A Winding Machine for Spaced-Turn Chokes
There 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.
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. As time permits, I will
be glad to scan articles for you. 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 sleeve.
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.
The spacing 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.
The circular 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.
The 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.
When 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 sections.
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.
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.