Hi Conrad,

Thanks for your message, I just answered it. 

Quote from: conradelektro on January 09, 2014, 05:38:23 PM

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

Quote from: gyulasun on January 09, 2014, 06:15:33 PM
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

Hi Conrad:

QuoteDo 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

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

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