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Author Topic: The bifilar pancake coil at its resonant frequency  (Read 231951 times)

Offline Magluvin

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Re: The bifilar pancake coil at its resonant frequency
« Reply #885 on: April 16, 2017, 04:15:40 AM »
Ok I see you made a correction in the inductance. Here is the sim shot with the update. The res freq is in the lower right.

Mags

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Offline Bob Smith

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Re: The bifilar pancake coil at its resonant frequency
« Reply #886 on: April 16, 2017, 04:18:55 AM »

Offline Magluvin

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Re: The bifilar pancake coil at its resonant frequency
« Reply #887 on: April 16, 2017, 04:19:03 AM »
Your inductance seems very low so that 588mhz range may be correct.

Mags

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Re: The bifilar pancake coil at its resonant frequency
« Reply #887 on: April 16, 2017, 04:19:03 AM »
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Offline Magluvin

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Re: The bifilar pancake coil at its resonant frequency
« Reply #888 on: April 16, 2017, 04:31:50 AM »
I might have expected tks coil to be higher in inductance than mine with twice the no of turns where on the other end, your inductance is far below what either of us have and yours has about half the turns of mine. Its funny how inductance can be odd with diff wire sizes vs no of turns etc. Much harder to gauge by the looks of the coil compared to guessing the resistance value. Weird stuff.

Mags

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Offline tinman

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Re: The bifilar pancake coil at its resonant frequency
« Reply #889 on: April 16, 2017, 04:34:35 AM »
Ok,so finding the resonant frequency the old way,seems like quite a task.
So what i did,was to put a diode on the positive input,so as the coil is only being pulsed by the top half(forward direction) of the sine.-->Blue trace =current--yellow =voltage across coil.

I managed to get the coil to ring,and so im guessing this ringing frequency is the resonant frequency of the bifilar coil--Yes?.

Will now try the single wound coil.


Brad

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Re: The bifilar pancake coil at its resonant frequency
« Reply #889 on: April 16, 2017, 04:34:35 AM »
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Offline tinman

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Re: The bifilar pancake coil at its resonant frequency
« Reply #890 on: April 16, 2017, 05:37:49 AM »
A couple of scope shots showing voltage across bifilar coil,and voltage across pickup coil.
Second shot showing current through bifilar coil,and voltage across pickup coil.

At this frequency,the voltage across the pickup coil is in phase with voltage across bifilar coil,and not in phase with current through bifilar coil.


Brad

Offline tinman

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Re: The bifilar pancake coil at its resonant frequency
« Reply #891 on: April 16, 2017, 05:58:35 AM »
The below scope shot,is to try and  get the single wound coil to ring-in the same way i did the bifilar.
As you can see,it rings down very quickly.
Tried a frequency sweep,and this was the best result.


Easy to see the bifilar coil rings down a lot more.

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Re: The bifilar pancake coil at its resonant frequency
« Reply #891 on: April 16, 2017, 05:58:35 AM »
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Offline TinselKoala

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Re: The bifilar pancake coil at its resonant frequency
« Reply #892 on: April 16, 2017, 05:58:45 AM »
I picked the wire for this some time ago when a RS store was going down for the count. was 29 bux and got 60%off so it was a no brainer. The flatness of the copper should give more surface area proximity than round wire per turn, that was my inspiration for going flat, to increase the capacitance.  Like imagine say 1in wide copper tape or even wider. A nice increase in both capacitance and induction in a similar sized dia with a thin insulation layer between turns.

I feel good with the outcome of the capacitance and the inductance. I was also happy with the 85khz as it clearly shows a decent amount of inductance and capacitance to not only go lower than the mhz range, but i wasnt expecting below 100khz. How many ohms is the series total on your coil?

Mags

The two bifilar pancake coils measure 3.90 ohms and 661 uH and 2.19 nF,  and 3.90 ohms and 685 uH and 2.20 nF, including the connections. The matching monofilar coil I wound yesterday (bitch!) measures 3.92 ohms and 712 uH. It may have a couple more turns on it than the bifilars, from the looks of it.

Still haven't measured resonant frequency, have been busy with other things this evening.

Offline TinselKoala

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Re: The bifilar pancake coil at its resonant frequency
« Reply #893 on: April 16, 2017, 06:05:06 AM »
So,when we say!self resonant frequency!,do we simply keep raising the frequency until maximum voltage amplitude is reached across the coil?

If so,who can calculate what mine should be,using the values given in my last post?.


Brad
With 1.22 nF and 0.02 nanoH I get 1.0189 GHz. With 0.06 nH I get 588 MHz.

But I must say those inductance values are implausible, even if you mistakenly connected the coil as a hairpin bifilar instead of a Tesla series bifilar.

Here's an inductance calculator that will show you that even the meter's connecting leads will probably have more inductance than that. You are talking short PC board traces, for that low inductance.

http://www.consultrsr.net/resources/eis/induct5.htm

How are you measuring the inductance to be that low?

Your recent scopeshot of the ringing appears to show about 6.2 oscillations in 5 microseconds (2 divisions) which should be a frequency of about 1.24 MHz.

So taking the 1.22 nF capacitance and that frequency and working backwards for inductance, we should have about 13.5 microHenry. Even that seems low to me.  The bifilar coils and the monofilar coil I showed earlier each have inductance of around 700 microHenry. Are you sure we are seeing the coil ringing, and not your probe leads?

http://www.1728.org/resfreq.htm


Are those two traces at 40.5 kHz at the resonant frequency point? (oops sorry I had MHz instead of kHz)

Can you expand the trace of the single winding coil so we can read the frequency from the graticule? At first pass it looks to be a bit higher ( around 2 oscillations in one microsecond)than what you got with the ringing of the other coil ( around 6.2 oscillations in 5 microseconds).

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Re: The bifilar pancake coil at its resonant frequency
« Reply #893 on: April 16, 2017, 06:05:06 AM »
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Offline tinman

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Re: The bifilar pancake coil at its resonant frequency
« Reply #894 on: April 16, 2017, 06:09:12 AM »
OK,regarding my last post and scope shots of ringing.

Some twat forgot to include the 1 ohm CVR in the single wound coil ring test.  ::)

Below is the amended single wound coil ringing scope shot ,alongside the bifilar coil.

Looks much better now.

Offline tinman

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Re: The bifilar pancake coil at its resonant frequency
« Reply #895 on: April 16, 2017, 06:11:35 AM »
At this point in time,i have to ask--what exactly are we looking for?

Found so far--> bifilar coil has lower self resonant frequency that that of the single wound coil.

Next is voltage difference between windings for both.


Brad

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Re: The bifilar pancake coil at its resonant frequency
« Reply #895 on: April 16, 2017, 06:11:35 AM »
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Offline tinman

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Re: The bifilar pancake coil at its resonant frequency
« Reply #896 on: April 16, 2017, 07:14:22 AM »
Each of my coils have 28 turns,and here is the voltages between turns of each coil

The single wound coil has 4VRMS across it,and starts with a voltage difference of 68mV RMS between the first and second turns (closest to center of coil)
As expected,the voltage increases between turns as we get closer to the outer most two turns.
At the outer two turns,the voltage difference between turns is 244mV RMS.

The bifilar coil has 4.24VRMS across it,and once again,as expected,has an average voltage between turns of 2.12VRMS--varying from 2.08 to 2.14VRMS between turns.

So far,nothing out of the ordinary between the two,except that the bifilar has a lower resonant frequency.


Brad

Offline TinselKoala

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Re: The bifilar pancake coil at its resonant frequency
« Reply #897 on: April 16, 2017, 07:33:45 AM »
At this point I'm not sure that disconnecting the series link and measuring the capacitance between the two disconnected files is giving us the correct answer for the distributed capacitance.

At least I get different answers from measuring this way, and from solving the resonant frequency equation for capacitance using the measured resonant frequency and inductance.

Offline tinman

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Re: The bifilar pancake coil at its resonant frequency
« Reply #898 on: April 16, 2017, 07:37:28 AM »


 

Here's an inductance calculator that will show you that even the meter's connecting leads will probably have more inductance than that. 

http://www.consultrsr.net/resources/eis/induct5.htm

How are you measuring the inductance to be that low?

Your recent scopeshot of the ringing appears to show about 6.2 oscillations in 5 microseconds (2 divisions) which should be a frequency of about 1.24 MHz.

So taking the 1.22 nF capacitance and that frequency and working backwards for inductance, we should have about 13.5 microHenry. Even that seems low to me.  The bifilar coils and the monofilar coil I showed earlier each have inductance of around 700 microHenry. Are you sure we are seeing the coil ringing, and not your probe leads?

http://www.1728.org/resfreq.htm


Are those two traces at 40.5 MHz at the resonant frequency point?

Can you expand the trace of the single winding coil so we can read the frequency from the graticule? At first pass it looks to be a bit higher ( around 2 oscillations in one microsecond)than what you got with the ringing of the other coil ( around 6.2 oscillations in 5 microseconds).

The end of one winding,returns back to the center,and is connected to the beginning of the other winding.

Quote
You are talking short PC board traces, for that low inductance.

I am using my DMM to measure the inductance value,and the leads are connected to the ends of the series coil pair.

Quote
With 1.22 nF and 0.02 nanoH I get 1.0189 GHz. With 0.06 nH I get 588 MHz.

The capacitance value was taken with the two windings disconnected from one another-as Mags asked.
Once the two windings are joined together(series connected),then would not that capacitance value change?,as no capacitor has there two plates joined together,as the bifilar coil dose.


Added
Ok,went and checked capacitance again.
Bifilar coil has .66nF capacitance,and get this, -(yes,minuse) 50uF when the two windings are hooked in series. So guess we cannot measure capacitance when they are linked together.
So what dose that do or say about our capacitance value when the coils are hooked in series,and we are carrying out tests based around the two windings open circuit capacitance value?.

Inductance still reads .06mH
If i join the two DMM leads together,i get .00--nothing.


Brad

Offline tinman

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Re: The bifilar pancake coil at its resonant frequency
« Reply #899 on: April 16, 2017, 07:40:15 AM »
At this point I'm not sure that disconnecting the series link and measuring the capacitance between the two disconnected files is giving us the correct answer for the distributed capacitance.

At least I get different answers from measuring this way, and from solving the resonant frequency equation for capacitance using the measured resonant frequency and inductance.

Ah,see you were posting as i was typing.
Seems we both think this is the wrong way to go about it.

Brad

 

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