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Re: [TCML] Trans-cap uf matching, over/under?



Hi Brian,

Actually, for static gap operation, LTR is 1.5 x Cres (so 0.008uF). However, some do like to use a little less in the 1.4 x Cres range. The reason is to take advantage of some slight resonant charging. The closer the cap value is to Cres, the greater the resonant charging will be. But there is a thing as going too close if your trying to keep the transformer and cap in good order. Also, the static gap will clamp the voltage at each conduction.

It is certainly ok to go slightly higher or lower. It comes down really to how your coil performs (does it like the higher bang energy with the larger cap size or does it prefer a faster charge rate with the smaller cap and lower bang energy?). I personally prefer the larger cap size and higher bang energy.

Keep in mind NST's are fragile and they don't last forever. All it takes is one nasty transient on one of the internal secondary windings to short out the winding and kill the NST. It happens, so don't be too bummed when/if this occurs. Thus, a Terry filter is a good thing to have with an NST.

For 2 NST's in parallel of the same output voltage, the impedance is based on the sum of the currents. So yes, two 15/30's is equivalent to a single 15/60 which has a static gap LTR value of 0.0159uF. If you choose 1.4 x Cres, then .0148uF. I personally would not go lower than that value.

Regards,
Bart

Brian Hall wrote:
I have a 15kv/30ma NST so the ideal capacitance is .0053 uf, but since I don't want it to match impedance with the transformer and risk frying out a leg, I have read that 1.3 to 1.4 x that capacitance is desirable, aka LTR (larger than resonant ... impedance)

So, that would give me a range of .0068 to .0074 uf for the tank capacitor (at around 30 to 40 kv)

My question is how would the performance be affected if I were to use 0.011 uf (a hair over 2 x resonant impedance)?
And what would happen if I go a bit under it, such as .0033 uf?  What effect would that have on the TC circuit or even the hv transformer?  Would it hurt anyting?

Lastly, if I were to connect 2 NSTs of the same value in parallell, such as two 15/30 NSTs, to basically get one 15/60 power supply, would the calculation for the impedance of the 2 NSTs of Z = E / I (to find the cap value) be the same as if I had a single 15/60 NST? if it were, then .011 uf would be a hair above resonance with the transformer, but not as high as 1.3 x above it ...


---------------------------------- Brian Hall



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