TX coil with Polypropylene spacer

[nick_f]

Two Teflon Wire Coils

Hi Tinkerer,

I got my teflon wire a few days ago and today I built two coils.
The first one is built with AWG22, silver multistrand wire, 19 strains. The coil has 21 turns. After the polypropylene spiral added (with no shielding) the inductance is 311uH (my LCR meter is able to measure capacitors with a precision < 1%, I hope it does about the same with the inductors). The resistance of this coil is 1.1ohm.
The inner diameter of the coil with the PP added is about 26cm, and the outer diameter is 27.8cm.
The second coil is based on your idea of using a spacer around the wire, to lower the interwinding capacitance. I used two twisted wires, AWG30 (or AWG26, I am not sure). Because of the twisted wires wound together, there is some air in betwen turns. The second coil has 301uH, 1.5ohm and also 21 turns.
Tomorrow I will measure the self-resonance of the coils at work and publish it in here. In about two weeks, I will have the Scotch24 shielding and I will measure them again, to calculate the other parameters, according to bbsailor instructions.


Regards,
Nicolae

[nick_f]

Note how the signal goes positive for the silver coin and how it goes negative for the ferrite.
Tinkerer

How about for iron? If the deviation is negative for iron, does it mean the circuit is able to do discrimination?

Regards,
Nicolae

[Tinkerer]

How about for iron? If the deviation is negative for iron, does it mean the circuit is able to do discrimination?

Regards,
Nicolae

Definitely.
But remember this only works with Induction Balanced coils.
It is the coil that does the discrimination. You just need to look at the right place in time for the signal.

Tinkerer

[Tinkerer]

Hi Tinkerer,

I got my teflon wire a few days ago and today I built two coils.
The first one is built with AWG22, silver multistrand wire, 19 strains. The coil has 21 turns. After the polypropylene spiral added (with no shielding) the inductance is 311uH (my LCR meter is able to measure capacitors with a precision < 1%, I hope it does about the same with the inductors). The resistance of this coil is 1.1ohm.
The inner diameter of the coil with the PP added is about 26cm, and the outer diameter is 27.8cm.
The second coil is based on your idea of using a spacer around the wire, to lower the interwinding capacitance. I used two twisted wires, AWG30 (or AWG26, I am not sure). Because of the twisted wires wound together, there is some air in betwen turns. The second coil has 301uH, 1.5ohm and also 21 turns.
Tomorrow I will measure the self-resonance of the coils at work and publish it in here. In about two weeks, I will have the Scotch24 shielding and I will measure them again, to calculate the other parameters, according to bbsailor instructions.


Regards,
Nicolae

Nice coils, good job.
I like to have about 5 to 8mm spacer between the coil winding and the shield.
You could wind the string around the coil a few times before adding the spiral wrap.

All the best

Tinkerer

[nick_f]

Self-resonance of coils

Hi Tinkerer,
I managed to measure the self-resonance frequency of my coils. I took some screenshots to show people how the resonance looks on a spectrum analyzer. You can see the peak level (which happens at resonance) is at 1.1974MHz for the orange coil and at 1.2147MHz for the twisted wire coil.
The self-resonance frequency is very close for the two coils. It looks like the twisted wire didn't improve at all the interwinding capacitance.
Based on Gary's calculator at chemelec, I found out the interwinding capacitance for the orange inductor (311uH). It is 56.8pF.
For the White-Black twisted coil (301uH), the interwinding capacitance is 57pF.

Funny thing is, it took me a while to measure the resonance. Initially I tried to use an oscilloscope and I connected the 1pF capacitor to the output of the signal generator (before the 1MOhm resistor, instead of after) and I couldn't identify the resonance frequency no matter how hard I looked. I also used an RF Voltmeter and I identified it eventually, but I knew something is wrong in the setup. After correcting it, all the above methods clearly showed the peak in signal.


Regards,
Nicolae

[nick_f]

Significant differences between almost similar coils

Hi All,

After I measured the resonance frequencies of the two teflon coils that I build (described previously), I decided to see how well they perform in HH circuit. I was expecting almost identical response of the pulse at TP4.
I couldn't have been more wrong. The coil built with twisted teflon wire 2x(AWG24 or AWG25) silver plated is much better than the one built with AWG22 silver plated wire.
You can see the results in the images below. You might be surprised to see I was using a coax cable with 70pF/1.45m (that's 4.7 feet -> 14.7pF/foot). Well, this great cable was lying in my junk box since almost ten years ago. It is white and I bought it from a cheap shop (the type of $1 stores in Sydney). It was pre-made with connectors, to be used for tv or VCR's. It has markings 3C-2V 75 ohm coaxial cable.

I added some sticky aluminium foil shielding to the twisted cable. The shield covers about 2/3 of the section of the coil and the shield capacitance is 160pF. For an increase of 160pF in capacitance, I had to lower the coil resistance from 2.66kohm to 1.90kohm, to prevent the self-oscillation of the circuit.

In conclusion, I think we can use silver plated wire, provided is not AWG22, multistranded with 19 strands. By using twised cable, I obtained much better results.

In order to identify what AWG cables I was actually using (rather than believing the seller description), I measured the insulated diameter of the twisted wire: 0.8mm and uninsulated diameter about 0.5mm. Surprisingly, that would make it AWG24 or AWG25 - and I used two in parallel). The seller described it as AWG30. It is also multistranded with 7 strands each.

For the cable described as AWG22, the outer diameter is 1.25mm and the uninsulated diameter is about 0.75mm. That would make it AWG20 or AWG21.


Regards,
Nicolae

[Tinkerer]

Hi All,

After I measured the resonance frequencies of the two teflon coils that I build (described previously), I decided to see how well they perform in HH circuit. I was expecting almost identical response of the pulse at TP4.
I couldn't have been more wrong. The coil built with twisted teflon wire 2x(AWG24 or AWG25) silver plated is much better than the one built with AWG22 silver plated wire.
You can see the results in the images below. You might be surprised to see I was using a coax cable with 70pF/1.45m (that's 4.7 feet -> 14.7pF/foot). Well, this great cable was lying in my junk box since almost ten years ago. It is white and I bought it from a cheap shop (the type of $1 stores in Sydney). It was pre-made with connectors, to be used for tv or VCR's. It has markings 3C-2V 75 ohm coaxial cable.

I added some sticky aluminium foil shielding to the twisted cable. The shield covers about 2/3 of the section of the coil and the shield capacitance is 160pF. For an increase of 160pF in capacitance, I had to lower the coil resistance from 2.66kohm to 1.90kohm, to prevent the self-oscillation of the circuit.

In conclusion, I think we can use silver plated wire, provided is not AWG22, multistranded with 19 strands. By using twised cable, I obtained much better results.

In order to identify what AWG cables I was actually using (rather than believing the seller description), I measured the insulated diameter of the twisted wire: 0.8mm and uninsulated diameter about 0.5mm. Surprisingly, that would make it AWG24 or AWG25 - and I used two in parallel). The seller described it as AWG30. It is also multistranded with 7 strands each.

For the cable described as AWG22, the outer diameter is 1.25mm and the uninsulated diameter is about 0.75mm. That would make it AWG20 or AWG21.


Regards,
Nicolae

Hi Nicolae,

could you give us the total DC resistance, including cable, Mosfet and coil, as well as TX pulse length and Voltage?

Tinkerer

[nick_f]

Hi Tinkerer,

Could you please tell me how can I measure the total DC resistance?

Following bbsailor's advice on thread http://www.geotech1.com/forums/showt...ed=1#post89195 now I understand why one of the coils is very bad compared with the other one.
The coil I made with thick multistrand wire is bad because the wire itself can be detected by a metal detector. The highly conductive silver plated strains act like a solid wire and it generates eddy currents through it. For this reason, it is much better to build the coil using more thin insulated cables in parallel, than only one thick wire. And it looks like silver plating indeed has a negative impact. I checked a few different multistranded wires that I have around (none silver plated) and they are thicker than my teflon wire and still they are not detected.

Regards,
Nicolae

[Tinkerer]

Hi Tinkerer,

Could you please tell me how can I measure the total DC resistance?

Following bbsailor's advice on thread http://www.geotech1.com/forums/showt...ed=1#post89195 now I understand why one of the coils is very bad compared with the other one.
The coil I made with thick multistrand wire is bad because the wire itself can be detected by a metal detector. The highly conductive silver plated strains act like a solid wire and it generates eddy currents through it. For this reason, it is much better to build the coil using more thin insulated cables in parallel, than only one thick wire. And it looks like silver plating indeed has a negative impact. I checked a few different multistranded wires that I have around (none silver plated) and they are thicker than my teflon wire and still they are not detected.

Regards,
Nicolae

measure the DC resistance of the coil with its cable with the Ohm meter, then add the ON resistance of your Mosfet, taken from the data sheet.

Thicker wire has more inter wire capacitance than thinner wire. Litz wire has individually insulated strands so it does not behave like multi stranded wire.
You can use several AWG 26 to 28 magnet wires twisted together to make your own Litz wire.

Tinkerer

[bbsailor]

Nicolae said:
"I added some sticky aluminium foil shielding to the twisted cable. The shield covers about 2/3 of the section of the coil and the shield capacitance is 160pF. For an increase of 160pF in capacitance, I had to lower the coil resistance from 2.66kohm to 1.90kohm, to prevent the self-oscillation of the circuit".

If you want to get a better understanding about the effect of adding a coil shield to the distributed capacitance of the coil, try this.

1. Measure the coil resonance at the end of your coax with an unshielded coil.

2. Add a shield around the whole coil, leaving a small gap and measure the coil-to-shield capacitance (with shield not connected to ground).

3. Measure the coil's self resonance with the shield added and grounded top the coax.

4. Calculate the total capacitance using an LC resonance calculator.

5. Compare the total capacitance in step 1 and step 3.

What you will find is that only a fraction of the measured coil-to-shield capacitance is actually imposed on the coil's resonance lowering.

My research shows that about 20% to 25% of the coil-to-shield capacitance is imposed on lowering the coil self resonance when using Scotch24.

When you make you coil, try this with a solid shield and Scotch24 and post your results (coil self resonance; calculated capacitance with solid shield and Scotch 24; and coil-to-shield measurements) so I can see if my findings can be easily replicated.

When keeping notes about coils, collect data about the following:

1. Coil Inductance
2. Coil Resistance (calculated by using AWG/SWG wire table for total coil
length). This is a more accurate way to obtain coil resistance than
with an ohm meter.
3. Total series resistance which includes: coil wire resistance, MOSFET
on-resistance, coax resistance and any series resistor in the TX circuit
to limit current. This data is necessary to calculate and understand
coil current rise TC.
4. Final shielded coil self resonance measured at the end of the coax.
5. Coil-to-shield capacitance
6. Pulse frequency (PPS)
7. Pulse Width (defines the percent of duty cycle and most of the battery
current draw.
8. Adjusted value of the damping resistor (Rd) which reflects interaction of
items 1 to 7 above.

Use a spreadsheet program to keep data about multiple coils.

Use the free on-line program MiscEL to see graphs of the current rise and compare this to what you measure. Use the value of Rd to graph the current discharge speed. Include the parallel value of Rin (R12 in the Hammerhead) with Rd.

The most important thing to discover is the relationship of making changes in coil design and to accurately anticipate the direction of the results. This means that you need to develop a good mental model of how all these variables interact. Once you start doing this and keeping good notes about your results, things will start falling into place. Then ask yourself: "what do I know better now than what I thought I knew before"?

Experimenting and understanding the results is where the real learning occurs.

It looks like you are having fun too!!!

bbsailor

[nick_f]

Hi bbsailor,

I was sort of trying to identify exactly what you told me, the influence that the extra capacity has to my pulse and how it influences the selfresonant frequency. I am surprised by what you are telling me, that the shield capacitance doesn't change the frequency as expected. It may be because the coil consist of a complex distributed ansamble of tiny capacitors, inductors and resistors and the shield affects the most only the closest layer of wire to it.
The indications you gave me about maintaining a spreadsheet are indeed very valuable, but it start to smell like work! But if I put this amount of time into it, I could go all the way with it. And it would be useful to all of us, to compare the results.

Best regards,
Nicolae

[EEE]

The goal was to produce a TX coil and read a sample with a delay of 3uS after the TX pulse switch OFF.

Below is a scope pix with the setting at 1uS. It shows the TX switch OFF and the output of the preamp.

TX pulse is 41uS and 12V, with a total DC resistance of about 2.8 Ohm so it is about 1.28A coil current at switch OFF

Tinkerer

Hi Tinkerer,

I just wonder what circuit you used to test the coil, HH?. Were the Tx and Rx coils in DD config? Did you add a bucking coil?

Thanks