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Home brew capacitor
Hi guys,
Been quite a while since i posted here. Due to the introduction of one 8 1/2
lb Glen Douglas Hopley on the 21st Of July and a simultaneous change of job.
I have been rather occupied and not able to do much work on my coil.
After the recent discussion on capacitors i thought i would give a quick
description of the one i am building at the moment. The design came about
after much studying of posts from Richard Quick and Richard Hull and many
other members of the group and hopefully incorporates all the major features
of a good homebrew cap.
The main elements are A4 Polypropelene document wallets and heavy duty
aluminium cooking foil. The wallets have a thickness of 2 Mils and after
trimming the punched hole edges off and cutting in half give a folded
rectangle of 8 1/4 X 5 7/8 inch. A sheet of aluminium foil was then cut to
be 1/4 inch inside the edges of the plastic and was slipped inside allowing
about 1 1/2 hanging out of one of the short sides.
___ ____
| ______________________________________________ |
| | Film | |
g | -------------------------------------------------------: u | |
| :
i | | | :
d | | | :
e | | | Foil :
| | | :
a | | | :
n | | | :
g | ------------------------------------------------------:
l | ___________________________________________ | e
s ^
Folded edge of film
Two of these plates were stacked with the overhanging edge of foil at
opposite sides, giving a double thickness of film and tested to destruction
under oil, (see previous post). It was determined that the plates failed at
aprox. 3.5KV. A safty margin of X3 was decided on so the cap was assumed to
have a working voltage of 1KV. A max. working voltage of 15KV was decided on
for the cap. at a size of 0.01mfd which allows finished units to be
paralleled up in 0.01 steps as coil input power is increased.
This Decreed that 15 X 0.15ufd caps needed to be built and connected in
series to achive the desired cap.
Each cap was built up with 52 plates stacked with the extended end hanging
out of each short side alternativly. To help buid the cap. a jig was built
using four lenghts of aluminium angle having an inside edge leghth of 1/4
inch. These were fastened to a board so that the corners of the plastic
sheet sat in to the inside corners of the angle and the foil was then spaced
inthe film by the edges of the angle.
The method of layering was as follows:-
1. A sheet of perspex, the same dims. As the film was laid on the
building board, to give the cap strength.
2. The 52 plates were then stacked alternativly on top of the perspex.
3. Another sheet of perspex was laid on top of the first cap then the
next cap was laid up on top as before. The first plate was arranged that
when the caps were bused together the plates either side of the
separator were connected together so that no electrical field was placed
across the sheet.
This was carried on untill all fifteen caps were stacked, then whole block
was bound using tywraps and lifted out of the jig.
The busing was done by folding the exposed edges of the foil on each
adjacent pair of caps. and crimping together using mole grips.
To enshure a constant pressure on the joint plastic spines from document
folders were slipped over the joint to hold it firm.
The input and output connections were made by folding a strip of 1 1/4 inch
copper strip over the folded foils of the end capacitors, and drilling and
bolting through, giving a copper tape lead out of 5/8 inch. Wide. The
connections are brought out of cap tank using 1/4 Brass studs.
unfortunatly a photograph could save a thousand words here but i dont have
access to a scaner anymore.
The entire finished cap measures 5 7/8 x 8 1/4 x 5 3/4 inch outside of its
tank. To finish it, it requires parralel resistors connecting across each
section to enshure even voltage distribution, than finaly dunking in a
cotainer of oil.
I have given it a quick (2 sec) burst of sparking with no oil at about 5 KV
with no primary connected and judging by the noise, brilliance of sparks
this is going to be a vast improvment on my old salt water cap.
If anybody considers building such a cap please note the following:-
The number of plates was determined by measurement with an LCR meter and
will change with the thickness of film.
Dont underestimate, and at the same time dont be put off by the amount of
time required to build one of these caps. The jig greatly assisted the
speed and accuracy that the cap could be built, but 780 plates and films
takes a long time to prepare. I used a paper guillotine to do all the
cutting. Spending about an hour or two at night when i could, it took me
about 5 weeks to layer up the cap. Hopefully it will be worth it when
finished as i bought enough materials two build two caps.
Total cost per cap so far? About 35 U.K. Pounds
*******************************************
Ian Hopley ----> i_hopley-at-wintermute.co.uk
Aberdeen
Scotland Callsign M M 1 A B A
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