The bifilar pancake coil at its resonant frequency

[TinselKoala]

OK, here is an example.

One of my TBF pancake coils measures 685 microHenry using my ProsKit inductance meter. When I disconnect the series connection and measure the two files for capacitance I get 2.75 nanoFarads. I have measured the resonant frequency in three different ways (single pulse ringdown, continuous pulsation ringing, and maximum voltage rise during frequency sweep) and get 242.7  kHz each way (within a few 100 Hz). Working the resonant frequency calculation backwards to solve for capacitance using the resonant frequency and the inductance, I get 0.628 nF (627.78 picoFarads) which actually seems more reasonable than the nearly 3 nF value.

The other TBF pancake coil measures 661 uH and the capacitance 2.19 nF. Resonant frequency is 273.2 kHz and solving the equation for capacitance I get 513.43 pF.

Both coils give huge voltage amplification on the resonant ringing. I pulse with a depleted 9V battery and get around 400 v p-p ringing oscillations.

[TinselKoala]

For the corresponding monofilar coil I wound yesterday, the measured inductance is 712 uH and the resonant frequency is 1.374 MHz. Solving the equation for capacitance I get 18.8 pF, which is right about the capacitance of my scope probe.

It also gives a good resonant voltage rise of a bit over 300 V from the 9v pulse.

[TinselKoala]

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

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

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

Ah-hah! The first time you said 0.06 nH.... nanoHenry! So when we worked the equation or did the sim we got the 588 MHz frequency.

Now, with 0.06 mH, or 60 microHenry--- that's a lot more plausible and I believe that a DMM could measure that value, whereas 0.06 nH is not really believable.

So, working the equation again using your frequency (my rough estimate!) of 1.24 MHz from the ringdown, and 60 uH, I get a capacitance of 274.5 pF which is entirely plausible for your coil. (plus your probe of around 20 pF) Call it 250 or 255 pF.

[tinman]

Ok,so what would happen if the winding capacitance was increased?
Would our resonant frequency come down in value?

Thought experiment.

If i pull apart say a 1uF cap,and made a transformer from the cap plates,instead of using copper wire--what would be the outcome of that transformer ?

Brad

[TinselKoala]

Ok,so what would happen if the winding capacitance was increased?
Would our resonant frequency come down in value?

As capacitance and/or inductance goes up, resonant frequency goes down. Bigger buckets take longer to fill and empty.

Thought experiment.

If i pull apart say a 1uF cap,and made a transformer from the cap plates,instead of using copper wire--what would be the outcome of that transformer ?

Brad

I have no idea.


I think the capacitance meters are being fooled by the relatively large inductance of the coils when we measure them that way. I think the meters basically work by creating a LC tank circuit and measuring the resonant frequency of the LC tank, then calculating either inductance or capacitance based on that. SO if the "capacitor" has substantial inductance, the resonant frequency will be lower than expected and so the calculated capacitance value will be higher...and wrong.