Inductance Reading ?

[swmomark]

I am building a new coil and I wasnt sure if the inductance reading on the new coil will change, depending on how tight the coil is wound. Thanks, Mark

[Tinkerer]

I am building a new coil and I wasnt sure if the inductance reading on the new coil will change, depending on how tight the coil is wound. Thanks, Mark

Hi Mark,

the inductance my vary by 10% if the coil is tightly wound or rather loose. The coil inter winding capacitance will increase too.

Tinkerer

[swmomark]

Thanks for the info

Appreciate the help.

[simonbaker]

I am building a new coil and I wasnt sure if the inductance reading on the new coil will change, depending on how tight the coil is wound. Thanks, Mark

I agree, it will go up as you tightly bind the wires together.

-SB

[Roughwater]

Coil Inductance

swmomark,

I agree that it will generally increase also as the coil is tightened. However, when building my first coil the inductance actually decreased after shielding my coil. But I believe it was because I wrapped the shielding around the coil without first wrapping the coil with spiral wrap or something similar and the shielding affected the coils inductance.

Terry

[wyndham]

Should there be some space between the windings of the coil such as an extra layer of insulating material or is it a better design to keep the coil as tight without extra insulation. Both cases the coil is tightly wrapped.
I'm still on the sideline learning all i can, Wyndham

[Roughwater]

Wyndham,

A very good question. One I wish I could answer but I cannot and hope Reg will come along and answer this one as I am curious as well.


I hope Reg will also answer my question below whitch is similar to yours:

I do know that it has been recommended from those with more knowledge than me to use teflon coated wire to build a coil. The teflon type insulated wire would of course put more insulation between the windings compared to say a coil you find in a motor which I believe has enamel paint as the insulator. I have read that capacitance has an effect on coild building and the tighter the coil is wound the better? So would not the enamaled wire ideally wind tigher and have less capacitance?
Either way, I do know the one coil I built using Teflon coated wire seemed to have worked quite well on the TDI.

Terry

[porkluvr]

I have read that capacitance has an effect on coild building and the tighter the coil is wound the better? So would not the enamaled wire ideally wind tigher and have less capacitance?
Terry

Coil capacitance is largely the result of interwinding capacitance which increases with proximity of the wires and with increasing dielectric constant of the insulation material. (A vacuum has D.C. of one, and air is only a tiny bit higher. The D.C. numbers are calculated on a linear scale).

There are different types of enamel, one of them being polyurethane which has a dielectric constant of 3.4. There are different compositions of polyurethane wire enamel, so this is only one example.

Comparing polyurethane (D.C. 3.4) with teflon having a dielectric constant of 2.1, you can see that if two coils were insulated with these two different materials of the same thickness, the teflon coil would have less capacitance.

On top of that, most teflon insulation on hook-up wire will generally be substantially thicker than the enamel insulation on magnet wire. You already knew that, but there you can see that we have two aspects, both contributing to less capacitance for the teflon wire coil.

The relative proportions of the two property changes is a complex thing to calculate - especially if the coil is scramble wound.

One way around some of the complex interaction is to make a planar, or disk shaped coil. That way each coil has major capacitance to its adjacent windings, and only minor capacitance to other windings. It's pretty easy to understand the generalities of coil winding, but when you want to quantify the inductance and capacitance changes that's when things get hairy.

Sorry, but I don't have the schooling to be able to give a more authoritative reply. Maybe Reg or bb can take over, or Carl?

[simonbaker]

Should there be some space between the windings of the coil such as an extra layer of insulating material or is it a better design to keep the coil as tight without extra insulation. Both cases the coil is tightly wrapped.
I'm still on the sideline learning all i can, Wyndham

It may depend on type of detector your coil is for. For Pulse Induction MDs, conventional wisdom says it's a goal to make capacitance very low, so people use techniques for spacing and dialectric choice as noted by others. For VLF type, capacitance may not matter much at all - there is usually a parallel capacitor that dominates the capacitance.

Regards,

-SB

[bbsailor]

Coil capacitance is largely the result of interwinding capacitance which increases with proximity of the wires and with increasing dielectric constant of the insulation material. (A vacuum has D.C. of one, and air is only a tiny bit higher. The D.C. numbers are calculated on a linear scale).

There are different types of enamel, one of them being polyurethane which has a dielectric constant of 3.4. There are different compositions of polyurethane wire enamel, so this is only one example.

Comparing polyurethane (D.C. 3.4) with teflon having a dielectric constant of 2.1, you can see that if two coils were insulated with these two different materials of the same thickness, the teflon coil would have less capacitance.

On top of that, most teflon insulation on hook-up wire will generally be substantially thicker than the enamel insulation on magnet wire. You already knew that, but there you can see that we have two aspects, both contributing to less capacitance for the teflon wire coil.

The relative proportions of the two property changes is a complex thing to calculate - especially if the coil is scramble wound.

One way around some of the complex interaction is to make a planar, or disk shaped coil. That way each coil has major capacitance to its adjacent windings, and only minor capacitance to other windings. It's pretty easy to understand the generalities of coil winding, but when you want to quantify the inductance and capacitance changes that's when things get hairy.

Sorry, but I don't have the schooling to be able to give a more authoritative reply. Maybe Reg or bb can take over, or Carl?

Porkluvr, and all

Yes, a disk or planar coil shape will have lower capacitance but there is a big however. However, shielding that shape coil adds back a lot more capacitance than you eliminated by using this coil shape.

The thicker the coil insulation of any type will reduce interwinding capacitance as well as reduce the inductance thus requiring an additional turn or turns to achieve the desired inductance.

The cross section of the winding bundle (non-planar type coils) defines the area that the coil shield will see when it is placed around the coil bundle. This coil shield will add another amount of capacitance that tends to lower the coil's self resonant frequency. Add the coax capacitance and the MOSFET COSS to this and you have the final capacitance value that the damping resistor (Rd) and Rin must damp.

Here are some universal tricks to keep in mind. The final coil wire bundle cross section bundle diameter for coils with 19 to 21 turns will have a bundle diameter 5 times the outside diameter (including the wire insulation) of the wire being used. The full coil to shield capacitance is not imposed on lowering the coil's self resonance. Only about 20 percent of the total coil to shield capacitance is imposed on lowering the coil resonance. This capacitance is not easy to calcuate but is very easy to measure with a signal generator and oscilloscope.

Damping occurs in three stages stages.

1. The MOSFET calmping occurs when the flyback voltage is above the MOSFET voltage rating. This tends to extend the coil discharge time by the amount of the clamping or length of the oscilloscope observed flat top.

2. The first stage of the damping resistor coil discharge curve is governed by the parallel value of Rd and Rin (input resistor to first amplifier stage) because the clamping diodes put Rin in parallel with Rd down to about 0.6V.

3. The second stage of damping coil discharge occurs when the voltage falls below 0.6V. when Rd alone governs the coil discharge curve. This portion of the discharge curve is steeper (more vertical).

These things are only important or get more important when you are trying to detect small targets that have lower time constants.

If you have a low capacitance coil and a higher value Rd, it is still in parallel with Rin. If you want to detect very small targets try increasing the value of Rin from the typical 1K value to 1.2K and any improvement (increased value) of Rd will have a better effect on improving (making it more vertical) the coil discharge curve. This improvement will only be visible on very small targets with about a 1us or 2us lower TC! Granted, higher values of Rin will tend to increase the noise in the amplifier stage but this may be a small tradeoff to attempt to detect very small targets with low time constants with minimal other design changes.

bbsailor

[Reg]

Hi wyndham,

If you use insulated stranded wire such as Teflon wire, you don't need to add any extra spacer material between windings. At least, I have never felt it did any good. Keep the windings tight so there can be no movement. I prefer when I have sufficient windings to lace the windings snug and then add a thin coating of silicone II to hold things together and fill spaces. This helps keep the windings from bunching when adding the spiral wrap.

Now, one can alter the final inductance and capacitance of a coil but changing both insulation type and voltage rating (which usually changes the thickness of the insulation). So, there will be differences if you change either the type or the voltage rating of the insulation.

This winding capacitance thing is quite complicated as bbsailor has indicated. To make it even more confusing , here is something I have not seen discussed that may seem strange. If you reduce the capacitance of the windings, you might find the decay curve actually appears to get longer or take more time to settle out when actually measuring with a scope on the detector. When this happens, the thing to do is to increase the damping resistance.

So, as you change something like the wire insulation or type, you may have to adjust other components for the best decay curve. This presents a problem since once that is done, then previously wound coils will work differently and may oscillate a little.

In other words, strange things can happen that may not have an explanation. I know, I have been there and done that.

So, it is best to try to ultimately find some consistency in the type of wire you will use to help minimize other unknowns that may crop up.

Now, over the years, Eric Foster and others have recommended a spacer between the windings and the shielding used, whether it be wrapping the windings or even using a conductive paint or other conductive material on the surface of the coil housing.

I personally have not tried to measure just which works best or figure out why. I just know it seems to work the best for both stability and consistency of my coils. At some time in the future when other projects are done, I may spend more time on this feature, but right now, other ideas are more pressing.

I hope this helps a little.

Reg

[wyndham]

I can see that I can over think this subject very easily I do have one additional question that I have read about but have not got a working idea of. That's "Q" I have seen on a Aussie ML forum where Woody talks about "Q". I seem to think that this is a balance ofmany different factors coming together to give a value but what is it and what does the range of values "generally" speak about. This is the deep end of the pond and my toes are not all touching bottom
Thanks Wyndham

[Reg]

Hi wyndham,

The Q of a coil is simply the XL/R or the inductive reactance of the coil at a particular frequency divided by the resistance of that coil. Lower the resistance and the Q goes up if the other factors remain a constant.

The problem is, the other factors will not remain a constant simply because the way to lower the resistance is to use larger wire. This will result in a different diameter winding, which will ultimately alter other factors.

Now, this is where it can be messy, the XL or inductive reactance is simply 2 times pi times the inductance times the frequency or (2)(pi)(f)(l). Now, on a PI, there is no resonant frequency per se at which it will be working. So, then we have to use then natural resonant frequency of the coil which will take into account the interwiring capacitance among other things.

So, there are many factors than can influence the Q or a coil. Also, keep in mind this is much more important when trying to build a coil for the ML PI"s than it is for most others.

I personally don't worry about the Q of the coil directly. Obviously, because I do select the type of insulation, wire size and voltage rating (which determines the insulation thickness), then indirectly the Q is taken into account. So, rather than worry about the Q itself, simply focus on the more obvious like characteristics of the wire used.

Personally, I wouldn't worry too much about all the difficulties you read in building a ML coil. Unless that is what you are doing, then a lot of the problems they have are non issues when building a coil for the TDI or some of the other PI's.

Reg

[simonbaker]

I can see that I can over think this subject very easily I do have one additional question that I have read about but have not got a working idea of. That's "Q" I have seen on a Aussie ML forum where Woody talks about "Q". I seem to think that this is a balance ofmany different factors coming together to give a value but what is it and what does the range of values "generally" speak about. This is the deep end of the pond and my toes are not all touching bottom
Thanks Wyndham

I'm working mainly on VLF detectors at the moment, which operate at a continuous sine wave frequency, and I'm interested in Q myself.

The "Q" of a resonant circuit is basically related to how "underdamped" the resonance is. So a high-Q circuit has very little damping, and will resonate very easily with just a little energy input. It is like a spring with a weight on it, where if you nudge it at the right frequency, you can build up a nice big motion. A low-Q circuit is like weight on a spring with the weight dragging in water.

With VLF detectors we don't use the natural resonant frequency of the coil itself (based on its parasitic capacitance) typically (but I'd like to try doing it -- if I could lift the coil!) -- rather we put a capacitor in parallel to set the "resonant frequency".

I believe high-Q can do two main things for us. With our TX (transmit) coil, high-Q can save us battery power because we can get more current sloshing back and forth with less nudging.

In the RX (receive) coil, it should mean the coil will naturally "amplify" any signal at the resonant frequency -- which sounds desirable since we're in the business of trying to detect miniscule signals induced in the target by our transmit coil. There is a little complication because we are also trying to detect the phase of the received signal using particular circuits, so we may not actually want to set the resonant frequency of our RX coil circuit right at the TX signal frequency.

I personally am interested in experimenting with very high-Q TX and RX coil circuits to see whether any and/or how much advantage could be gained, and get insight into any limitations of using such coils.

One downside of high-Q coils is you mainly need to use thicker wire to lower the resistance and raise the Q, so eventually the weight isn't practical. Silver wire would help a little, at a cost.

Regards,

-SB

[Reg]

Hi SB,

I believe wyndham is working on a PI detector and as such, the Q sort of takes on a different meaning since the idea is to dampen the coil anyway.

Now, you are correct, the Q of a coil can be very important, especially when working with VLF's. If too small of a wire is used for the transmit, there can be a large energy loss just trying to power the transmit circuitry. In other words, increasing the wire size in the transmit just may increase battery life considerably. That is why many manufacturers specify a minimum size wire be used for their TX coils.

As for the receive winding, a lot of manufacturers don't try to tune the receive so there it isn't as important. As such, a much smaller wire is normally used. For some reason, there is less desire to tune the receive. Since I have not done any experimenting in this area, I really can't say why this may be true.

Reg