Re: [TCML] Measuring "critical coupling" with an oscilloscope and signal generator?

[Futuret <futuret@xxxxxxx>]

Jon,

In a spark gap type Tesla coil, critical coupling is undesirable.  Critical
coupling is that coupling which occurs when the coupling is reduced
just to the point where there's no splitting of
frequencies.  This concept of critical coupling is more suited for
continuous wave applications such as radio circuits.  The degree
of coupling which results from critical coupling is a very low k value,
maybe around k = 0.01 or so (I don't remember exactly).  If such a
low coupling value is used, the coil is unlikely to produce any sparks
at all, or extremely weak sparks at best.  

For a spark gap Tesla coil, we want a coupling value which is much
higher than critical coupling.  We want the highest possible coupling
we can get without getting racing sparks on our secondary.  This is
usually around k = 0.12 or so.  

Some folks have obtained better sparks when they loosened their
coupling slightly more, even after loosening enough to stop racing
sparks, but I haven't seen this effect in my own work.  There may
be some relationship to how well the gap is quenching, or other
factors.  There are also so called "magic values" of coupling which
aim to cause the quench to occur at an RF notch or beat minimum
point, but again I haven't been able to capitalize on the magic value
effect in my Tesla coil work.  

Some folks may have created a new definition for "Critical Coupling"
and defined it as "the best degree of coupling for use in a Tesla coil".
I've heard some folks speak in those terms, but the traditional definition 
of critical coupling is as I described in the top paragraph above.

Cheers,
John

 



-----Original Message-----
From: Jon Danniken <danniken@xxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Wed, Dec 8, 2010 3:38 pm
Subject: [TCML] Measuring "critical coupling" with an oscilloscope and signal generator?


Hello,

I read the article on coupling written by D.C. Cox in the TCBA Volume 3, 

Number 2 (full article available for viewing on teslauniverse).  In that 

article, DC provides a graph to show the effects of coupling on the 

secondary current, as shown here:

http://teslacoils.home.comcast.net/CriticalCoupling.jpg

What I am wondering is if it is possible, using a signal generator and an 

oscilloscope, to actually determine the point at which critical coupling 

occurs.

Note: there is a page showing how to use a signal generator and an 

oscilloscope for other teslacoil measurements in the following link; would 

either of these methods work for determining the point of critical coupling? 

If so, what would I want to look for to investigate the effects of coupling, 

in an effort of achieving critical coupling?


http://www.hvtesla.com/tuning.html


Thanks for any suggestions on this,

Jon 



 
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