Re: Rotary Spark Gap Design
Tesla List wrote:
> >From MALCOLM-at-directorate.wnp.ac.nzTue May 14 12:53:13 1996
> Date: Tue, 14 May 1996 17:28:48 +1200
> From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
> To: tesla-at-pupman-dot-com
> Subject: Re: Rotary Spark Gap Design
> Hi all,
> Rod, I was interested to see the specs. you posted on the
> large coil. I would be interested to know the height of the secondary
> winding as near as poss. to calculate the theoretical maximum output
> which I suspect isn't even close 3.5MV. Reason? There is a reality
> check you can do :
> Assume that Cp is charged up to it's highest possible voltage
> and that ALL this energy gets transferred to the total secondary
> capacitance (Cself+Cterminal) (ignore gap losses), the theoretical
> Vs(pk) = Vp(pk) x SQRT(Cp/Cs). This is easily derived from a
> conservation of energy viewpoint. Also, since Cp.Lp = Cs.Ls (tuning
> requirement), Vs(pk) = Vp(pk) x SQRT(Ls/Lp). This second equation is
> given in two separate papers that I know of (one was Hoffman).
> On this basis I shall stick my neck out : Robert Golka isn't
> getting anything like 12+ million volts (I suspect he is doing a spark
> length estimation which is totally inaccurate for high frequency
> discharges). I would expect absolutely humungous sparks for 3.5MV
> at T.C. frequencies.
> I'll be interested to hear what others have to say.
> You wrote :
> > The original design was taken from Bill Wysock's
> > 3.5MV Super Model 9 Tesla Coil.
> > We have this coil in our National Science and Technology Centre
> > (Questacon), Canberra Australia.
> > Very impressive too!
> > Specs:
> > ------
> > Power Supply - 20KV -at- 12.5KVA
> > Current and Voltage Limited by variacs of some kind.
> > 2 x 0.1uF Caps in series = 0.2uF -at- 50KVDC each.
> > Rotating Spark Gap (see GIF file)
> > Primary - 5 turns 1/2" copper pipe, archimedes flat spiral
> > Outside Diameter - roughly 28" - 29"
> > Inside Diameter - 2" larger than secondary, ie, 1" spacing
> > between primary and secondary.
> > Secondary - 800 Turns of 3mm (appears plastic insulated)
> > Diameter - roughly 15"- 18"
> > 30" x 10" professionally made Alluminium toroid
> > Frequency - 80Khz
> > Output - 3.5MV (at full power)
> > Spark Lengths - roughly on average 6-10 feet.
You are attempting to deal with lumped constants when you try to say that
the secondary capacitance is c self + c term. I do not necessarily agree
with the voltage as specified but what if you deal with the secondary as
a wave guide or transmission line instead. It seems to me that the
capacitance in the secondary may be extremely small when referred to the
primary and the cv=cv equation will yield much higher voltages in the
secondary. This may also help to explain why higher Q secondaries yield
longer discharges. I'm not into the math on this but it seems to me that
I remember from college that the capacitance looking into a transmission
line or wave guide is not the sum total of all of the capacitance of the
line or guide.