Measured C and calculated L of thick and thin insulated coil winding wires ... questions

[Polymer]

I have measured two different coils @20 windings each.
One has "normal" insulation the other a very thick insulation.

I am somewhat puzzled by the results.
The measurements were made via resonant frequency.


Thin insulation:
650kHz with 150pF capacitor in measuring circuit.
1150kHz with no capacitor in measuring circuit.
The calculated distributed capacitance is 70pF
The calculated inductance is 272uH.

Thick insulation:
878kHz with 150pF cap.
1815kHz no cap.
The calculated distributed capacitance is 45pF
The calculated inductance is 168uH.

Maybe I've done something very wrong here. My interpretation is that to get back up to the "nominal"
inductance of 272uH I would have to put on MORE windings of the thick insulated wire, which in
turn would raise the self capacitance again ... this appears futile ...
Having read a lot here about winding spacing being the bee's knees for faster coils, this has me scratching my head.

So, what am I not getting here? Thanks!

[green]

I have measured two different coils @20 windings each.
One has "normal" insulation the other a very thick insulation.

I am somewhat puzzled by the results.
The measurements were made via resonant frequency.


Thin insulation:
650kHz with 150pF capacitor in measuring circuit.
1150kHz with no capacitor in measuring circuit.
The calculated distributed capacitance is 70pF
The calculated inductance is 272uH.

Thick insulation:
878kHz with 150pF cap.
1815kHz no cap.
The calculated distributed capacitance is 45pF
The calculated inductance is 168uH.

Maybe I've done something very wrong here. My interpretation is that to get back up to the "nominal"
inductance of 272uH I would have to put on MORE windings of the thick insulated wire, which in
turn would raise the self capacitance again ... this appears futile ...
Having read a lot here about winding spacing being the bee's knees for faster coils, this has me scratching my head.

So, what am I not getting here? Thanks!

Good to see someone try the method to get coil inductance, coil capacitance and SRF. I use a 10nf cog ceramic or 100nf polypropylene instead of 150pf across coil to calculate inductance. Minimize the effect of distributed capacitance. Do you have an inductance meter to compare with calculations? I don't so I have been using the calculations. The inductance calculation for the 10nf and 100nf capacitor are within 1% using capacitance measurements with my multimeter. Have been told I should be using an inductance meter instead of this method. Interested if we can prove the method good or bad.

What is the diameter of your coils?

[waltr]

I have measured two different coils @20 windings each.
One has "normal" insulation the other a very thick insulation.


Maybe I've done something very wrong here. My interpretation is that to get back up to the "nominal"
inductance of 272uH I would have to put on MORE windings of the thick insulated wire, which in
turn would raise the self capacitance again ... this appears futile ...
Having read a lot here about winding spacing being the bee's knees for faster coils, this has me scratching my head.

So, what am I not getting here? Thanks!

Sounds correct.
Play with the Coil calculator available at the top of this forum.
"Wire Thickness" is the parameter you changed. Change this in the coil calc and see if it predicts what you measured.
"Wire thickness" is that total diameter with insulation. When the wires in the coil are spaced apart the 'C' decreases but the 'L' also decreases (the magnet field from each wire is spaced apart so total is not as strong). These interactions are part of the engineering design compromises and also shows that there is no free lunch.

[bbsailor]

Sounds correct.
Play with the Coil calculator available at the top of this forum.
"Wire Thickness" is the parameter you changed. Change this in the coil calc and see if it predicts what you measured.
"Wire thickness" is that total diameter with insulation. When the wires in the coil are spaced apart the 'C' decreases but the 'L' also decreases (the magnet field from each wire is spaced apart so total is not as strong). These interactions are part of the engineering design compromises and also shows that there is no free lunch.

Another variable is the dielectric constant of the wire insulation with Teflon (PTFE) being near 2 while PVC is between 4 and 6. Another trade-off is the amount of area that the coil bundle represents to the shield around the coil bundle.

Then, when we do get the delays below 10uS the wire itself becomes a source of holding eddy currents beyond your delay. Using tinned stranded wire with Teflon insulation makes a good fast coil as the tinned strands have a slight resistance between the strands and hold the eddy currents less than strands with silver coating. For a high frequency PPS rate, a Teflon coated single strand AWG 30 makes a good coil to operate about 7uS. The reason why I like the Scotch24 wire mesh as a shield is because it has less surface area than a solid shield materiel. The key to using it is to use a single Scotch24 layer and cut it wide enough so it makes one complete turn around the cross section of the coil bundle without touching to not have a shorted conductivity around the cross section of the shield around the coil bundle. Put a gap in the coil shield where the coil end turns exit the coil. Compress this Scotch24 shield to the coil with a layer of tape or a layer PE spiral wrap.

Coil design is a true trade-off between many variables, especially at lower delays.

Joseph J. Rogowski

[Polymer]

Good to see someone try the method to get coil inductance, coil capacitance and SRF. I use a 10nf cog ceramic or 100nf polypropylene instead of 150pf across coil to calculate inductance. Minimize the effect of distributed capacitance. Do you have an inductance meter to compare with calculations? I don't so I have been using the calculations. The inductance calculation for the 10nf and 100nf capacitor are within 1% using capacitance measurements with my multimeter. Have been told I should be using an inductance meter instead of this method. Interested if we can prove the method good or bad.

What is the diameter of your coils?

Coil diameter at mid winding bulk is about 9 inches. Simple jumble wound no shield no frills.

I am using this circuit for testing, post #5 and the excel calculator:
https://www.geotech1.com/forums/show...947#post251947

I cannot go beyond 2200pF in the circuit as parallel capacitor, the oscillations don't work anymore - no amplitude.
How did you manage to get to 10nF or even 100nF, if you are using the same test circuit?

Tested the calculations with different relative capacitance in the lead wires to coil.
Lead wire in air well apart: 45cm length, calculated L 275uH, distributed capacitance 69.0pF
Lead wire as twisted pair: 40cm length, 30 twists, calculated L 275uH, distributed capacitance 82.5pF

I have read that twisted wire has around 1pF per Inch, so this measurement is OK to me and close enough.

What I do not know is the validity of the absolute values for the coil, as I do not have an inductance meter either.

I have looked at quite a few LCR meters and most measure at 100Hz or 1kHz, some higher and up to 750kHz.
Many do not have any specs. as to measurement frequency at all. Don't know what's really important here yet.

Having been told to use an inductance meter is nice, but which one especially for MD coils and why?

Have measured with different capacitors in parallel from 150pF to 3300pF, 2.5% and the calculated inductance is 270 to 280uH.
I'd say that's good, if the absolute value is correct. I think it is difficult to make really exact measurements as I've noticed the values
on the oscilloscope change on a day to day basis -humidity changes- or how close I am to the coil.

[Polymer]

Sounds correct.
Play with the Coil calculator available at the top of this forum.
"Wire Thickness" is the parameter you changed. Change this in the coil calc and see if it predicts what you measured.
"Wire thickness" is that total diameter with insulation. When the wires in the coil are spaced apart the 'C' decreases but the 'L' also decreases (the magnet field from each wire is spaced apart so total is not as strong). These interactions are part of the engineering design compromises and also shows that there is no free lunch.

I will see if I can find the calculator you mentioned to cross-check.
Finding specific information here often seems more difficult than finding treasures by the sea ...
Thanks for confirming my reasoning ... but ... what advantage(s) do complex wound coils actually have then, if any?

[waltr]

I will see if I can find the calculator you mentioned to cross-check.
Finding specific information here often seems more difficult than finding treasures by the sea ...
Thanks for confirming my reasoning ... but ... what advantage(s) do complex wound coils actually have then, if any?

The calculator is a Sticky as the first thread of this forum (coils).

Wish I could help with measuring coil parameter, LRC meter.
I use an EIS (Electro Impedance Spectroscopy) method at work. This applies a controlled AC current and measures the potential (Voltage) verse current at many single frequencies. The Bode plot (Impedance & Phase) is then fitted to inductance, resistance and capacitance values.

[green]

Coil diameter at mid winding bulk is about 9 inches. Simple jumble wound no shield no frills.

I am using this circuit for testing, post #5 and the excel calculator:
https://www.geotech1.com/forums/show...947#post251947

I cannot go beyond 2200pF in the circuit as parallel capacitor, the oscillations don't work anymore - no amplitude.
How did you manage to get to 10nF or even 100nF, if you are using the same test circuit?

Tested the calculations with different relative capacitance in the lead wires to coil.
Lead wire in air well apart: 45cm length, calculated L 275uH, distributed capacitance 69.0pF
Lead wire as twisted pair: 40cm length, 30 twists, calculated L 275uH, distributed capacitance 82.5pF

I have read that twisted wire has around 1pF per Inch, so this measurement is OK to me and close enough.

What I do not know is the validity of the absolute values for the coil, as I do not have an inductance meter either.

I have looked at quite a few LCR meters and most measure at 100Hz or 1kHz, some higher and up to 750kHz.
Many do not have any specs. as to measurement frequency at all. Don't know what's really important here yet.

Having been told to use an inductance meter is nice, but which one especially for MD coils and why?

Have measured with different capacitors in parallel from 150pF to 3300pF, 2.5% and the calculated inductance is 270 to 280uH.
I'd say that's good, if the absolute value is correct. I think it is difficult to make really exact measurements as I've noticed the values
on the oscilloscope change on a day to day basis -humidity changes- or how close I am to the coil.

https://www.geotech1.com/forums/show...090#post256090 Explains how I do it. Looks like were not exciting coil the same way. I did notice the capacitor calculations are labeled wrong, off by 1 million, should be pf not uf.

[green]

No one trying using a PI to excite resonance? I've been looking at amplifier out on my bench circuit. Tried clipping scope probe to ground lead to make a loop to pickup coil resonance. Probe laying on test coil. Don't need to open PI circuit to measure amplifier out. Maybe someone with a good inductance meter could compare with my method.

[green]

Hi Polymer, Wondering if you could do an experiment for me. Connect scope probe to ground lead to make a loop. Set scope amplitude to maybe 5mV/div. Lay scope probe on one of your coils(thin insulation). Position test coil near operating PI coil and read resonant frequency(SRF). Connect a capacitor across coil, something from 10 to 100nf(measure capacitance with a multimeter if you have one before connecting). Position test coil near operating coil and read resonant frequency. Calculate inductance. Repeat test with thick insulation coil.

[Polymer]

Hi Polymer, Wondering if you could do an experiment for me. Connect scope probe to ground lead to make a loop. Set scope amplitude to maybe 5mV/div. Lay scope probe on one of your coils(thin insulation). Position test coil near operating PI coil and read resonant frequency(SRF). Connect a capacitor across coil, something from 10 to 100nf(measure capacitance with a multimeter if you have one before connecting). Position test coil near operating coil and read resonant frequency. Calculate inductance. Repeat test with thick insulation coil.

Sooner done than asked

Grabbed my MPP E and excited the the blue and yellow coil with it. Thanks for the easy-to-follow description.

Amazing ... I get the same results with your method, within reason - with 10nf & 100nF - capacitors too.
This confirms the method I used prior. Good to know.
Anything within a few percent is great without having to get extra gear.

[green]

Sooner done than asked

Grabbed my MPP E and excited the the blue and yellow coil with it. Thanks for the easy-to-follow description.

Amazing ... I get the same results with your method, within reason - with 10nf & 100nF - capacitors too.
This confirms the method I used prior. Good to know.
Anything within a few percent is great without having to get extra gear.

Thanks, You got 272uH for thin and 168uH for thick insulation with 150pf capacitor. Is that what you got with 10 and 100nf capacitors. Wasn't expecting that much difference with insulation thickness. What is the total wire diameter for the thick and thin insulation? Was SRF the same with both methods?

[dbanner]

Your method described above, I will have a try.

[Polymer]

Thanks, You got 272uH for thin and 168uH for thick insulation with 150pf capacitor. Is that what you got with 10 and 100nf capacitors. Wasn't expecting that much difference with insulation thickness. What is the total wire diameter for the thick and thin insulation? Was SRF the same with both methods?

Measured again today. SRF measurements in the MHz realm are apparently very sensitive to all kinds of things. Hope the info helps..

Test coil Yellow excited with PI (your method):

2300kHz (no extra capacitor)
358kHz (w. 1000pF capacitor)
Calculated inductance 192uH, distributed capacitance 25pF


Test coil Blue excited with PI:
1270 kHz (no extra capacitor)
289 kHz (w. 1000pF capacitor)
Calculated inductance 287uH, distributed capacitance 55pF


Test coil Yellow in parallel resonance test circuit:
1829kHz (no extra capacitor)
373kHz (w. 1000pF capacitor)
Calculated inductance 175uH, distributed capacitance 43pF


Test coil Blue in parallel resonance test circuit:
1154kHz (no extra capacitor)
291kHz (w. 1000pF capacitor)
Calculated inductance 280uH, distributed capacitance 68pF


I assume the parallel resonant method has some extra ?c? due to stripboard, leads, etc..


Wire dimensions:

Yellow ? conductor diameter 0.5mm solid CuAg, outer diameter cable 2,7mm
Blue ? conductor diameter 0.6mm CuSn 7 strands, outer diameter cable 1,0mm.

[green]

I assume the parallel resonant method has some extra ?c? due to stripboard, leads, etc..


Wire dimensions:

Yellow ? conductor diameter 0.5mm solid CuAg, outer diameter cable 2,7mm
Blue ? conductor diameter 0.6mm CuSn 7 strands, outer diameter cable 1,0mm.

Thanks, Wanted to try the coil calculator. You posted about 9inch mean diameter. Calculator asks for inner radius(diameter). If you can, what is the inner diameter?