Re: Spark length vs. coupling results

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

Original poster: "Steve Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>


 > Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>"
<chunkyboy86-at-yahoo-dot-com>
 >
 > Hi,
 >
 > Thanks for running the test! Why was the DC voltage so low?

Because I knew that as I reduced the coupling the losses in the IGBTs would
get a lot bigger, and I didn't want to overheat them. I overestimated the
losses though and if I did it again I would start with at least 200V.

Why did you
 > chose 40 bps? Do you feel like runing a high bps test to see if streamer
 > growth affects anything?

I chose 40bps because I wanted a frequency high enough that my scope would
show a steady trace but low enough that streamer growth would be out of the
equation. Again there's no reason why I couldn't have used more at least in
short bursts.

It sucks that testing different frequencies is
 > hard. If you put a shorted turn around the secondary, so that it is loosly
 > coupled, it might reduce the inductance enough to raise the frequency, as
 > if it were a different secondary. Any thoughts on this idea?

Maybe but I doubt it would raise it by much. Replacing the topload for a
smaller one would probably be a better bet. The bowls that I made the
topload from come in three sizes :)
 >
 > It has been known forever that higher coupling helps spark gap coils (
yes,
 > even before tesla coils were invented ;-)), and it would make sense that
it
 > would also help OLTCs. In the case of DRSSTCs and SSTCs with untuned
 > primaries, longer rise time helps, because it allows more time for
 > transfer, and therefore, the peak current is less.

I'm confused now, in that case what would be the point of using a tuned
primary on a SSTC? I saw Richie Burnett's SSTC both with an untuned primary
and a series-tuned one. With the series-tuned primary it produced longer
sparks and didn't need a breakout point.

I think the main advantage is that the series-tuned primary works
efficiently with a looser coupling, so it doesn't need to be as close to the
secondary, and more voltage can be generated before it flashes over. However
the tuning is more critical especially since Richie's coil isn't a feedback
one.

 >
 > The waveforms look a little bit weird at first, because the peak doesn't
 > line up with the notch, but it makes sense. Because there is loss, the
 > point where energy in equals energy out occurs when the primary is still
 > adding energy. It looks like there is very little damping in the system,
 > how big were the streamers?

At the times the scope traces were taken there were no streamers at all,
just a very small bit of corona round the breakout point. I'm puzzled
because I once tried connecting an IGBT switch to my big coil (running at
much reduced voltage obv) and I got textbook waveforms all the way out to
the 4th notch. I think it is more probably losses in the _primary_ of the
OLTC that mess up the waveform.

Steve C.