PI coil capacitance

[Allan]

Re: PI coil capacitance

Reg, What I meant was to alternately have a long pulse and later sample / short pulse and early sample; to find the big bits / little bits. eg 7short / 3 long. Does that make sense now?
regards, Allan.

[Reg]

Re: PI coil capacitance

Hi Alan,

I am sorry about my misunderstanding of why the pulse change also.

I am using a very short pulse now and all testing I have done indicates I lose very little depth ,if any, due to pulse length on my gold nuggets I have which range from about 1 grain to a little over a half oz.

I suspect any depth loss due to a short pulse would be reflected in signal loss on very large nuggets ranging in the multiple oz range. However, a longer pulse should work better for increasing the depth on copper and silver objects. Unfortunately, I have done no testing in this area so I can't tell you just what to expect since I hunt strickly for gold nuggets with my PI.

Your idea of using two pulse lengths is a good one but one should build a second differential amp for amplifying the long pulse signal. Summation of the long pulse/short pulse received signals should be summed at a later point for best results.

Now, to keep the project simple, I would start out by selecting a reaonably short pulse length, maybe 50 usec to 100 usec, concentrate on reducing the sampling delay, reducing noise, maybe increasing the overall circuit gain, and refining the coils first before trying something really new like multiple sampling.

I would also recommend a person spend a little time experimenting with different sampling widths. Don't be afraid of trying widths as low as a couple of usecs.

Once these areas are maximized, then I would try a dual sampling project. This way, one will have a better means of measuring or noticing any side effects that will occur from dual sampling or any other idea one might try.

Reg

[Russ_NY]

Re: PI coil capacitance

Hi Reg
What type of hookup wire are you using? Number of strands and gauge? Tinned or untinned? Type of insulation? How about pre-Tinned wire where the strands are already soldered together, I suspect that is probably similar to solid hookup wire. Any recomendations on which manufacturer to use? I need to restock before I can do any serious coil winding and I really want to get the right wire so any help would be greatly appreciated.
Thanks,
Russ

[Norbert]

Re: PI coil capacitance

Hello Chris!

It's easy to measure the coil inductance.
U = L * I / t
so..
L = U * t / I

L in H
U in V (voltage you switch to the coil)
t in seconds
I in amps (peak-current)

[Chris]

Re: PI coil capacitance

Hi Norbert

yes, I know.... there is also another easy way. See Link.

Chris



Inductance Measuring Technique

[Reg]

Re: PI coil capacitance

Hi Russ,

First, I am not the great expert on PI's so don't take my word as golspel. Personally, I like to try different things and measure the results. This give me a better feel for how things interact.

Rather than buy a large quantity of any particular wire, I would probably try different types and see what happens if all other things are held constant. You may find that magnet wire works fine for your application. Magnet wire is definitely cheaper and if it works then I would use it. The key is to set an objective and then set out to reach it.

For me, that objective was to reduce the delay so I could detect smaller gold with a stronger signal. This required reducing the inductance and then the capacitance to be able to sample very quickly. Building a DD coil helps in this regard and careful alignment allows sampling even at shorter delays than with a mono coil.

Now, I found I could reduce the delay down to 12 to 15 usec without great difficulty. It did take some doing but nothing like dropping below 12 usec. The big factor was lowering the inductance to somewhere between 300 uh and 400 uh. The next key factor is the damping resistor. Also, don't use any compensation caps on the preamp.

Going to a DD coil helps a lot also, since the preamp doesn't saturate as bad. Also, the nulling of the coil really helps.

Now, to answer your question, I use a fairly small wire, about 29 awg stranded tin wire that has a teflon coating. But, I am making coils to use on Eric's Beachscan/GQ design and this PI doesn't draw as much coil current as Carl's project. I happened across this particular wire and got it cheap. That is probably the main reason I tried it.

I really need to build Carls PI and I will build it, once I am satisfied with the mods I am making on my present PI. I am getting close to that point now.

Teflon insulation is generally a little thinner than pvc insulation for the same voltage rating so the coil windings won't be quite as bulky. Unfortunately, Teflon is also quite expensive so I look for places that sell it surplus.

As for whether stranded, tinned, etc is better, I really can't say. I just noticed the coil capacitance was much lower when the wire has a little spacing between windings. In fact, the capacitance was about 1/2.

I have also tried Litz wire and it didn't work that bad. I didn't try served Litz which probably would have worked better for reducing the capacitance than the unserved type.

As for finding surplus wire, I found that Surplus Sales of Nebraska has some decent prices on teflon and Litz wire. The sizes are limited but that is ok if there is a size that works. I would probably try their served Litz on Carl's project and maybe either a 26 awg or 24 awg teflon.

World Wide Wire has a large inventory of wire with reasonable prices. The down side is they require a $100 minimum purchase.

[Russ_NY]

Re: PI coil capacitance( Thanks Reg

Reg
Thanks for the response. I have been using magnet wire as well as stranded PVS coated hook up wire, keeping the inductance in the 300uH range. No compensation caps on the amp. Reduced the gain to about 500 on the amp. I also use a series resistance between the coil and Mosfet of about 2.2 ohms to reduce the coil current and have the prf running at just over 2kHz with a 100usec on time. I spaced the shield away from the coil using spiral wrap, and still have trouble getting the delay under 20usec. I would like to try a DD coil but am not really clear on how to implement it into the circuit. I assume that one coil goes to each input of the amp, and that both coils are switched by seperate Mosfets at the same time. I have only seen one schematic for a dual coil PI and it was not documented at all. Any guidance on how to set up a DD coil into what is esentially Carl's circuit would be greatly appreciated.
Thanks again

Russ_NY

[Reg]

Re: PI coil capacitance( Thanks Reg

Hi Russ,

I am wondering about your decay time. I would think you should be able to get down to 15 usec without a major problem.

What do you have for a damping resistor and did your try higher values?

As for the DD coil setup, it is quite easy. You will need at least a 3 pin connector though. The third pin will be for the receive coil connection back to the pc board. Now, once the project is complete, the xmit will be isolated from the receive circuit.

1. You will have two separate isolated elliptical coils in the coil housing. One will be the xmit coil and the other will be the receive coil. Normally they overlap a little less than an inch or so. The actual overlap is determined by coil size, etc. One coil, the xmit coil, connects just like the mono coil. The receive coil will connect to the preamp circuit via R 12 (on Carl's PI).

2. You have to cut the trace between the xmit coil connection and the receive circuit to isolate the receive from the xmit. This means, cutting the trace from R 12 to the xmit coil connection. Now, R 12 will be open ended on the xmit side.

3. The new receive coil will now connect to the open end of R 12. If a plug is used, then a third pin will be used for this purpose to connect R 12 to the receive coil.

4. The commons of both coils should tie to the pc board common. They can tie together at the coil cable plug but I found it best to not tie them together in the coil.

5. You will need a separate damping resistor for the DD receive coil and that can be connected inside the coil housing or in the coil cable plug.

6. I found shielding both coils just like is done on the mono coil works fine. However, I am using a different shielding material that is not detected or if it is, is very slight.

7. The easiest way to to make the mono coils work with this setup is to add a jumper inside the coil cable plug to connect the xmit back to the receive. So, lets say you have a 3 pin plug with the following; pin 1 is the receive pin, pin 2 is the common, and pin 3 is the xmit pin. Now, on the mono coil, the coil connects to pin 3 and pin 2, and there will be a jumper between pin 3 and pin 1.

On a DD coil, the xmit connects to pin 2 and pin 3 in the plug. The receive connects to pin 1 and pin 2 in the plug. The receive damping resistor can connect to pin 1 and pin 2 or be mounted in the coil.

Remember, the plug should have at least 3 pins. One pin for common, one pin for the xmit and one pin for the receive. You will need a dual coax. I use a couple of different types. One that would probably work best for you is a Mogami cable. Look for the dual coax that has a low capacitance. The cheapest way to buy it is to buy the Mogami S video cable. Other SVHS cables I have tried have too much capacitance, so I have stayed with the Mogami.

I hope this is enough info to get you started.

Reg

[Russ_NY]

Re: Wow Reg Thanks

Reg
Thanks so much. You cleared up all of my questions concerning how to set up for the DD coil. I am currently using a 480 ohm damping resistor. I have tried higher values and the 480 was the highest value I tried where I did not get any ringing. If I remember correctly I tried a 520 and had what looked like about a half cycle of ringing. I will go back and check it to be sure. I don't recall having seen this Mogami cable but I will check around. I am pretty sure I have a few SVHS cables in my collection. I do have a fairly good supply of different types of coax cable. I pretty much cut 1 foot lengths of several and measured the capacitance and selected the lowest one. It happened to be some RG 59 I had on hand. I don't recall at the moment what it measured but I will check that out again also. As to the coil shielding I used the self adhesive aluminum tape. It is fairly heavy and not particularly easy to use. I had to use a drain wire spiraled around it to get a decent ground connection. I would like to try some sort of spray or paint on conductive coating but haven't tried to find any as yet. I well definitely try out the DD coil as soon possible.
Thanks again

Russ_NY

[Reg]

Re: S Video cable

Hi Russ,

You might try this website.
http://www.hometech.com/video/interconnects.html#MO-PPSV403

I purchased a 50 foot cable and just use the amount I need for each coil. It was easier and cheaper to do it this way than trying to find and then buying the cable by itself.

Reg

[Reg]

Re: Wow Reg Thanks

Hi Russ,

I suspect the heavy aluminum shielding is part of the problem. I know it will be if you build a DD coil.

What happens when you try a coil on the bench without the shielding? Do you still run into the same problem?

Reg

[Norbert]

Delay

Is it important to switch off the coil very fast to get a very short delay time?

HH
Norbert

[Reg]

Re: Delay

Hi Norbert,

Normally, yes you want to switch the coil off as quickly as possible.

Reg