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Re: [TCML] voltage

On 1/25/12 10:04 AM, Bob Svangren wrote:


Hi guys, I notice that there is a lot of conversation on measuring
the terminal voltages on these fantastic coils. I have read a lot of
books on the subject over the years and found that it takes very
complicated and costly equipment to measure these very high voltages.
Not necessarily. It is expensive to go build a general purpose piece of gear, yes, but for a single purpose application like this, a home built system is going to be the way to go. And that means you have to calibrate it yourself, but that's what we're discussing (and, as pointed out, Terry Fritz did a lot of this, with a similar discussion, 10 years or so ago). 





It seems that everything has a bearing on the accurate measurements
such as the electrostatic field, barometric pressure, humidity, AC-
DC. frequency and many other factors.
Well.. barometric pressure won't affect it, neither will humidity.. field shape and frequency DO have an effect on the measurement technique, but once you've got a basic technique, you can quantify the factors and move on. 



Of course we all like boasting

rights of our coil output and it's great to say that we are putting
out a million volts but in reality, no one is sure of this number
I think we can safely say that NOBODY has a conventional tesla coil that is putting out a megavolt. Even the big monster coils probably are not that high. Most coils probably have peak voltages in the few hundred kV range. It's all about radius of curvature and such. 




The

best experts cant agree on what an inch of arc is equal too but it
ranges from 10,000 volts per inch to 50,000 volts per inch of arc.
If you're talking about distance in a properly built sparkgap, then you're wrong.. A good spark gap, properly used, is probably good to around 5%, especially if you take into account air density and humidity according to the standard tables. For more, you can look at: 
http://home.earthlink.net/~jimlux/hv/sphgap.htm
for examples.
And if you're talking about "streamer length", then it's pretty well established that voltage isn't actually a big factor in length. Things like the capacitance and inductance of the terminals, the amount of energy to support the spark channel growth, and such are more important. Once you're looking at spark propagation in free air, the whole non-uniform electric field thing has a huge effect. (the standard work here is, of course, Bazelyan and Raizer's book "Spark Discharge") 


The makers of stun guns claim anywhere from 100,000 volts to a
million volts for less than a three inch arc as one example.
They are lying: bald faced lies for marketing purposes from vendors who have no assets and therefore have nothing to fear from the FTC coming after them. Any reputable manufacturer of such things (e.g. the Taser folks) doesn't make such claims.
The Taser folks talk about high initial voltage (20-30kV) but point out that this is like the 30kV electrostatic potential when you get a spark from shuffling across the carpet. There's no real energy there, and, as soon as there's any load, the voltage drops down to a kV or so. (their website has a whole host of journal article citations, by the way)
I note that those "stun guns" have pointed terminals, which sets a VERY low voltage on them (to actually have 30kV, you'd have to have terminals roughly 1cm in radius...). If they're measuring a high voltage, it's by immersing the device in an insulator to prevent air breakdown.
Note that automotive coils can produce several tens of kV *if properly insulated*, but cannot do that in open air.
More likely, the cheap units just copy the number from the 17th generation photocopy of their power supply design that they found on the internet.. and that was some sort of thing where they multiplied primary voltage by transformer turns ratio. 


 Who

would believe any of this to be fact. We do need our boasting rights
and I for one would be in favor of adopting our own voltage standard
which we could all understand and would do away with all this guess
work. I have determined that 20,000 volts would be a good and close
to accurate standard for us to use and we would all be on the same
page. Be it right or be it wrong the 20KV per inch should be close
for our use.
And by what science basis do you propose this? What "use" are you talking about? We brag on the basis of "spark length" not on the basis of voltage (because we're all aware that a) voltage isn't that important from a showmanship standpoint.. spark length is 
b) spark length isn't very well correlated to voltage
c) spark length is a whole lot easier to measure than voltage (as the discussion on field probes shows). (we can argue about how to measure length..do you go along the tortuous path, or end to end in straight line)
For spark length from spark gap TCs, there's a well established rule of thumb that the length of streamers, in inches, is roughly 1.7*sqrt(input power in watts).
For actual air breakdown, a number of 29-31kV/cm works fairly well for small gaps in a uniform field at usual atmospheric pressure and temperature. 


 If we should adopt the 20KV volts per inch standard, I

would be in favor of naming our new standard after the late and great
Mr. Harry Goldman who gave much of his life to making coiling fun and
interesting for all of us to enjoy.  We could simply call this the
Goldman standard.  A 50 inch arc would equal a million volts. Doesn't
that have a nice ring to it? So, the big question is, what do you
fellows think of this plan.
As you can tell, I think it's terrible.. It has no basis in science or tradition.
I judge science fairs, and I cringe every time I see a project with something where they are measuring the wrong thing, or worse measuring one thing and calling it another. The classic one is measuring voltage or current from some source (home-built generator, propeller on PM motor, solar cell) and calling that Power. Frankly, this kind of "spark length = voltage" idea is why Tesla coils make terrible projects for most kids. We should not be propagating bad science on this list. 
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