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Re: Idiot's Guide
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I would like to thank Malcolm Watts for his EXCELLENT Comments on the
Idiot's Guide. This is the kind of Feedback that is NEEDED! I certainly do
not want to be writing stuff that contains inaccuracies that don't Have to
be there. Such Feedback will enable All of US to be better informed. I have
added a few comments of my own interspersed with Malcom's comments.
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> From: Tesla List <tesla-at-poodle.pupman-dot-com>
> To: Tesla-list-subscribers-at-poodle.pupman-dot-com
> Subject: Re: Idiot's Guide
> Date: Friday, January 24, 1997 12:53 AM
>
> Subscriber: MALCOLM-at-directorate.wnp.ac.nz Thu Jan 23 22:35:08 1997
> Date: Fri, 24 Jan 1997 11:34:31 +1200
> From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
> To: tesla-at-pupman-dot-com
> Subject: Re: Idiot's Guide
>
> Thomas McGahee asks....
>
> > This is an On-Going Work In Progress.
> > See any errors? Did I leave something Out?
> > Anything that is Now Confusing You Even More Than Before?
>
> Like to make a couple of comments:
>
> <large snip>
> > Eventually almost ANY insulation can be broken down. Some people wind
the
> > Secondary wires further apart. That lowers the Q of the circuit.
>
> Actually, space winding can reduce winding losses by reducing the
> proximity effects in the winding. I have a spacewound coil that
> clocks in with an unloaded Q around 300 and is not particularly small.
> One thing you do lose is inductance for the same wire gauge.
>
Excellent observation. Tesla himself had some of his coils progressively
space wound. If you look at the old photos of his Magnifier you will see
that the windings on the secondaries are not spaced at regular intervals. I
remember reading somewhere that this was done because the electrical
stresses are not evenly distributed. Does that relate to what you are
calling the proximity effects, Malcolm, or is that something totally
different? I also recall that Tesla made some coils in which he actually
had several different diameters of wire used on the SAME secondary. I am
not sure however whether this was an experiment to Find Out if there was
any Value to doing that, or whether it was an Implementation based on his
having already determined that such an arrangement was desireable. I see
this technique applied on his Magnifier Coil design, but not on what we
might call his Classic Tesla Coil design. Anyone out there have more
information on any of this?
> > An Aside: Q is a general term that is applied to the relative
effectiveness
> > of a circuit or circuit element. Generally the Higher the Q factor, the
> > BETTER or more efficient a circuit or circuit element IS at what it is
> > supposed to be doing. For example, if a Primary is said to be a High Q
> > Primary, you can mentally decode that to say "This is a High Quality
> > Primary. It can conduct large instantaneous currents, and produce
> > Significant Magnetic Fields that encompass most of the Secondary." If
some
> > OTHER coil design was TWICE as Good at doing the same exact thing, you
> > could say that it had a Q that was twice as
> > much as the other coil's Q. The same term "Q" always relates to
Quality,
> > but the EXACT thing that Q MEASURES can be quite different. For
example,
> > the Q of the Spark Gap relates to its ability to go from a
non-conducting
> > to a conducting state in a Short Period of Time, Conduct LARGE
quantities
> > of current, and then turn OFF rapidly. Making ANY ONE OF THESE FACTORS
> > GREATER would increase the Q of the Spark Gap.
>
> I think the accepted engineering definition of Q is that it is the
> inverse of the dissipation factor of the circuit - in other words, it
> relates to circuit losses.
>
Malcolm is right. I was attempting to put things in very simple terms.
Maybe I got TOO simple. Maybe someone out there can e-mail me a Good Simple
explanation of Q that I can use instead of the one I gave. It needs to be
simple enough that a Beginner can understand it, and Exact enough that it
doesn't make Engineers Shake Their Heads in Dismay.
> > Spiral coils are generally better for the primary than a rising helix,
and
> > over-coupling is
> > less of a problem. The usual arrangement is what is sometimes called an
> > Archimedes or Archimedian Spiral (because the Greek mathemetician
> > Archimedes was the one who formulated its characteristics). In this
kind of
> > spiral the distance between adjacent turns is kept constant. If the
spiral
> > is kept flat, it is often referred to as a "pancake" coil. If the coil
is
> > not kept flat, but instead each turn of the spiral also includes a
RISE,
> > then you have the kind of primary coil that is so popular today.
> >
> > So why is it so popular? Because it works so well. Why does it work so
> > well? Because it creates a magnetic field that is large and encompases
> > (ideally) the entire secondary. You can actually SEE the beautiful
shape of
> > this field if you operate a powerful Tesla coil in the dark. The corona
> > discharge from the primary will engulf the entire secondary in a kind
of
> > inverted parabolic curve when the coupling and geometry are just right.
> > By the way, if you look at the photo of Chip Atkinson's coil, you can
SEE
> > exactly what I'm talking about.
> >
> > A Tesla coil is a 1/4 wave resonant device. When it is operating
properly
> > the base of the secondary has a low voltage and a high current, while
the
> > top of the secondary has a high voltage and a relatively low current.
It
> > may be useful if you think of it in terms of a standing wave: Imagine
one
> > cycle of a sine wave. It reaches its Peak value at 90 degrees. That is
a
> > quarter of a full wave. If you get a Tesla coil to operate at
resonance,
> > you have a standing wave in which the top of the secondary is operating
at
> > this 90 degree point. It will therefore have maximum voltage at that
point.
>
> IMO the primary by no means couples identically into all sections of
> the secondary winding. If it did, you would expect an equal current
> in all sections of the secondary at any particular instant in time
> (Y-N)? Perhaps that is not what is being said.
>
What I meant to say was that ONE of the things that a Good Primary should
do is couple the magnetic field to the secondary in such a way that as Much
of the energy As Possible is PROPERLY UTILIZED. Over-coupling, for example,
is not good because it over-stresses some parts of the secondary, promoting
breakdown and/or other things such as Wasted Power that we Don't Want. The
Size and Shape of the Primary, and its position Relative to the Secondary
ALL influence how the Secondary will behave. All three Primary types have
their Good Points and their Bad Points. Some have more Good Points than
Others in a Particular Implementation. The goal is not to couple all
sections Equally. This is not a Transformer, but a Tesla Coil. Resonance
and not Transformer action is the Star of the Show. There are aspects of
Both, but it is the Resonance that makes a Tesla Coil a Tesla Coil and not
something else. Because it is a 1/4 wave resonant device, the Base of the
Secondary should have the greatest current (and therefore the greatest
Coupling, maybe? This IS after all, where the Greatest Electrical Push is
being imparted.) But Sometimes a Perfectly Flat Pancake Coil Primary may
NOT be the ideal shape. (Actually it is always both shape and size that we
have to deal with). Some experimenters have reported that for them the
Rising Spiral works best. Others swear by the Flat Spiral. Why MIGHT a
Rising Spiral Sometimes be Best? Perhaps it is that MORE of the secondary
is Enclosed (is that the best word?) or Encompassed by the Primary's
Magnetic Field. Could it be that in Some cases the Rising Spiral's Magnetic
Field and Electrostatic Presence work AGAINST us reaching Perfection? Sure.
Some experimenters report that they prefer Rising Spirals for Medium Size
Coils, and Flat Spirals for Big Coils. Do the Ratios of the Width to Height
have any influence? Of Course. Will a Different Secondary Geometry
influence what is the BEST Primary for THIS Particular Coil? I believe so.
There are many nuances here. And that is one reason why it is useful to
gather Data about as MANY Tesla Coils as we can.
> > Ever notice that sometimes going near an operating Tesla Coil can cause
it
> > to change the size and even the NATURE of the Sparks? That is because
there
> > are interesting ELECTROSTATIC Effects Also At Work. The Secondary coil
has
> > a Capacitance that must be taken into account. The problem is the
> > Capacitance of the Secondary is affected by many physical parameters
such
> > as thickness of insulation, kind of insulation, number of turns,
exposed
> > surface area on the outside of the coil, height of the coil, width of
the
> > coil, closeness to other objects, and of course, ANYTHING YOU PUT ON
THE
> > TOP OF THE SECONDARY. PHEW! It's NO WONDER you can't just crank a lot
of
> > info into a computer program and expect it to spit out complete plans
for a
> > Coil that is guaranteed to work the First Time you fire it up!
>
> I have to disagree with that. Well tested formulae that can predict
> raw Cself have been posted on the list a number of times. It is true
> that one assumes a reasonable degree of isolation from other objects
> and also that toploads add to the static figure. Also that
> sparks/corona/ion clouds add to this figure when the coil is
> operating.
>
> Malcolm
My point was not to disparage the work of those who have tried to gather
the multitude of factors into mathematical formulae and try to produce
computer programs that tell us EXTREMELY USEFUL INFORMATION. I APPLAUD all
such efforts. We need them badly! Chalk it up to a bad attempt at humor on
my part. I was trying to make the point that there are MANY factors that
enter into the design of a Tesla Coil. Some of these Factors Change with
the Weather, if you know what I mean!
Malcolm, once again I want to thank you for your observations. I appreciate
the time you took to point out some of the deficiencies in my Posting.
Please CONTINUE to share such observations with me and the Rest of the
Tesla List in the future.
All Other comments from Interested Parties are MOST WELCOME! Let us Learn
from One Another!
If all else fails, try somethings else.
Fr. Tom McGahee