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Re: Sizing capacitors

Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

At 10:14 PM 6/16/2006, you wrote:

I've built coils before using caps that were obviously
"way STR" (e.g., 30nF on a 15kVA pig), and so never
bothered to really understand how one sizes a cap to
take maximum advantage of one's transformer (didn't
have room in my garage to do that with a pig anyway!),
but now I'm working on a coil based on a tiny 200VA PT
and find myself wondering how much cap the thing can
really take.  I know how to compute the cap size
needed to deliver, say, 200W to the coil based on a
particular firing voltage, however I'm guessing this
doesn't corelate well to the rated VA of the
transformer, which is based on the rated RMS voltage.


The coil's power is:

P = 1/2 x C x V^2 x BPS

Where:
P is the power in watts
C is the primary capacitance
V is the firing voltage
BPS is the breaks per second



The output voltage of my PT is 2400VAC (Yes,
2.4kV--this is for a SISG coil), so the peak voltage
would be 2400V x SQRT(2) = 3390V.  If I limit my
transformer to 200VA, how much cap can I charge to
this voltage in a half cycle?


200 = 1/2 x C x 3390^2 x 120      C = 290nF


As this is a
non-shunted transformer, I'm thinking LTR and STR have
less to do with the cap and more to do with the
ballast, yes?


Yes.


So if I compute the maximum cap size I
can charge in a half cycle at 200VA, then ballast for
300VA, I'm running STR, and if I were to ballast for
100VA, I'd be running LTR...?  How far off of
resonance does one have to be to meet the definitions
of STR and LTR?


Since you have control over the ballast, you can do anything you want.



If I go the LTR route, I'd like to know how much
voltage I can expect to develop on the cap in a half
cycle (less than 3390V, obviously) so that I know how
many SISG sections must be used.
LTR coil charge to the "same" firing voltage, they just charge larger cap values. However, since SISG coils are DC, the inductive kick effects are not present. You can't resonate "DC" so LTR has no meaning for DC coils. 


I know how I'd do
this for DC, but how about for 60Hz AC?

Lastly (in case Terry is reading this):  Is there any
reason why one wouldn't want to use fewer SIDACs in
one SISG section than in another within the same
"stack"?  For example, if my firing voltage divided by
900 has a 300V remainder, can I just use one SIDAC in
the last SISG section to take advantage of this
straggling 300V, or am I better off making all the
sections the same.  Can't think of why that'd be
critical, but just in case...
I have not tried it specifically but it "should" be fine. In my new coil, I just lowered all of them... I have just jumpered across the heatsinks for lower voltage effectively shorting them out and that works fine. It is a series charger so there should not be any timing problems. 



Sorry for any repeat Q's (I'm sure they all are!), but
while I see tables of STR and LTR values floating
around for NSTs and pigs, I'm afraid I simply have no
idea how those numbers were established!


You probably really don't want to know ;-))

Cheers,

        Terry




Thanks for reading all this :-)

Regards,
Aaron, N7OE