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Re: Capacitor Help
Original poster: "MalcolmTesla" <tesla@xxxxxxxxxxxxx>
----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Tuesday, December 13, 2005 7:29 PM
> 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
It's a 4.0" diameter. That is the largest I could find at the local
hardware stores.
> 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.
I have no problem adding more tubing. Do I just join it with a tiny piece
and solder them like I was fixing a water leak in the house? Will this
still work properly?
> 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?
The notches are 3/4" apart. I could remake the lid and use 1/2" notches or
I can just add more copper tubing. I have to purchase more anyway for the
strike ring. I like the idea of adding to the primary so I can keep my
secondary taller and reduce hitting the primary while keeping the sparks
longer.
> 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
Terry, thank you for all the help, I've answered some questions inline up
above. I noticed while reading the archieves that lots of people speak of
tuning the system for X lenght of sparks. How exactly do you tune the
system? Is it by adjusting where you tap into the primary, by adjusting the
spark gap? or a combination of the two? I guess I should ask before I just
fire the system up and ruin it but how big should I set the spark gap? What
determains if I need it bigger or smaller?
BTW - I'm pretty impressed by all the math up above and what you were able
to work out. I'm searching right now for Bart's various programs and others
on the net.
Thanks for all the help
Malcolm
> At 05:17 PM 12/13/2005, you wrote:
> >I started making my primary. I'll give a link to a picture below. It's
> >made from 25 feet of 1/4" ice maker copper tubing. There are 7 full
turns
> >and a little more on the sides. As you can see in the pic the overall
> >diameter is about 17 ~ 18" depending on where you measure. Inside
diameter
> >starts around 6 ~ 7".
> >The wire I've ordered for the secondary is 17 AWG (1.3 lbs on the way).
My
> >PVC is 4" diameter.
> >When winding my secondary should I pay more attention to counting turns
or
> >more attention as to how far up the PVC the windings go? Someone had
> >mentioned about 18", others have said 1300 to 1500 windings. Now that
you
> >see my primary and have more information what do you guys recommend?
Thanks
> >:)
> >
> >Oh and this pic is after spending some time bending the tubing, it's not
> >fastened down yet, just loosely laying there so it looks a little sloppy.
I
> >must say I have great respect for you guys that have such perfect looking
> >primary's. How on earth do you guys bend them up so neatly?
> >
> >http://www.v8-ranger.com/temp/tesla/7.jpg
> >
> >Thanks
> >Malcolm
> >
> >----- Original Message -----
> >From: "Tesla list" <tesla@xxxxxxxxxx>
> >To: <tesla@xxxxxxxxxx>
> >Sent: Tuesday, December 13, 2005 2:37 PM
> >Subject: Re: Capacitor Help
> >
> >
> > > Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>
> > >
> > > Hi,
> > >
> > > At 12:27 PM 12/13/2005, you wrote:
> > > >Hey guys,
> > > >
> > > >After much searching and calling around I finally got the 338-1175-nd
> >caps
> > > >from Digi. I got 14 so I'll have two extra incase of problems.
> > >
> > > Cool! They should be fine. It helps some to take stress off the
> > > caps by keeping the secondary frequency low (lots of secondary turns
> > > and big top terminal), but don't worry about it much.
> > >
> > > >Everywhere
> > > >I called only had 10 pound spools of wire and the cheapest I found
was $9
> >a
> > > >pound for 28 AWG. I was about to break down and pay the $90 from a
local
> > > >supplier when I got back in touch with surplussales.com to see if
> >anything
> > > >had changed since my last call to them and sure enough the guy had
1.3
> > > >pounds of 27 AWG for $13. Woo Hoo! He's shipping that out to me
right
> > > >away.
> > >
> > > Cool!!
> > >
> > >
> > > >Thanks guys.
> > > >
> > > >Oh and I have another quick question :) on the high voltage side I
see
> >some
> > > >people wire the capacitor in series with the primary and others put
the
> > > >spark gap in series and the capacitor in parrallel with the NST.
Which
> >way
> > > >is correct? or does it even matter? I was going to put the capacitor
in
> > > >series with the primary.
> > >
> > > The spark gap should be across the NST parallel with the NST. When
> > > the gap fires, it tends to put a short across the NST which helps to
> > > protect it greatly.
> > >
> > > Cheers,
> > >
> > > Terry
> > >
> > >
> > >
> > > >Thanks
> > > >Malcolm
> > >
> > >
> > >
> > >
>
>
>