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Topics - Coupling

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1
Dual Resonant Solid State Tesla coils (DRSSTC) / Another QCW Thread
« on: July 04, 2023, 11:22:47 AM »
I apologize because I'm sure this has been discussed in other threads, but I'm struggling to connect the dots.
I've made a system with pretty high coupling and I have achieved some okay spark lengths before my Agilent supply died...
I am struggling with tuning, though. It seems that my primary series capacitance doesn't make a difference. I can short it completely and the results don't seem substantially different.
Without a capacitor or with an oversize dc-block cap, my system locks at 420kHz. Adding a resonant cap doesn't seem to have any noticeable impact on performance. The system tends toward upper pole operation, which I believe only serves to cancel primary leakage inductance, which would have a diminishing effect as coupling increases. The secondary's standalone fres is 400kHz, so I know 420kHz isn't far off from where it should be once coupled with the primary.
I have tried many different capacitor values and I don't see an increase in current draw at any point. I only tried this at low voltage- I don't know if that makes a difference. Also, I expected to use something around the 20nF range, but capacitors of this size just raise the frequency a lot. Perhaps I need more primary turns.

Any direction would be appreciated.

EDIT: Picture attached

2
I am nearing completion of my FPGA-based QCW controller. I have tested it successfully in my QCWHFTBFR: https://highvoltageforum.net/index.php?topic=2503.0
But as I move my design to a standard DRSSTC, I am concerned about an issue I have had intermittently with my program.
The issue is with my state controller that causes it to get stuck in certain states. I have seen it get stuck in the ON state previously (worst case), thankfully at low power. This hasn't happened in a while.
Most recently I have seen it getting stuck in the OFF state. This problem goes away when I change innocuous things, for example like ramp slope and the clock at which I sample my ADC values.
The logic is quite simple and I am quite certain that the code itself is not bad, as I have gone over it many times. I have noticed that I have many unconstrained clocks and paths and frankly I don't know what to do.

I am currently using Verilog in Quartus II with a DE0-Nano development board. Unfortunately it seems Intel scrubbed the documentation for this all but deprecated piece of software, so I'm at a bit of a loss.
I'd like to open-source this project once I've worked out the bugs, so I was hoping someone here could help me work through it. If anyone is up to the task, please let me know.

3
Transformer (Ferrite Core) / QCWHFTBFR- A Tale of Hubris
« on: June 16, 2023, 05:14:08 PM »
Hello all.
This project has seen many iterations since starting it more than ten years ago. Overall, it was an attempt to emulate and perhaps exceed the performance of Steve Ward's handheld QCW coil. I wanted to do things a little differently, though, and as my knowledge developed across my professional career, I was able to ultimately design this from the ground up.
So first of all, yes, I did put this in the ferrite core subforum on purpose, because it isn't a Tesla coil. QCWHFTBFR stands for Quasi-Continuous Wave High Frequency Transformer Base Fed Resonator. The abbreviation is the same length as QCWDRSSTC, so I'm considering it acceptable.
The driver- custom FPGA-based controller with some peripherals, like buffers for the gate drive line outputs which incorporate cross-conduction prevention and high speed comparators for the OCD (primary) and ZCD (secondary) current feedbacks. Also ample filtering for ADC inputs for battery voltage and current monitoring. I am using the DE0-Nano FPGA development board, which has become somewhat hard to find nowadays, so I'd like to move to something else. I have two "shield"-style daughterboards mounted above and below the development board. They have ample ground planes and use all surface-mount components, save for the connectors. All logic is 3.3V. The unit is controlled by the trigger from a cordless drill I bought on Amazon and gutted.
The firmware contains many features but at its core it was made to work like Steve's predictive, resonance-locked phase shift converter.
The output, rather than going to an air-core primary and resonant capacitor, goes to the 10 turn primary of a large ferrite core transformer. The secondary is wound on a 3D printed ABS bobbin and contains 100 turns. This in turn, goes into the base of my 381kHz resonator consisting of 4.5" diameter PVC and... a whole bunch of turns of magnet wire. I wound it in college more than ten years ago and I'll update this once I've counted! It has two toroids and this was to lower the impedance of the coil in a vain attempt to push more power. More on that later...
The power stage is a development board from GaN Systems that can handle 400VDC at 50A. The double pulse test in the datasheet shows that this is a hard limit. It barely gets warm, but then I haven't really pushed it. Spoiler alert: 50A isn't enough. I have other options for immediate use as well as a SiC-based design that I am in the process of laying out.
The theory was that with a single-resonant system, I could transfer power more effectively, and with a ferrite core, I could lower my magnetizing currents. While I have likely achieved these goals, it did not translate to any degree of noticeable performance! At the end of the day, the goal is to have crazy long streamers. Perhaps if I design a transformer with more secondary turns or stack another 12 toroids on top, I could get closer to my 4-5 foot streamer target (vs my ~2ft achieved), but I have decided to give up on this experimental topology and to go the Tesla coil route. Overall, it won't be that much of a change to the design, but it will belong in a different subforum. Stay tuned.

Here is my final test. I didn't want to go above 340V because I have some spikes on the bridge and I didn't think it was worth adding in more bus capacitance when I knew I was pretty close to my voltage and current limits. The tiny streamers were the result of the OCD kicking in at around 45A, which is why turning the voltage down seems to give better, or at least more consistent results. Sorry about the noise- my eBay Agilent DC supply seems to crank its fans to full blast regardless of actual power output.

4
Transformer (Iron Core) / Pole Pig Refill
« on: March 10, 2022, 03:02:20 PM »
Hello all. I recently got a distribution transformer and I seem to have accidentally loosened the terminal on the HV insulator and then oil started seeping out due to capillary action! I have tightened it up and bought cat litter to clean up the mess but now I'd like to top the big guy off and I'm not really sure what kind of oil to use. I'm pretty sure the "High quality dielectric oil" that the instruction manual specifies is not something a consumer can buy 2 gallons of. I was looking at some higher temperature hydraulic oils like this one- https://www.grainger.com/product/MYSTIK-Hydraulic-Oil-Mineral-52XN97 but I'm not sure what viscosity, etc., I should be looking for.

Does anyone have any advice or recommendations?

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