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Re: Tuning for Class-E Re: [TCML] Tuning SSTC to avoid losing oscillation when I try and draw discharge



I got the SSTC tuned to what is (hopefully) quite close to Class-E using a current transformer on the primary lead.

(As per Terry Fritz's measurements -- http://deanostoybox.com/hot-streamer/TeslaCoils/MyPapers/modact/modact.html -- the current and voltage track quite closely in the primary, and I assume even more so in this case: it's a 1 turn SSTC primary.)

The coil ended up liking the 0.001uF capacitor with a one turn primary best. Tweaking the vertical position of the primary changed things very little.

I did note one thing that suggests further room for improvement: Though it ends up hitting zero just as the MOSFET is turning on again, the current decays much more quickly than in Steve Ward's simulated graphs (http://www.stevehv.4hv.org/classEsstc.htm). Perhaps increasing the size of the capacitor a bit would help, though going to .01uF killed the corona entirely. I have a .0047uF cap around here somewhere...


Regarding the strategy of using a diode-protected opamp to shield the 'scope from any disastrous misfortunes in the primary driver, I learned a few things. It turned out the MOSFET gate signal was rising faster than the LM358 in the diode-protected opamp scope probe buffer could follow; I swapped it out for a spare OPA134 and got a little more definition. I also learned the gate signal was clipping courtesy the buffer's protection diodes, doubling the buffer voltage helped a bit.





On Tue, 4 Sep 2012, Andreas Hahn wrote:

Update: Dropping the capacitor back down to 0.001uF got streamers flying again, as did unwinding the primary until just two turns were left. I also realized that what I was seeing on the scope screen was the 555-driven 30khz enabling/disabling of the MOSFET driver. The antenna was barely picking up anything from the secondary! Extending the antennna to about double its length fixed this.

However, class-E still appears out of reach. Here's how things look on the scope right now:

- The MOSFET gate goes high (+3.5V*) in about 1uS. (*approximate as I have a diode-protected opamp follower of unknown but probably low-ish input impedance between probe and scope, and there's a slim chance I built a voltage divider into it way back when I built it) - Just before the MOSFET gate reaches the peak, the primary sees a .3uS dI/dt spike (magnitude .6V, but this is horribly inaccurate due to the homemade shielded loop being pretty crude), which reaches zero just as the MOSFET gate reaches its peak. - The MOSFET gate drops almost but not quite down to zero in approximately 0.6uS, then rises back up to 3.5V in about 1uS, and the process repeats.




On Tue, 4 Sep 2012, Andreas Hahn wrote:

I rebuilt the MicroSSTC's driver to Steve Ward's Class-E SSTC schematic.

Using a shielded magnetic loop (which gives output proportional to dI/dt) scope probe on the primary lead suggests the primary is tuned way too high... with the main spike lasting well under 1/10th of the MOSFET turn-off / turn-on time. (the MOSFET gate goes HIGH for 10uS)

This seems odd, as I increased the capacitor value by a factor of 10 in the rebuilding (to 0.01uF), so I figure it should be tuned too low [capacitor taking longer to charge] if anything.

Tripling the number of turns on the primary and playing with coupling didn't change much either.

Any thoughts?


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