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. Components are so inexpensive that it's just not worth the trouble. If
you are one of the latter 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 all articles from Popular
Roll Your Own Capacitors
By Elbert Robberson
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.
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.
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 with alcohol.
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
Posted November 24, 2014