gifDIY Variable Air Capacitor

Variable Air Capacitor Mk I. Range: 30pF to 460pF

The small metal plate attached to the top of the front acrylic panel is for marking the capacitance value, but the numbers have not been stamped on yet.

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Variable air capacitors from the days of analogue radio are becoming increasingly rare and expensive items. Fortunately, making your own is not too difficult. There are many pages on the internet showing how to build your own variable air capacitor. This attempt was inspired by Anwar von Sroka and the instructions given in his article “Building A High Voltage (5KV) Variable, Plate Capacitor (17-220 pF)”. You can download this article as a PDF file here: https://www.qsl.net/k4dfh/varce.pdf

Another excellent guide is “Air Variable Capacitor From Scrap Aluminum Sheets” by Jezan which can be found at the following URL: https://www.instructables.com/member/Jezan/

Consult the above links for details. This prototype was built from scrap materials using minimal tools — a hand drill, a 3mm cobalt drill bit, a hack saw, tin snips, aluminium oxide abrasive paper, and lots of patience. The only purchased materials were M3 nuts and washers, some lengths of M3 threaded rod, and a 3mm motor connect plate for attaching the knob.

Blanks were cut from scrap material sourced from the base of an old VHS tape recorder:

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Each blank was marked and cut as below:

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Copious blanks were made because the quantity for the required capacitance of 220 pF is as yet unknown.

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Close-up after trimming:

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The remaining semi-circle for the rotor was cut out with a sharpened half-round of steel tubing; several blows with a hammer were required to cut through the metal. After cutting, the plates were hammered flat.

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The rotor pieces were then bolted together on a short piece of threaded rod ready for filing:

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After filing:

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The rotor was assembled on a length of M3 threaded rod. Each plate is separated by an M3 nut and two M3 washers. Corners were rounded off to stop sparks jumping from plates when used in high voltage projects:

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The size of the original stator plates was deemed too small, so new plates were cut:

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The stator plates were assembled and given a preliminary filing. To create the semi-circular recess, the majority of material was removed with a hack saw; then the profile was refined using aluminium oxide abrasive paper (No.80) wrapped around a length of dowel.

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Preliminary stator assembly:

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After examing stator plate alignment, a new bolt hole was drilled in the middle of the base edge: this is to minimize plate deflection and ensure the air gaps remain consistent:

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Assembled stator showing bolt arrangement:

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Assembled stator and rotor prototypes. Note: because the capacitance of the prototype is as yet unknown, the M3 threaded rod is left uncut, ready for the addition of new plates, if required.

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There are 13 stator plates and 12 rotor plates. Stator plate dimensions are 7cm x 3.7 cm.

After assembly, the measured capacitance with the rotor fully closed was 170 pF. At least 220 pF is required so plate quantity must be increased.

Extened rotor with nine additional plates. According to the above readings, this should give a capacitance of around 300 pF.

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Additional stator plates were sourced from scrap metal – electromagnetic shielding from a defunct cathode ray tube monitor:

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Additional stator plates after trimming with tin snips:

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Close-up of stator plate after trimming with tin snips:

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Stator plates were then hammered flat:

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Creating the semi-circular recess. An existing stator plate was used as a template and the edge marked with a Sharpie. The new plates were drilled and bolted together. As much material as possible was first removed with a file:

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The profile was then refined with aluminium oxide paper wrapped around a length of dowel (a wooden spoon handle):

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It takes some force to create the profile; gloves are preferable and be sure to wear a mask:

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Completed profile for the nine additional plates. Plate surfaces have yet to be cleaned up and polished:

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New stator assembly with the nine additional plates:

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New stator assembly showing bolts and threaded rods:

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Acrylic end plates were cut from the screen of a junk LCD monitor. Note: the plates of the rotor needed slight adjustment so that they lay in the middle of the air slots. This was simply a matter of turning the rotor 360°, checking for interferance, and making the necessary adjustments with some long nosed pliers.

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Checking for interference

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After assembly, the measured capacitance for 22 stator plates and 21 rotor plates with rotor fully closed was 307 pF.

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The rotor contact plate was cut from scrap metal; contact is maintained by a large spring between the plate and a washer on the rotor armature:

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The rotor contact plate leads out to a tab on the rear of the device:

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The rotor contact plate viewed through the rear acrylic panel:

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Rotor contact plate bent to shape and drilled for bolt fixing terminal. This contact should be connected to ground so that when user touches rotor knob, the user’s body does not affect the capacitance. The stator alone should be positive.

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By removing the washers and only using nuts as spacers, the plate interval decreases, leaving a much narrower air gap (dialectric). After assembly, the measured capacitance for 22 stator plates and 21 rotor plates with rotor fully closed was 451 pF - an increase of 144 pF. The narrower air gap requires higher tolerances during assembly, but has the added benefit of shortening the entire device. The spring contact on the rotor has also been removed as it is no longer required.

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In the shortened device, the rotor contact plate was too close to the stator, thus increasing the capacitance; therefore the contact plate was trimmed down. Ideally this would protrude through the rear acrylic plate within the circumference of the inner radius, thus the rotor contact plate would have as small a surface area as possible (to do).

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This shows the size of the trimmed rotor contact plate; the curve now matches the inner radius of the rotor plates:

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After the rotor contact plate was trimmed, the capacitance when fully closed decreased to 446 pF, so another stator and rotor plate was added to the front of the device. After assembly, the measured capacitance for 23 stator plates and 22 rotor plates with the rotor fully closed was 465 pF. The measured range when fully open was 28 pF.

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New rotor contact plate redesigned to cause least capacitance by running upward and away from stator plates. Inner view:

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New rotor contact plate. Outer view:

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