RE: Capacitor value not clear yet.

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Luke,

The primary current is a function of the primary resistance, frequency, and 
inductance.

If the resistance is 0.1 ohms and the inductance is say 100uH and we have a 
20000 Volt primary cap at 20nF, we can find the peak current.

The inductance will have a value Zl which is generally referred to as 
complex impedance or reactance.  This acts much like resistance.

Zl = 2 x pi x Fo x L

Fo = 1 / (2 x pi x SQRT(L x C)) = 1 / (2 x pi x SQRT(100e-6 x 20e-9) = 112540Hz

Thus  Zl = 2 x pi x 112450 x 100e-6 = 70.7 ohms

The total impedance is Z = SQRT(R^2 + Zl^2) = SQRT(0.1^2 + 70.7^2) = 70.7 
ohms.  The 0.1 ohms of resistance is very small compared to the 70.7 ohms 
so it really does not make any difference here (but it does matter when it 
comes to coil losses!).

The peak current is the voltage divided by the impedance:

20000 / 70.7 = 282.88 amps.

In a Tesla coil, the energy in the cap is going into streamers and gap 
heating so the energy of the system immediately starts to decrease.  Thus, 
the current looks like a "decaying sine wave":

http://hot-streamer-dot-com/TeslaCoils/MyPapers/modact/image171.gif

More info is in the paper at:

http://hot-streamer-dot-com/TeslaCoils/MyPapers/modact/modact.html

Normally, the sine wave decays as an exponential function but in Tesla 
coils it decays a linear function.


So it's messy ;-))  But just remember that the primary current is 
controlled by the primary's inductance and frequency which control it's 
impedance.  Impedance is used just like resistance so:

Current = Voltage / Impedance

Impedance = 2 x pi x Fo x Inductance

Cheers,

         Terry



At 07:08 AM 1/12/2004, you wrote:
>Ok I still would like input on all the questions!!  :)
>But I woke up this morning with what I thought might be the answer to
>part of my question.  Funny how the mind works stuff out while you are
>asleep and it's the only thing on your mind when your eyes pop open.
>Its kinda cool sometimes like being your own computer.  Throw in the
>data, go to sleep, and poof an analisys comes out.  Ok never mind the
>rambling, was just kinda cool to wake up that way again.
>
>Is the major current in the primary of the TC due to in major part a
>simple play with ohms law?  You have charged MC to a large voltage and
>now are discharging it into a very small resistance the primary.  If
>there is more at work id like to know what you all have to say,
>
>
>Luke Galyan
>Bluu-at-cox-dot-net
>
>-----Original Message-----
>From: Tesla list [mailto:tesla-at-pupman-dot-com]
>Sent: Sunday, January 11, 2004 7:27 PM
>To: tesla-at-pupman-dot-com
>Subject: Capacitor value not clear yet.
>
>Original poster: "Luke" <Bluu-at-cox-dot-net>
>
>Thank you very much for the replies on the toroid size question.  I will
>
>have to digest more of that after I tackle a two fold question that was
>brought up as a result of your replies.
>
>Question 1:
>I was planning on 36" arcs based on the formula I found somewhere
>[]
>where k is a value taken from a chart based on secondary dia.  in my
>case
>was .85.
>1800 watts  (2 nst's 15KV 60mA ea. Totaling 1800 watts)
>
>is this figure way off?
>The formula given to me in the toroid replies was
>spark length (inches) = 1.7*sqrt input power (wallplug watts)
>
>Which is the correct formula to use for an approx.?
>If I was going to drop to a single 15Kv 60Ma NST could I expect to
>generate
>3 foot arcs?
>
>Question 2:
>I would like to keep the discussion on cap selection limited to NST's
>since
>that's what I intend to use and to keep from getting too much info for
>my
>tiny little brain all at once.
>
>I understand the idea of resonant rise in the nst.  Or at least enough
>to
>see what you mean.
>The question is will the LTR cap (lower reactance) draw more current
>from
>my NST? Say a 15 60.
>I was under the impression they were current limited and no more than
>the
>rated 60mA would be drawn.  Resonant rise plays a role in this?
>
>Someone said that the current in the primary circuit can be in the
>hundreds
>of amps?  If so where in the -at-#$-at-#-at-~! Does it come from?
>
>Ok maybe I opened a can of worms but id like to know what is happening
>and
>why the current can be so high and still not hurt the nst and where all
>this high current comes from.
>
>Thanx again
>
>Luke Galyan
>Bluu-at-cox-dot-net