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Tesla Coil Builder 1/?

 * Carbons Sent to: usa-tesla-at-usa-dot-net
                    tesla-at-grendel.objinc-dot-com

Quoting Mattthew <cwolins-at-orion.it.luc.edu>:

> I recently bought a 15kV 60mA neon transformer.  I would
> really like  to build a Tesla coil, but every design source 
> I have set out to use  states different results and uses 
> different formulas. Every program that I have run to aid in 
> the design steps uses different coil types and I really am 
> getting confused.  What I really what is a good solid design  
> that will give me the most noticable output discharge.   

This is one of the most frequent questions I see, and one of the 
most difficult questions to answer; not because the answer is 
difficult, but because it is time and labor intensive.

The first thing I can tell you is that I am not using a lot math 
in my coil designing/building. I work with a set of modern, experi-
mentally developed, "rules of thumb". These guidelines have been 
developed in our "labs" over the last ten years using all modern 
materials and construction techniques. Much of the coil designs 
and techniques that have been employed by coilers in the past 70-
80 years have proven to be badly outdated when modern plastics and 
construction techniques are applied. Yet there are many books and 
plans around that draw heavily from this outdated material. 

Read some of the information below. You will be looking at a
6 inch diameter coil form for your project.

1) The first step in winding a coil is to select a coil form.  
The coil form should be a low loss material (we are talking 
about radio frequency losses in the 50 - 1000 kHz range) like 
polyethylene, polystyrene, or polypropylene, polycarbonate 
(Lexan), acrylic (Plexiglas), or even ABS: but the most common 
material used from the standpoint of cost and availability is PVC 
(poly-vinyl-chloride), which is VERY HIGH LOSS. PVC may be used 
if the material is properly prepared before winding on wire. 
Regardless of the material selected, the thinnest possible coil 
form should be used; avoid heavy walled or pressure rated tubing.

The ratio between the actual winding length and diameter is im-
portant. The ratio of the winding length to the winding diameter is 
known as the ASPECT RATIO (height : diameter), where the diameter 
always equals 1. Aspect ratios may be expressed by a single number 
such as "3.21". Please review the simple chart below when selecting 
a coil form and the proper wire gauge: (All dimensions are in U.S.A. 
measurements, inches, feet, AWG, etc..)

   Coil Form Diameter   Aspect Ratio     Winding Length

        3 inches           6:1              18 inches
        4 inches           5:1              20 inches
        5 inches         4.5:1            22.5 inches
        6 inches           4:1              24 inches
        7 inches         3.5:1            24.5 inches
        8 inches           3:1              24 inches
 larger than 8 inches      3:1       multiply the coil diam. by 3

This chart is based upon the physical characteristics of the actual 
winding. Do not assume that "six inch PVC drain pipe" actually 
measures six inches o.d., and be sure to allow a few inches of 
extra coil form length. When selecting and cutting the coil form 
you should allow at least an extra inch of coil form on each end, 
and I generally figure on cutting the coil form three inches longer 
than the actual winding length. After determining the length of the 
coil form required, measure twice, then cut. Make sure that both cut 
ends are square.

2) The coil form must be free of major surface imperfections. I wet 
sand my coil forms with #150 wet/dry sandpaper (or emory cloth) and 
water to remove markings, oxidation, scratches and cuts. After wet 
sanding, the coil form must be dried thoroughly. If PVC plastic is 
used the coil form should be dried in a drying box, gently baked 
under a heat lamp, or even placed in a very low temperature oven for 
a few days. PVC coil forms must then be sealed to negate the high 
RF loss factors that are inherent to this plastic. Sealing also pre-
vents PVC plastic from reabsorbing moisture. Using a sealer such as 
petroleum based polyurethane varnish, two-part clear epoxy paint, and 
some acrylic spray sealers is important. Avoid water based and milky 
"emulsion" type sealers. 

When I am ready to seal a PVC coil form, I mount the form up on a 
winding spindle. I work in a well ventilated area, and I turn on at 
least one fan to keep air moving over the work. I set up a few heat 
lamps or other spark-free radiant heat source. Then I begin a four 
hour coating process. I prefer a high-gloss polyurethane or epoxy 
sealer applied with a good varnish brush. Sealer is slopped on while 
the coil form is spun. I use the brush to apply and smooth the heavy 
coats, and to spread out drips and runs. Coats may be applied almost 
continuously in this fashion for an hour or two. After coating the 
coil form, it should be rotated on the winding spindle for a few hours 
while radiant heat and moving air are used to speed a cure. Using this 
method it is possible to build up a high-gloss finish which is free of 
runs, drips, and sags. If drips and sags do occur, then can be "grated" 
off with a body putty grater, or carefully trimmed away with a knife
while the sealant is still pliable. 

3) The coil should be close wound with a good quality magnet wire. I 
use double Formvar enamel coated magnet wire purchased surplus, but 
newer insulations such as Polyimide coatings are even better. Magnet 
wire gives the maximum inductance per unit volume of coil form. The 
coil should have over 800 turns, but not too many over 1000 turns. 
There is a little leeway here. Use the thickest gauge of magnet wire 
that will allow the correct aspect ratio, and between 800 - 1000 turns. 
It has been suggested that all coils be wound with at least No. 22 AWG, 
or larger, magnet wire. I would concur with this recommendation. 

I plug the ends of the coil form with a tight fitting wooden disk and 
run a dowel or threaded rod through a center hole so that it will spin. 
I set up the wire spool so that it will spin at one end of a pair of 
sawhorses, with the coil form at the other end. I wind the wire on by 
hand, making sure the windings are close-wound, tight, smooth, and even. 
Overlaps and gaps will adversely affect the performance. I use a dab of 
hot glue, epoxy, or tape to hold the first turns in place, and I make 
sure to leave a 3 foot tail of wire at both ends. Do not drill holes or 
permanently route wire inside of the coil form.

ROUGH FREQUENCY CALCULATIONS:

Assuming the information given in the text and chart above is used, 
a rough calculation of the resonate frequency is given below. In the 
chart below: OD = outside diameter in inches, Wire Gauge is in AWG, 
TPI = turns per inch, Turns = total number of turns of wire on the 
coil, Resonate Frequency is given in kilohertz and the figure is 
approximate only.


   Coil Form OD      Wire Gauge    TPI     Turns     Resonate Freq. 

       3                22         37       666         840 kHz
       4                22         37       740         540 kHz
       5                22         37       832.5       380 kHz    
       6                22         37       888         290 kHz
       7                22         37       906         238 kHz
       8                22         37       888         206 kHz
       9                22         37       999         163 kHz
      10                21         32       960         152 kHz


You can see by reading through the text and looking at the charts
that some design problems occur in coils with a small outside dia- 
meter. Coils under five inches in diameter must make sacrifices: 
either they must get long and skinny to obtain a sufficient number 
of turns; or they must be wound with smaller diameter wire. When
making this decision, after determining that a larger diameter coil
will not be satisfactory, it must be understood that 75% of the 
voltage produced by a 1/4 wave Tesla secondary results from pure
resonance; meaning that the "ratio of turns transformation" calc-
ulated with the primary coil is not responsible for the majority
of voltage gains: go with heavier wire as I have indicated rather 
than additional turns. On small diameter coils the aspect ratio may 
be increased somewhat rather than sacrifice turns or wire diameter.  


COILBLD1.GIF      6/7/95           Graphics and text prepared by:

Richard T. Quick II                  <richard.quick-at-slug-dot-org>
10028 Manchester Rd 
Suite 253
Glendale  MO  63122  USA

*************************************************************************

FREQUENTLY ASKED QUESTIONS:

> Question:

> How would someone go about drying a PVC or similarly hygro-
> scopic material before it was to be sealed? In the oven? What 
> temp? Is the interior of the form sealed as well?

Before I get to the drying stage, I go ahead and wet sand the
coil form with abrasive paper or emory cloth. I use a #150 grade
of abrasive and sand with water until all printing, oxidation,
and major dings (cuts and scratches) are removed.

One of the best ways to dry this plastic is to get a large
cardboard box. Cut a few small vent holes in the top and bottom
of the box. Thread some string or light cord through the top of
the box to form a sling to hold the coil form. You will need two
or three loops of cord inside the box to securely suspend the
coil form near the top vent holes.

Place a light bulb on a heat resistant insulator inside the box
directly below the coil form. Depending on the ambient temperature, 
the size of the box, and the number of vent holes, the wattage (or 
number) of bulbs may be varied. 

The heat from the bulb(s) will produce a warm air convection inside 
of the box that will effectively dry the coil form over a period of 
no more than three days. The temperature does not need to be very
high, temperatures between 120 - 150 degrees F are just fine.

Once removed from the drying box, the outside of the coil form
should be sealed immediately, then wound with wire. Once the coil
form is wound, the ends of the coil form are capped with plastic
disks glued down with epoxy to form a hermetic seal. 

However, if the coil form is removed from the drying box and
winding is delayed for some reason, one coat of sealer should be
applied to the inside of the coil form to prevent moisture from
reabsorbing. If the winding proceeds at an orderly pace without
delays, then the end caps will prevent air exchanges which
removes the need to seal the inside surface of the coil form.

> Question: 

> Besides polyurethane what is a good sealer for a secondary coil
> form and and where do I get this sealer?

Quoting Chip Atkinson:

The coating that I used is called 

Super Gloss 
Build 50

by Behr.

I got it at the hardware store. (Hugh M. Woods, aka Payless Cashways)
It is in two paint cans, shrink wrapped together.  The can wrappers are
orange.  The stuff is stocked in the varnish section, right next to the
polyurethane.  The potential problems are bubbles, which didn't seem to 
make much of a difference for me, and annular bulges.  The bulges appear 
if you don't get the coating on evenly and the form is turning.  I would
not recommend using Build 50 as a coating unless you feel confident in 
your abilities to apply it smoothly and evenly.  I ended up with some 
bulging, but it doesn't detract from the over all appearance.  It would 
however cause problems if one used it to seal a PVC pipe under the 
winding.  This would make the windings irregular and probably cause the
top coat to be very uneven as the stuff bulges and climb towards the 
high spots as the form turns.  Nevertheless, I still recommend Build 50 
since it goes on in one coat and is quite tough and hard. Chip Atkinson

Question:

> I got the 8" PVC today. Cost me $7.00 for 3 feet. It is solid wall
> about 5/16" thick. I am going to setup something to dry it later 
> today. I got a basic polyurethane varnish to coat it with.

No problems here. The coil form could be thinner walled. I always 
try to use the thinnest walled material possible for my coil forms.

> You said you did not want to put any holes on the coil form if
> necessary because of moisture seeping in. What about throwing a couple
> packets of silica gel in before sealing it up? Or maybe some other
> dessicant? 

Holes in the coil form are avoided primarily because holes compromise 
the electrical strength of the construction. The fact that a properly
sealed and capped secondary coil prevents re-absorbtion of moisture 
when using properly dried and prepared PVC is an added bonus. Desiccants 
such as calcium sulfate, silica gel, etc. will simply trap water. I 
would not use these desiccants with the idea that these compounds are 
going to end up as permanent parts of the coil. Desiccants can be em-
ployed as part of the drying process if desired, but these compounds 
should be removed (along with any absorbed moisture) and not left as 
part of the sealed construction.

> Oh, how did you cut the plexiglas disks in a circle? 

I use a band saw or a jig saw. A plastics dealer will be able to custom
cut these disks if required, and the same plastics dealer is usually 
able to supply the correct blades and cutting advice if you ask.

> And what about using a Silicon RTV type cement to attach the ends. 
> This stuff is deffinitly non-flamable and is hard to screw-up with.

Good Question!

Sounds great does it not..?  But... These adhesives leave a residual 
acetic acid vapor which is trapped in the coil form. The acetic acid 
vapor is highly conductive and ionizes readily. DO NOT USE THESE 
PRODUCTS in this particular application.

> I am also going to order the wire today. What gauge should I use?

I would used about 888 turns of number 22 AWG enameled magnet wire.
You will need about 3.75 pounds of wire to complete this project, so,
figure on buying four pounds of No. 22 AWG enamel covered magnet wire.

Richard Quick


... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12