Self running coil?

[gotoluc]

Hi Luc,

Thank you for the test.  I asked it because normally at such low audio frequencies toroidal cores give higher Q at a few kHz higher frequency (6kHz --> 8kHz) and, besides, higher frequency involves higher inductive reactance too (in the inductive reactance formula the reactance linearly increases with the frequency, (X_L=2*pi*f*L), so does the resonant impedance, Z=Q*X_L.

And if the resonant impedance is higher at a higher frequency wrt a lower frequency, the MOSFET will draw less current at its drain side when switches ON, this is the same as if you were using a ,say, a 50 kOhm resistor in the drain (instead of the coil) at 6kHz and then you were using ,say, a 70 kOhm resistor at 8.3kHz: obviously the the current draw would be less in the latter case.

And your split wound coil definitely must have a lower self capacitance with respect to the normally wound coil, I estimated the 533pF as being a close match to reality: your split wound coil may have 533pF LESS self capacitance than the normally wound.
This means that if you split the coil into ,say, four parts (for four quarters) you surely will have a coil with even less self capacitance. However, I can only GUESS whether the inductance in this case increases four times instead of the double value, maybe yes.

I think it also would be useful to test the two coils (the ones you kindly tested for me yesterday) in the following way:  please see Diagram 4 in this link: http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/experiment/lab/expt3/expt3.html

You could use the same C capacitor for both coils in parallel, its value could be anything between 1000 and 1200pF what you can find and combine, at 6kHz the 635mH coils should resonate with 1108pF.  The R resistor in the Diagram shows your coils DC resistance, no need for putting there anything.

It is possible you would have to use 470kOhm or even 1MegaOhm series resistor instead of the 100kOhm shown because the resonant impedances received from toroid coils should be in the several hundred kOHm range.

PLEASE use your Fluke voltage meter in AC to see the voltage V_out at the output , I do not know your Fluke AC input impedance (maybe 1MegaOhm with 20-50pF parallel capacitance, just use a 1-2pF series coupling capacitor at the V_out output to reduce any detuning and loading effect to a minimum.  USE the SINE wave setting on the gen (but if you curious, switch to square wave too  )

You would first search for the resonant maximum output voltage, starting with the normally wound, no magnet coil, paralled with the 10..pF cap. Try to adjust with the signal level control just 10V RMS at the resonant maxed output (input level actually not important now, only the output from the resonant circuit).  The reason why I ask the 10V RMS output at resonance is shown here:

http://www.allaboutcircuits.com/vol_2/chpt_6/6.html#22055.png

Now if you detune the signal gen first to a lower frequency so that the output voltage changes gradually from 10V to 7.07V RMS, you notice this frequency, ok?  It is possible you need to use a digital frequency meter to better read this lower frequency, if your dial on the generator is rather rough for finer tens or hundreds Hz details. Please tune also to an upper frequency where the output voltage also reduces gradually to 7.07 RMS, and notice this frequency too. 

Now if you substract the lower freq (say it is 5.8kHz) from the higher freq (say it is 6.2kHz), the difference is 6.2-5.8= .4kHz   The Q value is received if you divide the resonant frequency (say it was exactly 6kHz) with the difference:  Q=6/.4=15  it is much possible your toroidal coils will have a much narrower 3dB bandwidth, it means the Q values will be higher,  this is why a digital frequncy meter is a neccessity when you detune to the half power points on the resonace curve.

This way we could have a much better inside onto the two different coils, regarding their Q, resonant impedance and self capacitance. And this test setup creates a different circuit 'enviroment' for the coils, namely no nonlinear dynamic MOSFET capacitance.

If you do not understand anything wrt to this test, please ask.

rgds,  Gyula

Hi Guyla,

I don't have a frequency counter but I think I have a software that uses a computer sound card to do it. Do you think that would be good enough?

The other thing, can you tell me what doing this test will prove or disprove as I'm not even sure I understood the previous test of adding the capacitance proved.

Thanks

Luc

[skywatcher]

I think it's time to wind a new toroid. I hate winding toroids

Me too.   

Have you ever considered using cores like this one ?
http://img.alibaba.com/photo/209803177/UF_Mn_Zn_ferrite_core_magnetic.jpg

I don't think the core has to be round. It only must be closed.
So a rectangular shaped core should do the job too.
Also the coil could be anywhere and it doesn't have to cover the whole core.

Anyway, making and exchanging coils would be very easy with a separable core.

[gyulasun]

Hi Guyla,

I don't have a frequency counter but I think I have a software that uses a computer sound card to do it. Do you think that would be good enough?

The other thing, can you tell me what doing this test will prove or disprove as I'm not even sure I understood the previous test of adding the capacitance proved.

Thanks

Luc

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

[gotoluc]

Yes skywatcher,

I was thinking the same thing or cutting my toroid in half or just use the black sand I have to make my own with resin.

We shall see.

Luc

[gyulasun]

Luc,

But you have to insure the smooth mating surfaces to get the lowest airgap possible.