Self running coil?

[gotoluc]

I'm still looking at inductances with various setups - I noticed something which I cant explain, I'm hoping one of you can.

I wound a new toroid (Finemet nanocrystalline - small, about 14mm in diameter).  Once wound it has an inductance of 250mH.  As I approach my large 2" dia magnet, its inductance INCREASES to a peak, and then begins falling off as expected.  The maximum inductance increase is found when the toroid approaches the north or south pole of the magnet, with the toroid parallel with the magnet (inner hole facing the magnets pole).  The increase goes from 250mH to 500mH before falling off.

What mechanism is causing the toroid to GAIN inductance by the presence of the permanent magnets field?  I thought I understood the lowering of inductance, I just dont understand the gain.

Hi void109,

I think Inductance should be measured when there is no magnetic fields moving around. A moving magnet will induce an electrical current in the coil and interfere with the sampling frequency your inductance meter is sending through the coil to give you a reading.

That's what I think.

Luc

[void109]

Hi,

Mind answering some questions?   
Do you have the exact type of the core? 
What if you approach the core with a much smaller magnet in the same way?
What if you approach the core tangentially with a small or with the big magnet?  (Tangentially= simple let the magnet attract to side of the core on its outside perimeter.)

The answer to your question is probably in the way you place the core in parallel with the big magnet, flux surely is concentrated or collected first in the core more or less equally, without any chance for saturation. Then at closer and closer the core cannot 'collect' more flux and starts saturate: inductance falls.

rgds, Gyula

Hi Gyula,

The core is MP1305LF3T

http://www.metglas.com/products/page5_2_2_3_3.htm

A much smaller magnet does not increase the inductance, but it does reduce the inductance as expected.  I have various magnet sizes, I'll try with several more to see if there's a tipping point.

The increase does not seem to occur if approached tangentially  - but for maximum increase, you have to approach as I described.  My original numbers were wrong, it was from memory.  More precisely the inductance of the core is 25mH outside of the field.  After securing it to the magnet with electrical tapes using plastic guitar pics as spacers for an air gap, it now has a measured inductance of 71mH.  The highest I could get is 71.9, but its VERY touchy - it falls off very rapidly if I move it in any direction more than 1mm.

@Luc - I have it fixed in place, it is a constant inductance while within the magnets field.

With it fixed in place, I have tried finding its resonant point both with a 10nF and 100uF  capacitors.  No luck - I do the math, no dice - I tried to "eye ball" it on the scope, and I cant find resonance - it appears that resonance may be just outside of my function generators range as I can see the ringing of the circuit after a pulse, and it appears a bit higher than 4Mhz.

[gotoluc]

Hi Luc,

I am not sure on a correct answer for your last question above, it must have a relationship with the MOSFET switching on just or very near there in time.

When you replace the pulse coil with the sig gen, and supply the same amplitude at the same frequency, then the MOSFET capacitances are surely the same like with the pulse coil. The resonant pulse coil is a pure resistance in the oscillator mode, it is just like you place a several tens of kOhm resistor between the gate-source electrodes.
And when you connect the sig gen instead of the coil, it also has a pure resistance, though it is 50 Ohm, not many kOhms, this does not matter because the original resonant voltage is still there from the generator with the correct amplitude and frequency.
And when you slightly mistune the generator, then the amplitude stays the same but now the small frequency change causes a different phase relationship with the drain circuit that did not change. The FET gate-drain capacitance will have also a slighly different reactance now at this newer freq, alltogether these explain why the drain - gate waveshapes now can be shifted wrt each other. 
Maybe this latter could be done with the pulse coil in place if the gate-drain capacitance could be made a bit variable.  Have you placed ,say, a 20-30pF (or some more) capacitor between the gate-drain in a running oscillator to see if the waveforms shift away wrt each other?  It is possible that the gate-drain FET capacitance ought to be reduced and NOT increased: this can be possible only to use a big value tuneable inductance between the gate-drain but in series with a big uF capacitor to block DC. The latter method is called neutralization of the unwanted capacitance in an active device between its input and output electrodes, not often used in oscillators though, normally the active device is chosen for the oscillator with the smallest C_rss capacitance.

Thank you for the series 10kOhm test, I simply wanted to create an extreme value supply inner impedance for the oscillator. If you found more or less the same 'willingness' for oscillation with the 10kOhm like without it, then it is ok.

Will ponder on these...

rgds,  Gyula

Hi Gyula,

thanks for taking the time to post your explanation.

I did experiment some days back of adding a small pf capacitance between the drain and source and it has an effect on the pulse coil and main coil. It reduces the amplitude of the main coil and makes the pulse coil (top) fatter and rounder. It also causes a small advance timing shifts to the pulse coil and resulting in a small drop in current.

I don't believe the above to be a true benefit as it's kind of clear that a lower capacitance mosfet is the way to go. Yesterday I ordered some IRF510 which have a little lower capacitance then the IRFBC20 but a much lower gate resistance. I also bought 2 other 20v mosfet models irlml2502 and FDC637AN on ebay which have a low gate trigger voltage to see how they would performs also.

I attached all the pdf data sheet below for reviewing.

Luc

[gotoluc]

Hi Gyula,

The core is MP1305LF3T

http://www.metglas.com/products/page5_2_2_3_3.htm

A much smaller magnet does not increase the inductance, but it does reduce the inductance as expected.  I have various magnet sizes, I'll try with several more to see if there's a tipping point.

The increase does not seem to occur if approached tangentially  - but for maximum increase, you have to approach as I described.  My original numbers were wrong, it was from memory.  More precisely the inductance of the core is 25mH outside of the field.  After securing it to the magnet with electrical tapes using plastic guitar pics as spacers for an air gap, it now has a measured inductance of 71mH.  The highest I could get is 71.9, but its VERY touchy - it falls off very rapidly if I move it in any direction more than 1mm.

@Luc - I have it fixed in place, it is a constant inductance while within the magnets field.

With it fixed in place, I have tried finding its resonant point both with a 10nF and 100uF  capacitors.  No luck - I do the math, no dice - I tried to "eye ball" it on the scope, and I cant find resonance - it appears that resonance may be just outside of my function generators range as I can see the ringing of the circuit after a pulse, and it appears a bit higher than 4Mhz.

Thanks void109 for confirming this was happening with fixed positions.

In that case can anyone think of how we could use this close to 3 times increase in inductance to or benefit?

Thanks for sharing your findings

Luc

ADDED

void109, can you make a drawing of the position of your magnet an toroid so I can be clear on this. Thanks

ADDED

Never mind, I reread your post and it is clear but I tried it with my toroid and I don't get any increases that stay. Why the difference

[gotoluc]

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I have contacted Stefan about this user. All 14 of his posts are this kind of stuff

I don't have a feeling his stay here will be a long one

Luc