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Re: DRSSTC eye candy (sparks)

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

Hi Ken,

I will try to answer your questions below:

Firstly, see the new schematic i *just* posted after i realized i had
not updated the one on that page:


On Thu, 17 Mar 2005 19:10:46 -0700, Tesla list <tesla@xxxxxxxxxx> wrote: > Original poster: "K. C. Herrick" <kchdlh@xxxxxxx> > > Steve (& all)- > > Very nice, indeed; love those sparks! I hope your work will help restore > some of my motivation; I haven't had a running coil for a year or so, now. > > A few questions: > > 1. I take it the "1:100" c.t. is 2, 1:33 in tandem, as you mentioned > elsewhere--right?

Actually, right now on that coil i use 4 CTs total, in pairs of 2 1:33
CTs in cascades.  So for my feedback circuit i have the output of my
bridge pass through a 33 turn CT, then the output of that CT forms a
single turn into another 33 turn CT.  This in effect gives about a
1:1000 ratio, stepping the current down by 1000.  I use this exact
same method for feedback as well as the over current detection
circuit.  The only difference is that the feedback is terminated for a
"digital" signal (hi/low) and the overcurrent detection CT is
terminiated with a resistor for an analog representation of the
current as a voltage.  Hope that is clear, it should be if you look at
the schematic.

> 2.  What IGBTs do you use--and would they work adequately @ 100-130
> KHz?  (My 2 secondarys are 100 & 130 or so KHz. & I wouldn't have the
> energy to make a new one.)

Im using some mini-block IGBTs made by fairchild, part number
HGT1N40N60A4D.  They are hefty little guys rated 110A.  They work just
fine at 100-130khz (i used them at 300khz in my mini coil).  In fact,
before i wound my new secondary i was using one that operated at about
130khz (this meant tuning the primary to 95khz).  I pushed them to
over 1000A pulsed at 95khz with no failure or signs of stress.  You
might want to check out the IGBTs terry is using (hopefully terry will
post the part number again, i dont remember it off hand) since you can
currently buy them.  Fairchild seems to have discontinued the
particular IGBT i use, though i still see people sell them on ebay
from time to time.

> 3.  Why the disparity of 55 vs. 75 KHz, primary &
> secondary?  Expediency?  55 KHz to suit the IGBTs?

2 reasons for the drastic tuning difference.  Frequency splitting
causes there to be 2 new frequencies of preferred operation, one above
and one below the natural Fr.  In theory we could drive the coil at
the upper "pole", or at Fr, or at the lower "pole".  When tuned to Fr,
you will get a notched waveform, but tuning to the upper or lower
poles, you get a nice linear ring up in the primary and secondary
circuits (this i believe allows you to transfer energy without time
limit).  I chose the lower pole because streamer detuning will cause
the secondary to have a lower resonance.  Now i tune the coil even
LOWER than this lower pole frequency, due to streamer loading.  So
what happens is, corona forms during the first few cycles of drive,
but not enough to really pull the system into tune.  The primary
current and voltage build tremendously during this time until that
corona grows enough and the secondary frequency shifts.  Eventually
the 2 circuits are perfectly tuned (usually the last 3-4 cycles of
each "burst").  When this happens, all that built up energy in the
tank circuit gets sucked out by the secondary coil!  IF the secondary
doesnt break out (either insufficient voltage, or improper tuning)
then all the energy goes back into the filtering caps.

> 4.  Could you amplify on the circuits of your drawing DRSSTC Controller
> dated 12/18/04?

Oops, thats an old schematic and does not represent the circuit as it
is now!   See this new one (it is full size, should not have any
problem reading it, you may need to make your browser expand it):


Basically how the FF works is this.  The first interrupter pulse turns
the FF "off" making Q\ high (enabling the gate driver).  When the
interrupter goes low, the FF is active and waits for a change in the
clock, at which point Q\ goes low. That is until the next interrupter
pulse comes along several mS later to turn it back "off" (which is
really ON!).

Now there is a bit of an error here, and that is, what if there never
IS a clock input?  Well the FF is turned active (waiting to recieve
clock input) but it never gets it, so its never reset (the coil wont
do a thing in this case!).  What i did was send a delayed interrupter
pulse into the  set (PRE) pin so that if there never is a clock input,
the whole thing "resets" regardless.  This delayed pulse into PRE must
delayed by about 2X the length of an RF cycle so that this pulse
doesnt terminate the burst prematurely (that is, PRE changes before
the clock input changes), which would negate the benefit of the FF.

Hope that is clear.

Let me know if you have any other questions!

Steve Ward

I'm troubled that the J-K FF's /CLR is asynchronous with
> its CLK. Also, the J-K's /Q output goes high at /CLR=true, and so would
> seem to enable the drivers at that time. I'm confused. How do you manage
> to synchronize the gating with the primary current's z.c?
> Ken Herrick
> Oakland, CA
> Tesla list wrote:
> >Original poster: Steve Ward
> ><mailto:steve.ward@xxxxxxxxx><steve.ward@xxxxxxxxx>
> >
> >Hey guys,
> >
> >It was in the low 50's (temp) today so i cleaned up the garage a bit
> >and brought my DRSSTC out there for a run. I was happy to see it
> >pegging 5' and reaching out to 6' whenever it wasnt arcing to
> >something closer. You can see the pictures at the BOTTOM of this
> >page:
> >
> ><http://www.stevehv.4hv.org/DRSSTC1.htm>http://www.stevehv.4hv.org/DRSST C1.htm
> >
> >
> >Just a few notes about this run. I did blow the 15A fuse twice (the
> >fuse was all that failed). I think this might be partially due to my
> >VERY large filtering capacitors (22000uF each, 200V). The 7.5A variac
> >wasnt even warm after a long run, so im a bit skeptical if im really
> >using 15A rms. The peak primary current surged to about 700A during
> >stressful "short" arc conditions (like to the strike rail at about 2
> >feet). The longer sparks (most of the connected streamers are at 5'
> >length or better) did not trip the OCD. The sparks out to the wall on
> >the left were measured to 67", and i had a few reach out past 6'
> >(didnt get any on camera though). I will have to bring the coil
> >outside to see what it can really do... i never reached full input
> >voltage. I was running the coil at 120bps and about 150uS on period
> >for about 8.5 cycles. I found that increasing from 120uS to 150uS
> >made a considerable improvement. I even had it cranked up near 180uS
> >at one point where i was getting 5' sparks at maybe 65% input voltage!
> > Pretty much out of room in this garage though, so i will have to wait
> >till it gets dark and run it out in the back yard and see where this
> >guy tops off for spark length, my guess is about 80" will be the very
> >max.
> >
> >Comments and questions are welcome.
> >
> >Steve Ward
> >
> >
> >
> >