PSN-L Email List Message

Subject: the compact formless coil
From: sean@...........
Date: Wed, 9 May 2001 00:57:39 -0500 (CDT)

Here is a repeat of the design abd construction of a compact coil
to be used with the rare-earth magnet/soft iron washer magnet 
design. The original is from December 1999.

Regarding winding a compact coil for a seismometer as either a
direct velocity sensor or as a feedback force coil (using the
magnet assembly made with parts bought from McMaster-Carr).

One of the perplexities of making a home made seismometer is
the need to create an efficient coil and magnet transducer to
use either as a basic velocity sensor or as the forcing element
of a fedback broadband system. The resources of the amateur builder 
are constrained to making the best of "found" hardware that can be 
modified or or often ingeniously adapted. For a coil-magnet transducer,
the magnet is most likely to be used "as is", and a coil created
to fit it. But the coil itself needs to be created on a form that
provides the greatest number of turns actually inside the magnet
gap. But generally, a "found" FORM will take up a large portion of the 
available space in the gap. If one examines a speaker coil, the form is a
thin cylinder of craft-like paper and often aluminum (to dissipate heat)
that is no more than about 0.010" thick. THe rest of the circular gap of
about 0.093" is about evenly occupied by about 50% for the two or four
layers of the coil winding with the rest for clearances on each side. If the 
speaker coil IS used for the seismometer, the low resistance limits 
the velocity output. The tight clearance is a problem for linear motion 
of the coil for the partial arc movement of the seismometer boom.

So there is a need to be able to wind a coil with a much higher number
of turns that still fits the speaker magnet (or another magnet made with
found hardware). With enough searching, a smooth cylinder form with
the proper outside diameter (OD) can be found. For example, for the
10" speaker, the ID of the gap is 1 15/16  and the OD is 1 5/8", so the
ID of the coil is about 1 17/32" and the OD is 1 19/32". There are
40 turns per layer of each two layer, 4 ohm coil. Winding a new coil
with #36 magnet wire would get 8 layers of 90 turns each into the same
space, for a 4.5x increase of the output to about 50 volts/meter/second..

So I have found a way to wind a coil that consists entirely windings 
of the coil on a temporary tubular form, with epoxing of each layer. 
After the temporary form is removed, only windings remain to fill the
magnet gap.  This method is not easy, and will try your patience, but 
if you take your time, the result is predictable and rewarding.

The form is a very smooth tube of about 0.010" OD less than the 
required finished ID; the sources are everywhere; take your ruler
to the store. Pill jars might be used, but be careful of the taper
used in molding them.  Any very smooth tube will do. I have peeled
the label from  a CRazy-Glue tube for a 5/8" id form that I need for
a compact transducer using bought parts for the magnet.. (The dimensions
below apply to this coil).

The initial setup requires that the first layer be securely held in
place but in a manner that it will slide off the form. So a label is
installed >sticky side out< on the form.  Start with a paper label cut 
in length exactly to the circumference (2.0") but about somewhat wider
(1.0") than the planned winding (0.625") and installed sticky side out 
on the tube with strips of regular tape outside the planned winding area.
I fold under the ends of the tape to facilitate removal, as well as to
hold the free end of the starting turn.

Set up the wire roll (#36 Beldsol) ("Beldsol" wire has an enamel insulation
that melts free of the wire when it is soldered at 700deg.F) on a smooth
rod across the opening of a box so that it unwinds freely. Then pass it 
though a small pad of paper to control the tension by putting various weights
on top of the pad. Start the coil with the free end looped under the
temporary tape. I wind the coil by turning the top towards me so I
can see each winding. Turn the coil form by hand and make sure each winding
falls exactly next to the previous. I have found that using a portion of
the final coil mounting with a 1/4" shaft at each end of the forming tube
allows some amount of "twirling" to speed the winding process.

I get "magnet wire" as it is called from Newark (800-463-9275).
The #36 Beldsol is Belden # 8058, a 1/2 pound spool of 6400 ft,
which should make about 30 of the coils I described. (Or a single
2656 ohm coil at 415 ohms/1000 ft).

The Newark number is 36F1321, for $28.

(A more elaborate winding setup can be imagined, but may not be
worth the effort to create one or two coils. This proceedure takes
about 15 minutes per layer, including waiting on the epoxy. ... About
2 hours for an 8-layer coil, or one RAMS game for the 12-layer version).

One can go nuts trying to count the turns, so just use the winding length
divided by the wire size to determine the number of turns. A 0.625" winding
length of #36 wire of 0.0055" diameter is about 120 turns per layer. The
initial coil length  should be at least half the number of layers longer.

Once a layer is complete, I lay the form and coil on a tablet at a right
angle to the direction to the wire spool and use a pair of rulers and a 
weight to prop it up so the winding stays under tension and in place.
For epoxying the layer, I use the "five minute" epoxy by Devcon that comes
in the parallel syringe tubes. It takes about a pea size amount of each
resin; if it is over-mixed, it sets up too fast. I use a small screwdriver
to apply a thin layer to the winding layer, keeping the tension on the
free end to keep it in place. (This requires rotating the form so that
one additional turn results). Then I immediately wipe all the excess from
the winding with a finger, wiping it clean on a towel each time. This pushes 
the epoxy between the windings and results in a completely smooth outside
finish to the layer. Any delay will find the epoxy starting to gell: you
have about 1 minute to finish!. Prop up the coil again with the winding 
under tension and allow the epoxy to set for about 5 minutes. (Test the 
epoxy set-up on your mixing pad). (The un-set epoxy will wash off your
finger with warm soapy water).

The problem now arises in winding the second layer back towards the 
starting point: the wire is very difficult to see laying over itself. 
And if any winding is out of place: ie: not laying exactly adjacent to
the previous turn, dips or lumps in the winding will result, and
subsequent layers will take up too much room. So I have found that
laying three or four stripes of typing correcting film across the 
winding makes the new winding quite visible. I use the "Mono Correcting
film" for three stripes evenly spaced around the circumference. It then
becomes very easy to see each turn, and even back up a turn if white
space is seen between turns. I then wind back toward the starting
point, stopping one turn shy because a turn is added in applying
the epoxy. Each layer must be one turn less at each end in this
"formless" method construction. Again, the coil is propped up to keep
the wire under tension, the epoxy mixed, applied, and wiped smooth, as
above. I apply the next set of white film stripes offset from the
first so the slight added thickness is distributed evenly around the coil.

This process is repeated until the coil is built up to the desired OD
for clearance in the magnet gap. Mistakes will be amplified in later
layers, so don't make them. To date I have made a 12 layer coil of
about 1200 turns and 98 ohms. (5/8" ID by 3/4" OD; starting length 5/8",
finishing length (of 12th layer) 1/2". The output is 10.5 Newtons/Ampere
with the "bought" magnet I have described.

Once the coil is complete, let the epoxy cure for several hours. Then
remove the temporary taping and slide the coil off the form toward the
tapered end (even the KRazy glue tube is tapered slightly). Carefully
twisting the tube helps, but be careful not to damage the coil. Then
carefully remove the original sticky side up label from inside the
coil, being careful about the free ends of the winding. I then epoxy
the winding to a short piece of fiberglass tube with a 1/4" fiberglass
threaded stud in the center for mounting.

Regarding the direct solderability of Beldsol at 700 Deg F, I have
found that lightly scraping the enamel insulation with a razor-type
knife facilitates the removal of the insulation when soldering it to 
a terminal, which reduces the chances of melting the socket or pin or
whatever if it is  a PVC material. For temperature control, I use
the Weller WTCPT solder station that uses interchangeable tips with
different temperature cut-offs (where the curie point of the tip
is reached at a given temperature and it becomes non-magnetic and
releases a magnetic switch that turns off the heater current; if you
have one of these, this is why it clicks all the time). This Weller
soldering system is much more affordable than those with electronic

When I get some time to update the web site, I will post drawings and


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Larry Cochrane <cochrane@..............>