Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx> See interspersed comments.
Original poster: Crystal Clear Solutions <webmaster@xxxxxxxxxxxxxxxx> I have a 15kv/120ma NST, primary 120v/60hz 6" secondary 29.5" tall 22g wire sych gap 120 bps assume all protective circuits/safety gaps/filters are in placeI have read about Cres and Cltr. What is the best Cap to choose for the largest sparks? I've gotten Cres = 0.01uf and Cres for a sych gap = 0.026uf and Cltr = 0.055uf. Do these sound close?
The Cres for your NST at 60Hz is 21.2 nf. A recommended Cp value for a SRSG is 2.8*Cres, about 59 nf.
Here is a second question. If my Cp is fixed and I can retap my primary at any point to adjust for a new resonant frequency (Fres), how will varying the height/wire size of my secondary affect spark length? Will increasing the Ls and increasing Fres yield bigger sparks? Is there some trade-off?
Yes, most tap the primary to get in tune and there are many trade offs, many recommend a H/D ratio of 4-5 and to choose the wire size so to get around 1200-1500 turns. Lower profile coils can be vulnerable to primary hits from the topload. Taller coils can have excessive resistance that spoil the unloaded Q of the coil. For longer sparks, the rise time of the voltage on the topload seems to be important as well as the size of the toroid. Small toroids result in the E field dropping off too quickly as the distance from its surface increases. Discounting losses, the topload voltage will be proportional to the sqrt (Cp/Cs) or sqrt (Ls/Lp) because LpCp =LsCs
To keep gap losses at a minimum, many go for a large primary which takes an appropriately sized top load and secondary. If the inductance of the primary is too small, the peak current becomes too large, The gap losses are proportional to the I^2. I try to size my cap to the power source and type of spark gap. Then I experiment using JavaTC to see how big a toroid I can use and how many primary turns I will need. I try to get the biggest toroid possible and still have breakout. As the toroid size increases, the maximum topload voltage will go down and the breakout voltage will go up. A size that is at the intersection of these two curves can be considered a maximum. Once the toroid size is determined, Javatc will tell you how many turns of primary you will need. I would wind a primary with at least 3 more turns than indicated to allow for tuning with streamer loading and for any other errors in prediction. My experience with Javatc is it gets the tuning right usually within one turn.
If a secondary coil is too large for the power source and the primary is sized in proportion to the secondary, the top load must be made smaller to get tuning with the smaller Cp that the power source requires and it will look weird. In other words, many want to have the topload that aesthetically matches the coil size. This determines the frequency. If Cp is too small, the primary gets proportionally too large. If a coil is too small for the power source, secondary breakdown (racing arcs or secondary to primary strikes) often results. In general, larger coils with larger toroids have lower frequency and require a larger power source.
Third question: If one assumes Cp fixed and I can retap primary at any point to adjust for new Fres, how will increasing the toroid size affect spark length? From my reading increase the toroid size increases Cs so Fres will go down. Lp will have to increase and this will drop the secondary output voltage. However, increasing the toroid size, by increasing Cs will allow for a greater breakout voltage which should increase the spark size. Where is the "sweet spot"?
Gerry R.