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Re: Herrick's... SSTC

Original poster: "Kennan C Herrick by way of Terry Fritz <twftesla-at-qwest-dot-net>" <kcha1-at-juno-dot-com>

[responses from KCH interspersed]

On Fri, 18 May 2001 15:53:19 -0600 "Tesla list" <tesla-at-pupman-dot-com>
> Original poster: "Peter Lawrence by way of Terry Fritz 
> <twftesla-at-uswest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>
> Kennan,
>        am I correct in assuming that since your SSTC is synchronized 
> directly
> to the secondary (ie there is no primary res frequency to adjust) 
> that it
> will drive just about any secondary coil within reason.

Yes.  But "within reason" must take into account rise/fall times of the
primary excitation: At 140 KHz, each half cycle occupies ~3.5 us.  Within
that time period, one has to turn off & on all the MOSFETs, bucking the
Miller effect and suffering the dissipation during the turning-off times.
 Already, that's not much time so I'd think that ~200 KHz might be the
> What would be the practical upper frequency limit of your SSTC, 
> could I use it
> to drive my 3.5" x 12" mini TCs  (I've wound them with everything 
> from #24
> to #36 wire, Fres anywhere from 650kHz to 150kHz (those are approx, 
> I'm trying
> to remember this stuff off the top of my head, notes are at home))?

Hmmm...see above!
> Will it be possible to design a digital control that gives a 
> predetermined
> number of primary oscilations, and then shuts off until restarted 
> (by manual
> button, or by preset time delay)?

Oh, yes.  Currently I use a single CD4024 ripple counter to sequence the
interrupted mode.  One could easily add another one for a larger range, a
selector switch, and a stopping-circuit so as to do that.  Or perhaps
substitute either a decade-output counter or add a binary:decimal
converter, to get finer control.
> Could IGBTs (what Terry is using) be used for more voltage in the 
> primary,
> I'm wondering about using 1600v MOT power verses 160v to compare the 
> results
> of more verses fewer oscilations to breakout (ie build one machine 
> that could
> be run either way)?

My sense has been that IGBTs are slower turning on & off than MOSFETs but
maybe they've improved.  As to higher voltage, sure...but the transistors
must at the same time accomodate the higher current, of course.  As it
is, every 6 pairs of my MOSFETs handle all the primary current; they get
only moderately warm over perhaps 8-10 seconds of continuous running -at-
near-max. I-line.  That's only a rough statement since I've not done any
measuring; but I could probably do with 4 pairs, comfortably, instead of
the 6--absent any additional cooling, that is.
> My order for plans is in!
> thanks,
> Peter Lawrence.

I'll post a notice when I've finalized my offer.  Thanks for your
interest--and that of the others who will read this!

Ken Herrick

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