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Idiot's Guide
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THE ABSOLUTE IDIOT'S GUIDE TO TESLA COIL PRIMARIES
Here's a FIRST attempt at providing some information about Tesla coil
primaries. Please keep in mind that although this present article will of
Necessity talk about The Power Source, Transformers, Capacitors, Spark
Gaps, and Secondary Coils, our MAIN FOCUS in THIS ARTICLE is the TESLA
PRIMARY. So if I sort of don't give Lotsa Details about all those Other
Things, STAY COOL! I hope to post OTHER Articles that will Home In on Them,
Too. But Not NOW!
At its Simplest Level the Primary of a Tesla Coil is just a Dumb Old Coil
of wire that creates a magnetic field when you pump current through it.
Our Basic Goal as Coilers is to find ways to Create Repeating Pulses of
HIGH Current in the Tesla Primary. This will produce Intense Magnetic
Fields. If we can get these Magnetic Fields coupled to the Secondary and
operating at the Self-Resonant Point of the Secondary, then we can make
some Big Bolts. Yeah!
So here are the challenges:
Make the Primary Capable of handling Fast Rise and Decay Repetitive
Currents.
Create Huge Magnetic Fields that Rise and Decay RAPIDLY.
Shape the Resulting Magnetic Field so that MAXIMUM ENERGY is Transferred to
the Secondary.
Try to do All This and Not Implode Your Wallet.
Why do we want Fast Rise and Decay Repetitive Currents?
Well, you see, One of the Factors affecting the Strength of a Magnetic
Field is the Current. Current is defined in terms of the NUMBER of
electrons flowing Per Second. If we store the electrons up (in a capacitor)
over a fair period of time and then DUMP them ALL at Once, as FAST as we
Possibly Can, then even though our Average current flow May Be Small, we
can Still make the PEAK Value very High! At the INSTANT we dump all those
electrons into the Primary we can get really Obscene Peak Values IF we can
keep the Resistance of the Primary Really Really LOW.
OK, let's make that resistance LOW! We can make the Conduction Area
Greater. Now, with DC circuits we would want to increase the Cross
Sectional Area. But High Frequencies Flow Mostly over the Surface of
Conductors, NOT Through them. This is great, because if I make a Thin
Ribbon of Copper that is fairly Wide, it can readily carry Lots of High
Frequency currents wih only a fraction of the material that a Solid
Conductor would need. So with High Frequency circuits we want to maximize
the SURFACE Area.
Flat Ribbons have these Nasty Sharp Edges. We Dont't Like sharp endges on
RF conductors, because it promotes energy loss through corona discharge. So
we come up with an almost perfect Answer: A Conductor with Lots of Surface
Area and no wasted cross sectional area, and as Non Pointy as You Can Get
(and Still Be a Long Conductor). Behold Hollow Copper Tubing! Meets all the
criteria and is readily available to boot. Why Copper Tubing? Why not make
it out of Lead Pipe or Iron Pipe? Because we also need the lowest possible
DC resistance, and the metal should be NON-FerroMagnetic.
If it was FerroMagnetic then when it produced a Magnetic Field, the
Magnetic field would want to stay close to the Iron, because Iron Conducts
Magnetic Lines Quite Well. But WE want to PROJECT a Magnetic Field, not
Hoard it. Besides, If you DID use a FerroMagnetic Material the Energy Loss
due to Eddy Currents would be Horrendous. Much of the energy would be
expended in Heating the Pipe. Bad News, so we don't do that. We use Copper.
Copper is King when it comes to Tesla Coils.
For Smaller coils the 1/4 inch Copper Refrigerator Tubing is often used.
Many Medium to Large Size Tesla Coils employ 3/8 inch copper tubing. Really
Big Coils use 1/2 inch and Larger Copper Tubing. It is best if you can get
a single piece to form the Primary, but if not, then pieces can be soldered
or brazed together. If you put a sleeve on the outside to make a splice,
then you might have a corona problem later. An alternative is to stick a
short section of Thinner Copper Tubing Partially INSIDE both halves and
then solder. Use fine sand paper to remove any spots that are bumpy and
might cause corona problems. When bending Copper Tubing, you want to avoid
having the inside of the bend crimp or buckle. One method is to make sure
the inside bend is firmly pressing up against something and to only bend it
a small amount at a time.
We'll cover the topic of the Shape of the Primary Later in this article,
but we need to mention Inductive Reactance now. Whenever you have a coil,
it builds up a magnetic field when you pass current through it. Because of
the adjacent turns in the coil, self-induction exists in the coil. That is
the tendency of the coil to OPPOSE CHANGES in Current Flow. A Pulse would
normally cause a coil to first prevent current from increasing and then
actually Produce current when the Magnetic Field later Collapsed. This
tendency to oppose changes in current (it's called Inductive reactance) is
a function of the INDUCTANCE of a coil and the Effective Frequency of the
applied Pulse. I say the EFFECTIVE frequency, because a PULSE will always
appear to a coil as having a Frequency HIGHER than its Repetition Rate. A
Pure Sine Wave will have no pulses, and its frequency and repetition rate
are the same.
Oh Great! So we are supposed to keep the resistance of the Primary Coil as
LOW as Possible, and now we find that it's got this Stupid Inductive
Reactance Thing that makes it act like a Dumb Resistor. Sheesh! NOW WHAT?
How do you cancel out the effects of Inductance? It's really fairly
Simple. You Put the Coil in Series with a Capacitor that has an Effective
Resistance equal to the Coil's resistance AT THE EFFECTIVE FREQUENCY of the
Pulse. This works because Inductive Reactance and Capacitive Reactance are
180 out of phase with one another.
For any Given Inductor/Capacitor Combination, there is ONE Frequency where
The two Reactances are exactly opposite and EQUAL in Reactance value. This
Frequency is the Resonant Frequency. (It's the Same Frequency that we
ultimately want to ALSO equal the Resonant Frequency of the Secondary Coil.
Hey, what if we use the Same Exact Capacitor that we used for energy
storage to act as this Capacitive Reactance Thingie?? It happens to be
exactly the idea that Tesla had. (You are getting SO smart!)
By the way, do you see how all the parts constantly interact? The
Transformer and Capacitor are ALSO supposed to be matched for Maximum
Effeciency
Most of the Really Spectacular Things that a Tesla Coil can do occur only
when the tuning of the
Primary circuit Matches the Tuning of the Natural Self-Resonant Frequency
of
the Secondary circuit.
Consider coupling to be the way the primary's magnetic field encompasses
the secondary. If coupling is too loose, the coil is inefficient and wimpy.
If coupling is too tight, then you may over-stress the primary and have
voltage breakdown occuring along the secondary or between the primary and
the secondary coil. In olden days there was much emphasis on tight
coupling, because many coil builders erroneously thought that a Tesla coil
worked mainly by transformer action. T'aint so. A Tesla coil is a
transformer that is tuned to resonance. The resonance is just as important
as the magnetic effects. Even more so if you are talking about a Tesla
Magnifier! But that would get us off the topic, so 'nuff said about that
for now.
Many of the early Tesla coils used a rising helical primary that was
closely coupled to the secondary. Flashover from secondary to primary was
common with this design, and many a secondary coil was destroyed because of
the sparks destroying the insulation. When coupling is TOO Close, the
windings of the Secondary get OVER-STRESSED and the Secondary circuit can
experience Breakdown between windings or sparking between the primary and
the secondary. Some people try to compensate for this by cramming as much
solid or liquid insulation as they can around the bottom of the Secondary
(at least). That works somewhat, but it has its Limits and its Limitations.
Eventually almost ANY insulation can be broken down. Some people wind the
Secondary wires further apart. That lowers the Q of the circuit.
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.
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.
There is a simple experiment you can try with a child's jumprope that will
illustrate standing waves. Have someone hold one end of the rope TIGHTLY in
both hands, with their hands held tight against their stomach so that the
rope at that end will be anchored fairly securely. You grab the other end
of the rope tightly in one hand, and begin moving the rope up and down
about one foot. Start off slowly, and then gradually increase the speed at
which you are moving the rope. Keep the up and down DISTANCE you move as
constant as you can. Every time your hand moves the rope up and down it
will send a traveling wave down the rope. But you will reach one particular
rate at which you will no longer see a wave traveling down the rope.
Instead you will see that there is a point right at the center of the rope
that appears to be standing relatively still. At a distance halfway from
the center of the rope to where YOU are you will see that the rope is
whipping up and down pretty good. That is the 1/4 wave point. That is where
the greatest activity is. When a Tesla coil is operating at resonance, it
is operating at its 1/4 wavelength. Notice that you can get the rope to
have standing waves at HIGHER frequencies. Struggle really hard with the
rope and you might be able to get a standing wave with TWO nodes standing
still, each about one third of the way down the rope. Notice that it
requires a LOT more work on your part to maintain a standing wave with TWO
nodes, and that the AMPLITUDE of the wave is not as big as when you only
had ONE node. In the same way, you can FORCE a Tesla coil to operate at the
wrong frequency, but it is not achieving FULL resonance. If a Tesla coil is
improperly tuned the applied energy is wasted because returning waves
cancel the original waves.
The key, then is to achieve this 1/4 wave resonant point. And you want to
do it with efficiency. That's where all the little nitty gritty details
come into play. The Frequency we tune the primary circuit to should ideally
match the Natural Self-Resonant Frequency of the Secondary. Of course, in
Real Life nothing is quite as simple as that. The Secondary's Natural
Self-Resonant Frequency VARIES (in REAL TIME!)
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!
*****
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?
Ask and you shall receive.
Ask for Nothing, Receive Nothing.
Keep Quiet and you won't be heard.
If all else fails, try something else.
Fr. Thomas McGahee