PI metal detector for really small nuggets

[6666]

Are you using typical pi preamp ?

[6666]

My currently preferred method of driving an H-Bridge is with analog switches. While you can use 4053-types the DG469 is my favorite, 6 ohms max and up to 44V on the rails. Yes, I have run the TX supply that high. And the logic input is 3V-compatible. No level shifting or translation required.

There is a simple way of getting rid of the SPDTs on the NMOS devices, bonus points if anyone figures it out.

Hi Carl, I dont like asking for things, but I was wondering if there is any more to your h bridge circuit as I am interested to see how you are hooking up the pre amp and its configuration ? thanks.

[Carl-NC]

Asking is still the best way I know of for getting answers...

An obvious solution is to use a separate RX coil. It does not have to be induction balanced and offers the flexibility of optimizing the TX and RX inductances separately.

If you want to use a mono coil then options are limited. The circuit below works pretty well though I have never tried to push it into the realm of aggressive performance.

[6666]

Thanks Carl for the pre amp cct, much appreciated, I have got a basic very simple full bridge working with a single coil without letting out too much smoke , now I can add a preamp and see whats happening , cheers.

[green]

Tried the unipolar and bipolar circuit again. Bringing my hand next to coil caused a problem with the bipolar circuit. Thought coil volts not decaying to power supply volts might be causing problem. Tried next day and a few times after, no problem with bipolar circuit. Don't know why. Next is adding regulator to Tx power supply so can adjust for 1A peak at 160us Tx on time. Starting to like the bipolar circuit.

Maybe found the problem placing hand near coil. Sometimes I look at (A)integrator out, sometimes (B)differentiator out. Noticed when voltmeter was connected to (A) there was a problem bringing hand to coil. Connected a 10k resistor to (A) and 100nf capacitor to analog common to isolate meter from integrator. Problem solved. Maybe meter lead lead was picking up some Tx signal effecting the integrator?

Added regulator to Tx supply, looks good.

picture, one I posted awhile back

[green]

Tried bipolar awhile back. Ended with more diodes. Wondered if is necessary. D1 and D2 are MUR460, D3, D4, D5 and D6 are 5EWH06FN.
First column, with D1, D2, D3, D4, D5 andD6. Resonance with Rd disconnected, about 1MHz.
Second column, D3 and D4 removed and D5 and D6 shorted. Resonance with Rd disconnected, about 600kHz. Needed a lower Rd.
Coil(133mm fig8 ).
With extra diodes, resonance higher and could use a shorter delay. Not sure it's better yet. Need to do some more testing.
Top four pictures are with either M1 or M2 turning off.

Tried again today. Rd with all diodes(1200), Rd with 2 diodes Carl's circuit(650). 133mm figure8 Rx, separate Tx, looking at amplifier out. Carl's circuit looked better today.

[peyman 74]

hi carl . please help we for integrator circuit for bi polar . thanks

[darov922]

Asking is still the best way I know of for getting answers...

An obvious solution is to use a separate RX coil. It does not have to be induction balanced and offers the flexibility of optimizing the TX and RX inductances separately.

If you want to use a mono coil then options are limited. The circuit below works pretty well though I have never tried to push it into the realm of aggressive performance.



[ATTACH]55204[/ATTACH]

I know this thread has been inactive for several years. however the amount of information here is impressive to say the least. I have finished my first HH2 detector which I want to optimize for the detection of small gold nuggets under large stones in rivers. I am analyzing your bipolar circuit although I don't know how to adapt it to the circuit I already have... Is a pulse sent in one direction and then turned off and sent in the opposite direction with pulses of the same duration? What is the improvement over a unipolar pulse?
thank you for answering me and I will be very grateful for the answers. Dario

[Carl-NC]

Yes, it pulses in both directions. The advantage is that you do not need to use an EFE sample to cancel Earth field effect, it automatically cancels. Since there is no EFE sample, you can run at a much higher pulse rate.

I suspect that for river detecting you will need a design with ground balance. River rock is often very mineralized and because the rocks have come from many different places over 1000's of years they vary a lot in viscosity. You will probably get a lot of falsing. See Walt's GB addition to HH2 here.

[darov922]

This is not something I am currently doing. The last time I was working with a pot core was in 1998 when I obtained some 70mm diameter ones. Prior to that in 1989, I modified a PI frisker design to run at very short delays, using a 46mm pot core.

I was surprised to find these huge potcores such as 150mm and even up to 300mm by a German manufacturer called BLINZINGER. Hate to think what a 300mm would cost. No way am I going to fork out that sort of money.

Eric.
[ATTACH]54412[/ATTACH] [ATTACH]54413[/ATTACH] [ATTACH]54414[/ATTACH]

I am very interested in what you are telling, in my search I have found 69mm cores in China and 89, 114 and 150mm sold by the firm TDK... a coil built with one of these cores, in addition to allowing the plate to be placed electronics very close avoiding the parasitic capacity of the cable... will it increase the detection range by sending the entire magnetic field in a single direction? any other advantages?

Dario

[darov922]

Yes, it pulses in both directions. The advantage is that you do not need to use an EFE sample to cancel Earth field effect, it automatically cancels. Since there is no EFE sample, you can run at a much higher pulse rate.

I suspect that for river detecting you will need a design with ground balance. River rock is often very mineralized and because the rocks have come from many different places over 1000's of years they vary a lot in viscosity. You will probably get a lot of falsing. See Walt's GB addition to HH2 here.

Thanks for your answer Carl, I didn't imagine that you would respond so quickly. I used the HH2 with unshielded 3dss coil in rivers and although I didn't find gold, I didn't find hot stones, instead I found many nails, wires and even horseshoes. I am looking for a way to improve this design to make a HH2.5 version without DSP with display and that allows to adjust the parameters of the pulses and load some custom search profiles. But before that I want to get the maximum detection range and I will seek to orient all the "vectors" in the same direction, looking for that objective.

Dario

[lucifer]

I am very interested in what you are telling, in my search I have found 69mm cores in China and 89, 114 and 150mm sold by the firm TDK... a coil built with one of these cores, in addition to allowing the plate to be placed electronics very close avoiding the parasitic capacity of the cable... will it increase the detection range by sending the entire magnetic field in a single direction? any other advantages?

Dario

Can you post links to those?
The core won't increase detection depth because the entire field is in a single direction. The advantages are you can get same induction with less number of turns, hence more current and less parasitic capacitance. Also the coil is magnetically shielded from the other side. And yes, electronics can be placed close to the coil and eliminate the cable capacitance which is usually higher than that of the coil. So you can sample earlier this way. Of course the core material and its properties should be considered too

[darov922]

Green,
Here is a mental model to consider to solve your fast sampling question. When damping the initial flyback pulse the idea is to quench the flyback oscillations as quickly as possible to sample as soon as possible. The key question is:what causes these oscillations?

Think about it from the perspective of the total capacitance seen at the points where the damping resistor is located. All the energy in the oscillations that you are trying to damp comes from the total capacitance seen at that point and during the time it takes for these oscillations to stop or drop to a low enough value that you can begin to RX sample the target. Low TC targets give up their fully stimulated energy very quickly. The most creative solutions require identifying all the sources of capacitance seen as oscillations after damping and reducing or eliminating as many as you can. Here are some places to look.
1. Lower capacitance coax cable.
2. MOSFET coil driver COSS.
3. Coil winding capacitance.
4. Coil to shield capacitance.
5. Coil geometry, single coil, DD coil, figure 8 coil, etc.
6. Optimum coil size for your desired target size and TC.

I hope these things get you looking for the source of the unwanted capacitance to allow you to sample sooner.

Joseph J. Rogowski

P.S. A final thought. When sampling at very low times, the eddy currents in the coil wire may be detected as a target. Use wire with a lower TC than your smallest desired target. Litz wire with very fine strands and a thick outer insulation with a low dielectric constant is a good choice.

Hello, I found your post extremely interesting. I am analyzing each of the points that you propose in detail.
1- unshielded cable? What would be the electrical noise in the middle of the mountains?
2- Mosfet STF18N60M2 (600 V, 0.255 Ω typ., 13 A, Coss 40pF)
3-4-5- 3DSS coil with orthogonal crossings between turns, without shielding.
6- This is the point that seems most important to me now... how can I determine the TC of my target? Is there a table to find that information? I see it as very important to find the right wire to then design a coil of the appropriate size for that purpose.
I hope you can answer my questions and I am really very grateful for the tasty information you have published.
Dario

[darov922]

Can you post links to those?
The core won't increase detection depth because the entire field is in a single direction. The advantages are you can get same induction with less number of turns, hence more current and less parasitic capacitance. Also the coil is magnetically shielded from the other side. And yes, electronics can be placed close to the coil and eliminate the cable capacitance which is usually higher than that of the coil. So you can sample earlier this way. Of course the core material and its properties should be considered too

Thank you very much for the information, Lucifer, now I will publish the links that I mentioned, what do you think would be the most appropiate material for this use?
the core in aliexpress is GU69

[lucifer]

Thank you very much for the information, Lucifer, now I will publish the links that I mentioned, what do you think would be the most appropiate material for this use?
the core in aliexpress is GU69

I would say, just try and compare. I'm planning to produce my own cores for flat spiral coils but it will likely take few months before I can post results as my time is limited at the moment.