January 1956 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Here is another instance
that shows how much expectations have changed over the years. Except maybe for an
experimenter or someone set on reproducing original equipment as closely as possible,
nobody would even consider trying to build capacitors from scratch. Compared to
when this "Roll Your Own Capacitors" story appeared in a 1956 issue of
Popular Electronics magazine, components nowadays are
so inexpensive that it's just not worth the trouble. If you are one of the
type people, then this story is for you. Come to think of it, another use for this
article is to provide material for a physics class laboratory exercise where the
student calculates a predicted value for capacitance based on surface area, dielectric
constant, and plate separation distance.
See also "Reactivating
Leaky Electrolytic Capacitors" in this issue.
Roll Your Own Capacitors
By Elbert Robberson
Foil capacitors can be easily made from household "Reynolds-Wrap."
Slit two strips of aluminum foil to the desired length and width using a razor blade
or scissors. Also cut two waxed-paper strips, about one-quarter inch wider than
the foil. The paper will serve as the insulator between the turns of foil.
In radio control, "walkie-talkie" and other compact construction, builders need
not be handicapped because catalog capacitors won't always fit odd slices of available
space. Home-made capacitors can be made to fit almost anywhere. One example is the
screen bypass capacitor shown in the photographs, wrapped directly on the envelope
of the tube. Ordinary household waxed paper and aluminum foil are used, and two
strips approximately 1" wide and 12" long provide .001-μfd. capacity. Exact values
will depend upon paper thickness and the pressure with which the package is held
together, and can be varied by using strips of different dimensions.
To make the capacitor illustrated, lay a flat smooth piece of foil on a hard
surface, such as Masonite, and use a metal straight-edge to guide the cut. Slit
the foil with a sharp razor blade. A couple of tries may be required to find the
right angle to prevent tearing and bunching of the aluminum. Cut two strips 1" wide
by 12" long.
Next cut two strips of waxed paper, 1 1/4" x 12 1/4", to allow 1/8" margin around
the foil. Scissors can be used for both cutting operations, but in working on the
aluminum, be careful not to wrinkle it or leave jagged points between scissor cuts.
Before assembling the capacitor, leads must be soldered to the foil for the connections.
Use fairly active flux, and gently rub the tip of the iron on the foil to remove
aluminum oxide. The joint may not be very handsome, but it will carry current and
hold together. Just don't leave sharp points sticking up. Clean off excess flux
Solder a single strand from flexible hook-up wire to the ends
of both aluminum foils. Use very active flux and scrape soldering iron back and
forth across the foil to remove any of the aluminum oxide.
Stack the paper and foil alternately, and align so that paper
overlaps all way around. Use miniature tube as "former." Hold paper and foil stack
to tube with rubber cement, then wind up as shown at right. Secure end of stack
by cementing, and then hold in fixed position until dry.
Lay a flat strip of paper on the hard surface and put a drop of rubber cement
on one end. Put a strip of foil on top, aligning it so that the paper projects 1/8"
all around, and flatten the end down in the cement, wiping off any excess. Dab cement
on the same end of the aluminum strip, then lay another strip of paper on top of
it. Repeat the process with another foil and the last paper. Have both wire leads
on the cemented end.
Put a drop of cement on the side of the vacuum tube, and place it on the "stack,"
pin end of the tube near the capacitor "pigtail" side. Carefully roll it up, and
secure the ends with another touch of cement. Hold these cemented ends tightly for
about a minute, and then the capacitor will be on its own. The completed home-made
capacitor may then be connected for bypass or coupling.
The completed capacitor could be held together with cellophane
tape. If the tube is in use, it can be employed permanently as the base for the
capacitor. As mentioned in text, this method can be utilized to form two capacitors
in parallel, or two capacitors with common ground lead.
In many non-critical circuits, it is possible to form two capacitors around a
tube, Just keep leads on opposite sides of the tube to avoid mix-ups, and work the
circuit connections out so that a grounded foil is in the center of the wrapping.
Capacitors can also be made in ribbon form, or wound in flat packages and then
folded into almost any shape to fit irregular spaces, Soaking them in melted paraffin
after winding will increase their durability and prevent moisture absorption.
Posted July 2, 2021
(updated from original post on 11/24/2014)