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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Can you actually get that kind of performance without seriously reducing the life span of the coil? I don't mind stressing the components a bit, but I don't like to design tings that are destined to fail prematurely.
I think it could, and after all I did this with intention to see whether an ignition coils could be used as a power supply for a large tesla coil after being driven this way and rectified to DC.
People seem to punish ignition coils terribly with overvoltage (usually in pulsed mode) so that they arc over externally unless put under oil... My approach was to utilize a CW drive with square wave at 2kHz in order to get as much power from the coil without pushing the voltage too high... the result are arcs that start at about 1-2cm but stretch up to like 10.
One smaller coil I tried though (visible in background of the video) died almost immediately when I tried to drive it with a bridge... which is interesting because I never had my coils develop external tracking which is what most people easily acheive with flyback mode or pulsed style drivers, and this means it failed at unexpectedly low output voltage.
I suspect that the small coil had a fault of some sort already or it's insulation just sucked. I opened it up and it didn't seem like it had enough oil inside to insulate the top of the windings.
You will need to cool the coil actively if it's intended to run like this for extended period of time... at least a good muffin fan blowing over it should help a lot. For a jacob's ladder you won't be subjecting the coil to big open circuit voltage, even less than I did... so as long as it's properly cooled I think it could be ran pretty indefinitely that way. You can also use two coils in antiparallel (or antiseries, if you want to be really light on them) for higher voltage output if needed.
You don't even need TL494/SG3525 for this; you could even use your astable multivibrator circuit, but you'd need to find or construct two center tapped gate drive transformers which can run in low kHz which may not be the easiest task.
I used transformers scavenged from old modems for this. It'd be pretty easy to drive them with SG3525 directly.
Registered Member #3343
Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
Hi Fulmen!
I like the idea of discrete components astable.
Some time ago i was developing very similar circuit, and I use a SIDAC rated 230V to protect de FET from the primary auto induction voltage. Later, I put a 5W 120V in series with the another 110V SIDAC, and I could ajust the primary circuit in such way that the bulb does not glows. It is a protection/osciloscope "device" ....
I have used this "device" for more than one year, with a lot of coils made by myself and never fryed a fet. Voltage supply was 55~110V dc on top of the primary coil winding.
Regards
Newton
EDIT: a small neon bulb connected across the fet also can guive you a idea of voltage magnitude at the fet...
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
The discrete multivibrator was fun to build, but I never got good performance from it. I didn't have a scope back then, so I was working in the blind. Thinking about it I guess the problem was the same I had with this design, the low gain HV transistor didn't reach saturation fast enough. That's why I added a second transistor to give it a good kick. Now I'm giving stacked plate capacitors a try. I know MMC's are the way to go, but I feel I'm missing out if I don't try to make my own first. I'm using 0.1mm inkjet transparency sheets (PET) and plain aluminum foil, sandwiched with epoxy to seal it up. I know PET isn't good for high frequencies but the incredible breakdown voltage makes it easier to get a working result. It should also provide better adhesion for the epoxy. Once I have a working prototype it's easier to know what components I need to order.
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
This forum should have a warning sign: "May cause addiction and electrical burns".
The homemade cap worked like a charm, 8 sheets of 55x85mm aluminum foil and 0.1mm PET transparency sheet glued together with epoxy. Got 205pF, slightly more than calculated. Really increases power across the test gap, although the voltage dropped a bit. Threw together a crude tesla but haven't gotten it to work without strong coupling, so no impressive sparks yet. Small steps...
What has thrown me completely off is the ignition coil. I started the calculations with a primary resistance of 13ohms, and the tests seemed to confirm the theory. Then I suddenly realized that I used the LCR-meter which measures AC resistance, the DC resistance is actually 3ohms. But when I scope the primary the voltage flattens out after 1,5ms, suggesting a much higher resistance? The inductance was measured at 1kHz, so that should be correct, and I can't detect any significant drop in voltage from the PSU. Any suggestions to where the problem might be? With a 3ohm primary I should get a lot more power from this, shouldn't I?
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
The coil is moving along fine now, it resonates and produces 1-2cm streamers with a 30° primary and homemade caps. Not much, but it is working. Secondary is 300-350 rounds of .45mm wire (150mm high), top load a 70mm sphere.
Rectifying the iggy simplified things a lot, as I can run bigger caps. When doubling it from 220 to 440pF performance increased significantly, suggesting that power is my biggest limitation as of now. But I can't find any way to squeeze more power from the iggy without increasing the voltage. And I'd rather not do that for several reasons. Safety (and the charm of running stuff like this from battery) is one, besides I just don't like throwing more power at a problem. I'd much rather spend more time making the system more efficient.
I also need better caps, these are made from PET, so I suspect they dissipate a lot of power at this frequency ((appr 2GHz). Hard to find reasonably priced PP-caps around here, would ceramics work?
Next up is a better spark gap. For now I've used a piece of tripad stripboard, which works surprisingly well due to the multiple paths available. When set up just right the sparks will tend to jump around from pad to pad, suggesting good quenching. But I will try the "tubes in a tube" design with forced air and see how that performs.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I also need better caps, these are made from PET, so I suspect they dissipate a lot of power at this frequency ((appr 2GHz). Hard to find reasonably priced PP-caps around here, would ceramics work?
I'm pretty certain that your tesla coil doesn't resonate at 2Ghz!
Next up is a better spark gap. For now I've used a piece of tripad stripboard, which works surprisingly well due to the multiple paths available. When set up just right the sparks will tend to jump around from pad to pad, suggesting good quenching. But I will try the "tubes in a tube" design with forced air and see how that performs.
You must be kidding me about using stripboard as a spark gap right? That is going to char up at any significant power level and the carbon will render the gap useless, if it already hasn't.
The classic simple spark gap would consist of 10-20mm copper tube fittings, or small pieces of copper tube, soldered horizontally to some stripboard with carefuly aligned space between them of some 0.5mm (use a stack of paper sheets for alignment). Make sure to peel at least two stripboard traces underneath though to prevent any possibility of arcing over the stripboard surface. Use like 5-10 of such electrodes in series until you find the sweet spot. In emergency, you could use some large brass nuts the same way too.
Regarding the caps, you might best use some bottle caps for start, they should have up to some 1nF per bottle and you can use several in parallel.
Finally, check the rectifier polarity on your ignition coil. The coil produces a voltage pulse only in one polarity, so make sure it's not turned for the wrong one.
And after all, I don't see how would circuit become much more dangerous if you driven it with say 24 or 48V, touching the ignition coil is not recommended no matter how it's driven!
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
Dr. Pork wrote ... I'm pretty certain that your tesla coil doesn't resonate at 2Ghz!
My bad, should be 2 MHz
Don't know why I thought it would be in the GHz range, maybe PET isn't so bad after all.
You must be kidding me about using stripboard as a spark gap right? That is going to char up at any significant power level and the carbon will render the gap useless, if it already hasn't.
Nope, works fine for testing. As I said, I don't seem to get much power out of my coil, that might be part of the explanation. I know it won't last, but it did provide a quick solution. I'm already working on a proper spark gap, I just didn't feel like putting a lot of effort into things before I could get my bearing.
Regarding the caps, you might best use some bottle caps for start, they should have up to some 1nF per bottle and you can use several in parallel.
I might just do that. They seem so crude, but looks might deceive.
Finally, check the rectifier polarity on your ignition coil. The coil produces a voltage pulse only in one polarity, so make sure it's not turned for the wrong one.
I know. It actually seems to produce a bit of reverse voltage as well, with the rectifier I get twice the spark length. I'm guessing the rectifier works like a disruptor producing a flyback-effect in the secondary coil. Anyways, the coil has a safety spark gap, if the rectifier is connected wrong this fires.
And after all, I don't see how would circuit become much more dangerous if you driven it with say 24 or 48V, touching the ignition coil is not recommended no matter how it's driven!
A coil running at 12V kicks like h*ll, but it's not dangerous (don't ask how I know:-D). When do they become that? I don't know and I don't want to find out either. Besides, my PSU only delivers 15V, and I'd rather not make a new PSU right now.
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
Well, I've pondered different methods for squeezing more power out of the coil, but there isn't any way around increasing the voltage. So I have decided to "cheat" a bit. Rather than just increasing the input voltage (which would require another PSU) I realized I could build a PWM buck-boost with a simple 555-timer. It might seem like a complicated solution, but it does allow me to operate from battery power if needed. Also, this project is as much about learning as the result, and designing a boost converter is both useful and educational.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi -
One idea I never tried, but see no reason why it wouldn't work:
If you can find two large HID lamp ballast inductors, and two IRFP150 or similar mosfets, you could drive the coil by a royer/mazzili circuit.. (use a larger parallel cap, like a 10-20uf motor start cap). It might give better kick from the coil for low voltage supply since it'll then see about 45V with your supply.
In any case, no matter how you push more power into the coil, it's output will become more dangerous as result! So perhaps just better not touch it and use a mains powered halfbridge FTW.
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
Dr. Pork wrote ...
In any case, no matter how you push more power into the coil, it's output will become more dangerous as result! So perhaps just better not touch it and use a mains powered halfbridge FTW.
True, once I step up the power it will become more dangerous. And if HV was my only goal I might have done just that, but I'm more interested in learning more about electronics and circuit design. But I need something tangible to work with, and HV is one of the more amusing fields since it has little purpose By taking the long route I have figured out all by myself how to make a PWM out of a 555, I find that incredible rewarding compared to just running it off mains power.
I am searching for a decent NST to make a larger tesla, but I have no hurry. I first have to make a 10X probe for my scope, that's a good excuse for firing up the lathe and mill.
As for the Royer oscillator I need to research that more before I can make up my mind. Thanx for the tip though, looks interesting.
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Greetings and a small starter project (ignition coil / Jacobs Ladder)
