The bifilar pancake coil at its resonant frequency

[MileHigh]

Nelson:

The central issue is what is the reason for taking a bifilar pancake coil and exciting it at its self-resonant frequency?  Why do this?  What are you trying to accomplish?  What practical applications are there for this?  It's fun to "play" but what is the point behind the playing?  Those are the types of questions that are avoided and never posed and never answered by many experimenters.

Here is another set of questions:  Why a bifilar pancake coil?  Why not a regular pancake coil?  Does making it bifilar really do anything more than let the experimenter proclaim that there is a stronger electric field between the wires?  So what?  What about a solenoid bifilar coil, or even just an ordinary solenoid coil?  Both of the solenoid coils can be excited at their self-resonant frequencies also.   So there are four types of coils that I just mentioned.  What do you want to accomplish with the coil and what can you say about each of the four types of coils relative to what you want to accomplish?  We are talking about doing something practical with the coil.  I get the sense that these are difficult questions for many experimenters to answer.

A bifilar pancake coil, a similar pancake coil, and a bifilar solenoid coils and a regular solenoid coil with approximately the same amount of inductance as the pancake coils will all have different self-resonant frequencies and we know that for the two bifilar cases the self-resonant frequencies will be lower.  So what, what can you do with this information beyond making a frequency measurement and an observation?

Suppose you want your coil to act as a generic inductor in an electronics circuit.  Which of the four types of coil would be the best choice for this?   The answer is the regular solenoid coil.  The regular solenoid coil will meet the design goal with less wire and less resistance and less unwanted self-capacitance and have a higher working frequency bandwidth.

Suppose you want to make an air-core transformer.  Which of the four types of coil would be the best choice for this?  One more time, the answer is the regular solenoid coil.  The regular solenoid coil will meet the design goal with a more efficient coupling for power transfer, less wire used and less resistance and less unwanted self-capacitance and have a higher working frequency bandwidth.

When you look at a datasheet for a small coil that you might put onto a PCB they give you the maximum operating frequency for the coil and the self-resonant frequency for the coil.  Electronics designers avoid feeding the coil signals that go higher in frequency than the maximum operating frequency because above the maximum operating frequency you are approaching the self-resonant frequency for the coil and at those high frequencies the coil fails to function as a coil anymore.  As a general rule of thumb, electronics designers avoid the self-resonant frequencies of coils in their circuits.

A bifilar pancake coil is capable of holding more charge than a single wound coil and that is known .
When this coils are operated at resonance, the distributed capacitance on the bifilar  overcome the counter electromotive force that normal is find in conventional coils.

Not really, because the typical model for self-resonance of a coil is a parallel LC circuit.  A parallel LC circuit when connected in series with a load acts as an infinite impedance at the self-resonant frequency.  That means that the coil is acting like a 100% counter electromotive force device that opposes the excitation frequency with an equal and opposite voltage.

I will just repeat to you again:  I am telling you with 100% certainty that it is a major mistake to use a square wave when trying to find the self-resonant frequency of a coil.  What you should be doing is trying to understand for yourself why I am saying that.  Rejecting what I am telling you is the wrong course of action.

It's important for experimenters to realize that they simple can not assume something only because persons like you are not able to view more deeply some of aspects of bifilar pancake coil that are denied .
But persons like you that use the sentence  "carved in stone" to justify something without testing ,for sure will take their stubbornness to their grave stone too.

I probably just raised more questions about bifilar pancake coils and three other variations that you may never have even considered.  This is to get you and your peers to start critically thinking and examining all of the issues.  Chances are that I am viewing this issue more deeply than you have ever done before, and I am not even a hard-core electronics guy.  Sometimes things really are carved in stone.

I am challenging you and your peers to improve your wisdom.  If I was in your shoes I would test four coils, a pancake bifilar, a regular pancake coil, a bifilar solenoid coil, and a regular solenoid coil.  I would define some objectives, develop some test procedures, and arrive at some conclusions.  The general conclusion will be that the regular solenoid coil performs the best and offers the most inductance per unit of wire length and therefore has the least amount of resistive losses.  It also has the least unwanted self-capacitance that reduces the working bandwidth of the coil.  If you think a series bifilar coil is something special then you have to make tests and demonstrate what makes it special and why.

MileHigh

[gyulasun]

Gyula,

I just wound a single wire coil of the same gauge on an identical bobbin and measured the Ohms and Inductance. The test results show that you're right as regards the air core coils. I could only fit 4 Ohms worth of wire on the bobbin. The inductance was 4.3 mh. The serial bifilar at 5 Ohms delivered  5.81 mh. The ratio is pretty close at 1.162 mh per Ohm for the serial bifilar and 1.05 mh per Ohm for the single wire coil. There's clearly no where near the 100% difference Gotoluc measures with his ferrite toroid coils. nor the twice the magnetic strength my iron nail core serial bifilar demonstrated. The sloppy lash wrap of my single wire coil accounts for the slightly lower inductance to Ohms ratio. I concede that TK's measurements are correct and that his meter is functional.

It appears the ferrite core plays a critical role in the doubling of inductance with this Tesla bifilar connection. Maybe TK can run a ferrite core into his bore holes and make comparison measurements for us to help.

synchro1,

Very good you built the coil with single wire and obviously the fact that you did not have the same length of wire and had 4 Ohm DC resistance (instead of 5 Ohms) explains the 4.3mH inductance instead of the 5.81mH, no problem. 

You refer to Gotoluc's video, back then he used those toroidal coils in his circuit shown here:
http://overunity.com/8892/self-running-coil/msg233254/#msg233254 

so his toroidal core is not wound bifilarly but with one winding (in 5 layers) is wound on one half of the core and the other winding (also in 5 layers) is wound on the other half of the core, ok?
Such winding style is used for instance in so called common mode choke coils, see here one with single layer windings placed on the right and on the left hand side of the core:
http://wcmagnetics.com/product/50-amp-toroidal-common-mode-choke-507-series/

This type of winding can have double the inductance when the two windings are correctly connected in series (i.e. not to work as common mode choke) versus the same amount of wire (the two half windings have together) wound onto the same toroidal core in the normal way. However, we are now discussing Tesla style bifilar coils, not the winding style Luc used.

IT is not the ferrite core which playes the critical role but the winding style and position of the windings with respect to each other on the core (mutual inductance) that counts in the resulting L inductance of the series windings.

Gyula 

[Dog-One]

The regular solenoid coil will meet the design goal with a more efficient coupling for power transfer, less wire used and less resistance and less unwanted self-capacitance and have a higher working frequency bandwidth.

What if I'm not at all interested in power transfer?  What if I'm after something completely different?

Can you answer me what it is I could possibly be looking for?

If you have no clue, maybe it's time to cease and desist.


I'm quite certain Nelson knows what I'm referring to.   

[synchro1]

Quote from Milehigh:

"The central issue is what is the reason for taking a bifilar pancake coil and exciting it at its self-resonant frequency"?

Tesla used the bifilar pancake to transmit and recieve wireless power which is less efficient to do with the bifilar solenoid or the single wire pancake.

[synchro1]

synchro1,

Very good you built the coil with single wire and obviously the fact that you did not have the same length of wire and had 4 Ohm DC resistance (instead of 5 Ohms) explains the 4.3mH inductance instead of the 5.81mH, no problem. 

You refer to Gotoluc's video, back then he used those toroidal coils in his circuit shown here:
http://overunity.com/8892/self-running-coil/msg233254/#msg233254 

so his toroidal core is not wound bifilarly but with one winding (in 5 layers) is wound on one half of the core and the other winding (also in 5 layers) is wound on the other half of the core, ok?
Such winding style is used for instance in so called common mode choke coils, see here one with single layer windings placed on the right and on the left hand side of the core:
http://wcmagnetics.com/product/50-amp-toroidal-common-mode-choke-507-series/

This type of winding can have double the inductance when the two windings are correctly connected in series (i.e. not to work as common mode choke) versus the same amount of wire (the two half windings have together) wound onto the same toroidal core in the normal way. However, we are now discussing Tesla style bifilar coils, not the winding style Luc used.

IT is not the ferrite core which playes the critical role but the winding style and position of the windings with respect to each other on the core (mutual inductance) that counts in the resulting L inductance of the series windings.

Gyula

How would you explain the doubling of magnetic strength in my "Tesla Coil Builder" bifilar vs. single wire electro-magnet test with the trombone paper clips?