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Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158
I saw this super-cheap homemade capacitor, and calculated that it was about 10^-11 Farads per square centimeter. Not bad, considering it can go up to about 20kV for about .004 Joules per square centimeter. I've also seen this done with foil and wax paper, which I just so happen to have a lot of. Does wax paper give a similar voltage and energy density? I know that it was used for early 20th century capacitors. I want to try this now.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Yes the results should be comparable if your paper is about the same thickness, but remember that to make reliable capacitors is very difficult so don't expect too much.
There is a lot of information in the wiki about how to calculate the capacitance for different materials
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158
Bjørn wrote ...
There is a lot of information in the wiki about how to calculate the capacitance for different materials
There's nothing there for (wax) paper, but I'm assuming that because it's thicker, it'l have slightly less than the 10^-11 Farads per square centimeter that plastic film gives.
But, has anyone here built one of these homemade capacitors before? How well do they work?
Registered Member #3700
Joined: Sat Feb 19 2011, 12:59PM
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Yanom.
The Polypropilene constant dielectrid is about 2.2, and the dielectric strenght is about 20kV/mm (MKS}...
Assuming that the the capacitor dielectric is 0.75 mm thick (five sheets ) and a small air buble 0.1 to 0.2mm thich inside the dielectic or between the dielectric and the aluminun sheets, this buble may be ionezed when the the voltage reaches 4200V peak.
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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I was looking at this pulsed induction coilgun, where he has to use big, expensive polypropylene capacitors because he needs high energy and nonpolar. So I was wondering what sort of nonpolar capacitor gave the best cost per Joule, and I saw that you could make your own out of aluminium and plastic/wax paper for cheap.
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158
Tony Matt wrote ...
Yanom:
Please inform:
Cap AC working voltage, dc peak voltage & capacitance.
Cheers
I was looking for something in the tens of kilovolts range. Of course, that would mean building a means of generating HV, probably a C-W multiplier... but that's another story.
I'd like as much capacitance as I can get. How much do you think I could get with two whole rolls of aluminum foil, with wax paper in between?
Ultimately, though, I'm looking at cost per Joule. With aluminum electrolytics, it's about 20cents a joule, but if you want nonpolar stuff like what was used in that pulsed induction coilgun (which I'd like to emulate) it jumps to like $2 USD a joule. So I was looking at the cost-per-joule of homemade stuff.
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
Yanom wrote ...
Tony Matt wrote ...
Yanom:
Please inform:
Cap AC working voltage, dc peak voltage & capacitance.
Cheers
I was looking for something in the tens of kilovolts range. Of course, that would mean building a means of generating HV, probably a C-W multiplier... but that's another story.
I'd like as much capacitance as I can get. How much do you think I could get with two whole rolls of aluminum foil, with wax paper in between?
Ultimately, though, I'm looking at cost per Joule. With aluminum electrolytics, it's about 20cents a joule, but if you want nonpolar stuff like what was used in that pulsed induction coilgun (which I'd like to emulate) it jumps to like $2 USD a joule. So I was looking at the cost-per-joule of homemade stuff.
So the problem isn't just cost-per-joule; reliability is also important. Without proper vacuum potting, your dielectric is going to degrade real fast at 20KV, leading to many capacitor replacements.
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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Dr. ISOTOP wrote ...
So the problem isn't just cost-per-joule; reliability is also important. Without proper vacuum potting, your dielectric is going to degrade real fast at 20KV, leading to many capacitor replacements.
What's vacuum potting?
Also, how do I calculate the capacitance of a capacitor, given the plate area, dielectric thickness, and dielectric constant?
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Homemade foil capacitors
