Re: DRSSTC-3 VCO drive test (long)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "colin heath" <colin.heath4@xxxxxxxxxxxx>

hi steve,
            its interesting your getting good results with the VCO. with
unshielded electronics i have this trouble with my new drsstc if i runa big
topload. it can actually run it C/W!! not good at all. so the vco may be the
way to go with your big coil.
only trouble iv had with this approach is being off tune and hard switching
heating the igbt's but then this was running an induction heater C/W so not
surprising really.
cheers
colin heath
----- Original Message -----
From: Tesla list <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, February 02, 2005 11:42 PM
Subject: DRSSTC-3 VCO drive test (long)


> Original poster: Steve Ward <steve.ward@xxxxxxxxx>
>
> Hey Guys,
>
> Im not doing anything "new", but rather something new for me.  I made
> a nice little VCO box that can plug into my coils where the feedback
> normally plugs in.  I did a brief test and was pleasantly surprised by
> a few things.
>
> I set my spark length to 25" with my little DRSSTC-3.  Now looking at
> my recent records on this coil, i needed about 330W at 120bps, 100uS
> burst length, 400VDC buss, and 450Apk in the primary to get 25".  Now
> compare that to the VCO at the same settings (100uS, 100bps, 400VDC
> in) i find i only need 300W... not bad :).
>
> Also, tuning the VCO for best spark was really easy (im tuning it on
> the fly!).  I have my single turn pot along with a 12t trimmer, as
> well as several dip switches and timing capacitors so i can really
> dial the VCO in on what range i want to operate in.  The single turn
> pot (1k) will sweep the frequency from any range of 6khz to 30khz
> depending on how i set the other "stuff".  Anyway, i could vary the
> drive frequency by +/- 5khz in the "sweet spot" at full power.
> Detuning the coil too much at full power had no ill effects aside from
> a reduction in primary current and spark length.  But the VCO in its
> current state is limited to about 100uS on this setup.  The fact that
> the streamer detunes the coil is causing a bit of an issue.  My idea
> for a solution is a ramp generator synched with the interrupter to
> slowly down-shift the frequency during the burst.  I wish to control
> the amount of frequency shift as well.  I believe the VCO drive will
> be just perfect then.
>
> Things get a little better.  Tested it with heavy ground strikes and
> the system seems to respond very well to this.  The primary current
> actually goes DOWN (most likely due to the slight detuning and very
> low secondary Q at that point).  I like :-).  Any there were no real
> hints that the IGBTs were hard switching.  I could see switching noise
> on the primary current waveform, but they were pretty much by the zero
> crossings from what i could tell.
>
> Some other thoughts about any possible risks of hard switching in a
> VCO driven system...  It seems to me that when the DRSSTC performs
> best is when the drive frequency is the same as the primary tuned
> frequency, which is tuned to one of the poles (right now the upper
> pole seems to be the winner!  In fact this coil runs upper pole with
> feedback as well).  The fact that just a slight mis tuning of any
> "component" lessens the spark length considerably says to me that if
> its making big sparks, all is well.  So to get to my idea... there
> must be very little hard switching when the coil is operating at its
> best, because being in tune implies a situation of being very close to
> a ZCS condition.  Anyone have comments on that or some reason why im
> wrong?
>
> Just as a side note.. i believe what was killing the CM300s in my big
> DRSSTC was the devestating hard switching of 1200+ amps when the
> secondary feedback would go "bad" and lock to 3X Fo.  Thats the worst
> kind of hard switching possible i think ;-).  Now, a VCO system can't
> possibly have a situation like this, where everything is fine until
> say a cycle or 2 before the end of the burst when suddenly it starts
> switching at random.  And if the VCO is out of tune, the primary
> current never builds to super high levels anyway.
>
> Well, thats all for now, its time to get working on those VCO mod's to
> get this thing really tweaked up ;-).
>
> Seeya,
>
> Steve
>
>
>