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,
           Thankyou for doing the simulations. One or two 
comments/replies below:

> 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 ran a few simulations on Microsim today to investigate Malcolm's
> theory that the conduction time of the static spark gap could
> influence the resonant charging behaviour.  All the gory details
> follow...
> 
> I modelled a 10kv/100mA NST driving a 10nF capacitor from a 50Hz
> supply. Gary Lau's "Static gap model" was used to fire at a fixed
> voltage and conduct for an adjustable duration (quench time.)  I did
> not model the oscillatory behaviour of the TC primary or secondary - 
> just dumped all of the capacitor energy into the switch.  (The
> simulator doesn't mind ;-)
> 
> The aim was to see if a change in the quench time would influence how
> smoothly the static gap would fire.  I specifically chose a small tank
> capacitor to minimise the resonant voltage rise,  (otherwise the gap
> would still fire at 80kV !)   I also believe that a smaller tank cap
> would have its terminal voltage influenced to a greater extent for a
> given change in the stored ballast energy.
> 
> The firing of the static gap was very erratic,  and there were certain
> bands of breakdown voltages for which operation was regular and others
> where it was totally chaotic.  I _eventually_ managed to set the
> breakdown voltage so that the static gap would continue to fire if
> kicked with an initial breakdown.  However if the gap was to stop
> firing,  it could not restart by itself.  This is a condition I have
> observed in practice, and I believe this is the precarious balance
> that Malcolm intended ???

Exactly.
 
> I have posted two GIF files showing the results of two simulation
> results:
>  www.staff.ncl.ac.uk/r.e.burnett/mw01.gif
>  www.staff.ncl.ac.uk/r.e.burnett/mw02.gif
> 
> The mw01.gif graph shows the firing of the static gap with a breakdown
> voltage of 28kV and a conduction time of 100us.  The gap is
> "triggered" at 5ms and fires smoothly until t=42ms.  From t=42ms
> onwards the static gap does not fire as there is insufficient voltage
> to cause breakdown in this particular arrangement.  (In practice the
> transformer would just sit there growling.)

Perhaps until a serendipitous cosmic ray came by. ?
 
> The mw02.gif file shows exactly the same setup with the conduction
> time increased to 150us.  The spark gap is triggered again at 5ms, 
> but this time continues to fire smoothly until the end of the
> simulation !
> 
> Overlaying the two pictures does indeed show a slightly faster rate of
> charging in the case with a prolonged gap conduction.  Exactly what
> Malcolm predicted.   See: www.staff.ncl.ac.uk/r.e.burnett/mw03.gif
> 
> (I repeated this sim a few times with different component values and
> breakdown voltage.  It was often possible to affect the stability of
> the static gap by altering its quenching.)  If anyone would like to
> verify this behaviour with different component values then I have put
> the Microsim schematic on my web page:
>  www.staff.ncl.ac.uk/r.e.burnett/mw01.sch
> 
> I also ran a simulation with a 200BPS sync rotary in place of the
> static gap, because I'm not sure what is happening here !   In this
> case changing the conduction time from 100us to 150us (or even 200us)
> made no significant change to the power throughput.  The peak voltage
> actually dropped by around 1% for the later quench.

In practice, the rotary is going force firing as the electrodes move 
closer together, one reason I expect it works so well. Which means I 
*have* to build one.
 
> Personally,  I would not like to say whether this behaviour is really
> significant in terms of practical spark performance,  but the
> simulation does prove that gap quenching can influence the stability
> of a widely set static gap.
> 
> One additional consideration....  (Just to muddy the waters further
> ;-) In practice I have seen the breakdown voltage of static gaps vary
> wildly during repetitive firing.  I once used a storage scope to
> capture a half second of gap voltage in my coil and the breakdowns
> occurred at a wide range of voltages !  When tested with a one-shot
> method the breakdown voltage was uniform and repeatable,  but dropped
> considerably when running some real power.  I guess this effect would
> also act to maintain the light in an already active gap.

I would agree. Hot electrodes also conspiring. However, I did notice 
that running a coil with a static gap from a flyback cap charger led 
to rather more repeatable firing periods and voltages. In fact it was 
so stable I was able to see the firing rate on an ordinary scope with 
the timebase set to 5mS type rates.
 
> I think the decaying performance often reported from overheated TCBOR
> gaps may be due to this reduction in firing voltage rather than due to
> actual loss of quenching ???  You can hear the gap scream as the
> repetition rate increases ???

One rather poor coil of mine (very early I hasten to add) with a rock-
bottom Lp/Cp ratio in the primary had a tendency to almost totally 
power-arc the supply after the electrodes had been running a while.
 
> Well it's a big subject,  and I have rambled to much already.
> I'll listen closely for others thoughts on this...
> 
>        Cheers,
> 
>        -Richie,
> 
>        (Newcastle, UK)

Many thanks again. BTW - I enjoyed visiting you webpage. I recommend 
it to all.

Regards,
malcolm