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Re: Are those dumb questions?

Samuel,

If the TC uses a current limited xfrmer such as an NST or OBIT, then
the cap matching theory has some use.  But you're probably better
off using an LTR (larger than resonant) cap to keep the voltages low
on the NST.  Use a cap that is about twice the resonant size.

But if you use a non-current limited xfrmer, then the cap matching
has no real meaning, except for figuring the absolute largest cap
that can be used, and for calc'ing the ballast inductance needed.

I like to design a TC backwards; start with the desired spark length.
For instance 42", then select a toroid that is about 1/3 this size, or
about 13"  by about 4" thick.  Note that the toroid thickness
is 1/10th the spark length.  These are rules of thumb I use.  Next
use the formula:

    spark length (inches) = 1.7*sqrt power input (wallplug watts).

This formula will work only for efficient coils.  It's an empirical formula
that I made up for my coils, but it works well for many efficient coils.
We see that 600 watts works well for a ~ 42" sparks.  

Next we select a suitable transformer such as a 12kV, 30ma NST.
These are rated at 360 watts, but when used with an LTR cap, and
a step up type input variac, and a sync gap, 600 watts can be
coaxed into it.  

I have seen the best efficiencies at 120 bps, so I select the sync
gap for that bps.  Using the NST mentioned, some inductive kick
charging will occur, which will give a higher than expected output
voltage of perhaps 23kVpeak or so.

Some power will be wasted in the NST, so figure that about 78%
of the power might actually go into the caps, or about 470 watts.
If our cap is selected to be 0.0147uF, this comes to 3.9J at 23kV
peak, using J = 1/2CV^2.  Finally, multiplying 3.9J times 120bps
gives 468 watts.  So this agrees with the amount of power needed.

And indeed, by 12kV,30ma NST powered TC gives 42" sparks, using
about 600 watts input power.

Other factors:
It seems that quite a lot of inductance is needed in the secondary
to give a good impedance match to the streamers.  I like to use about
1600 turns in my secondary.  When I tried 660 turns, I lost 4" of spark.
Richie B. did a similar test and lost  ~11% spark length when he 
decreased his secondary from 1500 turns to 1000 turns.

Maybe TC's because of sub-optimal gap performance, and other
reasons, only give about 1/2 the spark lengths mentioned above.  
  
> I also read that a higher breaking rate of the spark gap increases the 
amount
> of power in the coil. But does the braking rate of the spark gap influence 
the
> frequency of the tank circuit? 

There's no relation between the bps and the TC's resonant frequency.
If you double the bps, the power input will double *IF* the voltage on
the caps remains the same....but it probably won't.  Generally higher
bps will give longer sparks, but at lower efficiency.  For best results,
the cap size must be matched to the bps.
  
> I want to limit the power of the TC at 1.8KV, which means 8 amps drawn on 
the
> house sector. How can I achieve that on a non-current limited transformer. 
(a
> resistor of 27.5 ohm??? but at 8 amps, that means that the 1.8KW are wasted
> here and nothing for the coil... Where is the mistake?)

You mean 1.8kVA.  VA = volt-amps, which is *sort* of a power 
measurement...if you ignore power factor.
For best efficiency, use inductive ballasting in series with one of the
incoming ac lines going to the HV transformer.  For instance, I use 
potential transformers rated at 3kVA for one of my TC's, but I choose to
run the TC at 600 watts, so I use about 12mH of inductive ballasting
which does the job.
  
> You might think that those are dumb question, but You would really help me 
if
> you could answer some of these questions, because it would help me to really
> understand how a TC works. And I already thank all of you who will bring the
> light to my brain... Thank you very much

We don't think these are dumb questions because we've been there !!
And we all still make our mistakes, and we all have more to learn.

If someone says they have all the answers....beware !!
  
Cheers,
John Freau

> Samuel >>