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1
Dual Resonant Solid State Tesla coils (DRSSTC) / Re: Idea for QCW DRSSTC
« Last post by davekni on Today at 04:30:28 AM »
Quote
The current is non sinusoidal due to extra breakdowns during the high voltage times of a cycle. That reduces phase shift. It is actually quite low. Later into the burst the current becomes sinusoidal and the phase shift settles at about 45 degrees.
Thank you for sharing simulation details.  Does indicate low (perhaps 20 degrees) phase shift.  If accurate, there would be weak negative feedback at the start with open-loop frequency ramping, likely making precise smooth control difficult (your initial thesis of this thread).

Quote
But don't trust these simulations wrt to phase. AFAIK nobody has ever made an arc current measurement for a QCW coil. A reality check is sorely missing.
Some day I hope to make such measurements with my fiber scope probe.  Need to build a QCW coil first.  Will also experiment with open-loop frequency ramping then.

Quote
I wish your hand gets well quickly and fully.
Thank you!
2
Spark Gap Tesla Coils (SGTC) / Keep blowing capacitors
« Last post by pmlapl on September 27, 2021, 11:21:43 PM »
Hi,
Working on my first SG coil. Started with a series of ceramic caps, which  died. Switched to film and they blew also. Finally bought a 30kv 10nf cap which worked about 5 minutes and then died. So I plan to get a larger cap but would like some help. My coil is 1300 plus turns at 4.68 OD x 10 inches with a 4.5 x 1.375 inch toroid. Primary is 3 turns at 4.5 inches OD. Capacitor needed is 10 NF. Coils are driven with an oil ignition transformer, 10kv x 23ma. Resonant frequency is 670kHz that yields about a 4 inch spark. Just wondering if I could use the 30kv cap but with a rotary spark gap to keep the load down. Perhaps a 40kv would work without the rotary SG or just go for two 40kv 20nf in series. Suggestions would be greatly appreciated.
 
3
Dual Resonant Solid State Tesla coils (DRSSTC) / Re: Idea for QCW DRSSTC
« Last post by Uspring on September 27, 2021, 06:47:52 PM »
Quote
What does your QCW arc model say about arc current phase near the beginning.  I suspect it is more capacitive than resistive at that point, which would reduce voltage faster..

Early into the burst a simulation looks like this:
See below, strangely the forum software moves my inline images to the end.

The current is non sinusoidal due to extra breakdowns during the high voltage times of a cycle. That reduces phase shift. It is actually quite low. Later into the burst the current becomes sinusoidal and the phase shift settles at about 45 degrees.
But don't trust these simulations wrt to phase. AFAIK nobody has ever made an arc current measurement for a QCW coil. A reality check is sorely missing.

Quote
Hope to experiment with all this eventually, once my hand finally heals and I get caught up with other pending projects.

I wish your hand gets well quickly and fully.
4
General Chat / Isogrid Features
« Last post by Patrick on September 27, 2021, 11:19:47 AM »
Ive been studying honey combs and I-beams for along time. Now Ive started looking at isogrids due to my new large 3D printer and have some questions.

Some of you know structures and calculus better than I so once again I return to the forum.  First, the vertical ribs are the shear web and the taller the rib the more stiff. I presume the more cells also increases stiffness, but is there a simple rule to describe a good starting density of cells and height of ribs ? or can it only be evaluated with calculus ? The ribs will be 0.6 mm thick maybe 15 mm tall. (Or there about, Im not building a bridge). I guess I could try several  of your recommended densities test them and report back, and go from there.

McDonnel Douglass wrote a in depth PDF on the matter. I understand there methods of manufacture but not the heinous quantity of math they use.
Here is a Link:  https://femci.gsfc.nasa.gov/isogrid/NASA-CR-124075_Isogrid_Design.pdf

Id like to use carbon fiber fabric backed with some 3D structures as this may make more survivable parts that might encounter the ground and reduce the cost of wet layup.
5
Oh, that makes sense as I'm creating my interrupts with an arduino and the off-time is quite short. But I still don't understand why the phase of the voltage on the primary shifts so suddenly from the feedback when the gate drive signal is off. Is there any way to keep the on times high and the off times low as I would like the maximum output from this during music playback (as it is a SSTC).
Thanks for the help!
6
Scope traces look roughly as would be expected.  I think most SSTCs are interrupted with longer off-times, so that ring-down is finished before the next enable edge.

For better understanding, I suggest learning an analog simulator (SPICE variant).  There are several free ones.  I use LTSpice.  There are simpler on-line simulators too.  No need to simulate your entire circuit.  Start with just a pulsed voltage source into a series L/C circuit.
7
I did some thinking and some research and my previous explanation doesn't really seem right. I read that adding those series capacitors cancels out the inductance of the bridge maybe? I'm now quite lost on what these capacitors do and if they will fix my problem at all, but as far as I can see this is the only fundamental difference between my design and Steve Ward's design, which implements interrupts. Here is a scope capture of what I was talking about in the previous post about voltage spikes.

Pink is the voltage across the drain and source of one of the Gate MOSFETs (I'm running a half-bridge)
Yellow is the gate to source voltage on the same MOSFET
Blue is the raw output from the CT on the secondary before any delays and filtering stuff is done.

You can see how after the yellow gate voltage shuts off, the voltage on the MOSFET gate to drain still oscillates due to, I'm guessing, it being back driven by the secondary that's oscillating by itself. But these oscillations are now suddenly shifted from where they were before with respect to the blue feedback from the secondary, which is why I think the voltage spikes at turn on are created as now the MOSFET switches at the wrong time and I'm guessing causing some sort of inductive discharge, but I'm sort of lost on this.
Any help will be appreciated, Thanks!
8
Hi, I'm back with a new problem and I don't think this warrants a new thread so I'll put it here.
I have rebuilt my whole bridge and driver circuit as the old version was quite bad in terms of parasitic inductances etc. But now, with the new bridge, it runs perfectly, and I do get zero current switching, with no voltage spikes on the turn off of an interrupt cycle. But on the turn-on, there are huge voltage spikes. I probed the voltage across the primary and the feedback signal from the secondary and, while the coil is running, see a 90 ish degree phase shift between the feedback and the primary voltage, which makes sense. But during the off-time of the interrupt, the primary voltage induced by the coupling between the secondary and primary has a 180 ish degree phase shift from the feedback signal, changing instantly from the original 90-degree shift the moment the coil turns off. I'm not exactly sure why this occurs, but my guess is that it has something to do with the series capacitors with the primary that are present in steve ward's coil design (I don't have them) linked here:
https://www.stevehv.4hv.org/CWcoil/PLLSchem.JPG
These capacitors look like they have a similar job to the capacitor between the current transformer and the clipping diodes. I don't exactly know what exactly it does though, my guess is that it provides a consistent ish load for the CT or in this case for the primary coil rather than just loading it when the voltage reaches the rails where the clipping diodes/ body diodes of the mosfets start conducting. Also, due to the large currents that flow through the primary and high duty cycles, I'm guessing I'll need a very high-performance capacitor with low ESR, and I don't have any such capacitors on hand. Also, concerning these caps, what voltage should they be rated at, do they need to be rated at the voltage the bridge runs on or lower since I'm pretty sure that voltage will never fully fall across the capacitor.
9
General Chat / Re: Tesla plate
« Last post by klugesmith on September 25, 2021, 11:09:30 PM »
Clever acquisition, you old coiler you!
In today's popular culture Tesla is just a brand of automobile, originally very novel and luxurious.

Not a kind of coil, or a unit of measurement in SI.  Or the name of a leader in polyphase AC electric power (which had won the second "current war" before 1895, around the time electrons were discovered).

Before the pandemic, were there any Teslathons that drew people who expected to see cars?

Speaking of units of measurement, there's a Volt in California sporting the license plate AMP*OHM. 
Probably means little to the masses.
My sister's Volt, acquired before her Tesla, has the same thing spelled backwards.  Not quite readable properly in a mirror view.   That brings up mirror-image license plate humor, like 3M TA3.
10
General Chat / Re: Measuring TC secondary parameters
« Last post by davekni on September 25, 2021, 09:51:49 PM »
Interesting way to measure impedance.  I wonder if Q measurement would be easier or more accurate looking for the +-45 degree phase between current and voltage rather than sqrt(2) impedance.

I've always measured secondary coil inductance at low frequency, then presumed it doesn't change too much with frequency.  Of course, both winding wire skin/proximity effect and capacitance from the coil reduce inductance some.
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