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Re: Capacitor Help



Original poster: m.j.watts@xxxxxxxxxxxx
On 13 Dec 2005, at 18:29, Tesla list wrote:

> Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>
>
> Hi,
>
> You should start thinking about "tuning".  The frequency of the
> primary circuit needs to be tuned to the same frequency as the
> secondary circuit.  Now is a good time to get a rough idea of the
> tuning to be sure you won't end up with a coil that will not
> tune.  So it is time to do the math or let Bart's program do it for you...
>
> We know that primary capacitance is 8.33nF.  From the details you
> give below, we can calculate the maximum inductance of the primary coil.
>
> Lf = (N x R)^2 / (8 x R + 11 x W)
> Lf = (7.5 x 6)^2 / (8 x 6 +11 x 6) = 17.8uH
>
> The frequency is:
>
> Fo = 1 / (2 x pi x SQRT(L x C))
> Fo = 1 / (2 x pi x SQRT(17.8e-6 x 8.3e-9)) = 414kHz
>
> So right now, the secondary has to tune to 414kHz or higher (using
> less primary turns will raise the frequency).
>
> We can make some guesses about the secondary.  For a 15/30 NST, the
> Freau formula gives a maximum spark length of 36 inches.  1/2 that
> (18 inches) should be reasonable to avoid a lot of primary to
> secondary arcing.  We can guess at a 18 x 4 topload centered 6 inches
> above the secondary.  It can be say 4.5 inches in diameter.  From
> these dimensions, we can use E-Tesla6 or Barts on-line program to
> guess at the secondary capacitance.  It will be roughly 25.5pF.
>
> The secondary inductor is say 18 inches of #27 wire on a 4.5 inch
> diameter form.  #27 wire winds 65 turns per inch
>
> Lh = (N x R)^2 / (9 x R + 10 x H)
> Lh = (18x69 x 2.25)^2 / (9 x 2.25 + 10 x 18) = 39mH
>
> The secondary frequency is:
>
> Fo = 1 / (2 x pi x SQRT(L x C))
> Fo = 1 / (2 x pi x SQRT(39e-3 x 25.5e-12)) = 159.6kHz
>
> So there is a significant problem.  The secondary will tune about
> 160kHz and the primary will tune about 400kHz....  We need to either
> raise the secondary frequency and/or lower the primary frequency.
>
>  From your picture, it looks like you could splice in more tubing to
> get say 11 turns at 24 inches.
>
> Lf = (N x R)^2 / (8 x R + 11 x W)
> Lf = (11 x 7.5)^2 / (8 x 7.5 +11 x 9) = 42.8uH
>
> That drops the primary frequency to:
>
> Fo = 1 / (2 x pi x SQRT(L x C))
> Fo = 1 / (2 x pi x SQRT(42.8e-6 x 8.3e-9)) = 267kHz
>
> That helps a lot!  See if you can stuff in more primary turns for sure!!!
>
> You might consider making the secondary shorter and or thinner to
> raise it's frequency.  A 12 inch secondary is a little short but with
> a top breakout point....  If it were 12 inches:
>
> Lh = (N x R)^2 / (9 x R + 10 x H)
> Lh = (12x69 x 2.25)^2 / (9 x 2.25 + 10 x 12) = 24.7mH
>
> Whish would raise the secondary frequency:
>
> Fo = 1 / (2 x pi x SQRT(L x C))
> Fo = 1 / (2 x pi x SQRT(25e-3 x 25.5e-12)) = 200kHz
>
> For a significantly shorter secondary the frequency raised some but
> not a great deal really.  You are probably best off buy finding a way
> to add a whole lot more turns to the primary.  It looks like you have
> about 1 turn/inch now.  Could you double the number of notches for 2
> turns/inch?
>
> It is probably not a good idea to go with a really skinny
> secondary.  It could be "space wound" but I would not recommend that
> to a beginner that does not have about a mile of extra wire...
>
> So...  I think you need to get more primary turns....
>
> Lp x Cp = Ls x Cs
> Lp x 8.3e-9 = 39e-3 x 25.5e-12   So Lp = 120uH
>
> So you need a little over 120uH of primary inductance....
>
> These numbers are just guesses out of the air, but it does certainly
> seem like you need a lot more primary turns!!!!
>
> Bart's program or many other programs can do the calculations.
>
> http://www.classictesla.com/fantc/fantc.html
>
> Beware that I tend to make math mistakes too.  So if something does
> not seem right, double check me...
>
>
> BTW - We can find the peak current estimate now...
>
> Ipeak = Vf / SQRT(Lp / Cp)
> Ipeak = 21000 / SQRT(120e-6 / 8e-9) = 171 amps.
>
> The caps you are getting are rated at 171 amps!!!!   You obviously
> had that all planned :o))))
>
> BBTW - Since your primary diameter is the same as the height of the
> coil, you coupling may be high (0.16).  Plan on being able to raise
> the secondary coil up some...
>
> Cheers,
>
>          Terry
<snip>

One way of boosting the primary inductance considerably with as
little extra material as possible is to use less spacing betwen
turns. As the number of turns increases, the volts/turn decreases.
      Another way is to stack two or even three pancake coils wired
in series with a layer of acrylic or polycarbonate separating each
layer. The first option is simplest and can score nearly as much
extra inductance as stacking a couple of coils.

Malcolm