Re: Capacitor matched size
First let me congratulate you on what looks to be a great find. Nice
You say your power source is good for about 2200 watts. At 220 V
that's about 10 amps. I'd guess the turns ratio of that transformer to
be about 60 to 1 so 10 amps divided by 60 gives 166 mA so that is what
you have to keep the thing limited to. As far as matching the cap, if
you want one of the computer programs that is designed to make a TC
with the tank cap mains resonant with the NST you need to make your
transformer look like a NST to the program. This can be done with
math, the same way it is done with parts, by adding an inductor whose
inductive reactance at mains frequency would be equal to (resonant
with)the capacitive reactance of the tank cap at the same frequency,
in series with the primary and the source.
If you used the 169mA-2200W,13kV to count the value of the capacitor,
and used a resistor of (220A/10A) = 22 Ohms to ballast the
transformer, and ran the system it would work. But you would not get
resonant charging to any significant degree because the resistor will
dissipate its energy as heat rather than store it as a magnetic field
then give it back to the cap. Also you could charge a much larger cap
than that efficiently and still keep the amp draw down to a reasonable
So I'm guessing that you want to design your coil, and want to play
with some numbers so you can figure what to do, so I'm guessing you
really want to know "What is the biggest cap I can charge wit this
transformer at this power level."
I think if you take the 2200 watt figure and divide that by the max
projected bang rate to get the Jules per bang, then lets see...
If I remember that formula(for going the other way) is J = C/2*V^2 so
if we move V^2 over J/V^2 = C/2, now the 2, (I was always slow with
algebra) 2J/V^2 = C.
So if I did that right, then lets try some numbers, say 60Hz mains
with a SRSG for 120BPS so 2200W/120BPS = 18.3 Jules/bang so
referring to my handy mouspad calculator 36.6/331240000
which works out to 110nF. Of course the system will have losses so the
actual cap size will be smaller.
When it comes to figuring the losses, however, it gets a bit more
complicated. For a resistive ballast you can just about guess that
half the energy is going to be burned in the ballast when running full
tilt, then there are the gap losses etc. so about the 50nF range could
be a good starting point for you.
Perhaps a better way to go about it is to select a cap size based on
the more commonly limiting factors such as budget restraints, or what
you have on hand and go from there. You can always fudge the computer
programs to give the desired cap size by giving the right voltage and
playing with the current entry till it gives the capacitance you want.
You can then adjust current limiting to get you where you need to be
I just use the "Ed Sonderman's Excel spreadsheet" available at the
pupman site, it does everything I need and doesn't have that trans-cap
match BS to deal with. I have modified my copy extensively by adding
formulas as I find them to be helpful, moving stuff around so I can
see what is going on here or there without scrolling when I change
whatever it is I am playing with, I added some wire tables as well and
changed the formula for the number of turns to 100% instead of 97%
because I use a lathe to wind and the numbers come out more accurately
for me that way. It is really all I use. I have tried almost all of
the computer programs out there that are free or have a free trial and
find them all to be a bit lacking in some respect. I found at one time
I had to have two or three programs running at the same time to be
able to play with numbers as I pleased. Then Ed put out the
spreadsheet, and I tried it. At the time all I had was a copy of
Microsoft Works that came with my first computer. It would open the
file but the formatting was wrong. The results boxes were black and no
results visible. I then found that all I had to do was change the
color of the cells and save the file and it was fixed. So was I. Now
I've got MSOffice2000 or some such and its more better than good.
----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
Sent: Monday, May 01, 2000 1:07 AM
Subject: Capacitor matched size
> Original Poster: "Kamil Kompa" <czlonek-at-polbox-dot-com>
> Hi !
> I have found a large 3000W transformer (primary 220V secondary 13kV)
> but I have a problem with finding matched value of tank capacitor.
> Every design program needs to input secondary voltage and current to
> count the size of capacitor. The transformer is not current limited
> I have to use a resitive ballast. My problem is that I can't get
> from my power line. I can take only about 2200W so the transformer
> not working
> with full power. I dont know if I schould use 230mA-3000W,13kV or
> 169mA-2200W,13kV to count the value of the capacitor?
> Thanks for all answers
> Kamil Kompa