D.I.Y. Variable Capacitor

Have you ever stuck in the middle of a project trying to find out the perfect size capacitor for your project. If yes then this is the project for you. If not then you can still stick here and you will enjoy the process.
So in this Instructable, I will be showing you how I created a Variable Capacitor with some regular aluminum foils and few 3D printed parts.

• Aluminum Foil
• Transparent Tape
• Double-Sided Tape
• 3 X Plane Rod 2mm Diameter & 90mm Length
• Bearing of 4mm ID and 12mm OD
• M4 Threaded Rod
• M4 Nuts
• Multi-Strand Wires
• Plane Paper
• Marker
• Access to a 3D Printer

Before starting with the build process I would like to brief you on how a capacitor works.
The basic principle behind the working of a capacitor is charge induced on parallel plates when voltage is applied. When battery or supply is connected between two parallel plates the negative charges i.e. electrons are attracted to the positive terminal of supply and thus creating positively charged plates that induce electrons on the plate opposite to it, making it a negatively charged plate.

In Capacitor the energy is stored in form of an electric field which is induced between the parallel plates.

C = ε * A / d

ε: depends on dielectric, A: Area of parallel plates, d: the distance between the plates

The formula of capacitance shows that the value of capacitance is dependent on the area of the parallel plates and the distance between them, also it depends on what kind of material is in between them which is known as the dielectric.

To make a variable capacitor we need to vary some parameters upon which the capacitance depends, as we saw in the previous step the capacitance value depends on the area and the distance been the parallel plates. We will change the area of interaction of two parallel plates to vary the capacitance.

Our solution will consist of two cylindrical aluminum foils whose area of interaction will be controlled by the rotation of a threaded rod. It will provide precision control over the range of the capacitance.

Only the area of cylindrical foils which are directly over each other will contribute to the capacitance.

There are two main and some side parts: The Left & Right Panel, Cap for Threaded Rod, and some parts to hold the foil. All the parts are printed without any supports with three wall perimeters.

I have also attached the Step file in case you wish to tweak the model to your needs.

Note: The bearing parts are optional if you have a bearing of 4mm ID and 12mm OD then you can skip printing the bearing parts or you can use the solution I have utilized by converting a roller bearing to these parameters.

First prepare the foils for the Inner and Outer cylinder (which will act as parallel sheets of the capacitor).

Draw two rectangles of width 35mm & Length equals to the circumference of the inner and outer cylinders, then cut them out.
As the aluminum sheet being only 20 Micron, it will be quite fragile so we will add a layer of paper to support it.

Mark and cut the paper for the support cylinder. The outer cylinder will have a width of 47 mm as 12mm of the sheet is going to be covered by the ring on the left panel, Rest will be covered by the Aluminum foil. The inner cylinder will be the same size as the aluminum foil.

Aluminum can't be soldered easily (unless you have aluminum flux)
Don't worry as we don't need such strong connections, sticking the wires with tape would do the work.

Just strip the insulation and spread the little strands of the wire. Lay it down on the foil and stick it with the tape.

As the inner and outer foils would face each other and thus we need some kind of insulation between them in order for capacitors to work. Also, the distance between two layers is inversely proportional to the capacitance so we need the insulating material to be as thin as possible.
I am going to utilize transparent tape to insulate the material, it will also let us stick the foils to the supporting paper pieces.

First, put a little double-sided tape on paper and position the foil correctly on paper. It will help in attaching the transparent tape. Stick the extending tapes behind the paper so the foil gets firmly attached to the sheet.

If you put the tape on foil without attaching it to the paper, due to static charge build on tape the foil gets randomly attracted to tape, ruining the foil lamination process.

First, attach the knob to the end of the threaded rod with the help of some plastic glue sticks or any epoxy.
If you have a bearing of 4mm inner diameter and 12mm outer diameter then you can use that instead of the roller pin bearing I am using.
In my case, I have inserted the bearing and a 3D printed washer of appropriate size and lock them up with a nut from another side such that the roller bearing is rotating freely.

There is a cavity of M4 sized nut to be inserted in the left panel. This nut will actuate the inner cylinder when the threaded rod rotates so the nut should be firmly fitted inside the cavity if it feels a little loose use some glue to make it fit rigidly.
Then stick some double-sided tape to the outer of the cylindrical structure as can be seen in the video and images.
Then is also a hole beside the hexagonal cavity which will be used for passing the wires from both the foils.

Remove the protective layer of double-sided tape and stick the outer foil such that aluminum foils lie at the inner surface. Also, keep in mind that only the exposed paper will stick on the double-sided tape.

After sticking the foils, cover the outer surface with some tape(I have used masking tape) to provide rigidity to the structure.

The Right Side of the Assembly consist of mainly two parts upon which the inner foils will be wrapped on.
Stick the double-sided tape on both the cylindrical 3d printed parts and wrap the foil around it.
Do note that the attached wire should be coming out from the side which will be going inside the outer cylindrical foil(as can be seen in the images) so that there is less strain on the wire while moving inside the assembly.

After wrapping the foils cover the joining part with some transparent tape so the structure becomes rigid and the foils don't get apart in case of movement.

The inner cylinder wire should also be passed through the same hole( on the Left Part) through which the outer cylinder wire was taken out.
As we have attached the foils passing the wires through bare hand would be a little difficult. So, take the help of tweezers to pass the wires.

There are holes on the exterior of the left and right panels.
The holes on the left part are slightly smaller than those on the right so that the support rods of 2mm fit snugly inside the left part and the right part can move freely on the support rod.

First, connect the threaded rod assembly on the right part( i.e. the one attached with the inner cylinder). The threaded rod should rotate freely when it is connected to the right part and the bearing should fit snugly in there. Now, fix the three support rods on the left part.

Note: if any of the connecting rods is free on the left assembly then use some Teflon tape to increase its diameter and then fit it rigidly inside the left part

Now connect the support rod to the right part and it should move freely on the rods & the inner cylinder should slide inside the outer cylinder.
Finally, thread in the threaded rod to the fix we secured into the left part.
With this, the assembly is finished.

Now let's measure the capacitance with the help of a multimeter.
The one we just made measures a range of 0-45 picoFarads.
If you want to make a larger one you can make multiple layers or increase the length of the cylinders.

Yeah! you made yourself a variable capacitor that is precise in control and provides a wide range.

If you face any difficulties in replicating it, do comment below. I will try to help with it.