Tesla's "COIL FOR ELECTRO-MAGNETS".

[gyulasun]

...

So it looks like this method http://www.qsl.net/in3otd/inductors.html is correct and that method is may be flawed http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm .

...

Hi Conrad,

Well, I would think just the opposite...  and your measurement results with the "exciter coil" method may confirm that. You could refine the measurement shown in the link you gave http://www.youtube.com/watch?v=BrY6Q4JCjXs by disconnecting the scope probe from the coil completely and just place the probe near enough to the bifi pancake and using the most sensitive input range (also with 1:1 probe setting) and see the "picked up" EM field then and retune the function generator upwards a little for the new maximum resonance (versus the case when the probe was connected to the bifi pancake). You could even use the monofilar pancake as the exciter coil to inductively drive the bifilar pancake.  And you may try to short the pobe pin to the probe's gnd clip to form a "wire loop" from the probe's grnd wire, serving as a kind of pick up coil (or you could also use a second air core coil like the size of the exciter coil and connect the probe to its ends).

I consider the measuring method shown in this link http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm   as one of the most accurate approach to estimate coils self capacitance because it mainly lets the actual self capacitance make its most influence on the parallel resonant frequency with the coils inductance (which you measured as 34uH).  This is especially so when you really use a very small CX  external capacitor to couple the top end of the coil to the scope (you used 3.4pF) and I know that using say 1pF as suggested the scope sensitivity may not be enough to see a convenient voltage maximum at resonance, still this CX could be reduced as small as practical because it improves the measurement accuracy when 1pF or even less. If you do not have a 1pF cap at hand, you can make one by twisting two enamelled copper wire of say 5 cm long each (wire OD is say 0.2 - 0.5 mm, any size) and using your C meter you could trim the total length to have near 1pF at any one of the twisted ends (the other ends are left floating and open of course).

Thanks for you kind efforts,
Gyula

[tim123]

... but I am now thinking about the diameter of the core (of the hole in the middle of the coil...

Hi Conrad.
  perhaps, due to the geometry of pancake coils, the core should not be inside the center, but instead, should be a circular plate behind it... Of course, you could have a coil on each face - so the core is between two pancakes...

Just a thought.

Regards, Tim

[conradelektro]

Hi Conrad.
  perhaps, due to the geometry of pancake coils, the core should not be inside the center, but instead, should be a circular plate behind it... Of course, you could have a coil on each face - so the core is between two pancakes...

Just a thought.

Regards, Tim

@Tim123: Thank you for your interest.

I was thinking about a helical coil as in the attached drawing. (A core for a pan cake coil is of course problematic).

I want to wind two identical helical coils (like a solenoid coil), one monofilar and the other bifilar.

Any ideas about the shape and the diameter of the core? The overal diameter of the coil will be about 60 mm, its length also about 60 mm.

Greetings, Conrad

[tim123]

Hi Conrad,
  I think to answer the question - "what would be a good pick up coil" - could be quite complicated... It would depend a lot on the rest of the circuit... There are far more knowledgeable folk than me around...

In my experience of ferrite-core coils, the core is usually about half the diameter of the coil...

I'm interested in how your experiments proceed... It think that the 'extra energy' stored in a tesla bifilar pancake coil is electrostatic (i.e. capacitance)- rather than magnetic (inductance) - but I have not done the experiments myself - perhaps it's both.

I'll read the whole thread when I get a chance...

Regards, Tim

[conradelektro]

I published a video http://www.youtube.com/watch?v=doJcDxpCvCo about measuring the "self resonance" of my two pan cake coils.

Please watch and let me know what I could do better?

At the end of the video I also try to calculate the "self capacitance" with the calculator from this web site http://www.1728.org/resfreq.htm.  May be it can not be done like that?

Greetings, Conrad