How do I improve a simple home-made capacitor?

Question

I was explaining to my son that a capacitor is simply 2 sheets of foil separated by a dielectric and rolled up and he said, "can we make one then?"

I'm giving it a go. I used household aluminum foil and some parchment paper. I cut the foil into two strips, about 100mm by 200mm, placed parchment paper between them, inserted a paper clip into each, and rolled it up tightly. I'm pretty pleased that I made one that tests 9 nF consistently.

But if I want to have something big enough that we can continue to light an LED with it for a noticeable amount of time once we remove the battery, 9 nF isn't nearly good enough. What should we try next to hopefully get us a few orders of magnitude more capacitance?

Answer 1

I cut the foil into two strips, about 100mm by 200mm, placed parchment paper between them, inserted a paper clip into each, and rolled it up tightly. I'm pretty pleased that I made one that tests 9 nF consistently.

I made a capacitor the same size as yours, but used cling film (aka 'Saran wrap') as the insulator, and got 37 nF. Cling film is made from a plastic called polyvinylidene chloride (PVDC), which has a dielectric constant of 3.9 at 1 kHz. After charging it to 9 V I got a noticable flash from a 5mm superbright white LED.

Answer 2

hopefully get us a few orders of magnitude more capacitance?

The suggestion about Saran wrap is a good one, but the easiest way to get a few orders of magnitude is to remember the capacitor equation and increase the area. 100mm x 200mm is all very well, but what if you used a whole roll of foil + insulator? With a bit of care you could put the two source rolls next to one another and roll evenly onto a target cardboard tube or spindle.

The next source of improvement is to acknowledge that capacitors (huge voltage curve) and LEDs (constant voltage) are very badly matched. If you built a small buck converter you could charge the capacitor to a higher voltage, thereby storing more energy for the same number of Farads. Best to keep it below 50V, though.

Answer 3

The equation for the capacitance of a plate capacitor is...
C = E0 * Er * A / D

C = Capacitance in Farads
E0 = Permittivity of free space = 8.854 pF / m
Er = Relative permittivity of dielectric
A = Area of plates in square meters
D = The distance between the plates in meters

To increase capacitance, you can do any of the following...

• Increase the area (use more foil)
• Decrease the distance (use a thinner insulator)
• Use a dielectric material with higher relative permittivity (Er)
• Use a wet dielectric

Theoretical capacitance of a 100mm x 200mm capacitor made from various materials.

• Seran Wrap (polyethylene): E0 x 2.3 100mm x 200mm / 12.7um = 32.1nF
• Wax paper: E0 x 2.5 100mm x 200mm / 25um = 17.7nF
• 20GSM Printer paper: E0 x 2.3 x 100mm x 200mm / 100um = 4.1nF

Its typically not possible (especially with a homemade capacitor) to make perfect contact between the foil and a solid dielectric material. This will have the effect of increasing D as well as reducing the effective Er. So, the real capacitance will probably be less than calculated above.

USE A WET DIELECTRIC
Using a wet dielectric allows the dielectric to make contact with the aluminum foil without any gaps due to surface roughness. Also, there are many readily available non-conductive liquids that have very high Er values.

Glycerin is a good choice. It has Er = 47-68, its non-toxic, it doesn't evaporate, and its readily available in places like Walmart in either the hand lotion or pharmacy sections. To use it, impregnate a thin piece of paper with the glycerin and then place the paper between your foil as usual.

• 20GSM Printer paper + glycerine: E0 x 47 x 100mm x 200mm / 100um = 83nF
• 10GSM Tissue paper + glycerine: E0 x 47 x 100mm x 200mm / 50um = 166nF
• 10GSM Tissue paper + glycerine: E0 x 47 x 100mm x 1m / 50um = 832nF
• 10GSM Tissue paper + glycerine: E0 x 47 x 100mm x 10m / 50um = 8.32uF

So, if you use glycerine impregnated tissue paper and make the foil longer (>1m) you can get values approaching 1uF or more.

Answer 4

You might find it better to make a battery, but that's not what you asked.

You could try making an electrolytic capacitor using aluminum foil and a paper separator. Maybe some engine antifreeze and borax for the electrolyte. Optimum mixture unknown (by me) but borax is fairly soluble.

You can roll it up, with the electrolyte saturating the separator, and immerse it in a cylindrical container.

You can apply current-limited voltage for a period of time to 'form' the oxide layer if it is not adequate. Store aluminum foil does not have the etching that commercial electrolytic caps have (which increases the effective oxide surface area immensely) so I wouldn't expect much but you may be able to get a noticeable flash out of a modern LED.

I have not tried this, but it should work.

The polarity will be that of the 'forming' voltage, but up to a volt or two it should be bipolar.

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Edit:

A brief try yielded a very conductive electrolyte (great) but way too much leakage (>1mA) which is a show-stopper here. I think it is necessary to use aluminum wire to get out of the electrolyte (which I did not do). That way an oxide layer can be anodized onto all the materials connected to the electrode.

Answer 5

You can use a thin layer of insulating varnish as the dielectric, use a sheet with increased area and roll it up tightly.

• Take 2 foils of large area, without ANY insulating coating.
• Dip one foil in insulating varnish, then let it dry. (this encapsulates that foil with a thin layer of varnish).
• Press the 2 foils together.
• Solder the leads and take them out.
• Roll it up very tightly over a thin insulating rod.
• Dip the whole thing in glue / varnish , without loosening it (this is just to maintain its shape and tightness).

Answer 6

Make it bigger and use a thinner foil. 2 m² polyethylene foil with 20 μm thickness should give you something just below 1 μF, probably enough to see a flash.

Answer 7

The old, soviet era capacitors I disassembled as a child were constructed similarly to yours, but with one very important difference. They were separated not with parchment, but rather with some very thin porous paper that was soaked in oil. There was no air in the package. They were also rolled up very, very tightly, after unrolling them and rolling them back I was in no way able to fit them back into the aluminum case.

Dimensions were also different, you have 10 x 20cm, ones I played with were more like 2 x 100 cm, rolled into a package 2cm high and significantly under 2cm in diameter.

If my memory serves me, strips with similar area as yours were in the order of tens or low hundreds nF.

Answer 8

Simply put: the thinner the dielectric, and the larger the area of the plates, the larger the capacitance.

Historically, a class of capacitors known as polythene used the approach described by the person suggesting Saran Wrap (UK: cling film); capacitances of a few microfarads were achieved.

The Wikipedia page has a good description of the physics and scaling with area and thickness.

Answer 9

The capacitance depends on three things, plate surface area, separation between plates, and the dielectric constant of the insulating material.
The easiest way to increase the capacitance is to increase the plate area, so you'd want longer, wider foil.
Having the plates closer together increases the capacitance, this also tends to lower the breakdown voltage but with low voltages such as a battery this shouldn't be a problem. You could try gluing the foil to the paper with a very thin layer of glue, that would keep it together tighter and might increase the capacitance. Alternatively you could just try rolling it tighter.
As for the dielectric constant, for an experimental cap you're kind of stuck with whatever you get with paper.

Answer 10

Great answers here on making a nice capacitor, but remember that big capacitors in reality are made up of a set of capacitors connected in parallel. In the image below, each rectangular box contains several cylindrical capacitors, and each four boxes are connected in parallel.

image source: https://commons.wikimedia.org/wiki/File:Condensor_bank_150kV_-_75MVAR.jpg

Answer 11

Kitchen aluminum foil is OK, but you need a thin insulator layer. Anodizing works well. For capacitor use, a thin oxide insulator is desired and thus you'd normally want Type 1 anodizing with chromic acid. That's a severely nasty thing to work with and requires special handling. Best forget about I even mentioned it. I don't even suggest the typical sulphuric acid, since that's not really fit for ad-hoc occasional use without a good safety protocol and PPE in place. Instead, use a 20-30% solution of NaHSO4 (sodium hydrogen sulfate), commonly used for lowering pH in pools. You can get it anywhere pool supplies are sold. It's less nasty than H2SO4, but still an irritant and you want goggles and gloves when working with it.

Dunk two well degreased pieces of aluminum foil into such solution, attached to a current limiting power supply, and run some DC current through it - a couple amps/sq.ft should be more than enough. The positive anode will get coated with oxide. You'll need some experimentation to get the thinnest possible oxide layer that doesn't short the two foils once you press them together. The oxide layer can be sealed by putting the anodized piece into boiling tap water for a couple of minutes. This may help make the capacitor more durable perhaps.