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Re: Capacitor Designs




From:	Alfred A. Skrocki [SMTP:alfred.skrocki-at-cybernetworking-dot-com]
Sent:	Wednesday, November 12, 1997 11:57 AM
To:	Tesla List
Cc:	megavolt-at-usa-dot-net
Subject:	Re: Capacitor Designs

On Tuesday, November 11, 1997 7:58 AM Matt Mills
[SMTP:megavolt-at-usa-dot-net] wrote;

>   I hear that rolled poly caps are the most ideal for a TC but in the
>   mean time before i go ahead and spend all my money building several
>   of these, I would like to make some salt water caps or vertically
>   stacked caps.

Actually some can argue that a flat plate capacitor made with polyethylene
film is better than the rolled design; it's easier to prevent bubbles, it's
easier to work with multible layers of polyethylene, it's easier to get out 
bubbles, and on and on.

>   In regards to salt water caps.....
>   
>   1.   Is salt best or is there some other chemical i can use (I have no
>        problem getting chemicals)

All the salt water is for ias to make the water a good conductor, if you've 
got something else that's cheaper and works better then use it.

>   2.   What shape bottle is best? I have aquired a dozen 2.5Litre acid
>        bottles (hydrochloric acid or solvents came in them) they are
>        approx. 12" high and 7" diameter with the neck starting about
>        2" down. They are dark coloured (I believe they are called
>        winchesters).

What you want in a bottle for making caps is smooth sides and no blemishes 
in the glass ie. no bubbles. outside of that you have a trade off on 
thickness of the bottles walls, the thinner the walls the higher the 
capacitance BUT if the walls get too thin you will invite deltectric 
breakdown and it will rupture the bottle. Most acid jugs I've see have too 
thick walls and you may be better off with something like beer bottles or 
pop bottles. Tesla used mineral water bottles in Colorado, in the few 
salt water/bottle capacitors I've made I used champane and wine bottles.

>   3.   How far down should the bolt be from the bottom? does size of
>        bottle and size of bolt change the capacitance/performance?
>        Are galvanised bolts ok?

The purpose of the bolt is to be a conductor connected to the salt water 
which is the internal conductor of you capacitor, so you want the bolt long 
enough so it's end will be below the level of the salt water in your 
bottles. The size of the bolt will have no effect on  the capacitance of 
your capacitor. As to using galvanised bolts it dependson how long you 
intend to keep these capacitors around. You are immersing the bolts in salt
water which will lead to corrousion, titanium will last almost forever but
will cost a fortune, you balance out the cost/ longevity trade off. The 
size of the bottle will directly effect the capacitance of your capacitor. 
The formula for capacitors is;

                 k x A
   C  = 08842 x ------- (n-1) x 10^ -7
                   d

   where C = capacitance in microfarads (ufd.)
         k = the dielectric constant (air = 1, glass = apx 6)
         A = area in sq centimeters
         d = seperation in centimeters
         n = number of plates

The calculation is streight forward for flat plate capacitors , more 
difficult for bottles. The area of each plate is effected by the size of 
the bottle and would be;

[3.14159 x diameter x depth filled with water] + 3.14159 x (.5 diameter)^2

NOTE you have to calculate this twice, once for the inside of the bottle 
and again for the outside of the bottle. The value d is the thickness of 
the bottles glass. And of course the number of plates is 2 times the number 
of bottles. From all of this hopefully you can see that ostensibly four 
things effect the resultant capacitance, namely the area of the plates,
the seperation of the plates, the dielectric constant, and the number of 
plates used. The size bottle as you can see effects two of those variables
simultaneously, the plate area and the seperation.

>   4.   Is any type of Oil ok?

The purpose of the oil is to prevent water evaporation and reduce coronal 
leakage. To serve that end you want an oil with as high a dielectric 
strength as you can, of course you don't want a high power factor oil like
castor because it will rob your system of power heating it up. Otherwise 
use any oil you like.

>   In regards to flat plate vertically stacked caps......
> 
>   1.   I hear these are quite good performance wise and simple to
>        make, How do you connect multiple plates in parallel to
>        increase capacitance?  Is there a good design out there?  I
>        seem to have found plenty of texts on rolled caps and also on
>        salt-water caps and only one on plate caps (cap3.txt).  Does
>        anyone have much expereience with these.  I can get hold of a
>        commercial vcuum pump if needed.
 
Personally I favor this design for many reasons; if you ever try rolling
a polyethylene cap with say 10 sheets of poly per layer and fight with it
to the point of frustration and then build the same value capacitor as a 
flat plate unit, you will quickly see the advantage in ease of 
construction, add to this that it is easier to prevent air bubbles and the 
very few that remain are easier to remove even with out a vacuum pump they 
will eventualy rise out of the capacitor if it is mounted on it's side. One 
way to connect the multible plates in parallel is to cut them with a tab or 
tongue and simple use a screw and a nut to clamp all the plates together. 
See ASCII diagram bellow.

        _______
        |  _  |         
        | (_) | <-- hole for
        |     |     bolt              _______________________
        |     |______________        |                       |
        |                   |        |                       |
        |                   |        |                       |
        |      FOIL         |        |     POLYETHYLENE      |
        |      PLATE        |        |         SHEET         |
        |                   |        |                       |
        |                   |        |                       |
        |                   |        |                       |
        |___________________|        |                       |
                                     |_______________________|

Note that the polyethylene sheet is bigger in all directions than the foil 
plate, this is to prevent arcing over at the edge, use atleast one inch 
larger dimensions all the way around on the polyethylene sheet.

> Recently I asked about using rotary vacuum pump oil in rolled caps and
> I'm not sure if anyone answered as my email server had problems and I
> lost 77 messages.  Did anyone reply?

Yup, I did back on Nov 6, 1997 heres another copy of same.

---------- Copy of Previously Transmitted Mail------------

To:               Tesla List <tesla-at-pupman-dot-com>
Subject:          Re: vacuum pump oil
Copies to:        megavolt-at-usa-dot-net
Date sent:        Thu, 6 Nov 1997 18:29:40 +0500

On Wednesday, November 05, 1997 2:27 AM Matt Mills
[SMTP:megavolt-at-usa-dot-net] wrote:

> Hi,
> 
>   I'm looking at making some capacitors and was wondering
>   whether rotary
> vacuum pump oil from mobil is any good as I can get 20litres
> (4-5gallon) for NZ$55 (about US$40

Matt, some vacuum pump oils like Sunsil are superior to any
'transformer' oils I have ever seen, but on this oil from Mobil
unless you find someone who has tried it or if you can get a
sample to test is a big unknown and could just as likely be a
total looser for capacitors. What you want in an oil for
capacitors is the highest dielectric constant and dielectric
strength you can get while having the lowest power factor you
can. You can measure the oils dielectric constant by making a
capacitor with it and measure it's copacitance then measure the
capacitors plate area and spacing then calculate the dielectric
constant. To find the oils dielectric strength you have to apply
an increasing test voltage untill you break down a sample. The
power factor is the hardest to measure, but you can get an idea

of the oils relative power factor by noteing how much it heats up
in a capacitor, the more it heats up the higher the power factor
and the less desirable the oil is.

-------End of Copy of Previously Transmitted Mail--------

                               Sincerely

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                        -----o00o-(_)-o00o-----
                           Alfred A. Skrocki
                   Alfred.Skrocki-at-CyberNetworking-dot-com
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