Choosing VTTC primary capacitance
megabyte, Tue Mar 22 2022, 03:52AMHi, I'm completing an old project for a dual 572B Vacuum Tube Tesla Coil.
For the primary tank circuit, I've seen capacitor values of 0.5nF to around 4nF and beyond.
In my case, I have a couple of TDK UHV-3A 4nF 20Kv caps, so I could easily make 2nF, 4nF or 8nF depending on their configuration.
Playing with online calculators, it looks like I could make any of these values work by altering the number of turns and diameter of the primary coil.
How do you decide on primary capacitance target? Lots of inductance vs lots of capacitance for the same Fres, what's the practical difference?
Re: Choosing VTTC primary capacitance
RoV IW3IPD, Tue Mar 22 2022, 10:19PM
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.
RoV IW3IPD, Tue Mar 22 2022, 10:19PM
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.
Re: Choosing VTTC primary capacitance
megabyte, Wed Mar 23 2022, 03:43AM
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.
megabyte, Wed Mar 23 2022, 03:43AM
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.
Re: Choosing VTTC primary capacitance
RoV IW3IPD, Wed Mar 23 2022, 09:02PM
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.
Roberto
RoV IW3IPD, Wed Mar 23 2022, 09:02PM
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.
Roberto
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