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Nice work there my friend ! good results indeed. I'm currently building my QCWDRSSTC too at the moment and having similar results with yours :) sooner i'll be posting my results too :) the only difference I think is that I don't have plan on selling anything heheh :) again nice QCW coil!
That sounds great! Was never planning on making the coil for sale though, but I will be posting up all my schematics and designs. I just thought that I had some extra boards leftover and maybe people will be interested, which also helps off-set a bit of my development cost. :) Simply selling at basically cost price (updated above). I get my purple PCBs at OSHpark which are quite pricy, but their fast turnaround and high quality makes them worth it I think, otherwise I send my boards for fab in China. I heard that APcircuits/4pcb has good student deals too.
Steve Conner wrote ...
Nice scope shots! What scope are you using? I'm thinking of replacing my ancient HP 54600B.
I highly recommend the Rigol DS1000 line! I bought the DS1074z a few months ago and it's fantastic, and probably the 2nd best value scope on the market right now ($550 for 4 channels 70Mhz 1Gs/s huge memory, beautiful screen, and hackable to 100Mhz). In fact, Rigol just released their DS1054z model just a few weeks ago for $399, which I think makes the -The Best- value scope on the market right now by far, and from the looks of it, it might be hackable to 100Mhz (usually $800+), since it simply looks like it could be simply a firmware change. For even better performance, I'd recommend the 2000 series. Their MSO line is great too if you do signal analysis. :)
I'm running it around 1Hz because I don't have a variac so my power line is going through a 25 Ohm resistor. This was to prevent blowing my breaker with the big bus cap, but limits my bps. I also found that some of the sparks don't show up or get cut in strange places, mostly because of the sensor readout. I'm currently winding a new coil and focusing on making a more polished setup.
Registered Member #42796
Joined: Mon Jan 13 2014, 06:34PM
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Posts: 195
loneoceans wrote ...
Indeed QCW is somewhat of a misnomer I guess. It's interesting to compare it with what I got for my Ramped SSTC 3 (or 'fake QCW!'). See my thread for that coil here:
one thing i didn't get from that thread: the buss cap needs to be omitted so that the coil produce swords like sparks?
Registered Member #3324
Joined: Sun Oct 17 2010, 06:57PM
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Posts: 1276
Still haven't got around to building a single tesla coil of my own.... This is really impressive, you may just inspire me to actually get round to doing so! :)
I haven't really played around with anything but it's running at the upper pole at the moment starting around 465kHz and dropping to 430kHz at the end of the pulse. Both the secondary and primary are about 400kHz independently of each other.
Interesting data. From the frequency values you provided, coupling seems to be about 0.26. The drop in operating frequency implies a drop of secondary res f from 400kHz to about 300kHz due to arc loading, i.e. effective top capacitance rises by about 80%!
I could run it in the lower pole by setting the primary to be lower frequency (this is just a normal feedback drivier so it automatically nudges itself to the upper or lower pole), but my reasonaing was that by running it on the upper pole, the spark will drop the frequency causing the coil to become more and more in tune as the spark grows, as opposed to the other case where it would become out of tune as the spark grows.
What matters wrt to tuning is the difference between operating frequency and secondary fres. Initially this is 465kHz - 400Khz = 65kHz. At the end we have 430kHz - 300kHz = 130KHz, so actually your coil goes out of tune at the end of the burst. This is probably not a big issue, since arc loading also widens the secondary resonance curve, so that the effect of detuning becomes less noticeable.
Say you tune your primary to 350kHz. You'll then run on the lower pole, which is initially located at 328kHz. The difference between operating and secondary fres is then 72kHz. Under arc load, the poles will be located at 283 and 385kHz. I'm not sure, whether the coil will stay at the lower pole, but if that's the case the difference will be 300Khz - 283KHz = 17 kHz, i.e. very well in tune. It might switch to the upper pole, though, where the difference is 85kHz. So lower pole operation would be the better choice, well, if it stays there.
Ideally I'd like to run the coil at the upper pole but tune the primary lower freq than the secondary.
You'll need special drivers to do that. Steve Conner has been doing that, but from recent posts I'm not sure, whether he still thinks it's a great idea, like I do.
Another note - I was never really sure what sort of load a high-Z tesla coil would be. The trace above of the primary current provides some insight. I realized that the envelop looked very much quadratic, and sure enough, I fitted a quadratic line to it and it looks almost perfect (pink line). Now with normal resonance at a fixed bus input, I noticed that the current rises basically linearly. Since the voltage in the primary tank goes up linearly, this implies the load should be constant. With the voltage now increasing from our buck converter, we can think of the voltage increase as a sum of an arithmetic series, which is quadratic (n^2+n /2), again suggesting that the load (tesla coil) is somewhat constant like a resistor.
I don't quite understand that. You have quite a lot of primary current initially, which suggests, that arc load doesn't play a big role and primary current is mostly limited by loss resistances in the primary. From then on primary current increases at a much slower rate than the increase of bus voltage implies, so that the primary looks more like a constant current sink. Primary current mostly picks up during the end of the burst, which might be due to it going more out of tune.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
wrote ... Ideally I'd like to run the coil at the upper pole but tune the primary lower freq than the secondary.
wrote ... Steve Conner has been doing that, but from recent posts I'm not sure, whether he still thinks it's a great idea
Yes, I am still doing it and I think I persuaded Steve Ward to start doing it too.
What I discovered is that in the usual kind of DRSSTC, with low impedance and short burst length, it doesn't make a huge difference. The burst has few enough cycles that both pole frequencies must be considered to be present, you can't say for sure that the system is operating on one or the other, and you can't really persuade a PLL to lock definitively to one or the other.
However, it most certainly makes a difference for high impedance DRSSTCs with long burst length, like my original Mjollnir, my newer AG2 and the coils Arc Attack have been building lately. It is also the best operating mode for QCWs.
And finally, it turned out to be the most stable operating point for Odin, though I think it would probably work exactly the same with less switching losses if I substituted a UD2.x for the PLL and let it switch at the zero crossings.
one thing i didn't get from that thread: the buss cap needs to be omitted so that the coil produce swords like sparks?
That's correct. This way the inverter will be powered directly from the rising voltage wave of the mains AC. My SSTC 3 board had provision for a bus capacitor on board because I wanted to make it a general purpose platform for normal SSTC use, as well as DRSSTC operation with the addition of an external resonant cap.
Uspring wrote ...
Interesting data. From the frequency values you provided, coupling seems to be about 0.26. The drop in operating frequency implies a drop of secondary res f from 400kHz to about 300kHz due to arc loading, i.e. effective top capacitance rises by about 80%! What matters wrt to tuning is the difference between operating frequency and secondary fres. Initially this is 465kHz - 400Khz = 65kHz. At the end we have 430kHz - 300kHz = 130KHz, so actually your coil goes out of tune at the end of the burst. This is probably not a big issue, since arc loading also widens the secondary resonance curve, so that the effect of detuning becomes less noticeable.
Say you tune your primary to 350kHz. You'll then run on the lower pole, which is initially located at 328kHz. The difference between operating and secondary fres is then 72kHz. Under arc load, the poles will be located at 283 and 385kHz. I'm not sure, whether the coil will stay at the lower pole, but if that's the case the difference will be 300Khz - 283KHz = 17 kHz, i.e. very well in tune. It might switch to the upper pole, though, where the difference is 85kHz. So lower pole operation would be the better choice, well, if it stays there.
Thanks for the useful analysis. I cannot claim to understand drsstc theory extremely well, but here are my thoughts. Also for more reference, my secondary + toroid resonates at 408kHz by itself, and the primary in its current configuration resonates at 392khz. Together, the system has poles at a measured 340 and 482kHz, close to the calculated values of 340 and 479 with my JavaTC k of 0.33. However at the start of the pulse, I did measure 465kHz (I'll go do another measurement nearer to front of the pulse in case it actually started higher), with 430kHz or so at peak current. I had assumed that tuning was referenced to the secondary frequency (400kHz), but (as you pointed out) I did not take into account capacitive loading dropping the secondary freq. This was the reason why I thought upper pole operation would be better but I'm glad you pointed this out. What you says makes sense if the secondary does indeed drop to 300kHz. By adding a 1m wire on the top of the toroid, the centre frequency of the system drops from 408kHz to 310kHz. I also measured the new system frequencies with my primary at 358khz, giving 327, 410, 457kHz. So my question is - How do you calculate the poles of a system with differing primary and secondary frequencies (not sure if I missed out some first order equation somewhere otherwise I'll just model it), and why would there be a tendency for the system to go to the higher pole?
Uspring wrote ...
You'll need special drivers to do that. Steve Conner has been doing that, but from recent posts I'm not sure, whether he still thinks it's a great idea, like I do.
What I plan to try is to make a small adjustable oscillator which feeds a weak signal into the UD feedback, which I will adjust to lower pole frequency. Hopefully this allows the coil to start off at the lower pole before the feedback is stronger and 'over-rides' the signal. I'm not sure if this will work but I'll try this out over the next few days. Finally, what you said about the primary current makes sense too; I'll continue thinking about it.
As requested, here's a short video of it running at about 1bps! Excuse the poor quality, will take more soon: Some sparks look smaller/disappear because of the camera sensor read-out speed. I promise the sparks sound much more quiet in a real life, like a thump instead of what it sounds like in the video.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Very nice QCW build, I also love your photography.
Your primary current waveshape is very intriguing. With a linear ramp on the input, my geuss is that your system impedance is low enough to start showing it's true nature. Normally you see a "close to linear" response of the primary current envelop with a linear change in modules voltage. This exponential growth of the envelop I bet is alway why you are seeing more branching than normal than with most other QCWs.
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Small QCW
What scope are you using? I'm thinking of replacing my ancient HP 54600B.
