning of this coil now.
Originally the primary was tuned somwhere in the middle of the 2
modes.  This worked well with very high coupling as normal, but i did
see some limit to my spark length (about 24" max).  I really liked the
idea of tuning to the lower pole, becuase when the coil starts making
streamers nearly 3X its length, its gonna detune considerably!  So i
started tuning the primary lower.  Instantly i noticed the primary
current had a much nicer linear ring up, instead of the choppy looking
current ring up when tuned in the middle.  I had to reduce my coupling
to about .15-.2 because arcs began racing up the side of the
secondary, and jumping to the primary nearby.  When tuned lower, it
took more input voltage to get it to produce real streamers, but once
streamers start forming, the result is like an explosion of streamers!
 Looking at the primary current, its forming a notch at the end of the
burst (current rises and then returns back to 0).  If i turned the
power even higher, a second "burst" began to form after the notch.
The notch occures at about 18 cycles or so... and im not sure what
causes it, but i think its a sign that the coils need to be tuned
better.  So i slapped on a turn of 12 awg at the base of my primary to
get more tuning room.  Now it takes even higher input voltage (about
60-70% input) but the resulting sparks are even longer!
Interestingly, primary current increases with voltage input, until
long streamers are formed, then the primary current doesnt increase
any more at all!  Right now im running about 420A.  But even still, as
im getting about 30" sparks, the notch is occuring again at the end of
the burst... need more primary L!  Its easy to see the improvement
that adding more primary inductance has.  Im tuned at least 1.5 turns
lower than before (the coil only had 5 turns to start with!),
obviously these streamers really do detune things a lot.

One other benefit i see to running at the lower pole is that you are
reducing switching losses.  You now have fewer RF cycles per burst
length, and also, any delays in the gate driver become less
significant as each half-cycle is now longer.  I think my primary
circuit is running at about 170khz, the secondary Fr is something like
220khz.  Hey, but it works great so far!

More tests (and numbers) to come later.

Steve W.