A Winding Machine for Spaced-Turn Chokes
December 1931 QST Article
are still a lot of people who wind their own coils, whether it be for
an amateur radio rig or for work in the lab. I know I've wound many
a coil around a drill bit or wooden dowel. This simple coil winding
machine that appeared in a 1931 edition of QST magazine would be a handy
addition to anyone's bag of tricks, especially if find yourself winding
single-layer coils that have a fixed space between the windings. The
home stores like Lowes and Home Depot sell small pieces of oak that
would be perfect for this kind of project. A little stain and a coat
of varnish would give it a real vintage look. Use your soldering iron
to burn your name onto the base.
December 1931 QST
These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged.
See all available
vintage QST articles.
A Winding Machine for Spaced-Turn Chokes
By W. H. Heathcote, ZT6X
Space-wound chokes are made easily
if one has a screw-cutting lathe, but these expensive items seldom form
part of a ham's equipment. The following description of a machine for
winding spaced chokes will, I trust, be of assistance to hams not in
possession of lathes. Most of the material will be found in the junk
box of the average ham, but even if all the material has to be purchased
the cost would be negligible. Since spacing the windings decreases the
distributed capacity of a choke and - more important - raises the breakdown
voltage at the end turns where the voltage per turn is always highest
in a transmitter of any power, the time spent in making the machine
is well worth while.
Referring to Fig. 1, it will be observed
that a traverse motion of the choke form along the horizontal rod is
obtained when the handle is turned in a clockwise direction. The nut
soldered to the circular plate through which the threaded spacing rod
screws moves the choke form along, the number of turns per inch being
dependent upon the number of threads per inch on the spacing rod.
A piece of wood approximately 16" x 4" x I" will serve nicely as a baseboard,
and to this two blocks of wood to carry the bearings are screwed; these
are 2 3/4" (in height) by 3" x 1/2". One is fixed about a half inch
from the edge of the board and the other 8 inches away from the first
one. Another block of wood 2 1/22 inches in height, also 3" x 1/2",
is mounted 4 inches distant from the second block, and to this last
block the circular plate is fixed. The main shaft is a piece of rod
12" in length threaded a half inch at one end and 1 1/2" at the other.
The purpose of the sleeve (see Fig. 1) is to enable the choke form to
be inserted and removed with a minimum of trouble. If the main shaft
is released from the socket on which the spacing rod is soldered, it
is only necessary to unscrew the wing nuts on the collar and the main
shaft can be instantly withdrawn, thus releasing the choke form. The
sleeve is 4" in length and of sufficient diameter to allow the main
shaft to pass through freely. A collar about 3/8" in length is soldered
over the end of the sleeve nearest the conical disc. Without this collar
the tubing is likely to cut into the conical disc if it is made of hard
rubber or other soft material, especially if a thin tube is used for
The bearings are both 1/2" in length. No.1 can be a piece similar to
that used for the sleeve. No.2 will have to be large enough in diameter
for the sleeve to pass through. The bearings are soldered to brass "saddles"
and screwed to their respective bearing blocks.
rods are four inches in length. An assortment of rods with different
thread pitches will allow a choice of different spacings between turns.
One end of each rod is soldered to a socket (which may be made from
an old binding post) as shown in Fig. 2. Care should be taken to see
that sufficient space is left, after soldering the rods to the sockets,
for the main bearing shaft to screw firmly in the socket. A simple way
to insure this is to screw a piece of wood halfway through the socket,
place it upright in a vise and after centering the spacing rod in the
socket run the solder into the surrounding cavity.
plate is a disc of 10 gauge brass 3" in diameter. Holes slightly larger
in diameter than the spacing rods are bored a half inch from the center.
Through the center bore a hole to enable the plate to be held in position
by means of the small bolt and wing nut on the block of wood on the
baseboard. Solder nuts corresponding to the gauges of the spacing rods
over the holes already bored for that purpose.
The handle is
very simple and needs little description. A nut is soldered on the side
nearest the sleeve. The one shown on the outer side acts as a locknut.
The collar is about 1/4" in thickness with holes bored and tapped at
opposite sides for the wingnuts. The guide (Fig. 2) can be made either
of wood or hard rubber. Notches are made with a file every 1/8" or 1/4"
along the top for holding the wire steady when the winder is in operation.
The conical discs can be made of tin, hard rubber or wood.
machine as described above will only make a winding 3" in length. Two
chokes could be wound and placed in series if it were necessary to wind
a choke on a form greater than two inches in diameter. Hard rubber or
fibre tubing cut into 4-inch lengths is used by the writer as choke
forms. After the shaft, discs and choke form have been placed in position
the wing nuts on the collar are tightened up and on turning the handle
pressure on the sleeve will center and tighten up the form, after which
the locknut at the handle end of the rod can be fitted.
winding chokes of say 100 turns, to decrease the distributed capacity
it is a good idea to use a spacing rod with about 50 turns to the inch
and after winding 25 turns remove the wire to the next notch cut on
the top of the guide. The same procedure is followed after each 25 turns
wound, the result being a spaced choke with additional spaces between
A number of improvements will suggest themselves to
hams; in fact I have made several myself, but to make things as clear
as possible I have shown and described the machine originally built.
Many amateurs do not
realize that there is often a real advantage in the use of tuned filter
circuits for power supplies, since the amount of inductance and capacity
necessary for a given degree of filtering is much less than that required
in the more common filter arrangements. Here is some interesting information
from Franklin Offner, W8AJZ-W9FTO:
"A few days ago I was working
with W3AH, measuring the ripple voltage from various condenser-choke
combinations. The ripple voltage was measured with a one-ma. rectifier-type
voltmeter in series with a 2-μfd. condenser. Our results, though
only an indication of what can be done, lead me to believe that hams
can do a lot better than merely pile on the microfarads and henrys in
single or multiple section filters.
"We tried various combinations of the following: a 1-μfd. condenser,
a 2-μfd. condenser, an RCA double 30-henry 80-mil choke and a couple
of Stromberg-Carlson 250-mil 4-henry chokes (43¢ each) all used simply
because they were available. With the best combination of chokes in
the brute-force arrangement using 1 μfd. on the input and 2 on the
output, the ripple was around 6 volts, from a 550-volt (each side) transformer,
full wave, at 100-mil load. Then the circuit of Fig. 3 was hit upon,
and the ripple voltage output from this combination was only 0.8 volts.
Evidently the two 4-henry chokes and the 1-μfd. condenser
were series resonant, since either adding to or subtracting from capacity
or inductance caused a large increase in ripple output. It is probable
that by varying the values of the condenser and choke in the series
resonant portion better filtering would be obtained and also by more
careful adjustment of the inductance of the choke, "A," possibly by
using one with variable air gap, in order to make the combination resonate
at exactly 120 cycles. Obviously the choke used at "A" may be of low
current-carrying capacity, since it carries no d.c. This point is the
big advantage of this circuit over one using tuned traps in series with
the output - that is, the chokes carry no d.c. and therefore their inductance
does not vary with the load drawn.
"We intend to do more work on this circuit, and would appreciate hearing
from anyone else trying it."
In the October Experimenters' Section two diagrams
were shown for switching feeder condensers from series to parallel,
in one of which, Fig. 7, a connection was unfortunately omitted. The
right-hand feeder should be connected to the right-hand switch blade;
if this is not done the diagram will not work when the switch is thrown
to the" parallel" position.
Several letters were received from
readers who caught this mistake, with Clem Wolford, W8ENH, and Robert
A. McConnell, W8FJ, both suggesting a switching arrangement which is
quite the simplest we have seen. Fig. 4 is the diagram. The switch is
a double-pole single-throw affair, the condensers being in series when
it is open and in parallel when closed.