Re: proven design
Subject: Re: proven design
From: chip (Chip Atkinson)
Date: Tue, 19 Sep 1995 12:33:31 +0700
Given the supplies that you have I don't have a proven design for
you. However, I can give you a past posting that may help a lot.
I'll also tell you what I have found with the 4 coils that I have
built. The best coil that I have built is the current one, which
is 4" in diameter. I used the tips and techniques
suggested/endorsed by Richard Quick. The second best performing
coil is a 3"x12" coil. It has 750 turns of 26 gauge wire and a
12x3 toroid. The primary is a 12 turn flat spiral of 2 ga.
grounding wire (solid soft Cu). The capacitor is 0.0177uF rolled
The 4" coil was made with little if any math. I basically used
Richard's suggestions and produced the secondary. Then I made a
primary that was 1" larger on the inside and consisted of 16 turns
of 3/8" Cu tubing spaced 3/8" in. apart.
The 3" coil was made using the design specs. from the TCBA newsletter
article about Gary Legel's 3" coil.
So, in summary, I would read the re-posting at the end of this reply,
and make sure that when you build the primary you put in enough
turns (about 16) to allow you to tune the thing.
... If all else fails... Defer to Richard Quick! :-)
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), or acrylic (Plexiglas): 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 dia. 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 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 gently baked under a heat lamp or even placed in
a very low temperature oven overnight. PVC coil forms must then be
sealed to negate the high RF loss factors that are inherent to this
plastic. Sealing also prevents 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 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 contin-
uously 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 rad-
iant 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 and
drips. If drips and sags do occur, then can be "grated" off with a body
putty grater, or carefully trimmed away with a knife.
3) The coil should be close wound with a good quality magnet wire. I use
double Formvar enamel coated magnet wire. 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 coil
be wound with at least No. 22 AWG, or larger, magnet wire. I would concu
with this recommendation.
I plug the ends of the coil form with a tight fitting wooden disk and ru
a dowel or threaded rod through a center hole so that it will spin. I se
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 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
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 90% 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 substantial
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
10028 Manchester Rd
Glendale MO 63122 USA