Announcement

Collapse
No announcement yet.

Opinion needed for optimal PI coil for gold hunting

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Opinion needed for optimal PI coil for gold hunting

    I have Surfmaster PI board and would need opinion for optimal coil setup for prospecting natural gold. I am thinking quite small diameter and very thin copper wire. The environment is Lapland in Finland where the soil is bit less infected with black sand than what I have heard being elsewhere, like in Australia.

    I have made modification to front end, original transistor is gone and faster FET is used. Also frequency, pulse width and r6 flyback shunt resistor are made adjustable for optimal triggering.

    Any good ideas?

    Thanks,
    Markus

  • #2
    Originally posted by msysta View Post
    I have Surfmaster PI board and would need opinion for optimal coil setup for prospecting natural gold. I am thinking quite small diameter and very thin copper wire. The environment is Lapland in Finland where the soil is bit less infected with black sand than what I have heard being elsewhere, like in Australia.

    I have made modification to front end, original transistor is gone and faster FET is used. Also frequency, pulse width and r6 flyback shunt resistor are made adjustable for optimal triggering.

    Any good ideas?

    Thanks,
    Markus
    Hi Markus.
    Not only the coil is problem for gold prospecting. Using PI principle, you should minimize the cable capacitance. Read this and if you have questions, I will try to answer:

    http://australianelectronicgoldprosp...2385/#msg12385

    http://australianelectronicgoldprosp...3669/#msg13669

    http://australianelectronicgoldprosp...2346/#msg12346

    Comment


    • #3
      Assuming you are looking primarily for gram-sized nuggets and not ounces, I would make the coil 10-15cm in diameter. The wire gauge is a trade-off... small reduces parasitic C, large reduces series R.

      The SMPI is not a good nugget hunting platform. While changing the PNP to a PMOS will help, a P-switch is still not as good as an N-switch. Also, the single-stage preamp really needs to be a 2-stage preamp to make it faster.

      I would try running the frequency up to 5kHz with a 100us TX pulse. Try to get down to 8-10us delay (absolutely requires a 2 stage preamp and low-C coil).

      - Carl

      Comment


      • #4
        Carl! What's with the two stage Preamp? What is the Benefit? Wouldn't that introduce More noise to deal with?

        I have a Sirf Kit unconstructed I too would like to make into a hand held gold probe for Crevicing. I was thinking of a Etched coil of about 4" for the job.

        I messed with a Garrett x-500 getting it down to 7us and it can see a 1/2 gram at 5".

        What limits the Surf Kit from Silverdog?

        Here in New Mexico working Bed Rock that cold work.

        Comment


        • #5
          The SMPI uses a single-stage preamp with a gain of 1000. The GBW is ~10MHz, so that means the BW=10kHz or a tau of 100us. That's too slow for a fast-response gold detector. Split the preamp into two stages, each with a gain of 33, and now the tau is 3.3us. The noise increase is minimal, and simply switching the first stage opamp to one with a lower noise will beat the single-stage NE5534.

          Comment


          • #6
            Big thanks for the answers.

            Carl, is the slow preamp baddest shortcome in SM, gold prospecting wise? It should not be a problem to split the single stage, in any ways I have been thinking of replacing the analog timer system with microcontroller for more sophisticated adjustment. What kind of amp you recommend for second stage in such setup? The microcontroller I have in shelf is AVR's ATMega. In such case it would also be easy to change the polarity of the FET. How much faster might the N-switch be compared to P-switch?

            Mikebg, thanks for the interesting point and for the links. I will get on to them as soon as my registration to the bulletin board is accepted.

            Thanks,
            Markus

            Comment


            • #7
              If you fix the preamp, and you don't need ground balance, and you make some timing changes, and you speed up the TX drive, then the SMPI will probably do fine for prospecting. Changing the PMOS to NMOS is not so easy, it usually requires a complete change of how power supplies are designed. My suggestion is leave the PMOS, but get one with a high breakdown to improve response speed. You might also want to look at the speed of the gate buffer.

              Comment


              • #8
                Thanks,

                About the original SM preamp, I guess it is only the Q1 (2N3906)? So do we want to split this component if we build 2-stage preamp setup? I am not very good with the terminology...

                Markus

                Comment


                • #9
                  I will take a stab at this
                  I think what carl is saying
                  change this single preamp
                  to two amps
                  something like this
                  Attached Files

                  Comment


                  • #10
                    Oh, makes it clearer :-)

                    I was stupid enough to think only Tx side...

                    Now I have to think how does this affect and how to implement the change.

                    Thanks,
                    Markus

                    Comment


                    • #11
                      I would make the coil 10-15cm in diameter.
                      Carl what inductance would you suggest to make the coil ?
                      Thanks

                      Comment


                      • #12
                        Originally posted by 6666 View Post
                        Carl what inductance would you suggest to make the coil ?
                        Thanks
                        The correct question is:
                        What resonance frequency should have the tank circuit formed by coil ?

                        Comment


                        • #13
                          Originally posted by Carl-NC View Post
                          The SMPI uses a single-stage preamp with a gain of 1000. The GBW is ~10MHz, so that means the BW=10kHz or a tau of 100us. That's too slow for a fast-response gold detector. Split the preamp into two stages, each with a gain of 33, and now the tau is 3.3us. The noise increase is minimal, and simply switching the first stage opamp to one with a lower noise will beat the single-stage NE5534.
                          I would like to understand this a bit more in detail. Is there any reference I could study to get into the terms GBW, BW and tau? In general I dont get the connection between preamp gain and speed.

                          Thanks,
                          Markus

                          Comment


                          • #14
                            Originally posted by msysta View Post
                            I would like to understand this a bit more in detail. Is there any reference I could study to get into the terms GBW, BW and tau? In general I dont get the connection between preamp gain and speed.

                            Thanks,
                            Markus
                            BW= Band Width the amp is able to produce.

                            GBW= Gain of said bandwidth

                            tau= Think of tau as how long it takes the amp to produce that response.

                            All frequency may not be equal.


                            Some of the Usable signal may be lost (Disguised) due to Phase shift and or delay.

                            Comment


                            • #15
                              Originally posted by homefire View Post
                              BW= Band Width the amp is able to produce.

                              GBW= Gain of said bandwidth

                              tau= Think of tau as how long it takes the amp to produce that response.

                              All frequency may not be equal.


                              Some of the Usable signal may be lost (Disguised) due to Phase shift and or delay.
                              The GBW (Gain Bandwidth) of an opamp is the bandwidth at unity gain (gain of 1). Strictly speaking, this is called the Gain Bandwidth Product, or small-signal bandwidth. For example, if your opamp has a GBW of 4MHz, then it will have a BW (bandwidth) of 400kHz when configured for a gain of 10. You simply divide the GBW by the gain. As a second example, the NE5534 has a GBW of 10MHz. If configured for a gain of 1000, the bandwidth of the circuit becomes 10kHz. By splitting the pre-amp into two stages (each with a gain of 33) you can boost the usable bandwidth to 303kHz.

                              Tau is the time required for the voltage to rise from zero to 63.2% of its final value.

                              Comment

                              Working...
                              X