Introduction: How Not to Roll an Aluminum Foil Capacitor
In my second Instructable I am going to talk about the fun I had this afternoon.
Specifically, I will descibe the making of a large value rolled aluminum foil/plastic wrap film capacitor.
Though this Instructable would otherwise be eligible to the Epic Fail contest, I am not from any of the countries listed in the rules so I can't apply for the contest. But I still want to share my experience because sometimes published negative results may save more resources than positive results.
During the course of the Instructable I will give suggestions and make plans for constructing a working device for rolling capacitors.
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Step 1: Materials
A capacitor needs conductors and a dielectric material.
The conductor here is aluminum foil, a 29 cm wide roll of 10 meters.
The dielectric is plastic wrap, 28 cm wide and 30 meters long, 15 meters of which will be used. It is quite thin, Wikipedia says it may be 12,5 micrometers thick. The material is probably LDPE with additives.
A successful capacitor of this type would be used in an induction heating circuit, with a 12 V power supply (car battery), so the voltage rating would be enough. It would have a high current rating, since the thickness of the Al foil is relatively large. Being a large value capacitor, the oscillating frequency of the circuit would be lower than usual. But those are all dreams yet to be realised.
The rack I made out of wood. I used low quality wood, which contributed to the failure of my project.
The core of the rolled capacitor was also wooden and rectangular in section. Rectangular is the worst choice for rolling purposes. It magnifies rolling errors and doesn't allow for tight rolling. I chose rectangular because end connections could be made in a comfortable way (pressure clamps).
A PVC pipe would probably be a better choice.
Step 2: The Rack
The Rack of Enourmous Fun would be a good name.
It is a cheap, lightweight and graceful contraption.
It has a holder for the capacitor in process of being rolled but the capacitor-to-be is held in your hands when you roll. I hoped that by holding it I would be able to correct the positioning by hand.
The weak points of this rack are its bad materials and an even worse constructional quality
A better rack would be made of straight good quality wood or even aluminum.
The roll holder would hold a circular cross-section roll which stays in place during rolling.
Step 3: Collapsibility
I decided to dedicate a whole step in honour of the collapsibility of my rolling rack.
That is because this feature is in contradiction to the whole philosophy of doing things right. Collapsibility excludes stability but stability is the key to good quality industrial machines.
A good rolling device would be constructed at stable right angles.
The fact that you can fit your device under your bed next to your collapsible training machines doesn't outweigh its uselessness.
Step 4: Positioning the Raw Material Rolls
The rolls should be parallel in all dimensions to the pipe onto which the capacitor will be rolled. A single degree of deviation will mean a huge deviation during the rolling process.
There should be mechanisms which act as brakes for the rolls. This increases tightness of the capacitor. The wrap requires force to come off its roll but the aluminum comes off freely.
At first I had better ideas for increasing rolling tightness but abandoned them in favour of simplicity. It was a mistake.
Step 5: Preparing the Roll
I wound electrical tape onto the wood. This increased the irregularity of the substrate, generating errors in the roll, though these errors were nothing when compared to the errors generated by other construction flaws.
PVC pipe wouldn't need any preparation of this sort.
The only preparation would be marking the places for the foils on the capacitor roll. This ensures good positioning.
Step 6: The Rolling Itself
I don't have pictures for this step because I got carried away and in the end I was laughing very hard at the mess I had caused.
Words will have to do.
First I attached the raw materials to the capacitor roll one by one. This was made harder by the fact that I hadn't previously cut off the unusable parts of the materials. Also the positioning wasn't precise enough.
Then I started rolling and all hell broke loose. The low tightness of the capacitor wasn't the worst thing. The worst thing was that the materials started shifting on the roll and there was no way for me to regulate the system.
So I just rolled and hoped that I had left enough surplus in the positioning of the dielectric. Of course I hadn't, the foils were shorted when I tested them afterwards.
The take-home advice here is that small errors in early stages of construction generate large errors in the end product.
Step 7: The End Product
It is a mess and I think the pictures speak for themselves. I still burst out laughing when I look at it.
Step 8: Torch the Rack
I disassembled the horrible thing immediately and burned it.
Step 9: Conclusion
Though I had enormous fun while building the device and very much enjoyed rolling my first non-capacitor, some mistakes should only be made once. Next time I try something like this, I will spend a lot more time on constructional quality, because in my head everything worked out nice and the failure was due to the clash of ideas and reality.