Post by: alan sailer on March 09, 2024, 10:40:11 PM
challenges of making a coil. I don't have a fancy machine shop or 3D printer and I like figuring out how to make
parts with a table saw, belt sander and drill press.
Second, like many coil makers, I would love to have a two inch high coil that shoots out twenty foot sparks.
My last coil is about three feet high (including electronics) and has about two foot sparks. Not my dream coil.
A while back I ran across Arc Attacks commercial "Thundermouse". It's about 18 inches high and puts out 24 inch sparks.
That's more like it.
The most interesting aspect of the coil to me is the curved primary.
[ Invalid Attachment ]
I have few questions about this design. First I assume the curved primary allows higher coupling while keeping
the chances of a secondary flash-over under control. Is this true?
The second question is the coupling of this design. According to the web-page it's 0.308 which I understand
is very high for a DRSSTC. I tried modeling the coil in JAVATC using a helical winding (ie bottom radius smaller
than the upper radius) and got 0.311 coupling. I also tried modeling the primary as a cylindrical coil
with the radius the average of the curved primary and again got a similar coupling factor.
So I guess the question is, is a DRRSTC with such high coupling realistic?
Cheers.
Post by: davekni on March 10, 2024, 03:42:28 AM
Quote
So I guess the question is, is a DRRSTC with such high coupling realistic?Short answer: Yes, I think so.
AFAIK, DRSSTCs are better with as high coupling as possible without creating racing sparks along lower part of secondary. Low secondary aspect ratios (height/diameter) allow higher coupling without racing sparks. Low aspect ratios are common for short coils such as this. I think I've seen several such small DRSSTC examples with similar coupling on the forum, but don't recall any specific ones to mention.
Post by: alan sailer on March 10, 2024, 04:50:13 AM
Thanks for the input. I did some searching to find examples but I don't have the best talent for picking
search terms that give me the results I want. Since you believe examples of short, high coupling DRSSTC are in here
somewhere I'll try again later.
Cheers.
Post by: flyingperson23 on March 10, 2024, 07:10:22 AM
Post by: alan sailer on March 10, 2024, 02:16:25 PM
Thanks for the inputs. I am willing to experience mild disappointment. I'm not totally fixated on arc length which is a good thing.
All of my coils are based on other peoples designs and usually get about 70-80% of their performance. I think this is because I
am a poor tuner, too afraid of killing FETs.
I'll check out those two builders.
David,
I tried another search term last night and ran across OneKone (a designer I have copied before). He has a short coil with high
coupling that I plan to study and compare to the ThunderMouse.
Cheers.
Post by: davekni on March 10, 2024, 06:49:36 PM
Quote
Looking at the thundermouse's page, it lists on time up to 20ms. This is more likely why its max arc length is so good.You may not need 20ms, but long on times match well with high coupling. The high coupling allows primary-to-secondary energy transfer for longer (over a wider frequency range as arc loading reduces secondary frequency). That's the key advantage of high coupling.
Post by: TMaxElectronics on March 11, 2024, 12:37:52 AM
I'm in the process of open sourcing it too, including the mechanical design.
It uses a standard flat primary without a high coupling coefficient but still gets 60" sparks from a 12" secondary (total coil height is around 24") at 1ms ontime (as you can see here: https://www.youtube.com/shorts/LmHMwYoh9tM). The biggest issue that I've been having with the current setup is primary strikes (no doubt due to the flat primary ;) ) which are so energetic that they ionize the air in the space between the strikerail and the outer most winding and start an arc there too. I'm currently working on reducing the footprint of the secondary as well as adding a current ramp to the UD3 even in normal drsstc mode which apparently helps with starting the arc at the breakout instead of at some random location around the topload (both things that the at least the thundermouse already has).
Post by: alan sailer on March 11, 2024, 02:30:11 AM
That is awesome. I'll definitely check it out. I'm sure to learn something even if I don't try to clone it.
Cheers.
Post by: flyingperson23 on March 11, 2024, 05:01:03 AM
60" sparks from a 12" secondary
Can it sustain that? Does it not cook the coil running at 20kW?
Post by: TMaxElectronics on March 11, 2024, 12:50:34 PM
Quote
Can it sustain that? Does it not cook the coil running at 20kW?
The issue is the power input. That can't really handle 20kw constantly. Running the coil at a slightly lower power level (10kw or so) is perfectly fine in terms of thermals though. I've also run the coil at low power (2-3kw) without the fan on for a few minutes without the temperature going crazy. So it seems to be decently efficient.
Post by: alan sailer on March 16, 2024, 12:36:02 AM
in what looks like the development/tuning stage. The primary leads going to the tank capacitor and the driver
can be quite long and the videos/photo of the set-up shows a great spark length.
Then a later picture shows the set-up all buttoned up with shorter leads still working great. I have always
assumed that the unit needs to be re-tuned after the primary is re-packaged. Or maybe people just roll up
the excess wire and call it good.
My goal has always been to get the coil working, longer leads and all, but not tuned for maximum output.
Then I package it nicely with a short as possible connections and do the final tuning.
It there a better way?
Cheers.
Post by: Edi on March 16, 2024, 02:22:28 PM
I need some help with GDT. Im using FGY75N60SMD with min treshold voltage of 3.5 V and max 6.5V. GE volage is rated fo +-20V. How should I wind my GDT if my UD2.7C driver has an output of +-24V for GDT.
Post by: Hydron on March 16, 2024, 02:42:39 PM
Post by: Edi on March 16, 2024, 03:30:27 PM
Post by: Hydron on March 16, 2024, 08:21:44 PM
Then just run the UD2.7 off a lower voltage (change the regulator or just supply the correct voltage) and adjust the UVLO to suit (a few volts below the nominal drive voltage).
Unless you're pushing very large currents well beyond the IGBT rating (probably not a great idea with TO-247 parts) you won't need more than 15-18V. I've run the same parts up to at least their Icm rating of 225A at 300+kHz (though you need to be careful about hard-switching at those currents!)
Post by: Unrealeous on March 16, 2024, 08:46:20 PM
Hi guys
I need some help with GDT. Im using FGY75N60SMD with min treshold voltage of 3.5 V and max 6.5V. GE volage is rated fo +-20V. How should I wind my GDT if my UD2.7C driver has an output of +-24V for GDT.
Page 1 of the datasheet https://www.onsemi.jp/download/data-sheet/pdf/fgy75n60smd-d.pdf shows Vges +/- 20 with Vges transient +- 30
This looks like a pretty normal IGBT when it comes to GE voltage. I think you might be getting confused with the Vge (threashold) voltage of 6.5 max on page 2. The threshold voltage is the minimum voltage to turn the IGBT on, but will not give you it's maximum current. You want to drive at Vges at +/-20V.
(http://Vgethreshold.png)
As Hydron says, I would use a one:one ratio and using a 30v zenner between gate and emitter, like most designs out there (see loneoceans, kaiserpower etc). You can read about why everyone drives at slightly over the rated voltage (24V instead of 20V) at https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/igbts/
Post by: flyingperson23 on March 17, 2024, 12:33:06 AM
Post by: alan sailer on March 25, 2024, 04:27:22 PM
with flash-overs from the bottom of the top-load to the strike rail. One of the features that is puzzling
to me is that at higher rep-rates (~300Hz) there is no flashover but at low pulse rates (~40Hz) I'm
getting flash-overs.
Other than raising the top-load I'm not sure what to do about this.
Cheers.
[ Invalid Attachment ]
Post by: flyingperson23 on March 25, 2024, 05:44:25 PM
Other than that, I can't think of anything besides smoothing any rough edges or changing the coil geometry. On one of my coils, when the coupling was too high I'd get arcing over at <10hz that went away with higher bps.
Post by: alan sailer on March 25, 2024, 06:45:46 PM
The coupling on this coil is very high, almost QCW levels. I can't model it directly using JVATC but using two methods
the coupling is about 0.3.
I'll try another breakout. I am assuming sharper is best.
Cheers.
Post by: flyingperson23 on March 25, 2024, 08:05:02 PM
Post by: davekni on March 26, 2024, 04:16:24 AM
Quote
Other than raising the top-load I'm not sure what to do about this.Looks like raising top-load a bit might be good for another reason, to avoid shorting upper turns of secondary. A bit of gap between top of secondary and top load allows secondary magnetic field to exit top and return to bottom. Will reduce secondary frequency a bit, so may need retuning.
Post by: alan sailer on March 26, 2024, 03:16:03 PM
These all should move the top load away from the primary/strike ring.
I have done no real tuning as of yet. I have to finish getting the coil into it's final mechanically stable condition.
I am curious if anyone has an idea how effective strike rings and that de-coupling cap on the bridge are?
Do they prevent bridge death or just make it less likely?
Once again, the help is very useful.
Cheers.
Post by: flyingperson23 on March 26, 2024, 05:02:18 PM
Post by: alan sailer on March 26, 2024, 08:46:59 PM
All of the strike rings I have seen are smooth rings of metal. Another protective device is a lightening rod, which
uses a sharp point to create a potential concentration which directs the lightening return strike to a ground rod.
The only reason I can think of that this effect is not used in tesla coils is that it would encourage secondary to
ground arcs.
Cheers.
Post by: Benjamin Lockhart on March 26, 2024, 11:07:20 PM
I would add a much larger toroid sitting on top of your existing one if tuning allows for it. This will probably help the most, aside from winding a taller secondary coil.
Larger diameter tubing for the strike rail might help but not much.
I try to avoid coil designs where the toroid is smaller than the strike rail diameter, especially when they are super close together like this.
The ramped nature of the QCW coils with similar dimensions as yours helps a lot with controlling the flashover, as well as those coils have a pretty low topload voltage.
Post by: alan sailer on March 27, 2024, 12:04:34 AM
Don't know if I have enough primary inductance to have a larger toroid. I will do some JavaTC.
I have two QCW and as you say the toroids are much larger.
The picture I took may not show it but my toroid and the strike rail are darn close to the same diameter.
I di however, raise the torrid up about 3/4 of an inch. I also made the breakout about an inch longer.
I spent a little while coming up with a simple but effective way to measure arc length to do tuning. It is
set-up right now and I was able to make my first arc measurements. The results surprised me. The
primary is set at some guess for decent initial tune. For a 130usec interrupter pulse and 150Hz I am getting
solid 22 inch/56 cm arcs at 150VAC input.
I'm pretty amazed. The Vce waveforms are so clean that they look like a textbook.
A slow motion video of the coil hitting 22 inches.
https://www.youtube.com/shorts/E11fc_pDfyw
Cheers.
Post by: davekni on March 27, 2024, 04:41:59 AM
Quote
The only reason I can think of that this effect is not used in tesla coils is that it would encourage secondary toYes. Before finding this forum, my initial DRSSTC construction had a strike ring with small points. Exactly that problem, strikes from part way up secondary to strike ring.
ground arcs.
Post by: alan sailer on March 29, 2024, 10:08:41 PM
I am just going to avoid low pulse rates.
I am super pleased with the arcs. 24 inches is more than I expected by about 50%.
I am thinking about adding a soft start circuit to the coil so that I don't need a variac to run the thing. I did a quick
(15 minute) search for some circuit ideas and didn't run across anything likely. Does anyone have a circuit that they like?
If not, I have a few time delay relays that I'll look into...
Cheers.
Post by: Benjamin Lockhart on March 30, 2024, 12:10:13 AM
The 24V power supply has about a half second turn on delay and the 12 ohm resistor limits inrush to 20A which is enough to mostly charge the bus before the power supply turns on.
It's only 2 extra parts and has worked flawlessly for me.
Post by: alan sailer on March 30, 2024, 12:27:50 AM
Cheers.
Post by: Benjamin Lockhart on March 30, 2024, 12:45:17 AM