Re: drsstc questions

["Tesla list" <tesla@xxxxxxxxxx>]

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

Hi Mike,

Comments:


On Sun, 27 Mar 2005 22:37:54 -0700, Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: "Mike" <mike.marcum@xxxxxxxxxxxx>
>
> I was thinking of jumping on the bandwagon and i was wondering several 
things:
>
> 1. Could these things be ran w/o the big electrolytic caps if it was ran
> from a 720v Li ion battery pack (5A max continuous current draw)?

Afraid not.

 Or are
> they more than just for filtering?

Indeed they are!  They have to supply several hundred amps for a few
hundred uS (typically 150uS for smaller coils).  I dont know if a
battery will do that.  Also, compare the ESR of a giant electrolytic
with that of a battery.  I dont know the ESR of those kind of
batteries, but i have a feeling its pretty high if its only rated 5A
continuous.

 I figured if I'm gonna do this might as
> well make it portable than can be ran anywhere since it'll be alot lighter
> than 100 lbs of nst's and a variac. I don't plan to go over around 2kW.

Use the batteries *AND* a big bank of lytics!  No worries then ;-).
It is starting to get a bit heavy though!

>
> 2. Has anyone figured out the most efficient on/off times/bps for a given
> power to get the longest spark?

Ive been trying to figure this out.  So far, with my coils, it seems
that my smaller coil (500W max, 36" sparks) prefers about 140uS ON and
8mS or so OFF.  My next size up (up to 1500W, 72" sparks) seems to be
working great at about 200uS ON and again, about 120bps (or a bit less
than 8.33mS OFF).  My BIG DRSSTC (4000W, 11' sparks) needs about 300uS
to do its thing.

But these numbers are slightly irrelevant unless you select the right
tank impedance!  The tank impedance will determine the slope of the
primary ring up, and also your peak power.  Higher peak power means
shorter time, lower peak power, longer time.  I found that either
extreme doesnt perform so well.  For a comparison, my 1500W coil uses
about 800A peak at 400VDC.

> I figured if I got a half bridge of those Powerex 1200v 600A bricks I could
> run them at 5% duty w/o blowing them up with only 2kW.

Easily.

 With that in mind I
> came up with a back-of -the-envelope table from 10us on/190us off/5000 bps
> on down to 1ms on/19ms off/50bps at max power.

I find that 120bps gives the best efficiency still.  Actually, 100bps
works quite well too, but the streamers just lack a bit of intensity.

 Obviously the extremes of
> that won't work since the spark will die before the next pulse or the caps
> will from high rms current. I figured once I found the optimal pulse width
> I could vary the power by varying the bps/off time as long as I don't
> exceed the 5%. Sounds like an experiment for Terry :).

Again, probably no more than 400uS ON time.  I usually dont see a
benefit past about 200uS on the smaller sized coils.

>
> 3. Did an ebay search of the above bricks and didn't find any. How often
> are those there?

I see them quite a bit, but it really fluctuates.  Sometimes there are
several auctions for them, right now there dont appear to be any.
Just have to wait for salvagers to find them and list them!

Feel free to email me if you have any other questions.

Steve Ward

>
> Mike
>
>