Improvements to my medium drsstc

[Giuliocassano97]

Hi everyone! I'm new in the forum, Mads suggested me to register here.

I'm an Italian tesla coil maker and I have already build a medium drsstc that is working under a full bridge inverter with FF200 powered by a couple of 6800uf 450v bus capacitors in a voltage doubler configuration due to make it work with 680vdc on the bus, secondary resonant frequency with topload is around 110khz, the primary circuit is composed of a 150nf mmc and flat primary coil tuned at 105 kHz.
The system now can get 1.30 mt spark length to ceiling discharge.

Driver is 1.3b without phase lead but I've already bought a 2.7 pcb to assembly the better driver with phase lead, and I hope that when I will replace the old driver with this new it will give me best results.

My question now comes from a try that I made, I've increased the mmc from the old 95nf to new at 292 nf that let me reduce the number of turns to the primary coil, when I tested the drsstc I've seen that the result was very worst.. The length of the sparks was reduced to less than 1mt...
After this somebody suggested me to see how other people do so I look at loneoceans drsstc 2  that is similar to mine, so I reduced the capacitance to 150nf like him and the results was very better!
So my question is, how can I find the right "rapport" between capacitance and inductance in order to the same frequency?
Thanks Giulio
P.S. If someone show me how to post video I will happy to show you all my coil 

[oneKone]

Hi Giuliocassano97,
 
Welcome to the forum. Unfortunately I don't have the knowledge to help you with your drsstc primary/capacitance.

But if your video is on YouTube you can just past the link in the text box and it will appear once posted.

[Giuliocassano97]

Thanks! I only have the video on my smartphone :/

[Uspring]

The question about the right choice of MMC values has been dicussed here https://highvoltageforum.net/index.php?topic=113.0 . Note that Vmax refers to the peak output voltage of the bridge, i.e. the case, where the primary tank is fed alternatingly +Vmax and -Vmax as in a half bridge with a +Vmax and a -Vmax rail. In the case of a full bridge with the same power supply, Vmax would  be doubled.

[Mads Barnkob]

Welcome to the forum

With only 150 nF on such a large coil, you are running a with a high primary impedance, how long on-time are you using to get 130 cm sparks at only 300 Apeak? It must be quite long I guess.

I think you should experiment with higher OCD setting, maybe try around 500 Apeak and you are properly still nursing them a little, 750-800 Apeak would be pushing them.

How much time did you spend on re-tuning after you tried 300 nF MMC? In lower impedance DRSSTC it is often seen that the required lowering of the primary resonance will prevent long arcs forming before you are at almost full input voltage, as the arc load has to pull the secondary circuit down to hit the lowered primary frequency. The lower impedance primary circuit, the more volatile, hard to tune and adjust it will get.

Please provide some pictures of the bridge, inverter, primary coil, MMC, secondary coil and so on, so we got an idea of how you built it. Maybe it all just comes down to a coupling issue.

[Giuliocassano97]

Probably you're right, I've spent few time to tuning the primary with the MMC at 300nf and my error was tuning it at the same frequency of the secondary circuit and I forgotten that the streamers decrease the secondary resonant frequency, but i thought that the problem was the too small number of turns in the primary... So do you think that I should still make some experiments  with 300nf?
Also if the the frequency is tapped on few turns? I don't remember but I think it was tapped around only 3 turns..
However now I'm returning back home from job, when I'm at home I'll try to post some pictures of my coil, and during weekend I'll probably let you know about some experiments whit right information about on time, ocd setting ecc.. Thanks