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Author Topic: Tesla's "COIL FOR ELECTRO-MAGNETS".  (Read 459845 times)

Offline gyulasun

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #525 on: January 09, 2014, 04:36:44 PM »

...

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

Offline tim123

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #526 on: January 09, 2014, 09:05:05 PM »
... 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

Offline conradelektro

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #527 on: January 09, 2014, 09:26:11 PM »
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

Offline tim123

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #528 on: January 09, 2014, 10:12:13 PM »
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

Offline conradelektro

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #529 on: January 09, 2014, 11:38:23 PM »
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

Offline gyulasun

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #530 on: January 10, 2014, 12:15:33 AM »
Hi Conrad,

Thanks for your message, I just answered it. 


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?


IT can be done like that, no problem with that.  As I wrote, use a series coupling cap of 1-2 pF  between the scope probe and one of the outputs of the pancake so that the 115 pF input cap of the probe should not detune the pancake coil.

Greetings, Gyula

Offline conradelektro

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #531 on: January 10, 2014, 12:27:59 AM »
Hi Conrad,

Thanks for your message, I just answered it. 

IT can be done like that, no problem with that.  As I wrote, use a series coupling cap of 1-2 pF  between the scope probe and one of the outputs of the pancake so that the 115 pF input cap of the probe should not detune the pancake coil.

Greetings, Gyula

@Gyula: Thank you for the many hints and for your help.

I will try the decoupling of the scope probe with a hand made 1 pF capacitor tomorrow.

I will also keep a greater distance between the exciter coil and the pan cake coil.

Might have to redo the video with this improvement. The video needs zooming to the scope screen and function generator display anyway.

With all the help I get here I might be able to finally do some decent measurements.

Good night, Conrad

Offline MileHigh

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #532 on: January 10, 2014, 02:57:52 AM »
Hi Conrad:

Quote
Do you think that the core of the coil I want to wind should have a diameter of 25 mm, because the spinning magnet has a diameter of 25 mm? (I wanted to have a core of 10 mm, because I have a Ferrite which would fit.)

Keep in mind that your radially magnetized disk still generates a magnetic field that is quite similar to a bar magnet of similar dimensions.  Of course you can make an inner diameter of 10 mm for the core of the coil if you want to.  By the same token, you don't need to have a rectangular core for your pick-up coils.  Nothing is stopping you from doing that, whatever is your preference should be fine.  It's very hard to go wrong when you make a pick-up coil.

Note that the individual inner turns of wire that might have a very small diameter will cut proportionally less changing magnetic flux at a given distance as compared to the larger turns of wire at the same distance, and therefore pick up less energy.  If you make a regular set of coils (bifilar and regular) with a "satisfactory" inner core diameter (so that you can easily insert different things into the core) that you are happy with then you should be fine.

You can always experiment to see what the magnetic field pattern of the spinning disk magnet looks like with a simple sensor coil.  Say 20 turns of fine wire that are say 2 cm in diameter for a sensor coil.  Just turn on the motor and get the magnet disk spinning and scope the waveform from the sensor coil as you move it around.

MileHigh

Offline MileHigh

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #533 on: January 10, 2014, 03:51:23 AM »
Conrad and Gyula!

I watched the clip!  Conrad tried the exciter coil concept and it worked!  I am flattered and relieved that it actually worked!  lol

I have some comments for your consideration.  Hard core analog designers will tell you that just lightly touching a component on a PCB with the tip of your finger will add about 10 picofarads of capacitance to the component.  The message being that when you are dealing with very small capacitances in the tens or hundreds of picofarads range many things can affect your measurements, simply because the "stray" or "parasitic" capacitances in the immediate environment are comparable in size.

Gyula mentioned that the "classic" model for a coil is the inductor in parallel with the intrinsic capacitance.  That is a parallel LC circuit and that means at the resonant frequency the impedance of the parallel LC circuit is infinity.

Let's look at the classic test setup:  signal generator -> series resistor -> coil under test -> ground.

The scope probe is placed across the coil and you tune for maximum amplitude to find the self-resonant frequency.  Interestingly enough, many beginners might think that means that the coils is "outputting a high signal at resonance" but in fact it's just the opposite.  The coil is *blocking* the signal at resonance and the "high output" is actually just the fact that since no current is flowing, there is no voltage drop across the series resistance.  So you are simply measuring the signal generator voltage at resonance, and not any kind of "output" from the coil.

And here is my main point:  You are loading down the coil with the scope probe and it's associated capacitance and "disturbing" the measurement because of the extra capacitance.

However, you may see the possibilities for an interesting measurement trick that takes the scope probe load off of the coil completely.

The trick is to just measure the voltage across the series resistor, and leave the coil untouched and open-circuited.  When you do the test you look for the *minimum* voltage across the series resistor.  That is telling you that no current is flowing and the unloaded coil is being excited at its self-resonant frequency.

My first thought is that you need to do one final "small trick."  You put the series resistor in line with the signal generator ground and not the signal generator "hot."   Then monitor the voltage across the series resistor and when it is at a minimum, you know that the voltage across the coil is at a maximum.   And just to emphasize - the coil is *unloaded*!

I hope that you are still having fun.  I apologize because I can't follow 100% of the discussion because sometimes I have insomnia and I am too tired when I get home.

Conrad, you are doing a great series of experiments, my compliments.

MileHigh

Offline MileHigh

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #534 on: January 10, 2014, 05:07:00 AM »
A small addendum:

In my previous posting where I discuss moving the scope probe across the resistor and off of the coil under test, I realize with a second look that that's not exactly "unloaded."   You still have the "hot" or signal output from the function generator directly connected to one side of the coil, and the resistor to the signal generator ground connected to the other side of the coil.  So although the signal generator is driving the coil, at the same time that also represents a load on the coil.

Here is a compromise, it might be worth investigating:   You put a 50 kohm resistor in series with the "hot" from the function generator.  You also put a 50 kohm resistor in series with the ground return to the function generator.  Then sweep the frequency of the generator and monitor the voltage across the low-side 50 kohm resistor.

It's not perfect, but perhaps the coil is "quite undisturbed" when you do this test and you will be able to find the near-true self-resonant frequency of the coil like this.

Life is a compromise.

MileHigh

Offline TinselKoala

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #535 on: January 10, 2014, 05:21:53 AM »
"Exciter coil concept"..... the way I determine resonance frequency of big aircore inductors (Tesla coil secondaries) is to use a single turn "exciter coil" in series with a 50 ohm resistor, hooked to the FG output. This is simply wrapped loosely around the big inductor. The FG is not hooked to the secondary at all, just to the single turn exciter. The scope probe is then hooked to the top of the big coil thru 1 megohm or even 10 megs, with the scope probe ground at the base of the big coil. Then one tunes the FG for peak response in the scope, as usual. I've used this technique for years.



Offline conradelektro

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #536 on: January 10, 2014, 12:58:25 PM »
@Gyula
@MileHigh
@TinselKoala

Gyula made me aware of the "decoupling" of the scope probe with a self made 1 pF cap. (My scope probe has an input capacitance of about 100 pF.)

See the attached measurement circuit where I put a 1 pF cap before the scope probe. The rest is as in the video.

Now, with this 1 pF in the measurement circuit the self resonance frequencies have gone up. It should be like that because the ~100 pF of the scope probe are now hidden behind the 1 pF cap. (Two caps in series results in a little bit less capacitance than the smaller cap.)

Self resonance frequency of the monofilar pan cake coil 9.6 MHz --> self capacitance ~ 8 pF (calculator http://www.1728.org/resfreq.htm )

Self resonance frequency of the bifilar pan cake coil 4.04 MHz --> self capacitance ~ 45 pF (calculator http://www.1728.org/resfreq.htm )


It is difficult for the scope to register the signal through the 1 pF cap, but after some fiddling it always works. The exciter coil has to be as close as possible to the centre of the pan cake coil to make it work (otherwise the induced signal is to weak to be picked up by my scope).

If you look at this post http://www.overunity.com/13460/teslas-coil-for-electro-magnets/msg382210/#msg382210 and the measurement I did then with the circuit proposed at http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm you will see a great agreement of the measurement I did just now (with the improve exciter coil circuit) and then:

Monofilar pan cake coil: self resonace 8.5 MHz and self capacitance 7 pF

Bifilar pan cake coil; self resonance 3.8 MHz and self capacitance 44 pF

So Gyula was quite right to believe in the web page http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm . It did not matter much that I used a ~3 pF decoupling cap then.

Since the self capacitance of the coil is ~7 pF (monofilar coil) and ~45 pF (bifilar coil) the input capacitance of ~100 pF of my probe falsifies the results if not hidden behind a 1 pF decoupling cap. Gyula, you did a great job of teaching me that, I did not catch on at first but now I learned a very important lesson.


And here is my main point:  You are loading down the coil with the scope probe and it's associated capacitance and "disturbing" the measurement because of the extra capacitance.

However, you may see the possibilities for an interesting measurement trick that takes the scope probe load off of the coil completely.

The trick is to just measure the voltage across the series resistor, and leave the coil untouched and open-circuited.  When you do the test you look for the *minimum* voltage across the series resistor.  That is telling you that no current is flowing and the unloaded coil is being excited at its self-resonant frequency.

My first thought is that you need to do one final "small trick."  You put the series resistor in line with the signal generator ground and not the signal generator "hot."   Then monitor the voltage across the series resistor and when it is at a minimum, you know that the voltage across the coil is at a maximum.   And just to emphasize - the coil is *unloaded*!

MileHigh

@MileHigh: thank you for the explanation, I will try this "resistor trick". Also, thank you for the helpful explanations concerning the pick up coil dimensions which will help me to build the next pair of test coils (helical coils, monofilar and bifilar).

"Exciter coil concept"..... the way I determine resonance frequency of big aircore inductors (Tesla coil secondaries) is to use a single turn "exciter coil" in series with a 50 ohm resistor, hooked to the FG output. This is simply wrapped loosely around the big inductor. The FG is not hooked to the secondary at all, just to the single turn exciter. The scope probe is then hooked to the top of the big coil thru 1 megohm or even 10 megs, with the scope probe ground at the base of the big coil. Then one tunes the FG for peak response in the scope, as usual. I've used this technique for years.

@TinselKoala: I will try that as well.

After these initial tests I want to draw a first conclusion:

A measurable difference between a monofilar and a bifilar pan cake coil seems to be its self capacitance (monofilar has lower self capacitance than bifilar) which then results in different self resonance frequencies (monofilar has higher self resonance frequency than bifilar).

I will now try a few more measurments ("resistor trick" from MileHigh and "single turn Exciter coil" from TinselKoala) and then I will write up a little "document" with the measurement circuits that worked for me. May be I make a new video with the correct measurements. Also the tank circuit measurements have to be redone with the 1 pF decoupling cap (to hide the scope probe capacitance).

I regard the attached measurement circuit as one of the good ones. Also the circuit from  http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm seems to be good. And the results (self capacitance) agree very much.

It means that my progress will slow down, please be patient.

Greetings, Conrad

Offline conradelektro

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #537 on: January 10, 2014, 02:56:04 PM »
I just understood that the approach from http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm is indeed the classical approach (function generator -  resistor - coil to be tested - GND; scope over "coil to be tested") with the "Gyula improvement" in the circuit (namely the 1 pF cap to shade the 100 pF of my scope probe).

See the attached circuit diagram. And it worked just fine.

Monofilar pan cake coil, self resonance around 9 MHz.

Bifilar pan cake coil, self resonance around 4 MHz.

So, my mistake, I thought that the approach from http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm was something new, but it is the classical approach with a 1 pF cap to shade the scope probe. Might be nothing new for the experts, but it took a while till I got it.

Greetings, Conrad

P.S.: Working on MileHigh "resistor trick" in the classical approach.

Offline conradelektro

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #538 on: January 10, 2014, 03:50:41 PM »
Very strange effect with the MileHigh "resistor" trick:

The resistor trick works with the bifilar pan cake coil (Voltage over resistor is lowest at about 4 MHz) but it does not work with the monofilar pan cake coil.

May be the "resistor trick" only works with a certain self capacitance of the coil. The self capacitance of the monofilar coil is only 7 pF (self capacitance of bifilar coil 44 pF).

With the monofilar coil the Voltage over the resistor is higher at lower frequencies and continuously goes lower up to 20 MHz (the limit of my function generator).

May be someone has an idea what is going on. See the attached circuit diagram. The resistor has to be between coil and GND (in the classical set up it does not work even with the bifilar coil).

Greetings, Conrad

Offline synchro1

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Re: Tesla's "COIL FOR ELECTRO-MAGNETS".
« Reply #539 on: January 10, 2014, 08:15:15 PM »
@Conradelektro,

This thread is entitled Tesla's "COIL FOR ELECTRO-MAGNETS". When do you you plan to test and compare the coils as electro-magnets?