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Coil Update

Quoting SE-CRAWSHAW-at-wpg.uwe.ac.uk:

 > I have three Spark Gaps in series, two of which are 1" diam.   
 > carbon rods and 1 of which is 1" diameter brass rod... Fired   
 > it up, gaps too wide, adjusted gaps to a total distance of     
 > roughly 7mm and fired again at a tapping of about 17 turns.    
 > Approximately 5" spark :-( Anyway the final max tapping was    
 > *20 TURNS* This seems pretty big compared to the normal 8-12   
 > turns quoted on this list. I assume this is due to my rel-     
 > atively small primary cap. I wanted to try a more accurate     
 > tune but I was unable to see the spark properly, one of those  
 > rare bright days in the UK. I guess this will have to wait     
 > until i can do a nightime run, as my coil is run outside.
 > The final output was maxed out at about 1ft but THIN SPINDLY 
 > sparks. I figure this to be due to either a lossy cap, gaps or
 > both. I'm limited as to my cap but I could try only 1 gap, 
 > maybe just the brass one, what do people think of this 
 > strategy?

Now that we have a better handle on the transformer issue I would
say that you need to improve your gaps, perhaps even changing
your gap types. This transformer, even at 1 kVA, needs an active
breaking mechanism of some type to deliver better peak powers. If
the gap does not achieve a nice sharp break, then energy back-
flows in the system until impedance/resistance and corona losses
sap the energy until the gap is able to make a break. When this
happens the instantaneous peak powers are quite low = less spark. 

I posted about this just last week. In order to obtain the
highest tank circuit Q factors you MUST have a plastic film
dielectric capacitor; but even with the best commercially made
cap, your instantaneous peak power will only be as good as the
quench time of the tank circuit spark gap that switches the cap.

The difference in this case here is with the step-up transformer
construction. Most of us are accustomed to internally current
limited transformers at this power level (neons, oil burners).
But even with larger internally current limited transformers
(bombardiers) simple static gaps that are well designed and
ruggedly built work quite well. Externally current limited
transformers, or those transformers running without any current
limiting at all, require more heroic efforts in the spark gap to
get the system to switch properly.

Richard Quick


... If all else fails... Throw another megavolt across it!
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