Home made capacitors

[Christer]

You can usually find data about paper and various other
dielectrica in introductory textbooks, some component
catalogues etc. However, paper is a very broad concept
and paper comes in so many types and varieties that it
is probably dangerous to use such data.

[baggystevo82]

Yes, specially with coated paper and recycled paper etc... but i was still surprised how close the actual capacitance measured came to matching my calculated capacitance Even so, I think I'll talk nicely to some of the lecturers up at uni and see if they have any thoughts on testing...
Steve

[markp]

I used to make caps out of mylar sheets and aluminum foil back in highschool(20yrs ago). The voltage rating is determined by the dielectric material and thickness. To give you an idea, we used to use the covers of those folders used to turn in written reports, a fairly thick plastic and regular aluminum foil. We were able to charge it with a Van De Graf generator at over 10000 volts with no problem. We'd leave it on the desk of some unsuspecting soul and they would pick it up out of curiosity and BANG! No deaths occurred

[baggystevo82]

Hehe, oh the old days (only about 5 years ago for me ) An old trick for us was to turn monitors on and off over and over again with your feet off the ground and a hand on the screen.......then touch someone no idea how that one worked though. Hmm, maybe one sheet of 0.12mm paper will be up to it then...will try and find out.
Steve

[cogsncogs]

Originaly posted by: baggystevo82
So temperature should really be the main issue here then?

Yes it's an important issue, I may be wrong, but I think the leakage current would play an important role here in determining the maximum voltage rating.
I've DIY'ed a few caps with aluminum foil and good ol' Scotch Magic Transparent tape, the green plaid stuff not the red! Even tried the foil/paper liners from cigarette packs! The edges where the challenging part. But I never used them where they were exposed to high voltage.
And also getting an even and tight winding is another factor.

Wayne

[annex666]

Originally posted by baggystevo82
Hehe, oh the old days (only about 5 years ago for me ) An old trick for us was to turn monitors on and off over and over again with your feet off the ground and a hand on the screen.......then touch someone no idea how that one worked though. Hmm, maybe one sheet of 0.12mm paper will be up to it then...will try and find out.
Steve

Have you ever seen the long strips of tinsel that are sold at christmas time? (not the furry rope type tinsel, but the single strand stuff)

Take two of these and cross them over on the screen of a CRT TV (large is preferable) and get someone to put their finger in the centre - then simply turn the TV on.

Also holding the spark plug of a lawnmower while someone is pulling the starter cord has the same effect.


Remember kids DON'T TRY THIS AT HOME elastic-trickery is extremely dangerous! We wouldn't want anyone getting killed!

[annex666]

...and with regards to the homemade cap. I have a simple suggestion.

Capacitance is proportional to both the dielectric constant of the medium between the plates, the area of the plates and the distance between the plates.

Increasing the area and dielectric constant will increase the capacitance and, conversely, increasing the distance will decrease the capacitance.

C= (Epsilon x Area)/Distance

My idea was to lay a sheet of tin foil out as flat as possible, then spray it with a few coats of laquer, then lay the other sheet on top and roll it up - this will hopefully allow you to minimise the distance between the plates. If laquer doesn't work you can try all manner of dielectrics - a little research will help.

[baggystevo82]

Hehe, lawnmowers would be VERY dodgy! As many of these small garden engines have weird electronic ignitions that sometimes have very high outputs at low revs.

The edges where the challenging part

In my one i just made sure that the paper was about an inch wider than the foil, and then just tried to lay the second sheet of foil in line with the first as best i could.

...and with regards to the homemade cap. I have a simple suggestion.

Capacitance is proportional to both the dielectric constant of the medium between the plates, the area of the plates and the distance between the plates.

Increasing the area and dielectric constant will increase the capacitance and, conversely, increasing the distance will decrease the capacitance.

I did consider cling film for a while, but decided it would change its properties too much when it gets hot. I think tracing paper might work well, or just very lightweight normal paper, assuming it will still cope with the pd accross it ok.

Steve

[markp]

Make the foils 1/2" wider than the paper an offset one to the right and the other to the left. This will give you an easy way to connect the ends then wrap the whole thing in a larger wrapper to cover it all. Roll it up as tightly as you can using a small rod as a former. Tape the foils to the dielectric on the inside end of the roll so they dont move when you roll it. Mylar sheeting is better than paper if you can get it thin enough. Keep the roll tight to maximize the value.

[baggystevo82]

Ahhh didnt think of offsetting bigger sheets of foil for connection...nice one!
Steve