Auto tuning primary?

[Overlordcheese]

I’ve had this idea for a while: what if, in addition to the primary tank capacitor, we had a system of… MMMCs I guess (triple m intended). However, they aren’t directly connected in parallel to the tank capacitor, instead a switch in the form of two mosfets or igbts connected so that it acts like a bipolar switch. At the beginning of every pulse, a microcontroller would cycle through all the possible capacitor combinations, and using secondary feedback, would measure which one produces the highest output voltage. This way, when the environment changes, the coil still performs
But people laughed at me on discord so I’m assuming there’s a stupid part to it

[Mads Barnkob]

There are some fundamental issues with changing the capacitance, rather than changing the inductance.

- The sweet spot of tuning a primary LC circuit is within centimeters on the primary coil. And it all depends on the spark load, this is also the issue you want to adress.
- The speed at which you have to do these calculations and switching, how much time do you have to get a good enough sampling rate, do calculations and then do switching?
- Switching on the MMC, you would properly need a lot of switches and end up with a very complex matrix that requires expensive control (fast)

I wont laugh at it, but simply say its a very over complicated solution to a not-very-problematic problem.

[klugesmith]

Start by figuring the electrical requirements of your capacitor tuning switches.
Maximum RF current (bidirectional) and resistance or Vf in a switch that is ON.
Maximum RF voltage (bipolar) across a switch that is OFF.

I'm not seeing any obvious semiconductors that are a good fit.
Perhaps vacuum relays. https://www.ebay.com/itm/254791558120 or
https://www.rossengineeringcorp.com/products/control/single-pole-hv-vacuum-contactors

Or home-made mechanical contactors, like moving a tap on primary coil to adjust inductance.
But with MMC the steps are discrete and coarse.

[davekni]

But people laughed at me on discord so I’m assuming there’s a stupid part to it

There are some fundamental issues with changing the capacitance, rather than changing the inductance.

- The sweet spot of tuning a primary LC circuit is within centimeters on the primary coil. And it all depends on the spark load, this is also the issue you want to adress.

Both my SRSGTC and DRSSTC use MMC changes for tuning.  Primary coils are litz wire and not adjustable.  However, adjustments are all manual, connecting or removing capacitor sets.  All have connectors, so no soldering involved.  My SRSGTC has coupling factor (primary to secondary) lower than optimum and required relatively fine adjustment as Mads mentioned.  My DRSSTC is much less sensitive to tuning.  7% steps are fine.  I change from 500nF for music to 700nF for maximum spark length.  Might be related to my relatively-low impedances and high-current H-bridge.

Start by figuring the electrical requirements of your capacitor tuning switches.

Yes, this is the key.  Definitely not worth the work for normal tuning.

However, I am slowly building a crazy experiment to do something like you suggest.  Not for initial tuning.  Instead to adjust tuning within the duration of each enable pulse.  The idea is to add primary capacitance as the arc adds secondary capacitance.  That way primary remains in tune with secondary for longer, hopefully allowing arcs to grow longer than is possible with fixed tuning.

My design uses 448 BTB16 TRIACS as 6 banks of 4S18P.  These will short out parts of the MMC in 6 steps.  Remaining series parts of MMC will have higher capacitance when not in series with the shorted sections.  Thus the 4S18P sections never see full MMC voltage.  Trick will be precise timing of TRIAC gate drive.  Any mistake that turns on TRIACs when voltage is not close to zero will dump MMC energy into TRIACs and make a bunch of melted silicon    So, this is an experiment that may be interesting and may fail catastrophically.

[Mads Barnkob]

Or home-made mechanical contactors, like moving a tap on primary coil to adjust inductance.
But with MMC the steps are discrete and coarse.

Finn Hammer built a "auto-tuning" or "tune-on-the-fly" primary coil DRSSTC called Thumper. Pictures can no longer be opened on 4hv, but you can get the idea here: https://4hv.org/e107_plugins/forum/forum_viewtopic.php?p=12&id=68820#post-87712

He never got it to work like he imagined, the small changes on the primary was, as Dave also states, not really needed (judging by spark output) and it was too slow to do any real time optimization. I seem to recall a chat with him some years ago where he mentioned he did not really use it after the experiments, but it was an easy way to tune, just not worth the huge engineering effort