Frequently Asked Questions-Technical

1.Where can I get a set of plans to build a Tesla coil?

This is perhaps the single most common request I get.
And one that is the most difficult to answer.

Tesla coils do not lend themselves to duplication. They
are not like building a radio or a model airplane. Every
coil is an individual and no two are exactly alike.

Take a look at some of the various coils on some of the
web pages. You will see that everyones coil is a bit
different.

What you need to do is create your own set of "plans"
around the materials and components you have or can easily get.
For instance if I specify a 12kv/30ma neon in a coil design and
all you can find is a 9kv/60ma what do you do then? Or perhaps
instead of AWG 24 wire for the secondary you happen to have a big
spool of AWG 26. Then what?

In each of those cases you can use the transformer and wire you have
but both changes require major modifications to the coil design.

So you see a set of coil plans does no good as you are not likely to
have or need the exact components called for in the design.

This also means that plans from companies like Information Unlimited
are not worth the paper they are printed on unless you have the exact
components called for.

2. So how do I start?

First order of business is to locate a suitable supply transformer.
This is most commonly a neon sign transformer. Get out the yellow
pages and look up all the sign companies in your area and ask for
used neon sign transformers. (NST)
Look for 9kv/30ma or 12kv/30ma neons to start with. Avoid
15kv units for now unless you have no other choice. 15kv units are
harder to build reliable capacitors for.

Also useful are oil burner ignition transformers. (OBIT)
These are used to ignite the burners on oil burning furnaces.
They have ratings of 6kv/23ma and 10kv/23ma. They have very
little resale value so can often be had for free from furnace contractors.

Once you have found a suitable transformer then you can begin
to design a Tesla coil around it.

3. What is a pole pig and can I use it instead of a neon transformer?

A pole pig is a slang term for the utility company power distribution
transformer that is feeding power to your home. They step down the
high distribution voltage to 240 VAC for your home. The distribution
voltage varies from system to system but is generally 7200 or 14400 volts.

A pig can be run backwards inputting 240 VAC into the secondary
and getting 7200 or 14400 volts out the primary. There are rugged
transformers capable of serious overload without problems.

They have however serious drawbacks.

First they are supremely dangerous. While you may survive a shock
from a neon sign transformer a pole pig will surely kill you instantly.
The power level is high enough that body parts can be burnt right off.

Second they require some form of external current limiting and control.
While you can just plug a neon in you cannot do this with a pig.
A large variac and inductive/resistive ballast are REQUIRED to
operate a pig.

Third, pole pigs are quite heavy. 350 pounds is not uncommon for
a 10kva unit.

NSTs on the other hand are small and relatively lightweight.
They are inexpensive and have built in current limiting.
Several can be paralleled for higher power levels.

4. What about a transformer from a microwave oven?

Microwave oven transformers (MOT) are quite common and readily
available but suffer from some serious problems making them
a poor choice for a Tesla coil. Their output voltage is typically
2200 volts which is far too low for spark gap operation. Also
the output current is too high with ranges from 300-500 ma being
common. Trying to use such a transformer will only lead to overheated
and burnt gaps and little or no output.

Two MOTs can be wired in series to to get you around 4400 volts
and while this is better for a spark gap it does nothing about the
excessive current.

Some builders have wired 3 or more MOTs in series in an effort
to raise the voltage further but this will comprimise the insulation
on the transformers and lead to failure.

Much easier and far safer to use neon sign transformers.
They are the most foolproof and trouble free power supply
for small to medium size coils.

5. Where can I get a capacitor?

Capacitors of all types and sizes abound in electronic equipment
all around us but high voltage caps suitable for Tesla coil use
are quite uncommon. Even some of the more common high voltage
caps are not suitable. Caps from microwave ovens for instance
usually have mylar dielectrics. Under RF power from a Tesla coil
these types of caps will overheat and possibly explode. Ceramic
caps in disc or doorknob form also have very poor RF performance.

What is needed is a cap with a high voltage rating, low inductance
and low RF dissipation. Unfortunately such caps are exceedingly
expensive. Thats why most coilers build their own.

The most suitable caps will use either polyethylene or polypropylene
dielectric. They can be built in either a stacked plate configuration
or rolled up. The rolled type is more compact while the flat plate
gives the best performance.



6. How do I know what size to make the main capacitor?

The main capacitor should sized to make best use of the available
current from your NST. This is why you need the NST first. You
wont know what size cap you need until you have your transformer.

The proper cap is one that has the same impedance at 60hz as
the secondary winding of the NST. This insures maximum transfer
of energy from the transformer to the main cap.

The math required to determine this is very straightforward.

 First determine your neon sign transformer impedence:

                      E
               Z  =  ---
                      I

    Z = Impedence in ohms
    E = volts
    I = current in Amps
    
Example:

A 12kv/30ma neon has an impedance of:

Z = 12000/.03
Z = 400000 ohms


Now we need to find what value capacitor has an impedance of
400K ohms at 60 hz.

                           1
             C =   -------------------
                   2 x pi x Z x .00006


     C = capacitance in microfarads needed for primary capacitor.
     Z = transformer impedence from above
    pi = 3.14
    Note: The .00006 is the 60 Hz AC, if you live outside the US then
          substitute your cycle rate.

Example:

 C = 1/6.28 x 400000 x .00006
 C = 1/150.72
 C = .0066 mfd


7. What size secondary coil should I make?
   Shouldnt it have as many turns as possible?

The size of your secondary coil depends on the power level you intend to run.
The power level depends on the size of your supply transformer. (NST)
So again this is a decision that must wait until you have located
a suitable transformer.

For a low power coil running from a single 6kv/20 or 7500V/30 or even
a single 9kv/30ma neon a 3 inch diameter secondary would be fine.

For a higher power coil up to maybe 1500 watts a 4 inch diameter secondary
would be appropriate.

For power levels above 1500 watts you would want a 6 inch diameter secondary.

The length of the secondary winding should be from 3 to 6 times the
diameter of the secondary. The best H/D (height/diameter ratio) varies
from 6 to 1 for a 3 inch diameter secondary to 3 to 1 for a 10 inch
secondary. Here are some examples using standard PVC pipe:

PVC     actual OD    Best H/D   winding length

3       3.5 inches    6:1       21 inches
4       4.5 inches    5:1       22.5 inches
6       6.5 inches    4:1       26 inches
10      10.5 inches   3:1       31.5 inches.

Considerable variation is allowed. For instance my 4.5 inch diameter
secondary has a 21 inch winding and works very well.

The proper number of turns is from 700-1000. Less than 700 may not
allow the secondary to reach its full potential. (pun intended)
More than 1000 dont contribute much to the output and make the
resonant frequency unnecessarily low.

Since we have determined the winding length we then select the largest
guage wire that will give us the required number of turns. This turns
out to be AWG 24 for 4 inch secondary and AWG 22 for a 6 incher.


8. What size should my primary coil be and what should
   it be made from?

The primary coil needs to have sufficient inductance to resonate
with your chosen main cap at the same resonant frequency as your secondary
coil. It should have additional turns beyond this to allow bringing the coil
back to resonance at a lower frequency once a toroid is added to the system.

The most common mistake is making a primary coil with too few turns.
I made this same mistake myself and had to add turns twice.

The primary coil can be of 3 basic shapes. A helical solenoid
form, a flat Archemidean spiral or a shallow saucer shape.
The 3 forms differ only in the amount of coupling they provide.
The solenoid provides the most coupling and the flat spiral the least.
The saucer is a comprimise and offers a degree of coupling between the
other two. One shape is not really any better than another but their
characteristics make them best suited for some types of coils.

The solenoid is best for small coils that often need all the coupling
they can get. High power coils are best with a flat spiral to minimize
overcoupling problems and secondary/primary flashovers. Medium sized
coils do best with the saucer shape at a 15-30 degree slope. The angle
is not critical. The flatter you make it the less coupling it provides.

A solenoid type should have a diameter more than twice the diameter
of your secondary. If the outside diameter of your secondary is 4.5 inches
then a 10 inch diameter primary would work well.

For the flat spiral and saucer types the inside turn should be spaced
far enough away from the secondary to prevent arcing. For 12kv or less
1 inch spacing is adequate. For 15kv 1.5 inches is necessary.
So if your secondary is 4.5 inches OD and you are using 15kv neons the
innermost turn should have a diameter of 7.5 inches. For all primary types
you should have a minimum of 12 turns. I recommend 14-15 turns.

Use heavy wire or copper tubing. For a small coil AWG 12 solid copper
house wire is adequate. For larger coils 1/4 inch diameter copper tubing
is good for power levels up to 2 KW. Above that 3/8 inch copper tubing
should be used.