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Registered Member #64746
Joined: Sat Sept 08 2018, 09:22PM
Location: Italy
Posts: 27
By changing the number of primary turns, you change the impedance loading the tube. The capacitance is changed accordingly, to set double resonance. Depending on your tube and your secondary coil, you'll have an optimum loading that allows transfer of the maximum power from the tube to the coil. So, it's a matter of impedance matching.
Registered Member #4953
Joined: Sat May 19 2012, 11:56PM
Location: Australia
Posts: 12
Thanks RoV, I was thinking about impedance matching and how a lower C value with higher L value might typically have higher impedance for any given Fres, but at resonance the impedance of a tank circuit trends towards infinity anyway.
I wonder if the impedance is more affected by the relative turns ratio from primary to secondary, in which case more primary turns would lower the turns ratio and increase impedance seen on the primary.
I also went through the calculations for Q factor in tank circuits and it looks like a combination of more inductance and lower capacitance will decrease the Q factor, which could make it work across a wider frequency range as arcs load the secondary Fres. Though of course this is at the expense of efficiency at resonance.
I'm happy to build a few primary coils and test with trial and error, but I'm struggling to determine what heuristics define the sweet spot for capacitance in the primary tank circuit.
Registered Member #64746
Joined: Sat Sept 08 2018, 09:22PM
Location: Italy
Posts: 27
According to me, it's not easy to estimate/simulate in advance the tube loading for a given primary+coil combination. And it depends on the desired working mode, particularly between regular discharges and QCW: in the first case the load impedance is more or less the same that can be measured at low level e.g. with a VNA or with signal generator + scope; in the second case, when the spark is on and growing, loading changes a lot. However, I think you don't need to test many configurations if you can take measurements of anode voltage and current waveforms: you test one configuration, evaluate the loading and then decide: less turns -> higher current.
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Choosing VTTC primary capacitance
