PI metal detector for really small nuggets

[Ferric Toes]

Hi Eclipse. I powered up a different board designed for single mono coil and it still works, so here are some relevant results.

Coil is 4inches diameter with a shield (can't remember what material). Cable is 1.5m RG179.

TX pulse is 25uS and repetition rate 23,000pps.

Delay and samples are 5uS

Power is 12V at 120mA

It will detect a 0.08gm flat gold nugget at 2 inches and a 1.00gm flat nugget at 6 inches.

Tomorrow I will shorten the cable to 30cm and see if I can also reduce the delay to 2.5uS. Damping is critical and I have a cermet preset to adjust for the fastest recovery with no ringing. OPA1161 works fine. Previously it was AD8055.

Eric.

[green]

That post was originally by Carl.
The 56k resistor should be from the MOSFET drain (and the HV diode) to system ground (or maybe should be across MOSFET's Drain to Source).
It is needed since the diode blocks discharging the MOSFET's Drain charge.

Is it needed? Looks to me to be better without a discharge resistor.

[Elliot]

That post was originally by Carl.
The 56k resistor should be from the MOSFET drain (and the HV diode) to system ground (or maybe should be across MOSFET's Drain to Source).
It is needed since the diode blocks discharging the MOSFET's Drain charge.

Something like in the dashed blue line - ?

[Ferric Toes]

Is it needed? Looks to me to be better without a discharge resistor.

Agree. Latest thinking is that it is best to leave the drain/diode junction charged as it reverse biases both the mosfet and the diode, which results in the coil seeing less parasitic capacitance in the receive period.

Eric.

[DIRCEU]

Agree. Latest thinking is that it is best to leave the drain/diode junction charged as it reverse biases both the mosfet and the diode, which results in the coil seeing less parasitic capacitance in the receive period.

Eric.

Hi! Eric

Can I apply This trick on the Baracuda or Surf? What diode to use and where I connect it.
Thanks in advance

Dirceu

[6666]

Hi! Eric

Can I apply This trick on the Baracuda or Surf? What diode to use and where I connect it.
Thanks in advance

Dirceu

Yes HEP208,

[Ferric Toes]

Yes HEP208,

Should be HER208.

[Qiaozhi]

Agree. Latest thinking is that it is best to leave the drain/diode junction charged as it reverse biases both the mosfet and the diode, which results in the coil seeing less parasitic capacitance in the receive period.

Eric.

True. Adding the resistor makes things worse, as it increases the time before you can sample.

[6666]

Should be HER208.

Dont know why I mistyped that, pity we dont have more time to edit here

[eclipse]

I did go to one of the sites (a creek with lots of rocks) with a regular VLF (Vista Gold) and it failed miserably.
Did not ground balance very well (3/10) points for GB.
On the positive side it does detect small items in air better than Golden Mask 1 & 4 and Minelab FT16000 & Goldquest.
I will definitely need a custom built machine to hunt over there.

[ZED]

Would leakage from the drain diode create any noise while it was reversed biased ?

[6666]

I did go to one of the sites (a creek with lots of rocks) with a regular VLF (Vista Gold) and it failed miserably.
Did not ground balance very well (3/10) points for GB.
On the positive side it does detect small items in air better than Golden Mask 1 & 4 and Minelab FT16000 & Goldquest.
I will definitely need a custom built machine to hunt over there.

[ATTACH]50886[/ATTACH]

Did you do any test pans ? Did you bring home a bucket of dirt for testing at home.

[Ferric Toes]

Hi Eclipse,
I have been doing a few more tests on coils suitable for small nuggets and have to revise my suggestions for coil specs. I tried a 25cm mono coil made for another application and it worked fine for a 5uS delay. The coil was bundle wound with 0.1mm single conductor PTFE insulated wire and measured 207uH and 4.3Ω resistance. Winding is wrapped with polyethylene cable wrap and then overlaid with conductive fabric tape as a shield. Cable from electronics to coil is 35cm of RG179 which measures 16.8pF. Tx Mosfet is IRFD110 with a series resistance to the coil of 27Ω. Initially with no series diode I could sample comfortably at 5uS. With a 1N4148 series diode, I could improve it to 4uS at this setting with damping optimised with the aid of a trimpot. I tried a HER208 diode, but in this application it did not improve on 5uS for some reason.

Back emf was 110V with a small sign of avalanche on the IRFD110 drain. the drain capacitance held the 110V flat to the next Tx pulse and the pulse rate was 25,000pps.

Preamp is a single AD8055 which appears to recover faster than the OPA1611 in this application.

Detection range on audio was 12inches for a nickel and 7.5inches for a 1gm flat nugget; 1inch at coil edge for a 0.08gm flat nugget. Current draw for whole board was 110mA at 12V. A quarter only gives about 7inches range due to very short pulses.

Looking at the conditions in your picture of a creek, I think a 20cm coil would be about right, and maybe of the dual field type to improve the sensitivity to the smaller nuggets at the coil centre. Another possibility is a ferrite cored probe to get in between rocks and in crevices.

Eric.

[6666]

Hi Eclipse,
I have been doing a few more tests on coils suitable for small nuggets and have to revise my suggestions for coil specs. I tried a 25cm mono coil made for another application and it worked fine for a 5uS delay. The coil was bundle wound with 0.1mm single conductor PTFE insulated wire and measured 207uH and 4.3Ω resistance. Winding is wrapped with polyethylene cable wrap and then overlaid with conductive fabric tape as a shield. Cable from electronics to coil is 35cm of RG179 which measures 16.8pF. Tx Mosfet is IRFD110 with a series resistance to the coil of 27Ω. Initially with no series diode I could sample comfortably at 5uS. With a 1N4148 series diode, I could improve it to 4uS at this setting with damping optimised with the aid of a trimpot. I tried a HER208 diode, but in this application it did not improve on 5uS for some reason.

Back emf was 110V with a small sign of avalanche on the IRFD110 drain. the drain capacitance held the 110V flat to the next Tx pulse and the pulse rate was 25,000pps.

Preamp is a single AD8055 which appears to recover faster than the OPA1611 in this application.

Detection range on audio was 12inches for a nickel and 7.5inches for a 1gm flat nugget; 1inch at coil edge for a 0.08gm flat nugget. Current draw for whole board was 110mA at 12V. A quarter only gives about 7inches range due to very short pulses.

Looking at the conditions in your picture of a creek, I think a 20cm coil would be about right, and maybe of the dual field type to improve the sensitivity to the smaller nuggets at the coil centre. Another possibility is a ferrite cored probe to get in between rocks and in crevices.

Eric.

Hi Eric, when you made the coil you mention with conductive fabric tape as shield , do you remember when you wound the tape, did you over lap the tape so edges were touching, or did you leave a small space between the wraps ? thanks

[Elliot]

Hi Eric, What pulse width were you using and what delay was the EFE delay at?

Elliot