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Registered Member #1642
Joined: Sun Aug 17 2008, 11:36PM
Location: Black Canyon City
Posts: 96
No, it's true. Back then I built a similar coil using a six-pack of 4CX250B's but due to only having a 60A electric service to the house, I was only able to obtain measly 68 footers..nowhere near as efficient or extreme as Chris's two-tuber. FCC shut me down, coil was desroyed in a hurricane and I haven't been able to acheive similar results since...rotten shame.
Cool thing about the 4CX250's is that the screen grid can be amplitude modulated for cool audio discharge. (Also known as "swamp modulation")
Registered Member #1956
Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172
Holy crap!! I'm 64 feet tall. A spark that long shall make a spectacular noise!! What is the size of the coil to make sparks +68' ? Back then you guys were able to achieve what resonator/spark length ratio?
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
Thanks for the reply on the coupling question, Richie mensioned that coupling and power are the two things that grow sparks in his website. Do you think that it is frequency or method of modulation ie. drsstc, vttc, sgtc ect. Dr. Spark and you have found that frequency is a factor for sword sparks and I have had others feal it is modulation that is responsible for sword sparks. Do you think that ramping the energy at a lower resonsnt freq like 100k might give simular results as your coil, if so perhaps a larger coil with larger igbt could be utilized.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
teravolt wrote ...
Do you think that ramping the energy at a lower resonsnt freq like 100k might give simular results as your coil, if so perhaps a larger coil with larger igbt could be utilized.
I think the size of the resonator (resonant freq.) is pretty much the determining factor for sward-like sparks but not the only one. One of my fastest coil I have ever made was 220KHz and it never showed sward-like streamers
On the other hand, I also had a coil which resonant frequency was 380KHz, but because of slow transistors I had to drive it in 1/2 resonant freq – in that case it never showed sward like streamers , but the same coil once driven in full resonant freq. showed swards (unfortunately it was just a short run which destroyed the IGBT)
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
thanks mates genraly all the coils that I have seen that have swod sparks have been small and therfore have higher frequency. The coils of this type that have streight sword sparks have some sort of AC modulation IE VTTC and others that have no filter caps and or are driven by half wave AC power sources. I am wondering if there is any corilation since Steve seems to be able to have some controle over the shape of his sparks.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
In my experience it is the modulation envelope of the RF burst that mostly determines the spark appearance.
1. A very fast rate of rise of the RF envelope gives rise to multiple branches and a pretty typical lightening-like appearance to the sparks. Like in a conventional spark-gap TC. (Primary-side losses in spark-gap TC's dictate that the secondary voltage must always rise quickly over a small number of RF cycles. So they almost always display thin jagged sparks with multiple branches.)
2. A slow rate of rise of the RF envelope shape tends to produce fat sparks that grow in a straight line without branching. Nothing kills the straight sword-like sparks better than an abrupt increase in terminal voltage though, as this gives rise to branching instead of steady growth. As mentioned VTTC's and SSTCs running off half-wave rectified mains frequently display sword-like sparks due to the slow rate of rise of the AC half-cycles that define the RF envelope. However, if they are mistuned and suddently pull into tune with corona loading, the straight swordlike sparks will be replaced by gnarly sounding branched sparks.
3. Prolonged RF bursts seem to make the spark appearance more bushy and fuzzy as the conductive channel presumably moves around due to heating effects, air current and what have you. Just prolonging the application of a fixed level of RF doesn't seem to make the sparks grow any longer just get hotter and move around, so it doesn't seem worthwhile if you just want long sparks.
As for what is most efficient for growing the longest sparks possible regardless of shape I really haven't got a clue. Since Steve W is very relatively long 16ms bursts, and I would have thought there was quite a lot of energy in each of the bursts. So it's probably not really that efficient from a spark length vs bang energy input point of view, but the spark shape and brightness is very impressive.
The long bursts are quite hard on the power electronics because there is considerable power being dissipated over a significant amount of time. In DRSSTC's with short burst lengths you can run tremendous currents because the burst lengths are usually shorter than the thermal time constant of the semiconductor dies. One you start generating 16ms bursts, the dies are definitely going to heat up due to conduction and switching losses over that time!
It helps if you can keep the devices as cool as possible before the bursts so you have more headroom for heating during the transient overload during the burst. Maybe that is why Steve used water cooling!?!?
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
GeordieBoy wrote ...
The long bursts are quite hard on the power electronics because there is considerable power being dissipated over a significant amount of time. In DRSSTC's with short burst lengths you can run tremendous currents because the burst lengths are usually shorter than the thermal time constant of the semiconductor dies. One you start generating 16ms bursts, the dies are definitely going to heat up due to conduction and switching losses over that time!
Thanks Richie, I would submit that a standard DRSSTC is less efishent than Steves coil because his coupling is .5-.6 there for more enegy makes it into the spark. If you are right a coil that uses large bricks at 68khz is also able to make sword sparks with a ramping supply voltage. I think that at 16ms ramp the devices are actualy doing some work verses a transient switch in a DRSSTC wich accualy may add to the power that you deliver to the tesla this also adds to the spark length with the greater coupling. Adding a D type amp is positivly brilaint hats off Steve, along time ago I thought about it but didn't know how
It helps if you can keep the devices as cool as possible before the bursts so you have more headroom for heating during the transient overload during the burst. Maybe that is why Steve used water cooling!?!?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Dex wrote
If the frequency is below 500 khz,no matter what you do,and what driving mode you use,you will not get more than 5 foot long air sparks with less than 100 kV.That's not possible at normal altitude.That's what I claim
Its a curious thing. I suspect that you can set up proper conditions that allow the E-field to become high enough at the tip of the streamer only, which allows long distance propagation with fairly low voltages. This would rely on the existing streamer channel to be a very low voltage drop, and providing sufficient power to the spark channel to keep pushing charge out to the tip so you can establish the required E-field to further propagate breakdown. If there is any truth to what i say, i dont yet understand the time constants involved with these charges moving through the plasma channel. Identifying the time constants involved with tesla coil sparks, i believe, would unlock a huge potential for modeling and resolving a lot of questions about how to best make long sparks. Basically, spark impedance changes dynamically in time vs voltage and apparently the frequency of the source. In the case of the QCW coil, the spark impedance would appear to be much lower than in the case of "transient" coils that operate over just 10's of uS.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
Hi Steve, to bad you can't do a"time warp" with a stop action camera with simultaneous current monitoring of the secondary to see how the ramped rails and wave shape of the current in the secnondary affects the spark. It seems to me that since your coupling is around .5 you have a lot more control over the power being transferred to the spark than a transient type coil. If the coupling was any lower say .2 the spark shape would be more dependant on the energy stored in the reactance of the tesla and secondary ramp up. The QCW is more like a cross between a regular transformer because you have relatively high coupling witch allows for more direct energy transfer and a tesla wich uses Q for voltage amplification.
Is it harder to tune and keep tuned because the notches are sharper and farther apart with the extra cupling?
Registered Member #2566
Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147
Steve Ward wrote ...
Dex wrote
If the frequency is below 500 khz,no matter what you do,and what driving mode you use,you will not get more than 5 foot long air sparks with less than 100 kV.That's not possible at normal altitude.That's what I claim
Its a curious thing. I suspect that you can set up proper conditions that allow the E-field to become high enough at the tip of the streamer only, which allows long distance propagation with fairly low voltages. This would rely on the existing streamer channel to be a very low voltage drop, and providing sufficient power to the spark channel to keep pushing charge out to the tip so you can establish the required E-field to further propagate breakdown. If there is any truth to what i say, i dont yet understand the time constants involved with these charges moving through the plasma channel.
The time constants and many other things that affect shape of high frequency sparks are poorly understood.What has been well researched and known is the phenomenon of gap breakdown for "leader free" HF sparks .100 kV or less means that you are dealing with leader free propagation of HF sparks.For such sparks tip radically differs from leader sparks and moving of the charges along the spark channel is pretty uniform while tip has the form of HF brush discharge of stremers.For 100 KV and 300-500 kHz ,max breakdown gap distance is about 5 feet (point to point).Which means that your experiments are already close to that limit.
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Steve's Q.C.W coil design
