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Old 22nd May 2004, 05:58 PM   #1
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Default Home made capacitors

Hello,
We got a little bored today, with nothing better to do on a Saturday afternoon, and decided to try making a capacitor from some paper and some tin foil. Using the capacitance formula (taking into account the distance between plates, area, and dialectric constant) I found an A4 sized pair of plates (kitchen foil) and paper dialectric (0.12mm thick, just ordinary A4 paper) should give around 16nF ish.
Well we made it, and the best way we could think of testing it was to use it in an RC high pass filter. We tried it and according to the java program on this page the actual capacitance was around 20nF. I think that's quite encouraging and was wondering about the feasibility of actually building some nice usable caps, maybe making some nice cans for them to sit in and using copper foil. It seems these would be ideal for interstage coupling caps in tube circuits (my amp has 100nF here, and a 100pF in the NFB loop)
Just a few questions, what effect would it have using oil (and what type of oil to use) like for example Audio Note PIO's.
Also is there any way of telling what kind of voltage they would be able to take? This is the main thing I'm scared of with using these. I guess the thickness of the paper would be the main issue here? Maybe the oil helps in PIO's?
Cheers for all your help,
Steve
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Old 22nd May 2004, 06:07 PM   #2
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I have got to say that this is the ultimate DIY. Making your own components is definately the pinnacle. Congratulations on your success. I can't think of any reason why your scheme using mineral oil would not work. You would have to do some tests to determine the working voltage though.
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Old 22nd May 2004, 06:19 PM   #3
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Thanks
Hmmmm, any ideas on how to determine the voltage capabilities of it without resorting to destructive testing? which i dont really fancy to be honest.
Thanks,
Steve
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Old 22nd May 2004, 06:25 PM   #4
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That is the DIY spirit, but you are not the first one to make them.
My father used to make his own caps in about the same way
when I was a child. To be even more cheap, he even
reused the foil wrapped around pieces of chocolate.
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Old 22nd May 2004, 06:26 PM   #5
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Measure the core temperature onder load, then you just read the voltage when the cap is at the max temperature you dare

Just to follow the forum rules about safety...do this with the cap behind something solid and heat resistant!!!

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Old 22nd May 2004, 06:31 PM   #6
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Hi,
So temperature should really be the main issue here then?
Is it ok to use dry paper? or is the oil just used to stop the copper tarnishing? and/or change the dialectric constant?
Any clues as to what thickness copper foil would be good, though i'm guessing that depends on factors like voltage rating. I also need a source for copper foil or i could just keep on using kitchen foil....
Cheers again,
Steve
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Old 22nd May 2004, 06:36 PM   #7
Magura is offline Magura  Denmark
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The main purpose of the oil is to increase the heat transfer. The voltage is determined by heat and when the cap sparks over. The foil thickness determines the load the cap can take...I think.

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Old 22nd May 2004, 06:38 PM   #8
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No, I don't think temperature determines the voltage capability
here. Rather it is the type of dielectric and its thickness. Since
you are using paper, humidity also enters the equation so I
think it would matter alot if you can dry them and then seal
them compared to just using them as they are. While temperature
is not important, you will probably not use such a capacitor in
a place where temperature will be the limiting factor. It seldom
is for film capacitors.
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Old 22nd May 2004, 06:40 PM   #9
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Hello,
Well the wima's that are in that position in my amp at the moment are fairly small (polystyrene btw) and they aren't noticably hotter than their surroundings to the touch. Are there any hard and fast rules anyone knows of for what thickness paper I would need to use for a given voltage?
Steve
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Old 22nd May 2004, 06:44 PM   #10
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http://uk.farnell.com/jsp/catalog/vi...?prodId=187896
Any thoughts as to how well this stuff would work?
Steve
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