Download tesfaq 27KB Aug 20 2001 09:27:16 AM - hot

What is a Tesla coil?
---------------------------A Tesla coil is a high voltage, high frequency resonant air core
transformer. It was developed by the Serbian born inventor Nikola Tesla
in the 1880s.
It was originally created to serve as a transmitter for the wireless
transmission
of electrical power. The idea of wireless power transmission never
caught on
mainly due to lack of funds. The true Tesla coil had three coils, a
primary,
a secondary, and an extra coil which is called the Tesla magnifier.
Today Tesla coils serve little practical purpose. There are a
few
practical purposes for them however. They are used to simulate lightning
strikes to test aircraft hulls, and power distribution equipment. They
are
also used in the film industry wherever good lightning like discharges
are
needed. An example of this would be in the movie "Terminator 2". Most
coils
built today are 1/4 wave coils which do not make use of the Tesla
magnifier
coil. Tesla coils are fairly easy to build with readily available
components.
The normal hobby Tesla coil setup consists of a high voltage step-up
transformer which has a secondary voltage of 5000-15000 volts AC. Neon
sign
and oil burner ignition transformers are commonly used. Almost all of
the
other components are made by the builder.
The theory of operation of a Tesla coil is not too difficult to
understand. The most basic Tesla coil system consists of a high voltage
stepup transformer, sparkgap, capacitor, primary, secondary, and output
terminal.
First the capacitor is charged until it the air around the sparkgap
breaks
down and forms a conducting channel. The capacitor discharges rapidly
into
the primary coil and a magnetic flux is built up around the primary coil.
The
flux then collapses and charges up the capacitor again. The capacitor
then
discharges into the primary again. This forms what's called an LC (L
stands
for inductance and C stands for capacitance) oscillator. This cycle goes
on
until there is not enough power left to jump the gap. The capacitor then
is
charged by the high voltage transformer and the cycle repeats over and
over
again. You can think of the primary, sparkgap, and capacitor as a radio
transmitter. The secondary can be thought of as a radio receiver. The
resonant frequency of the secondary is determined by it's capacitance
which is
derived from the output terminal, the capacitance of the secondary
winding,
and the inductance of the secondary. If the resonant frequencies of both
the
primary, capacitor, and sparkgap circuit (also called a "tank" circuit)
match
with the resonant frequency of the secondary coil then high voltages will
be
induced in the secondary due to the effect of VSWR, or the standing waves
in the coil. What happens is there is sparking that occurs at
the output terminal. Understanding this is not required to build a
sucessful
Tesla coil, although it does help to know some of the theory.
-Safety concerns
The skin effect
Legal Concerns
-Types Of Tesla Coils
---------------------------1) Classic 1/4 wave monopolar Tesla coils
Classic 1/4 wave Tesla coils are what most people think of when
they
hear "Tesla coil". They usually have a discharge terminal on top of the
secondary coil, Primary coil near the base of the secondary coil. They
require heavy RF grounding at the base of the secondary coil.
2) 1/2 Wave Bipolar Tesla coils
Bipolar Tesla coils are Tesla Coils have 2 discharge terminals.
They
are connected to each end of the secondary. Discharges eminate from one
terminal to the other. The primary coil is placed in the center of the
secondary coil. The secondary coil is 1/2 wavelength long. The
discharges
from each terminal are 180 out of phase, so the discharges attract each
other. They do not necessarily require grounding, the secondary coil can
be
left to work against itself.
3)
The magnifier coil is the true Tesla coil. This is what Tesla
experimented with in Colorado Spings. It consists of three coils, a
primary,
a secondary, and an extra coil or "magnifier". The extra coil is not in
any
way coupled to the primary's field flux. The primary and secondary make
up a driver
stage. The secondary is not like that of a 1/4 wave Tesla coil. It is
normally very
large and is wound with heavy wire ofnot many turns. The primary and
secondary are
coupled very closely. The magnifier is placed at a distance to prevent
the primary's
field from destructively interfering with the extra coil. The extra coil
is base fed
with a large conductor of decent surface area. This conductor is
connected
to the top of the secondary and to the bottom of the extra coil. What
this
does is allow the extra coil to develop very high voltages since it is
allowed
to oscillate freely. Magnifiers do not make a good first coil project.
4)
The solid-state Tesla coil is driven by a solid-state driver at a
frequency in which the secondary is excited at. Solid-state Tesla coils
do
not require a sparkgap, a high voltage transformer, or a high voltage
capacitor. They normally use bipolar transistors or field effect
transistors
(FETs). They are beyond the scope of this FAQ for now, however.
5)
Vacuum tube Tesla coils are very much like the solid-state designs.
Since tubes can handle higher powers they make better drivers for large
Tesla
coils. Tube types commonly used are 805s are 811s. They too, like their
solid-state relatives will not be touched upon in this FAQ now anyways.
What are common power supplies that are used for Tesla coils?
------------------------------------------------------------1. Neon sign transformers
The most common power supply for powering a Tesla coil is the neon
sign transformer. These step up the 120 VAC from the wall socket
to the several thousands of volts necessary for spark gap operation.
The most common neon transformer voltage ratings are 6000, 7500, 9000,
12000,
and 15000 volts with current ratings of 30 or 60 ma. Occasionally you
will find 120ma neon transformers but these are not very common.
These transformers are ideal for Tesla coils as they are internally
current limited with magnetic shunts. This means that neons
can take direct short circuits with no ill effect.
Generally speaking the higher the transformer voltage and current rating
the more output you can expect from your Tesla coil.
The most useful neons for Tesla coils are the 9000 and 12000 volt models.
The 15000 volt ones are quite hard on capacitors.
For a first coil project 9000 volt neons are recommended.
2. Where to obtain these transformers
First thing to do is to get out the yellow pages. Look under signs
and take note of all the sign companies in your area. Make special
notice of any that advertise sales and service of neon signs.
Now take your list and go to these sign companies and ask them
for used neon sign transformers. Tell them what you want them for.
This will often generate some interest.
If you strike out at the first place go to the next one.
Expect to pay about $20 for a used 30ma neon and $35 for a
60ma neon. If they want considerably more than this move on
to the next place. Sometimes you can get lucky and find a place
with a pile of transformers in unknown condition. Negotiate
a price and take your chances or see if you can do some
testing on the spot. You can check the secondary windings
for continuity with an ohmeter. The resistance will vary considerably
from transformer to transformer. Just check for opens or short circuits.
Or take along an AC line cord and plug the units in. Attempt
to draw an arc from each high voltage terminal to the case. Each side
should
show an equal arc. Careful you dont get shocked. The smaller neons
can deliver painful shocks and the larger ones can kill.
3. Unpotting and repair of bad transformers
How about getting a Tesla power supply for free?
Check out those same sign companies in your area and ask for dead
neon transformers. Any place the services signs will have bad
transformers.
Some places pile them up while others pitch them into the the trash
dumpster. Check these places out and ask them to save their bad units for
you.
What good will dead transformers do? You can unpot these dead units
and disassemble the transformers. Often just unpotting them will
revive them. Neons transformers are potted in tar. Tar is a cheap
and not very effective insulator. Over time the tar dries out.
High voltage arcs within the tar block slowly carbonizes some of
this tar. Once carbonized it becomes more conductive inviting more
arcing and more carbonization. Eventually high voltage secondary winding
shorts out. Because of the current limiting in neons the shorted
secondary
is normally not damaged.
So removing the carbonized potting compound completely restores
the neons to operation. Other problems like broken wires and connections
can also be repaired once unpotted. New insulation like hot glue
or silicone RTV is used to insulate the secondary windings from
the core. Or the transformer can be put into a container of mineral or
transformer oil.
4. Oil burner transformers
Oil Burner Ignition Transformers (OBIT) are also useful as
Tesla coil power supplies. These transformers are used to ignite
the burners on old style oil burning furnaces.
They are magnetically shunted like neons so may be short circuited
without damage. They are not as rugged as neons not being designed
for continous duty operation. However they do serve well.
They are most commonly available in 10KV/23 ma and 6kv/23 ma
ratings. The 10kv units are what would most interest us.
You may find these at heating/airconditioning contractors.
Often you may find such places may have a couple old furnaces
ready to be scrapped. The transformers can be removed and
Sometimes be had for free.
Some places may have a couple old transformers on hand.
Most heating contractors dont place much value on these used
transformers so will usually give them away for the asking.
New units can be ordered but are expensive. Before buying a new
OBIT consider going to a neon transformer.
> Power distribution transformers
> Where to obtain these transformers
> Typical price range for this transformer
> Current limiting
> Inductive
> Arc welders
> Variacs
> Resistive
> Heating elements
> Water tanks
> Microwave oven transformers
> Where to obtain these transformers
> Typical price range for this transformer
> Minature Tesla coil power supplies
> Flyback transformers
> Ignition coils
What type of primary is best?
> Helical primary
> What is a helical primary?
> What are they suited for?
> Inverted cone primary
> What is an inverted cone primary?
> What are they suited for?
> What degree of rise should I use?
> Flat pancake primary
> What is a flat pancake primary?
> What are they suited for?
> How do I determine how many turns will I need?
> What spacing should I use?
> What type of wire works best?
> What should the primary coil forms be made of preferably?
> Should I insulate my primary wires?
What type of capacitor will I need?
> What the capacitor does in the Tesla tank circuit.
> Determining capacitor size needed based on transformer
characteristics.
> Dielectric concerns
> Why is polyethylene used in capacitor construction?
> Why are glass and ceramic types undesirable?
> Voltage ratings
> Where to get commercially made capacitors
> Saltwater types
> What is a saltwater capacitor?
> What does the saltwater do?
> How are they made?
> Materials needed
> Flat plate
> Rolled plate
> Advantages/disadvantages of certain capacitors
What is a spark gap?
-----------------------------The spark gap is the heart and soul of a Tesla coil. It is the single
most important component and yet one that gets little attention
from beginners. More coils have shown disappointing results due
to poor gaps than any other single cause.
Think of the spark gap as a high speed high power switch.
It appears as an open circuit while the main capacitor
is being charged by the power supply transformer.
Once the cap is fully charged the gap ionizes the air
between the electrodes and begins to conduct.
Once conducting it becomes a short circuit and allows
the cap to dump its charge into the primary coil. Once the cap
discharges the gap must quench (go out) and become non-conductive
once again so that the cap may begin charging once again.
There are several type of gaps and each has its own
advantages and disadtanges.
1. Static gaps.
Static gaps can take several forms. The most basic is just two electrodes
spaced a small distance apart. This can be the heads of two heavy bolts
or two brass knobs from the hardware store.
This is an easy gap to implement and will serve Ok for the beginner.
The spacing between the electrodes depends on the power supply voltage
from your transformer. 9kv neons do well with about .150 inch of gap
while 12kv neons can use up to .200 inch. 15kv neons can use up to .250
inch. Wider gaps than this should be avoided as they put a great deal of
stressn your capacitor and transformer.
The single static gap while easy to build is a poor performer.
All the heat is dissipated in one area and the gap doesnt want to quench.
The hot ions in between the gap electrodes tend to keep the gap lit
at lower voltages. Blowing air through the gap with a fan or blower
will improve the operation of this type of gap by removing these hot
ions.
A much better approach is to break
of smaller gaps in series. This is
divides
the arc up into smaller parts that
efficiently. This type of gap also
remove hot ions.
up the single large gap into a series
called a series static gap. This
develop less heat and operate more
benefits from a little forced air to
One such gap popularized by Richard Quick and often referred to as the
RQ cylinder gap consists of multiple electrodes made from lengths of
hard copper tubing. The electrodes are usually made from 3/4 or 1 inch
copper tubing cut from 2 to 4 inches long. Larger and longer elctrodes
provide more thermal mass and handle the heat better. These electrodes
are
mounted inside a piece of PVC pipe and spaced about .025-.03 inch apart.
7 such electrodes providing 6 gaps between then are normally used for 9kv
neons
and 9 electrodes providing 8 gaps are used for 12kv neons.
Up to 11 electrodes with 10 gaps can be used for 15kv neons.
A small muffin fan is mounted on the end of the PVC pipe holding the
electrodes
to provide cooling airflow.
2. Air blast gaps
Blowing a high speed column of air through the teeth of a
static gap greatly improves the operation by stretching and cooling
the arc and removing hot ions from between the electrodes.
Playing the output of an air compressor between the electrode faces
can produce some good effects. This type of gap is loud and requires
a constant supply of high speed air to be effective.
3. Vacuum gaps
Instead of blowing high speed air through a static gap the vacuum
gap sucks air through the gap. Use a shop vac or purchase a used
or surplus cannister type vacuum cleaner motor for the purpose.
This type of gap is very effective. In fact the vacuum cleaner motor
can replace the muffin fan on a RQ style cylinder gap and increase the
performance of that type of gap.
4. Quench gap.
A special form of static gap is known as the quench gap. In this gap
the electrodes are wide flat plates with their faces parallel and
spaced very close. The electrodes are spaced from each other with
mica rings that enclose the gap area. The gap areas between electrodes
become sealed chambers and the oxygen in these chambers is used up
quickly when the gap is first put into operation. This type of gap
works fairly well at lower voltages than other gaps and provide
a nearly continous wave (CW) output. These were used in spark radio
transmitters
until spark transmitters became obsolete with the invention of the triode
vacuum tube in the 1910s. This gap design is difficult to implement
due to the sealed nature of the gaps and lack of suitable material
for the spacers. The mica rings normally used are difficult to find
and expensive
5. Rotary gaps
A rotary gap is a motor driven gap that has a solid disk with
electrodes mounted around its periphery. These electrodes come
into line with stationary electrodes to form the gap. As the
disk rotates each set of electrodes comes into play in sucession.
With a large number of spinning electrodes and a high rotational speed
the number of gap presentations can become quite high. And quenching
is quite good with a rotary as the electrodes are in firing position for
only a short time.
Rotary gaps work well with higher power systems but are unnecessary with
small to medium power systems running from neon sign transformers.
In fact neons work better with one of the multiple static gap systems and
can actually be damaged by a rotary gap.
What type of spark gap will I need?
------------------------------------The type of gap you should use depends a great deal on the kind
of transformers you are using and the power level you are operating at.
Low power operation of less than 500 watts can often do well with a
single
static gap using a pair of bolts or brass knobs.
Even here though a series string of gaps like the RQ style cylinder
gap will improve efficiency.
Medium power systems up to 1500 watts using neon sign transformers
do very well with a RQ style cylinder gap. Above this power level
plenty of forced air should be used for cooling or you can go to
a vacuum gap. The vacuum gap works quite well up to 2500 watts.
Above this power level rotary gaps become necessary and are often
used in series with static gaps.
Rotary gaps should generally not be used with neon sign powered coils
as the forced break causes a great deal of strain that neons dont
handle well.
What is Quenching?
----------------------No discussion of spark gaps is complete without at least a rough
definition of "quenching". This term is commonly thrown around
when talking about spark gaps.
Quenching refers to the ability of an established arc to be
extinguished. It is much easier to start a gap firing than it is to
put it out once it has fired. In Tesla coils quenching the arc is
critical to good coil performance.
A cold, non-firing, spark gap is "clean". It contains no plasma
or hot ions. On applying voltage to the gap, a tension is established between the electrode faces. Once the tension becomes high
enough to ionize the air between the gap the resistance drops rapidly.
This breakdown ionizes the gas between the electrodes and the arc
begins to ablate and ionize the metal electrodes themselves.
This mixture of ions forms a highly conductive plasma between
the gap electrodes. Without this highly conductive channel through
the gap, efficient tank circuit oscillation would not be possible.
But this plasma also shorts the gap out. A gap choked with hot ions
does not want to open and allow the capacitors to recharge for the
next pulse. The gap is "dirty" with hot ionized gases and must be
cleaned so it can be extinguished.
To quench the hot gap the built up ions must be removed. This can be
done with a flow of air from a fan, blower or vacuum cleaner motor.
Once the excess ions are removed and the gap is cooled by the airflow
it can quench much easier.
Anything that reduces the amount of heat in the gap will aid quenching.
Series static gaps reduce the heat in each gap by breaking up the gap
in smaller segments. Quenching becomes easier.
What is the secondary coil for?
> Aspect ratios
> Determining needed turns for a specific frequency
> Commonly used form materials
> Form preparation
> Winding
> Ground contacts
> Can I feed my wire throught holes?
> How should I plug the ends?
> Why do the ends have to be plugged?
> Instead of using epoxy to glue the end plugs in, Can I use silicone
sealant?
What is the purpose of the discharge terminal?
> How do I connect the discharge terminal to the secondary?
> What is the advangtage of a toroid over a sphere?
> What does adding a terminal do to the secondary's resonant
frequency and why?
> How is a discharge terminal constructed?
> Materials needed
Power supply protection circuits and devices.
> Bypass caps
> What is their purpose?
> Do I need them on my coil?
> Safety gaps
> What do they do?
> How does it work?
> Do I need it on my coil?
> RF chokes
> What do they do?
> How do they work?
What type grounding do I need for my coil?
> Why is grounding is needed?
> Do I really need a good ground for my bipolar Tesla coil?
> What type of ground strap is preferable?
> Can I use my house ground?
Tuning a Tesla Coil
> How do you tune a tesla coil?
> What is the purpose of tuning a tesla coil?
> What is the best method of tuning?
> My alligator clip is heating up when I use it as a tap, What should
I do?
How can I help eliminate RFI/EMI?
> Line filters
> Grounding
> Faraday Cages
Tesla coil schematic:
_______
(_______) OT
T1
RFC1
|
||(-----+--------******---+---------+
(
||(
|
|
|
(
+---+
||(
o
|
|
(
<---|
|------)||(
o SG1
|
|
(
|
|
)||(
|
o
| L1
(
120VAC |LF1|
)||(-----+
|
) (
|
|
)||(
|
o MG
+-->) ( L2
<---|
|------)||(
o
|
) (
+---+
||(
o SG2
|
) (
|
||(
|
RFC2
|
| | C1
) (
|
||(-----+--------******---+---| |------) (
--+-|
| |
|
----+---+-----LF1 is a commercial line filter wired in reverse.
connected
Input (120VAC) is
to the "load" side and T1 is connected to the "line" side.
The case
is
grounded to the normal electrical ground.
This helps keep RF energy
out
of the house electrical system.
T1 is a shunted transformer. Neon sign transformers and oil burner
ignition
fit this description. The core is grounded to the dedicated RF
ground.
SG1/SG2 are safety gaps to protect T1 from RF "kickbacks". They must
fire
occasionally for them to be effective. Their spacing is slightly
wider than the main spark gap (MG).
RFC1/RFC2 are RF chokes. They should be in the vincinity of 5-10mH each.
MG is the main spark gap. It should consist of multiple gaps made of a
heavy
material such as hard copper water pipe. The number of gaps required
is
determined by your power supply.
C1 is the high voltage, pulse discharge rated tank capacitor.
be
a good match with your transformer for best results.
It should
L1 is the Tesla primary coil. It should be constructed of a heavy
material.
Soft copper tubing is typically used, however heavy litz wire or
stranded
wire without a twist in it can be used. It should have a large
surface area
to minimize the skin effect. It is typically around 10-15 turns and
has
an adjustable tap for tuning.
L2 is the Tesla secondary coil. It is normally around 500-1000 turns of
enameled magnet wire closewound on a sealed PVC or low loss coil form.
OT is the ouput terminal. Toroid (torus shaped) terminals should be
used, but
a sphere will also work, but not as well.
Formulas
------------------------------------Ohm's law
------------------------------------E = I * R
I = E / R
R = E / I
E = voltage (volts)
I = current (amperes)
R = resistance (ohms)
Finding a transformer's impedance
------------------------------------This formula is useful for finding the input transformer's impedance so
a
capacitor can be matched with it.
Z = E / I
Z = impedance in ohms
E = secondary voltage output
I = secondary current ouput in amps (divide milliamps by 1000 to get
amps)
Matching capacitor size to transformer
-------------------------------------C =
1
------------------2 x pi x Z x .00006
C = capacitance
Z = Transformer
pi = 3.141592654
Note: The .00006
substitute
in microfarads needed for primary capacitor.
Impedence
is the 60 Hz AC, if you live outside the US then
your cycle rate.
Watt's law
------------------------------------This formula is useful to find a transformer's power output based on
it's
output voltage and current or to find other values such as current or
voltage
output based on it's power rating.
P = E * I
E = P / I
I = P / E
P = power in watts or volt-amperes (VA)
E = voltage in volts
I = current in amps
Inductive reactance
------------------------------------This formula finds the reactance (AC impedance) of a given inductor at
a
given frequency.
X(l) = 2 * pi * f * L
X(l) = AC impedance (reactance) in ohms
pi = 3.1415
f = frequency in hertz
L = inductance in henries (divide by 1,000,000 to convert microhenries
to
henries)
Capacitive reactance
------------------------------------This formula is the same as the inductive reactance formula except for
one
point, the value is inverted. It determines the reactance (AC
impedance)
of a capacitor at a given frequency.
1
X(c) = -------------2 * pi * f * C
X(c) = AC impedance (reactance) in ohms
pi = 3.1415
f = frequency in hertz
C = capacitance in farads (divide by 1,000,000 to convert microfarads
to
Farads)
Frequecy of LC circuit
------------------------------------This formula determines the resonant frequency of an LC tank circuit.
1
-------------______________
f = \/2 * pi * L * C
f = frequency in hertz
pi = 3.1415
L = inductance in henries (divide by 1,000,000 to convert microhenries
to
henries)
C = capacitance in farads (divide by 1,000,000 to convert microfarads
to
farads)
Q (quality) of an inductor
------------------------------------This finds the quality (how good it is) of an inductor.
it's
resistance and it's reactance.
Q = X(l) / R
It's based on
Q = quality (there is no unit for quality)
X(l) = inductor's reactance in ohms
R = the inductor's resistance in ohms