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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. 

> 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. 

> 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.

> 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