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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I'd say 10x the bridge output voltage is way too low. More likely 30x. For a peak primary Q of 10 (which is a relatively common value for a DRSSTC), the peak voltage already rings up to 13 times the bridge output voltage. And the DC rating of the caps should really be at least 2 times this.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
In a DRSSTC you can limit the voltage across the tank capacitor to whatever you want by setting the current limiter appropriately. The question is then whether the coil will make the size of spark you wanted at that level of primary current.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Thanks guys, I guess I'll keep looking then.
Cheers, Matt
SEARCH UPDATE:
Back in the day when I was building a SGTC I read that doorknob capacitors were a great choice. I am assuming the same goes for DRSSTCs? I found the following capacitors on ebay that the seller claims to be a "doorknob capacitor": I will be driving my coil from mains adjusted by a variac and rectified, so I would expect between 0 and 170VDC on the primary (ignoring ringup). Multiply that by 30 and I get 5100V. The caps in the link, if paralleled, will give me 20nF rated for 5kv. I'm thinking that would be close enough? I'm also not sure about the material, whether it has a high dissipation factor or not. I'm asking because I know that doorknob capacitors were recommended for Tesla Coils and this Russian capacitor is supposedly a type of "doorknob" capacitor.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Hi folks,
It's been 48 hours, so according to the ToS I can post again with new info
I pulled a few transformer cores out of an old ATX power supply and a large amplifier that I think are just perfect for this project. The two identical ones are from the amp, and the other one is from the PSU:
I had a long ethernet cable just lying around that I picked up somewhere for a couple bucks so I decided I would salvage the neatly-twisted pairs of wires from it. The current transformer is a 1:1024 one made by following the instructions provided in this video:
Here's the finished (though not soldered/heat shrunk yet) product:
The GDT was created using the last core. 1 8-turn coil (the copper-colored wire) was used for the primary, and two pairs of wire (green) were used for each secondary. The secondaries are out-of-phase with each other. Together, they will drive each of the 4 IGBTs:
I recently drew up an updated version of the schematic, this time using Eagle. Eagle, unfortunately, did not have a model for the UCC27425, so in order to represent it I used the 4420 and 4429 models along with some AND/NAND gates:
Here are the specs I have (calculated) so far:
Resonant frequency: 237.76 kHz Secondary height: 20 in. Secondary diameter: 4.5 in. Secondary wire gauge: 26 AWG Secondary # of turns: 1257 turns Topload minor diameter: 3 in. Topload major diameter: 10 in. Primary diameter: 5.5 in. Primary wire gauge: 12 AWG Primary # of turns: ~10 turns adjustable Primary inductance: 22.404 uH Tank capacitor(s): 2x 10nF, 5kv paralleled Transistors: 4x HGTG20N60A4D (600V 70A@25C w/ built-in flyback diode) GDT: Pentifilar coil, 8 turns each. The two pairs of secondaries are out of phase with each other Feedback coil: 1:1024 (16 turns of paired wires connected in series, passed through second (identical) transformer. This gives 32 windings on each transformer, 32*32 = 1024 multiplier.
Does it look like I'm (finally) on the right track? I'm on vacation at the moment but I will be heading back to work soon, so I will probably not be able to complete this Tesla coil for another month or two.
Thanks again everyone for your help. I've definitely learned a lot even in this short period of time.
One quick note; in my experience anything less than 10 turns on a GDT will have poor results. I always had excellent results with 14-16turns, and I've had several customers have to remake their GDT when their coils didn't operate well to match.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Sigurthr wrote ...
One quick note; in my experience anything less than 10 turns on a GDT will have poor results. I always had excellent results with 14-16turns, and I've had several customers have to remake their GDT when their coils didn't operate well to match.
Good note, thanks very much. I kept it at 8 turns simply due to the size of the core, but if I can find a larger one I very well may increase the number of turns. Thanks a lot for the heads-up. I found several designs that only used 8 turns, so I assumed it would be ok, but I will definitely keep this in mind. I have plenty of left over wire (the ethernet cable was 25-foot, I think) so I definitely don't lack the materials
You can't just make a GDT by winding any number of turns on any core. It depends on a variety of factors - what frequency are you running the coil at, what sort of power etc. There is no rule of thumb for number of turns either, but if you have too many turns [corrected: too few turns], your core will saturate! You would actually want to use the minimum number of turns without saturating the core. This all depends on the characteristics of your toroid material etc. Finally, I'm pretty sure most power supply toroids will not work especially if they are powered iron. You would want a ferrite core with a high permeability.
If you don't have the datasheet for the cores (which it looks like you don't), you can do a quick test with a scope and a signal generator. Send a square wave at the expected running freq in the primary, and see if you get a suitable output (which looks more or less square out - it will be very obvious that something's going on if you try that on an iron core). Most cores in power supplies will be too lossy at tesla coil frequencies so you cannot use those. Lastly, it looks like you separated your primary and secondary windings for your GDT into sections. That will probably work fine but I would twist them all together instead to reduce the leakage inductance.
[Edit] you can't use iron cores for your feedback either, so make sure they're the right material. If you're going to buy toroids, just make sure they're of the right material (AL value of around 4000, 5000 or so). I'd recommend buying a few so you can use them to make other gdts or current transformers. :)
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
loneoceans wrote ...
You can't just make a GDT by winding any number of turns on any core. It depends on a variety of factors - what frequency are you running the coil at, what sort of power etc. There is no rule of thumb for number of turns either, but if you have too many turns, the core saturates. You would actually want to use the minimum number of turns without saturating the core. This all depends on the characteristics of your toroid material etc. Finally, I'm pretty sure most power supply toroids will probably not work especially if they are powered iron. You would want a ferrite core with a high permeability.
If you don't have the datasheet for the cores which it looks like you don't, you can do a quick test with a scope and a signal generator. Send a square wave at the expected running freq in the primary, and see if you get a suitable output. Most cores in power supplies will be too lossy at tesla coil frequencies so you cannot use those. Lastly, it looks like you separated your primary and secondary windings for your GDT into sections. That will probably work fine but I would twist them all together instead to reduce the leakage inductance.
I knew that some cores (namely powdered iron) would not be effective in Tesla Coil operation. I came across something once, though, saying that a core that almost looks ceramic is the type I would want, which is exactly what I have. Unfortunately I don't have the datasheet but I will certainly do some testing.
As for winding the coils themselves, would you recommend I wind one over another, instead of winding them into sections? I imagine that would make twisting the leads together when finished.
Let me run some tests on this core and I'll get back to you.
Regards, Matt
EDIT:
Unfortunately my oscilloscopes and signal generators are all quite old and they don't run very well at high (>10 kHz) frequencies. I could test them at work, but I won't be back there until next week. Is there a well-known source for gate drive transformer cores that is popular among the Tesla coil community? In the mean time, I'll keep looking.
SECOND EDIT:
I've been looking on mag-inc.com at their model ZJ42508TC (link downloads PDF datasheet): It has a permeability of 5000, and an AL value of 4,830 nH/T^2, which I believe fits in with your recommendations. I just need to be sure it's the right material ("J") and that it will not have significant losses at ~240 kHz.
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First SSTC design - Need some critique
