Re: Even More DRSSTC numbers

["Tesla list" <tesla@xxxxxxxxxx>]

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

Hey Steve,



On Mon, 24 Jan 2005 08:02:29 -0700, Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>
>
> >Looks like the best i
> >can do is K=1.57, vs the 1.7 figure typically seen, and thats at 70uS
> >burst lengths (only 20" spark though).
>
> That's on your little DRSSTC-3 though right? Small coils do seem to be less
> efficient than big ones.

yes, the DRSSTC-3, the little guy.

>
>
> >One interesting thing is that the peak currents dropped lower as i
> >increased the burst length!
>
> That's not really surprising since you were decreasing the AC input voltage
> as you increased the burst length. Why did you do this, was it to keep a
> constant spark length or something?

I was so caught up in the fact that i had the numbers, the obvious
fact that lower buss voltage implied lower peak currents had not
occured to me until a bit after i sent this email...  *smacks
forhead*.

Yes, i wanted to leave the target distance constant.  Leaving it set
in one exact postion and fiddling with the controls at MY end is much
easier.  I need to also take figures at several more spark lengths (vs
voltage input) to generate a nice curve for each individual uS
setting.  Boy, lots of numbers to write down... i might start getting
hand cramps ;-).  I need to (more importantly) start making graphs of
this data, so its much easier to see the implications.

>
> If you had kept the input voltage constant, I would have expected the peak
> current to go up as you increased the burst length, or stay constant,
> depending on the tuning and coupling.

The current didnt seem to grow any higher after about 100uS...
interestingly enough, this is about the most "efficient" burst
length... coincidence? i think not.

>
>
> >I'm pondering either converting the setup to a full
> >bridge, or altering the tank Z
>
> This would be an interesting experiment sure enough. Like you say, halving
> the tank Zo or doubling the drive voltage should have the same effect and
> should roughly double the peak power. (halving the tank Zo would double the
> peak current though and you're at 400A already)

On that subject, one other test is too see how far these IGBTs and be
pushed reliably ;-). If i do half the Zo, i will of course have to
reduce the ON time accordingly as to not go thermal ;-).  Im expecting
that with such tank impedance, it will achieve higher peak powers, and
i will need much less time to ring up the primary to achieve longer
sparks.  My guess is that with 1/2 the Zo i might only need 50uS or so
to get the same 25" sparks at 100% input voltage.  Im thinking the
IGBTs should be just happy at say 600A, maybe 800A.  Jimmy would run
them at 1200A or something at laugh at us (remember, he pushed 150A
IGBTs to 1700A or something).

>
> No matter what the result of this experiment we will learn something
> interesting about streamer loading. If the coil does not get any more
> efficient then that suggests the toroid capacitance is the limiting factor.

Right.  I think the Q of this particular secondary is "ok" so it
should probably come down to the toroid, or possibly the coupling
factor, but im not sure just how crucial that is for efficiency (it
does seem very important for long sparks though, at higher powers
accordingly).

> Maybe the root cause of low performance in small coils, is that a small
> toroid just doesn't have enough charge on tap to meet the demands of a
> decent sized streamer. After all we know that a streamer grows by jumps
> drawing large HF pulsed currents as it goes and these HF pulses can only
> come from the toroid capacitance itself.

Right.  Bigger toroids seem to brighten the sparks as well, another benefit 
;-).

>
> So I would suggest another experiment- leave the tank Zo constant, use the
> same secondary, but add a ridiculously large toroid.

That may be impractical at the moment, i already trimmed off my extra
primary turns in order to reduce flash over between the coils.  After
i change the Zo to a lower value i will have enough tuning room for a
bigger toroid i suspect.  I think even if it works better it may be
undesirable still, since the bigger toroid makes the sparks look
smaller in comparison.

Steve Ward


>
> Steve C.
>
>