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Coil to Spark Length Ratio

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Coronafix

Wed Dec 20 2006, 12:12PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Just wondering what the best spark length to coil length ratio people are getting.
My little coil has been pushed to approx. 15" sparks (to ground point) out of its
8 1/2" length. I reckon that is pretty good, but I know that the DRSSTCs get
better than that. So any way, just approximately, my ratio is 1.7.
I know that it all depends on power input, but really, how far can you push a little coil???

Steve Conner

Wed Dec 20 2006, 12:32PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

As far as I know, I have the record for this, with 5ft from a roughly 13" tall secondary (a ratio of about 4.4)

Coronafix

Wed Dec 20 2006, 12:38PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Wow, that's hardcore!

A lot more than I expected!

Marko

Wed Dec 20 2006, 02:00PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Haha. Steve definitely pwns noobs in this part.

It's all about putting a small secondary on oversized bridge and trying to get sparks as much as possible away from the coil.

I don't know if that counts, but Steve Ward got 5,14 resonator lengths (21inch coil, 9 foot spark) with his large magnifier setup; biggest ratio I know yet.

(Maybe a driver should be in count also?) Dan McCauley also got something 4-5 secondary lengths IIRC, with small coil, but at that point his secondaries started blowing up.

From all that, it seems that it's pretty plausible to get ~5 resonator lengths, if spark is moved (by any means) away from hitting the driver itself.

Hazmatt_(The Underdog)

Wed Dec 20 2006, 08:42PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Spark length I don't really think is a good metric because there is more to a system that gives you that spark length.

I'm getting about 40-48" out of my 24" coil, so its roughly 2:1, but that's not nearly enough information. What is to stop me from using my PT and doubling that length, nothing. This is why I don't use this kind of measurement. I use length/Joule. My factor for length/Joule is approximately 3 feet/Joule of bang energy. This eliminates all other factors. It doesn't care wether you use a PT, NST, MOT, FBT, because it is the core element of the tank.

This also stresses another point, an efficiently matched tank/supply. I've got my simulation and numbers now that I know what the best match really is and its not pi/2*Cresonant (That only gives you a 90% match). My point is that I can drive a little over a joule with an NST efficiently. I'd like to see more people get their systems calculated right, use only the needed amount of power, and get their spark length based on 3'/Joule.

Steve Conner

Wed Dec 20 2006, 10:02PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I don't see how length per joule is a good metric either, since most people believe that the spark length from a Tesla coil is proportional to the square root of the power input, so there's a good case for arguing that it's proportional to the square root of the bang energy too. It should be length per root joule or something.

Did you measure the power input to your coil for a 48" spark? What is it? The record for "Freau Factor" is about 1.7 (inches per root watt) so if you beat that, we're impressed. Me especially because I've never managed it :( There's a whole load of discussion on these various metrics in the Pupman archives since the year 0.

P.S. I tried to coin the name "Watts Number" for (spark length/coil length) but it never caught on the same as "Freau Factor". My current thinking is that you can't have both high at once: if you design for a high Watts number you get a low Freau number and vice versa. John Freau's original coil that set the 1.7 benchmark only had a Watts number of about 1.5 IIRC.

Tesladownunder

Wed Dec 20 2006, 11:49PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

For a SGTC my 6 inch coil Watt number is 3.0 with a clear photo showing this. (32 inch sec, 96 inch spark) I have not seen better than this with photographic proof. Clearly the DRSSTC's do better. Magnifiers need to take into account the height of the secondary as well as the tertiary coil and then become just ordinary in performance.
I was thinking the other day that I now have the tank cap and power gear to get a lot better performance out of that coil as it was very well behaved and didn't have any racing sparks or obvious impending failures. It is not just a case of doubling power giving better performance of course - something will "break". The big double toroid and high break rate of 1100 BPS were probably the key features in the performance.

Peter

Steve Ward

Thu Dec 21 2006, 01:16AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Most of my high performance coils get around 3-3.3X the winding length. The harder you push on them, the less efficient they seem to become at making long sparks.

I dont count my magnifiers unless you want to add secondary and tertiary length. In which case i think that was 22" + 17" for 39" winding length for 108" streamers which is only 2.76X or so.

Hazmatt, the only idea here is to see what spark length a secondary can support. It says nothing about the efficiency of the setup. I personally think its a pretty cool measurement, because it relates the output performance vs size.

Hazmatt_(The Underdog)

Thu Dec 21 2006, 01:36AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

That's fine that you guys want some kind of overall performance factor, I don't have a problem with that except that I believe its a bit misleading and not really representative of the system.

I have been taking note of the Joule energy of coils and 3'/Joule is pretty reliable. I met a coilier at a swapmeet who is an engineer, using a large pulse discharge capacitor and repition rate of 3s/bang. His 12' spark length and 4 Joule input confirm what im finding. So wether others believe it or not doesn't really matter to me, since my findings are holding true. All I'm trying to suggest to all is that this is a more realistic approach to characterizing spark length, at least for a SGTC. If you don't or don't want to believe me, that's fine, but I would suggest you test the theory first.

I can't give you exact figures for Wattage because that transformer is dead now. I do have a clamp-on that will give me what you're looking for, but the coil is at college right now getting 'prettied' up. My input NST was a 12/60, the capacitor was I think 16nF, but it could've been 18nF. It's hard to remember because it was a while back. What I have been working on is the real system simulations rather then the model. The real system uses a 9kv/120, and the model was a 12/60 as I've already stated. So basically I've been doing more preparatory work for the finished system, and its spark length should be 6.3' from my Joule calculation.

teravolt

Thu Dec 21 2006, 04:37AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

when drssts are operating why do sparks have a tendancy to reach out ? is this a phenomina of drsstc's ?

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