PI coil capacitance

[Chris]

Hi everybody

What is a typical inductance value range for a PI coil? I saw a few coil values in some designs but there were big difference between the designs.

And does someone know the typical capacitance range of commercial PI's coil between the windings and the shield? I measure something between 2.6nF and 3nF at my DIY coils. I measured between the shield and one end of the coil. Could it be correct (in range), wrong or ..... I don't know .... I'm not 100% sure. Need some help on this.....thanks in advance.
Another question is .... my coils are a shielded mono coil design (34wdg/10inch dia/AWG24). Where should I join shield and coil (GND). In front at the search head or at the electronic (PCB). Which point is better and why?

Regards
Chris

[Carl]

Re: PI coil capacitance

I've seen as low as 100uH, up to a couple of mH. Most commercial detectors with the standard front-end design, seem to be a few hundred uH at the most.

I measured my 8" coil, it's about 600pF, so you're in the ballpark. You didn't touch the probes or the coil while measuring, did you? That'll throw it off a bit.

Good question on the shield connection... I assumed that everyone ties it inside the coil. Probably doesn't make much difference, but I don't know for sure.

- Carl

[Reg]

Re: PI coil capacitance

Hi Chis,

I am guessing your coil has an inductance of something between 750 and 850 uh which is somewhat high for high pulse current applications. Are you using magnet wire or stranded hookup wire for your windings?

The higher the inductance value, the harder it will be to reduce the delay because it will take longer for the coil signal to decay.

The figures I have seen for PI coils are inductances between 300 uh and 1 mh. Most of the higher powered units operate with a lower inductance.

Without knowing the application or what you want to search for, I would not want to guess what yours should be.

Now, the capacitance between the coil windings and the shield of your coil seems a little high. What are you using for shielding?

My guess is your shielding is right on top of the windings with no space at all between the windings and the shield. Eric Foster mentioned he uses spirawrap around the windings as a spacer between the windings and the shield. This seems to work just fine. Other people have also indicated a spacer is a good idea.

I measure about 100 pf or so between the shield and the windings with my BK LCR meter. have the shield spaced out from the windings using spirawrap.

I can't say where is the best place to connect the shield, but I connect the shield with one lead of the coil to the shield of the coax used to connect the coil to the control. This effectively ties the shield to the common of the electronics. Doing it this way simplifies wiring between the coil and the control unit and it seems to work just fine.

I hope this helps.

Reg

[Reg]

Re: PI coil capacitance

Hi Carl,

What are you using for shielding and how are you spacing it from the windings?

Reg

[Carl]

Re: PI coil capacitance

>What are you using for shielding and how are you spacing it from the windings?

Adhesive foil, what's used for A/C ductwork. I did not add a spacer.

Before I shielded the coil, I measured the inductance, and put the decay curve on the o-scope, and air-tested a couple of targets. I did the same after shielding, got no change in inductance, decay looked the same, and air tests seemed the same. Except for measuring the inductance, this is all subjective. Next time I wind up a coil, I'll try to look at this a little closer.

- Carl

[Chris]

Re: PI coil capacitance

Hi Reg & Carl,

thanks for your kind reply. I want to make a good general purpose PI coil. No special application in mind. I'm using magnetic wire (enamelled copper wire) and for the housing the coil shells from Hayes Electronics.
I don't know the inductance of my coil. I don't have a LCR meter.
I measured the coil capacitance with my digi. multimeter and compared the reading with a second one (diff. model) but with the same reading without touching the probes. The value of 2.6nF includes the 1.8m cable as well. Maybe the cable I used is not the best to use as a search coil cable. The cable is a shielded twisted pair cable used for S-VIDEO application (US$4/meter).

For the shield around the coil I use lead tape from 3M (suggested by Eric Foster). My coil is nicely wounded (layer by layer) and there is no spacer between the windings and the shield. The shield is connected together with "Signal-Ground" at the PCB side.

Is stranded hookup wire better than enamelled copper wire to use in PI coils? If yes, why? And what is with HF-wire (high frequency wire)?

Chris

[Eric Foster]

Re: PI coil capacitance

Hi Carl,

What metal is the foil? What I have seen over here is usually aluminium. This is OK as long as the foil is thin enough, otherwise you generate eddy currents in the shield which mask those of the target. Comparing the coil recovery times of identical shielded and unshielded coils, shows to what degree this is happening.

Eric.

[Chris]

Re: PI coil capacitance

Hi Reg,

I measured the inductance of my coil with a scope today.....your guess of 750 to 850uH was quite good. I measured 870uH with the scope.
Is this to much for a general purpose coil? Should I reduce the number of windings?
Somewhere I read that are special search coils for gold around (coiltec). What is so special or different? Different (higher) frequency range, more or less inductance than general purpose PI coils, or lower coil capacitance?

Regards
Chris

[Reg]

Re: PI coil capacitance

Hi Chris,

Since you do not have a special application, I would suspect the wire you use would be ok.

Now, if you want to try to use the PI for detecting small gold, then some adjustments probably should be made. If coins and general metal objects are the main objects to be detected, then things like a very short delay isn't that critical.

As for your question about enameled wire versus stranded wire, the answer to that lies in the application also.

One of the things you can do to get an idea of how the coil is effected by using different wire is to simply build two and see how they oscillate. This is a simple process.

Just build the coil but don't connect it to the PI. Instead, have a coil already on the PI. Now, what you will do is to place the new coil close to the operating PI coil and look at the oscillation that occurs by connecting your scope to the new coil leads. This new coil will have a spike imposed upon it from the operating detector, which should ring the new coil at its natural frequency. This frequency is determined by the inductance internal capacitance.

If you know the inductance of the coil and measure the frequency of oscillation, then you can calculate the capacitance. However, I am lazy and do not have a spreadsheet set up with the formulas to calculate the capacitance so what I did was to simulate the LC circuit using a spice program. I knew the frequency of oscillation, I knew the inductance, and I had an idea of what the capacitance might be so I just plugged in values for the capacitor until I got the same frequency. This is simple, sloppy but effective.

I wound a couple of coils using stranded insulated wire on one and basically enameled wire on another. The oscillating frequency was quite different between the two indicating there was a significant difference in capacitance. The stranded hookup wire oscillated at a much higher frequency indicating the capacitance was much less.

This lower capacitance really only becomes important when you want to have the coil decay as fast as possible so you canoperate with a very short delay. This is necessary for looking for very small gold objects. I would think it would be a non issue for coin type objects.

I am not sure of your question about HF-wire. What I have found is some of the so called special wire built for high frequency operation has quite a bit of capacitance built in. In fact, when I was trying to find a very low cap wire for my DD coil applications, I tried a few different SVHS cables and found the cheaper ones to be worthless for my application. The Mogami cable seemed to be the best I could find.

For your application I would simply use RG 58 AU coax or something similar. This should work quite well. As for your coil shield, I would simply tie it to the coax shield along with one lead from the coil. I doubt you will see any improvement by taking a separate lead back to the pc board. If you do, let me know.

Now, I haven't given much thought as to how important the capacitance of the shield is on the operation of the PI. I will have to think about that one for a while.

I also have not taken time to see just what the impact of having the shield closer or farther away from the windings. I really should do some experimenting to see. I can see where it might be more critical to be sure the shield has a small gap due to the coil's ability to detect the shielding. What I don't know is just what the capacitance of this shielding will have on the detection of the shield or the over all decay time.

Reg

[Chris]

Re: PI coil capacitance

Hi Reg

A spacer between the coil and shield reduce the capacitance for sure...as everybody know C is proportional to 1/distance (between the capacitor plates). You can read this in every electronic reference book (capacitor formula).

Like you suggested I'll do some experiments to see the difference in stranded and enmalled wire in coils.

I looked also at the data sheet of the cable that I used for my coil. The capacitance of my cable (57pF/Ft) is much higher than RG58AU coax cable (100pF/m). I'll replace it as well.

And another question....you wrote....

"As for your question about enameled wire versus stranded wire, the answer to that lies in the application also."

What do you mean? Which wire would you take for which for application if you want to design a coil?

Chris

[Reg]

Re: PI coil capacitance

Hi Chris,

I will try to give you and example based upon my PI which isn't drawing a lot of coil current during the pulse on time. This makes a difference also.

In my application I wanted to detect very small gold which which produces a weak signal that decays very rapidly.

Now, I had two choices, first to increase coil current which should produce a stronger signal or I could try to reduce the delay before sample. This would allow me to sample when the signal was stronger.

Obviously, one could do both but that was not an easy option for me due to other constraints.

Also, the size of gold I wanted to detect required the sample delay be short simply because the signal would diminish to nothing very quickly.

So, I needed to reduce the coil inductance and capacitance to allow me to sample sooner. The capacitance becomes important when trying to squeeze the delay down a couple of usec's which I needed to do to allow for the fastest decay possible.

In this case I tied to minimize coil and coil cable capacitance and pick an inductance value that would also allow for a faster decay. The reduction in capacitance in the windings and in the coil cable would allow for a higher resistance value damping resistor which would dampen the coil without oscillation. Together, the decay time would be shorter which then would allow for sampling sooner.

All of this is a balancing act to try to determine the right inductance, etc for the specific application.

So, for applications where the you want to reduce the decay time, you probably should select the wire that provides the least capacitance.

Now, for applications where it is not necessary to sample extremely early, then any wire should probably work just fine. I can't tell you the critical point where the wire becomes a problem. My guess is as long as the sampling is delay is more than 12 to 15 usec or so, then the wire is not a main issue. Inductance and coil current become a problem then.

For applications such as beach hunting where rings and coins are the primary objective, then a longer delay should be ok to a point. If thin gold rings are to be detected, then the minimum delay becomes reasonably important again.

One should also remember that as the delay is shortened then all low conductive items become targets that can be heard. Things like tin foil in gum wrappers can produce a strong signal.

If coin hunting is the objective, then the minimum delay isn't as critical. Pulse on time may become a factor for maximum depth.

If you are looking for iron objects, then the short delay is of little value. In this case, a short delay will cause more problems because there will be more ground signal to contend with.

So, as you can tell, it is difficult to give you a specific answer to your question. Everything is a tradeoff. Reduce the delay time, you can detect small gold better but you will also have stronger ground signals, detect more junk, etc.

Lengthen the delay and the junk can be ignored but some other objects that may be desireable will be ignored also.

Reg

[Reg]

Re: PI coil capacitance

Hi Carl,

Thanks for the input. I have never tried the foil you mentioned. I do have a couple of small pieces of the stuff and find there are different thickness of the foil. At least, there is on the ones that I have.

I will have to try the foil also if I can find the thin stuff. I have tried most everything else. One of the strangest things I tried was a plain old decorative ribbon I got from a Wal Mart store. The ribbon has a thin aluminized layer and is very conductive.

It seemed to work but I really never did do any extensive testing. Instead, I went back to my special shielding tape, the 3M 1190 copper coated polyester foil. This seems to work fine for my applications but it is a little expensive and hard to find.

Also, when time permits, I need to see just what obvious difference there is to having the shield right on the windings and spaced out a little.


Reg

[Allan]

Re: PI coil capacitance

Carl, Eric, Reg, Chris and others on this great site. I'm learning heaps thanks to all your help. Here is my first maybe dumb idea, for a low capacitance coil.
Remove the pvc outer from however many metres of coax required for your coil. Then pull out the inner wire with insulation or unravel the braid. The inner conductor with its thick insulation (air cored for low weight) would wind a low capacitance coil with the end connected back to the shield. You then could even use the braid wrapped around the coil as the shield.
Second maybe dumb idea, after the discussion about ground balance on the PI site,
and target masking. Pertaining to Carls PI-1.
Tap off the 555 pulse output, divide with a 4017, and flip-flop to toggle the spare 4066's and another resistor across the sample delay and pulse width resistors. I realise this will increase noise in the integrator requiring a slower time constant and therfore a slower sweep speed.Probably need a dual 555 timer circuit to eliminate the pulse width/pulse speed interaction but this could fit in with a active fet turn off to help with the quick sample.Does this make sense? regards ,Allan.

[Carl]

Re: PI coil capacitance

It's aluminum, a bit thicker than household foil. I did check decay and detection depth before & after the shield, but I did not do a side-by-side comparison. I need to do this.

- Carl

[Reg]

Re: PI coil capacitance

Hi Allan,

Sorry I missed your post. One could do what you recommend to try to reduce the capacitance of the coil but the coil would get pretty fat in a hurry. I am not sure it would be as much of a help as taking more care in the winding techniques.

Besides, I have found that just using hookup wire reduces the capacitance significantly. It seems to be enough for most applications and I don't have to be that touchy in winding the coil.

Now, as for your idea of switching the pulse delay, I am assuming you mean to do this for a quick check of the target by increasing the delay. Your idea should work, but I am doing something like that now, but simply use a simple toggle switch to change the delay. Thi circuit I am using is a little different so I am not sure a simple switch wouldn't introduce just a little noise, though. My change allows a quick check of a target to see if it is gold or not.

I suspect that if you do not slow everything down, you will not have to adjust the sweep speed.

One of the more unique ways to change things would be to build types of coils such as the figure 8 receive coil. This way you would have noise cancelling and ground cancelling. Recitify the outputs so you eliminate the half positive and half negative signals. Then you could use the second sample normally used for subtracting the earth field effect, to check a target at a later delay.

I am planning on doing more testing using a little different technique on my DD coils. I also plan on trying a concentric type coil again. I tried this once but really wasn't set up right at the time. This time I will try a couple things that are a little different.

As you can tell, there are lots of ideas to play with.

Reg