### Author Topic: Low frequency coil arrangement  (Read 295 times)

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#### LaserTazerPhaser

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##### Low frequency coil arrangement
« on: September 22, 2019, 03:40:48 am »
Is there a particular arrangement/winding method for aircore coils yielding very low LC frequency =<2khz with the L being =>1000uH without using any external capacitors to lower the frequency?
« Last Edit: September 22, 2019, 03:43:07 am by LaserTazerPhaser »

#### magic

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##### Re: Low frequency coil arrangement
« Reply #1 on: September 22, 2019, 07:49:48 am »
In general, increasing diameter and decreasing length both have a double effect of increasing inductance and parasitic capacitance.
I have not a faintest clue how far you can take it in practice.

edit
You know the drill: 1/f=2π·sqrt(LC)
To get below 2kHz, you need L·C of at least a few nano if my math is right. That may be hard since capacitance of any reasonable coil is probably in the pico- range.
« Last Edit: September 22, 2019, 08:09:35 am by magic »

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##### Re: Low frequency coil arrangement
« Reply #2 on: September 22, 2019, 08:08:53 am »
Use coil diameter equals to the coil length, it gives best coil parameters

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#### soldar

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##### Re: Low frequency coil arrangement
« Reply #3 on: September 22, 2019, 08:09:22 am »
I am not sure I understand what you are asking but a rough calculation tells me a coil with 4 cm external diameter, 6 cm length and 800 turns has about 13000 uH and coupled with a 1 uF capacitor it would resonate at 1.4 KHz.

At such low frequency skin effect should not be a consideration and I cannot think any other reason you cannot just wind it. Except that I have not calculated if you can physically construct such many turns in that space.
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#### Kleinstein

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##### Re: Low frequency coil arrangement
« Reply #4 on: September 22, 2019, 09:00:20 am »
Normally one want a high self resonance frequency, thus minimize the parasitic capacitance.
To get self resonance at only 2 kHz would need a lot of parasitic capacitance. More like using material normally used to build a capacitor, like thin foils to make a 1 µF cap with some mH inductance.  It may work if you think large, that is 10s of meters in diameter and using lots of wires in the "wrong" ordering. That would than be more like inductance in the H range and some nF of parasitic capacitance.

I would go for an external capacitor.

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#### Zero999

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##### Re: Low frequency coil arrangement
« Reply #5 on: September 22, 2019, 10:08:44 am »
Winding the coil in flat spirals, sandwiched together will most likely give a high capacitance, why not add an external capacitor? Relying on the parasitic capacitance means the inductance and therefore effective series resistance will also need to be higher, thus a lower Q, than using a separate inductor and capacitor.

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#### LaserTazerPhaser

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##### Re: Low frequency coil arrangement
« Reply #6 on: September 22, 2019, 09:34:13 pm »
Winding the coil in flat spirals, sandwiched together will most likely give a high capacitance, why not add an external capacitor? Relying on the parasitic capacitance means the inductance and therefore effective series resistance will also need to be higher, thus a lower Q, than using a separate inductor and capacitor.
External capacitor is prone to damage from high voltage and obtaining just the right capacitance won't be the same over a duration however the a coils wire diameter does not vary its capacitance unlike an external capacitor.

Wire diameter does have specific capacity but weres the table depicting capacity per length of wire size?
« Last Edit: September 22, 2019, 09:36:42 pm by LaserTazerPhaser »

#### MagicSmoker

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##### Re: Low frequency coil arrangement
« Reply #7 on: September 22, 2019, 09:53:16 pm »
...External capacitor is prone to damage from high voltage...

And wire insulation isn't just as prone to damage from high voltage?

...and obtaining just the right capacitance won't be the same over a duration however the a coils wire diameter does not vary its capacitance unlike an external capacitor.

Whut? Are you saying that it would be harder to tune a resonant LC circuit using an external capacitor than relying on the stray capacitance of the inductor itself?

Wire diameter does have specific capacity but weres the table depicting capacity per length of wire size?

That table - were it to even exist - would need a near infinite number of entries because capacitance depends on area *and* separation distance *and* permittivity.

#### LaserTazerPhaser

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##### Re: Low frequency coil arrangement
« Reply #8 on: September 22, 2019, 10:13:14 pm »
And wire insulation isn't just as prone to damage from high voltage?

Tuning a coil without a cap is more reliable any coil insulation can handle =>400v and doesn't have issues with ripple current - a capacitor is prone to destruction from conducting large currents regardless of the type it is. Magnet wire is well over 1kv in insulation.

Whut? Are you saying that it would be harder to tune a resonant LC circuit using an external capacitor than relying on the stray capacitance of the inductor itself?
Its by far easier to simply wire the coil properly to attain the proper LC frequency and not use troublesome capacitors which would incur damage and degrade rapidly from transients, heat and conducting large currents.

Use coil diameter equals to the coil length, it gives best coil parameters
What does keeping diameter and wire length acheive for increased capacitance?
« Last Edit: September 22, 2019, 11:51:52 pm by LaserTazerPhaser »

#### magic

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##### Re: Low frequency coil arrangement
« Reply #9 on: September 23, 2019, 07:18:17 am »
This gives a formula for capacitance of two parallel wires, supposedly from some book on power electronics. Probably capacitance isn't affected much by the wires being bent, as long as they are parallel.

There is also a bunch of air core inductor calculators on the Internet, have fun.

You will probably find you need something like 3 tightly spaced turns and 10m diameter.

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