Re: LTR vs. more primary turns

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

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

Hi Dave,

On 5 Dec 2002, at 11:17, Tesla list wrote:

 > Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
<dgoodfellow-at-highstream-dot-net>
 >
 > If you start on the transformer end of the project, let's say you are
 > building a coil based on a 12/60 neon matched up with a .02uf capacitor
 > bank, then by what means do you determine the ideal parameters to construct
 > the secondary from?

The considerations that should be applied: First off, why 0.02uF? Is
that going to give you the maximum energy per gap fire that your
transformer is capable of delivering without opening the gap wider
than the transformer's peak voltage rating? (hint - visit Richie
Burnett's website for insights). Knowing how much energy per bang you
have available is the starting point. Cp and Vp are decided.

Second(ary)ly: decide on a target output voltage. Run the figures on
it. Start with the assumption that you are going to get an 80%
transfer of initial energy.
      Example - suppose you decide on 1MV. How much capacitance is the
secondary+topload going to have (you will quickly see it is an
unrealistically low figure for your supply ;) Choosing 100kV is just
as bad - you are missing out on spark reach. I do not have a hard
formulaic answer for this (yet) but I expect it to be one thing which
pops out of the TSSP research project. In my case, it is based on
observation. So choose, and calculate. The higher the voltage, the
taller the secondary must be to withstand it. As the secondary gets
taller (and its topload gets larger), its capacitance increases and
output voltage drops. ROC of the topload figures in this as well. It
is desirable to operate on the edge of breakout to maximize secondary
voltage before it lets go.

       Out of this mix drops Hs, Cs, Ctot and Ds. Now it is down to a
wire choice to get a balance between secondary Q and as low an
operating frequency as possible (to minimize primary losses). Hence
the tradeoff is primary vs secondary Q. I'd go for a balance, say,
wire being no less than 2 and preferably 3 skin depths thick at the
lowest frequency of operation. Assuming the coil is closewound, you
now have Ns, Ls and Fr decided. All that remains is to build a
primary with sufficient L to resonate at, say, 0.9Fr with the primary
cap. To a large extent, primary losses are now defined as peak Ip is
now defined but in following the above procedure, you would be hard
pressed to improve on this.

     Lastly, gap design enters the picture. There are many types and
designs and all can be made to work, some better and less lossy than
others. Ultimately, you want reliable firing at Vp. In order to
minimize losses, you want as little gap spacing as it takes with as
few gaps in series as it takes (there is a voltage drop associated
with each). Again, Richie's website summarizes gap characteristics
well.

      This is the basic formula I am following in the tabletop
competition (which will be completed as soon as other more pressing
matters are dealt with). The primary is now wound, the secondary is
now chosen (a suitable one was one on hand - not perfect, but
suitable).

Regards,
Malcolm

**Engineering is the art of compromise**

  I am very interested in how you determine this, because
 > I had a situation where I thought that I was powering a coil with a 12/60,
 > but it turned out to really be a 9/60. With the 9/60, the coil was producing
 > sparks about 30" long. When I realized my mistake, I installed my 12/60 to
 > the system, expecting even better performance. If there was a performance
 > improvement, it was about impossible to see. Effectively, the output looked
 > identical despite my experimenting with the tap point and the spark gap. I
 > felt that the only way to longer sparks on that particular instance, without
 > changing out the secondary, would have been by using a larger toroid. The
 > coil was fashioned after John Freau's tt 42 as far as turns, height,
 > diameter. I still think that he uses some kind of Voodoo magic to get 42"
 > sparks from a 12/30 neon, (grumble grumble).
 >      If you have a way to determine the optimal secondary for a given
 > transformer, I am all ears!
 > Thanks, Dave
 >
 >
 >
 >
 >
 >
 >  > What John said is true. Starting from a given secondary is not the
 >  > best way to design a good coil. You should be starting from the
 >  > capabilities of your chosen supply. That is exactly how I design
 >  > coils. Once you've wound the secondary, you're stuck with it for
 >  > better or worse.
 >  >
 >  > Regards,
 >  > Malcolm
 >
 >
 >