Re: Homemade stacked plate cap. *Need* a vacuum pump?
Sent: Thursday, October 30, 1997 9:43 PM
Subject: Re: Homemade stacked plate cap. *Need* a vacuum pump?
Yes building your own capacitor is feasible, but don't use a single
.090" poly between plates, plastic specifications do state a large
voltage can be applied to .090" , but that is short time duration. The
transients and ringing from an oscillating Tesla coil will puncture the
poly. Use at least two capacitors in series to bring the voltage down
accross the poly, the more the better. Corona will also destroy the
plastic, that is why pulse capacitors work as they do, they have many
separate capacitors in series all in one enclosure using very thin poly
film/paper/foil in oil or other impregnate.
I have built many capacitors using polyethylene and polyester (mylar).
All single layer caps failed using only a 9kV 60mA neon transformer. The
one design capacitor that does work and work well is multiple layers of
thin poly such as .006" sheeting from building supply centers. I use
poly/paper/foil for each layer to build a large value cap, then series
others to bring the voltage rating up. The paper is standard printer
paper saturated in oil before assembly. To this date I have made a .002
uF and .007uF cap running in a synchronous coil with a 9 kV neon 60 mA
with 36" sparks and holding.
4 other caps using .060" poly and aluminum or foil in oil have had 10kW
run in them for over a year with no problems, my Capacitor Product cap on
the other hand failed running in parallel with them. The home made caps
can be repaired if they fail, manufactured caps can't. I am in the same
ballpark as you, I can't spend the big bucks for ready made pulse caps.
The .05uF Condenser Product cap cost $350 and lasted almost a year before
a strike bypassed the guard ring and hit the primary coil.
Good luck, it took me some time to hit the magic combination of layers
and packaging to work right. The near future sees a .1uF poly cap being
built using the above method. Also I never used a vacuum pump but need
one. Bring the voltage up slowly in stages running a Tesla coil and over
several weeks most of the air with find its way out.
On Wed, 29 Oct 1997 23:15:49 -0600 Tesla List <tesla-at-pupman-dot-com> writes:
>Sent: Wednesday, October 29, 1997 8:26 PM
>To: tesla list
>Subject: Homemade stacked plate cap. *Need* a vacuum pump?
>After reviewing several capacitor options for my 8" coil setup, I have
>found only one solution that I can realistically afford: A stacked
>capacitor. I know a maxwell cap would be nice, but I can't justify to
>myself or to my family that it is fiscally responsible to pay
>for a 0.025uF 50kV capacitor. I have hand build every other aspect of
>coil with the exception of the neon transformers ($30/each), so why
>I considered building the rolled LDPE and Flashing capacitor, but this
>too seemed cost prohibitive, primarly with regards to time and money
>spent finding the right materials. So, I have settled on the
>flat plate cap, to fit an existing rubbermaid container that I have
>already (15"W by 21"L by 5"H - $4.99):
>Dielectric: 25 layers of LDPE sheet (14.75" by 19.2") Each layer is
> 90 mils thick (3 * 30mils) Total= 5 30mil sheets -at-
>Plates: 26 (12" by 20.2") pieces of Reynolds Heavy Duty
> Three inches will stick out each end of the capacitor
> connection to the buss wire. $1.99 for one 50ft roll.
>Fill: 2 gallons of Mineral Oil. $???
>The plates will have a 1.375" border on each side, and 2" on the end
>does not connect to the terminal. Thus, there is always at least 2"
>distance from aluminum to aluminum without going through the PE sheet.
>Total overlap area of the plates will be 12" by 15.2". I have choosen
>use 90mil of PE, as I will be running at 12kV or 15kV input, and I
>want to build this thing once.
>My calculations tell me this cap will measure between 0.022 and 0.025
>depending on whether I choose 2.0 or 2.2 for the dielectric constant
>PE. Total cost should be under $100.
>OK, here's the dilemma: I have no vacuum pump, and I don't intend to
>one. Is this going to be a big problem? Is there a way do get the air
>bubbles out of the cap without one? According to my Electromagnetics
>Textbook, it would take 90,000V peak to punch through 90mil of
>polyethylene. Shouldn't I have enough of a saftey margin here that a
>little air bubble isn't going to break the thing? Also, wouldn't a
>pump just crush my rubbermaid box down and break the seal? This thing
>going to have a good 2" of airspace above it when I'm done (I could
>find a box shorter than 5").
>Right now, I plan to just put as much weight as I can on the cap (to
>squeeze air out), fill the box enough to cover the cap with 1" of oil,
>release the pressure and wait a few weeks for the oil penetrate as
>as possible. If anybody else has a better idea, please let me know!
>Epoch, Inc. Digital Music Project
>www.tiac-dot-net/users/absmith/ MP3 Demo Tracks Now