Text file for COILBLD1.GIF, graphical instructions for
construction of high performance 1/4 wave Tesla resonators.
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@slug.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.
=========================================================================
Question:
> A brief suggestion regarding polystyrene as a form material: it
> is polystyrene as is the plastic used to make model cars. Why
> not use airplane glue? It will act as a solvent welding
> activator, is very water and humidity resistant, and if used
> properly, will resist at least the same temperatures as the
> form material? I have no idea what the electrical properties
> are, but it is cheap and possibly worth a try.
I am not exactly sure what you propose using airplane glue for,
but I can tell you from experience, you don't want to use this
material anywhere on or near a Tesla coil. The bottle is clearly
labeled "DO NOT USE NEAR SPARK OR FLAME".
If you were to use a solvent based adhesive to cap the ends of
the secondary coil form for instance, and the coil fails
internally, the resulting explosion will be hazardous. Trapped
solvent fumes mixed with air in a sealed tube provides enough
energy for a pretty decent explosion. I have had this happen more
than once. The windings will prevent the entire coil form from
splitting, but the ends of the coil form between the winding and
the cap will shred and throw shards of coil form "shrapnel" about
the room. The discharge terminal will be thrown, and you can
almost count that it will receive damage when it strikes the
floor or the tank circuit.
Nowhere in my instructions do I recommend using solvent based
adhesives in the secondary coil construction.
======================================================================
Importing text written by Steve Crawshaw:
I've just intercepted RQ's request for info on secondary coatings, and
I thought I'd throw my info in here. In UK there is a component
distributor called RS Components who supply all sorts of electronics
gear. They have a number of products for HV insulation. I have got
hold of a tin of the following
Anti Corona Lacquer 569-290 $2.86 400ml Aerosol
This has a dielectric strength of 48Kv/mm with a working temp of up to
150 degrees C. I intend to follow RQ's preparation and sealing method
for secondary construction on a 4.5 inch Diameter Perspex (Lexan)
former, but use this stuff instead of polyurethane.
Unfortunately the perspex has been extruded, not moulded, so there
are small striations / lines running along the length of the tube.
This is why I intend to wet & dry it before winding. Also being
extruded, it is not completely round :( I have some 0.4 mm enammeled
Cu winding wire and I have calculated 1150 turns with a 4:1 aspect
ratio. On top of this goes my 12" x 3.5" stainless steel toroid. I
have just had this shotblasted and it is double sexy. I have silver
soldered a 1/16" thick circular steel plate into the centre of this to
support the toroid while on top of the coil. I have asked this before,
but I can't remember if I got an answer. When constructing the
secondary coil, I seem to recall that RQ recommends sealing end caps
on the former. Why is this?
Anyway, back to the point in question. There are some more products of
interest in the same section of the RS book:
modified silicone conformal coating
90Kv/mm dielectric strength
red and transparent colours
fluoresces under UV (good for tracing corona leakage?)
temperature -70 - 200C
best cure 24 hours @ 100C; 200ml aerosol 494-714 $6.20
--------------------
Tropicalised varnish
Acrylic varnish with UV tracer
touch dry in 10 mins
recommended drying 24 hours @ 21C + 4 hours @ 60C
temp -60 - 130C
dielectric constant 2-45 @ 10KHz @ 21C
dissipation factor 0.028 @ 10kHz @ 21C
500ml tin 188-627 $6.29
-----------------------
Hope this info is of use. As I say this is a UK source but no doubt
there are equivalents in USA. Prices are in pounds sterling but I
can't make my keyboard do Pound signs. Cultural imperialism or what?
Quoting Steve with ">"
> I have asked this before, but I can't remember if I got an
> answer. When constructing the secondary coil, I seem to recall
> that RQ recommends sealing end caps on the former. Why is this?
In a high performance design, properly driven, the secondary coil
is easily capable of producing sparks that greatly exceed the
physical height of the secondary coil. In other words, your coil
may be 20 inches tall producing sparks that are 30 inches long.
Under these conditions an "open" or uncapped coil will almost
certainly fail. All of the electrical failures that I have seen
occurred inside of the coil form. The failures resulted in
sidewall perforations, arc scoring, and carbonization of the coil
form. I have never been able to successfully repair a coil once
it has failed in this manner.
To prevent electrical breakdowns in the secondary coil the
following points are important:
1) Never drill holes into the sidewall of the coil form.
2) Do not allow wire (or any conductor) inside of the coil form.
3) Both the top & bottom of the coil must be capped air tight.
4) Solvent based adhesives must not be used for capping the coil.
If these simple guidelines are met, the coil will have an
enormous electrical strength and failure is highly unlikely.
Coils wound on all plastic coil forms that are constructed
following the simple guidelines above can produce sparks that
exceed the physical winding height by a factor of 5 to 1 without
failure or breakdown in the construction.
Every set of printed coil plans that I have seen shows two holes
drilled into the coil form sidewall. The wire used to wind the
coil is fed through the hole near the intended base. The wire is
then routed inside of the coil form to a ground terminal. The
coil is wound, and the other end of the wire is fed through the
hole near the intended top. The wire is again routed inside of
the coil form through an insulator mounted on the top of the
coil. If end caps are even used, wood is frequently recommended.
While these instructions will work fine on an open wooden or
cardboard coil form that is space wound with d.c.c. (double
cotton covered) wire a-la 50 years ago, if followed today using
close wound magnet wire on a high-Q plastic form it is lethal.
Modern high-Q construction materials demand a change in con-
struction practices and techniques. A high inductance design
(close wound magnet wire on a plastic coil form) will produce a
very potent secondary winding which will promptly smoke when
coupled to an efficient tank circuit unless care is taken to
prevent internal arcing and sidewall failure of the coil form.
If the wire is never allowed to enter inside of the coil form,
no holes are drilled into the sidewalls, and both ends of the
coil form are properly capped, there is no way the spark can get
inside of the coil. The result is that the coil will not fail.
BTW, the reason I have recommended that solvent based adhesives
NOT be used when capping the coil form is because the explosive
vapors are trapped inside of the coil. If points #1-3 are
followed then point #4 should not be too critical, but if an
internal failure of the secondary coil does occur, you can count
on any trapped vapors igniting and an explosion will result. This
is not just an idle warning either. I have had it happen and it
was nothing but dumb luck that a serious accident did not result.
================================================================
Question, quoting Ed H:
> On the subject of coatings:
> Richard Hull (or TCBOR) mentions in the guide to the Colorado
> Springs notes that he/they don't use secondary coatings
> anymore-or at least they were using only a single thin coat.
> He says they avoid breakdown from the coil by proper terminal
> placement.
> Any comments on Hull's method?
Yes, Hull's method works just fine. Many times I have test fired
a coil with no coating whatsoever. While coating is not an
absolute necessity, it is worthwhile for several reasons.
1) Coatings prevent the winding from loosening and slipping off
the coil form.
2) Coatings decrease the chances of a coil suffering permanent
damage as a result of over-coupling and "splitting".
3) Coatings provide physical protection to the winding and help
prevent serious damage to a coil that may result from bumps,
dings, cuts, falls, etc.
4) During experimentation a coiler may choose to deliberately
over-drive the coil, the coil may be overdriven accidently, or
the coil may be fired in unusual configurations where the
electrical stress may be high (Tesla Magnifier and bi-polar
configurations for example). The coating does reduce corona
losses and reduces the incidents of electrical breakdowns.
5) A well applied coating looks good, and can be maintained.
I guess the argument here is that if you are winding a coil to
become part of a permanent system where changes are rarely or
infrequently made (like a museum or display coil) one or two
light coats of sealant, or just enough to stick the windings
down, would be fine. Once tuned and adjusted with a proper toroid
the coil will function well, and without excessive corona losses.
But on the other hand if you are like me; a person who frequently
removes/replaces/stores coils, changes primary configurations,
gaps, caps, dischargers, fires an occasional Magnifier or bi-
polar system, etc.. The benefits of a good heavy coating are
clear, if for no other reason than protecting the winding from
accidental physical abuse.
I have found that the benefits of a heavy coating on the
secondary far outweigh the advantages of a light or non-
existent coating. Also, there are a lot of beginners out
there. Beginners especially need the physical and electrical
ruggedness that a heavy coating provides.
Richard Quick