Re: TESLA COIL REVISED

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

Original poster: "Chris Arnold" <chris_arnold-at-msn-dot-com> 

Jaro,

I have been debating whether or not to reply to this thread for a while 
now, and I wouold like to add my two cents worth here.  But please, avoid 
personal attacks on members of the list at all costs and stick around for a 
little longer to see who really knows what they are talking about before 
dealing out such harsh criticism.  My comments are spaced throughout your post.

<snip>

>Gary,
>and I thought that you are some kind of a Tesla coil expert. What you
>said doesn't add up. Please see below.

<snip>

> >I said no such thing.  A 50-turn secondary would result in a very high
> >resonant frequency.  Since AC resistance increases with frequency, such
> >a coil would have a higher resistance than one operating at a lower
> >frequency.  The lower Q would probably result in inferior performance.

>NOT SO. Let's compare a 1000-turn 3" diam. thin wire secondary with a
>50-turn 12" diam. thick wire secondary. The 1000-turn coil has
>inductance of about 9 mH, and the 50-turn one has 255 uh inductance.
>And since that makes the inductance of the 50-turn coil, 35 times
>LOWER, it will have THE SAME reactance at 3.5 MHz, as the 1000-turn
>coil has at 100kHz.

This assumption is entirely disregarding the effect of increasing frequency 
on real world losses in the secondary system.

>So you can see that the reactance of the 1000-turn coil is about 5600
>ohms at 100kHz, and the reactance of the 50-turn coil is ALSO 5600 ohms
>at its resonant frequency of 3.5 MHz.

It is indeed very true that the reactive impedance due to secondary 
inductance is the same in the two systems that you have described, but this 
system also has a self capacitance to be contended with, as well as 
resistive components of the impedance.  Skin effect losses greatly increase 
the effective resistance of a given material at increasing 
frequencies.  See Gary's inquiry into the question of primary losses on his 
web site.  While the currents are much lower in the secondary system, they 
are still present, and since the secondary wire length is comparatively 
long, even in a 50 turn design such as you have proposed, the secondary 
resistive losses will have to be considered.  I don't have the time right 
now to run the numbers assuming a copper conductor in the two cases 
discussed here, but be assured that the AC resistance of any material at 
3.5 MHz is far from negligible.

>So what you said is a MYTH. The 50-turn coil resonates at higher
>frequency, but it WOULD NOT have a higher resistance than one operating at 
>a lower frequency.
>
>Now, since the output voltage of CLASSIC TC depends on the L2/L1 ratio
>instead of a resonant rise, this design wouldn't be good for classic TC
>because the output voltage would be rather low. BUT, it WOULD be
>perfect for a SOLID-STATE Tesla coil, because this coil produces
>RESONANT RISE (as you yourself mentioned).

I also need to interject here too.  A resonant frequency of 3.5 MHz is 
hardy a frequency that most solid state tesla coil builders can consider in 
their design.  To date I am only aware of one person, Dan McCauley, who has 
successfully built a solid state coil with an Fres in the megahertz range 
with any substantial (>100 W) input power without almost instantly 
destroying the power circuitry.  Most of us don't have the skillz (yes, 
with a z) necessary to pull this feat off.  Now this operating frequency is 
not out of the question with tube coils, but dealing with power supply 
design becomes much more difficult as frequency increases when any stray 
loop or conductive object close to the circuit causes changes in operation.

>So now that we know that a 50-turn secondary doesn't have higher losses
>than a 1000-turn one, even though it resonates at much higher frequency
>(several MHz), we can see that the 50-turn secondary is a superior
>design when used in SOLID-STATE Tesla Coils.

Superior only in one regard - higher *apparent* Q on paper neglecting real 
world performance differences due to skin effect and other resistive losses.

>And that's because while the output voltage of 1000-turn coils is
>limited by the L2/L1 ratio, the 50-turn solid-state TC has no such
>limitations, and will build up the output voltage to ANY LEVEL that the
>insulation will allow.

While theoretical discussions are good, especially for increasing one's 
understanding of a subject and consequently one's ability to construct a 
working Tesla coil, diagnose problems, and effectively eliminate/reduce 
those negative effects, it is not helpful to completely ignore obvious 
limitations, i.e. funding, practicality, safety, human error, etc.

>Of course the higher frequency will result in shorter sparks, so this
>coil would be more for people who want to experiment with very high
>frequencies and perhaps experience beams or walls of light (brush-like
>discharge), instead of the sparks. And that brush-like discharge would
>probably be more plasma-like than the usual low-frequency sparks.

Erm, if the definition of plasma is a fourth state of matter more energetic 
than the gasseous state in which electrons have been stripped off of their 
respective atoms creating a conductive, almost certainly luminescent fluid, 
then "the usual low-frequency sparks" look a whole lot like plasma to me ;)

>Jaro
>
> >
> >Regards, Gary Lau
> >MA, USA
> ><snip>

Regards,

Chris Arnold