Self running coil?

[chadj]

Hi Jules,

I have some IRF9640 P channel mosfets and they don't seem to work with the pulse coil. I tried them before and tried them now and I can't make it self pulse.

I don't know what kind of circuit I need since I don't know much about electronics. All I know is, if I had a circuit that could be powered by 3vdc at 10u and capable of producing 7vpp sine wave (square wave may work) at around 10KHz with adjustable frequency, then this Toroid coil should give OU from what I can tell.

If you can provide a circuit that can do this,  will be yours!

Thanks for sharing.

Luc

Luc,

The circuit in its present condition leaves very little power for auxiliary circuits. At this point I think it is more important to understand how to increase the output. There is probably a lot of power being being burned through the copper losses but who knows because the current is zero? Can this be done with less windings? Would a gap or shunt make a difference? What happens when you change the angle of the magnet? Can you do this with an E core?

Ideally we need some computer modeling of the magnetic field. This data could be compared with static tests of the toroid to compare for accuracy. I have a lot going on right now and I may get to this but it will not be for a while.

Thanks!
Jules.

[void109]

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.

[gyulasun]

At everyone,

I think I found something important that is preventing the circuit to show its full potential.

I decided to reproduce as close as I can the sine wave signal the self pulsing coil produces when connected between the gate and source of the mosfet. Using my Signal Generator in sine wave I matched the frequency and adjusted the output to the exact vpp the pulse coil produces. Once I had them matched up as best I can I pulled out the pulse coil and connected only the generator. To my surprise the current draws is exactly the same as when the Pulse coil is connected and self pulsing. I would of though it would use less current since the switching is now powered by the generator! However this is not the case and I also found something even more important.

See the first 2 scope shots below to see first the pulse coil (self pulse mode) shot and next the generator pulsing the circuit.
All tests are using the same 3vdc feed and IRF640 mosfet with same dual coil toroid @ 1075mH (no magnet)
Green scope probe is between mosfet drain and source and Yellow scope probe is between mosfet gate and source.

Why is this important!... because ever since I've been using the pulse coil I lost the ability to control the mosfet switch on timing. If you look at all the scope shots I posted before, when using the pulse coil you will see it has this delay. However if I use the generators I can adjust the on time timing.

So without changing the setup I only turned the micro frequency adjustment on the generator to make the mosfet turn on time sooner and bingo!... no current used and as I keep going it starts to send back current (up to a certain point). See those scope shots below the first two.

It is clear that the pulse coil is delayed too much. We will not get it to use no current or send back current this way. We need the mosfet on pulse trigger to be earlier for this to work.

What do you think?... any ideas how to solve this?

@Gyula, please notice that the main coil bump is still there (without the pulse coil) so this is not what we think it is!... notice it goes away when on time is advanced to starting to come back but in negative as it send most current back. What could this be?

Luc

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

[gyulasun]

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,

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

[gotoluc]

Luc,

The circuit in its present condition leaves very little power for auxiliary circuits. At this point I think it is more important to understand how to increase the output. There is probably a lot of power being being burned through the copper losses but who knows because the current is zero? Can this be done with less windings? Would a gap or shunt make a difference? What happens when you change the angle of the magnet? Can you do this with an E core?

Ideally we need some computer modeling of the magnetic field. This data could be compared with static tests of the toroid to compare for accuracy. I have a lot going on right now and I may get to this but it will not be for a while.

Thanks!
Jules.

Hi Jules,

Q: Can this be done with less windings?

A: from everything I have seen and done I would say no!... Actually, my conclusion so far is that more windings are needed. If the inductance is raised that means more PM flux can be applied which if I understand this correctly should give a stronger kick back I think.

Q: Would a gap or shunt make a difference?

A: I don't understand this question

Q: What happens when you change the angle of the magnet?

A: No difference. There is nothing special about the magnet position, distance and quantity of magnets. It all about how much PM flux the core should have for the proper balance between it and the coils flux when switched on. I'm quite sure the core should not be saturated by the PM. I would say once the PM is applied the core needs to have enough flux space left over for when the coil is energized that its flux can go in and create a compression effect against the PM flux. This is how I understand it at this time.

Q: Can you do this with an E core?

A: I have not tried it but I think a Toroid wins on most inductance created for the wire length. This I think is the most important, since the result will be less resistance and reactance time.

Hope this answer your questions.

Luc