Self running coil?

[skywatcher]

Cutting ferrite is difficult. It's very hard.
You need diamond or tungsten carbide tools to cut it, and in most cases the material will break apart from the vibrations.

[gotoluc]

Hi Luc,

Yes I think the software with the audio card would be fine, though you would have to attenuate the 10V RMS at the Fluke output to the audio card input so that it should not overload.

Just occured to me: what if you would use your scope for monitoring the 10V voltage also at the Fluke input and measure the frequency with it at the same time?  Maybe the resolution will be enough (could it be increased in the scope software?).   

By the way, the 10V RMS sounds too high if comparing it to the so far 'usual' 10-11V peak to peak voltages across these toroidal coils, of course you can reduce it to around 3-4V RMS instead, but then you have to multiply the actual max resonance voltage value by .707 to know the lower and upper amplitudes for detuning. (say you adjust 3.4V RMS, then 3.4*.707=2.4V so you detune from the peak to the left and to right side till the amplitude reduces to 2.4V from the 3.4V)

From this test

1) the unloaded Q quality factor for the split and normal wound coils can be learned.
2) it will turn out if so far the MOSFET caused the distortion in the sinewave or the core. You would wish to see beautiful sine waves from these resonant circuits, not distorted.
3) the self capacitance of the split wound coil could be checked again.

With this test I ahave no intention to prove or disprove anything.

Regarding the sense of the previous test I asked: Besides what I already wrote, I suspected that the resonant impedance of the split wound coil (with the magnet) has increased at 8.3kHz (where you found its resonance) with respect to the normal wound coil's 6kHz resonance and at this increased impedance the MOSFET may get "overdriven", I mean it cannot 'give' its output power to the load due the too high resonant impedance and it reflects back.
It is the same if you have an audio amplifier and you drive it normally and use a 4 Ohm loudspeaker, and it works ok. Then you replace the 4 Ohm with a 16 Ohm loudspeaker and drive it with the same power, you will probably hear distortion because the load impedance is not optimum any more, it is too high for taking up all the available output power, the amplifying device or devices inside the amplifier will "produce" distortion (unwanted frequencies) from the reflected and not used up power and these may find their way back toward the power supply or elsewhere (and may cause harm in the active devices).
Something similar may happen in your case at the higher frequencies, of course at a much lower, just microWatt, power level. But I am not certain here whether this output 'reflection' really happens, it is a possibility.

Thanks,  Gyula

Thanks Gyula for the additional information and explanation of what these tests are for.

Luc

[gotoluc]

Cutting ferrite is difficult. It's very hard.
You need diamond or tungsten carbide tools to cut it, and in most cases the material will break apart from the vibrations.

Yes I agree!  very difficult stuff to cut

Luc

[skywatcher]

By the way, i think the most optimal core would be a 'pot core' like this:
http://www.oppermann-electronic.de/assets/images/Bild2642.jpg
http://www.jogis-roehrenbude.de/Bastelschule/RIM-Piccolo58-Spulensatz.jpg

Such cores have the lowest losses, are also closed like a toroid, but even more closed  and the coil can be wound very easily because it's a normal coil, you can wind it with a drilling machine in one minute. The magnets can be placed on the outside.

I think i have some pot-cores somewhere, if i find them i will make some experiments...

[Eastov]

Hi guys,

   There's a 2.5 inch ferrite toroid core (Type W, 800 (medium) Permeability) for $5 at mouser.com and here's the link: 61mm exact size...

http://mouser.com/ProductDetail/Wurth-Electronics/74270097/?qs=5twSNpOB8IC3x1MNWE5eDg%3d%3d   

  There's also high permeability (slightly more expensive) here:
http://www.cwsbytemark.com/

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