Re: Tesla Coil Blunderbusses

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Richie,
           Thought your interest might be piqued sooner or later ;)

On 18 Apr 01, at 16:40, Tesla list wrote:

> Original poster: "R.E.Burnett by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <R.E.Burnett-at-newcastle.ac.uk>
> 
> 
> Hi Malcolm, all,
> 
> I have been pretty busy recently with real work,  but I am following
> this thread with great interest !
> 
> Over the last two years I have done quite a lot of simulation work on
> AC resonant charging behaviour.  My observations have been that the
> ballast inductor (or leakage inductance of NST) is primarily charged
> during the first portion of the capacitor charging cycle, (when the
> tank capacitor is empty.)  Then during the later portion of the cycle,
> the ballast inductor becomes a "source" and releases this energy to
> charge the tank capacitor to a higher voltage.
> 
> (For those who find this explanation a little too concise,  there are
> some graphics and explanations of this effect on my web site.)
> 
> I understand what you are saying Malcolm,  about the gap conduction
> re-directing the energy from the supply into the ballast during that
> time. In my opinion the amount of energy stored in the ballast as a
> result of the gap being "closed" would be comparatively small compared
> to say a 5ms charging time.  However if your static gap is firing at a
> high repetition rate,  then I appreciate that the duration the gap is
> closed could become significant.

I did hesitatingly say that the difference might be marginal 
considering the time involved.
 
> My gut feeling is that the boost due to the gap conduction time won't
> make more than a few percent difference to the peak voltage.  But, it
> is definately this "inductive-kick" from the ballast which allows
> widely spaced static gaps to fire smoothly.  The switch-on transient
> can be enough to initially breakdown the gap,  and then the inductive
> kick from the previous firing of the gap boosts the voltage to achieve
> the next gap firing !

I do think this is definitely at work in marginally set systems (like 
one of mine for example). 

> I have not had time to do so yet,  but I will run some Microsim 
> simulations to check out this theory about the quench time.  The
> results are of great interest to me,  as I had always considered the
> charging system and the coupled resonant circuits as two seperate
> entities during simulations so it would run quicker.  Now you're
> telling me they might interact ! ;-))

Thanks for offering to do a run.
 
> Looking at the double-resonant behaviour,  I did find one interesting
> fact when experimenting with tuning and coupling some time ago.  I
> found that if the primary and secondary resonant circuits WERE NOT
> tuned accurately to the same resonant frequencies,  then the energy
> transfer from primary to secondary was incomplete.  (This was in the
> simulation world,  without any accounting for corona loading,  etc.)

I'd forgotten about the incomplete transfer. Thanks for the reminder. 
Now the waters deepen a bit with regard to the results I got!

> The result of this inaccurate tuning,  was that the primary current
> notches were not as deep.  Specifically,  the envelope of the RF
> current in the primary circuit went close to zero amplitude,  but did
> not quite reach zero.  My conclusion at the time,  was that accurate
> tuning is important to get a deep primary notch,  and therefore give
> the spark gap a decent chance of interrupting the current.
> 
> All of this was without considering the drop if Fsec due to corona
> capacitance.  I also thought that good spark breakout loads the
> secondary heavily.  As a result good spark breakout actually
> dissipates the secondary energy quickly, and little is coupled back to
> the primary circuit.  My conclusion here is that good spark breakout
> leads to an early quench in the primary gap,  (not the other way
> around !)  If you suppress breakout then the primary gap does not
> quench ???

Absolutely agree. Emptying the secondary quenches the gap and not the 
other way around. Couldn't agree more. 

> Out of interest,  my CW inverter driven TC also displays intermittent
> spark breakout when the driver is tuned too low compared to the
> natural frequency of the resonator.  I have noticed that the Fres
> drops by 3 or 4% during CW spark loading (8" spark.)  But if the
> driver is detuned further, then sometimes the resonator would ring-up
> and pull itself into tune, resulting in a spark.  Other times it would
> not ring-up so it doesn't pull into tune,  and no spark would
> breakout.  This says to me that some detuning occurs BEFORE the
> visible spark breaks out ???

I understand what you are saying. This is turning into a can of worms.
 
> I'm not sure if the SSTC behaviour is really the same behaviour you
> are discussing,  but there may be some clues in there somewhere ;-)
> 
> A very interesting area.  I will let you know what the sims turn up.
> 
>       Cheers,
> 
>       -Richie Burnett,
>       (Newcastle, UK)

Thanks Richie!

Regards,
Malcolm