Re: Sizing capacitors

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>

Thanks, Terry!  Very helpful as usual.  I suppose my
doubts about the transformer <==> capacitor
formulation come about because it seems to me that a
capacitive load on the transformer is going to have a
crummy power factor, thus rendering the transformer VA
incompatible with the coil Watts value.

Is that true?  In other words, to use 290nF would mean
200W effective power from the transformer, but the VA
would be higher, right?  If I'm way off, just shut me
up! :-))  Otherwise, what I guess I was asking was
this:  How would one go about choosing a cap for a
given VA, or otherwise so as to avoid exceeding some
rated peak current.  Again, this is a PT and probably
can take a few times its rated 200VA...guess I'm just
interested in "getting the math" instead of surviving
on "rules of thumb" as I have for years now.

Regarding "resonance" and "DC"...duh!  My brain was
obviously already in bed when I went off on LTR/STR.
I'd actually *love* to know how those values were
computed for NSTs, etc., but if you recall sending it
to the list or posting it somewhere, I'll just look
harder :-)

Good night (for me!) -
Aaron, N7OE

--- Tesla list <tesla@xxxxxxxxxx> wrote:

> 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
>
>
>