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Re: xformer/cap - match

HI all,
           I'm sure that what I have to say probably isn't new but if it 
isn't, this might turn out to be timely for those starting out:

> Original Poster: "Herwig Roscher" <herwig.roscher-at-gmx.de> 
> 
> Hi all!
> 
> Im not sure to understand the influence of the xformer/cap-match on
> the system performance. Therefore:
> 
> For current-limited xformers like NSTs and OBITs it is recommended to
> use resonant charging of the main cap.		    1/(2 * pi * f * C) = V/I
> 
> 1. Capacitance is like calculated:
> The cap will reach nearly the peak voltage. The gap is adjusted to
> break down just below the peak voltage. 
> 
> 2. Capacitance is larger than calculated:
> The cap will not reach the peak voltage. Therefore the gap has to be
> closed down a bit in order to render possible discharges. The "bang"
> is smaller than optimal.
> 
> 3. Capacitance is smaller than calculated:
> If the gap was adjusted to a breakdown voltage lower than peak
> voltage, the cap will be loaded several times during each halfcycle,
> resulting in several breaks. This higher breakrate causes longer
> discharges as the ions will not cool down between the breaks. So, if a
> correct capacitance isn't available, it is advisable, to use a
> *smaller* one.
> 
> Are these assumptions correct? Any comments are welcome!

      A few days ago, I fired a smallish coil on the workbench using a 
variety of NSTs.  Having been plagued with erratic and poor 
operation of simple static gaps using caps sized around the 
resonant charging value, I decided to go for a VMLTR (very much 
larger than resonant :)  approach - in fact four times the resonant 
value for one transformer I used.  The results - nice sparks with 
good stretch, especially when the primary was tuned somewhat 
lower than the secondary and the stretching sparks "pulled" the coil 
into tune and satisfactory quench without air blowing, gap cooling 
etc.  HOWEVER, do not assume that using caps of this magnitude 
will prevent the transformer from reaching some pretty high voltages. 
I metered the gap using the transformer with no cap attached and 
found that I was starting to open it wider than this setting in 
operation, even with a large cap attached. I appeared to be getting a 
charging resonance at some mains sub-multiple.

Regards,
Malcolm