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  • Originally posted by mickstv View Post
    Hi Tinkerer, no worries. Just wondering if you could post up some specific details for the coil ie:- diameter + turns ratio etc. Will it be suited to smaller coils 8 to 12"



    Cheers
    Mick
    The peak coil current is about 4A and the TX power consumption is about 100mA or 1.2W out of 12 battery, so the coil wire needs to be thick.

    About AWG14 or AWG12 and about 300uH, not much different from the well known ML coils.

    The main difference is that the coils are induction balanced.

    OK, I will calculate some different coil specifications and post them in the days to come, say 40cm, 30cm and 20cm.

    For a general purpose coil, I suggest about 40cm diameter.

    Smaller coils will give more sensitivity and less detecting depth.

    Tinkerer

    Comment


    • Originally posted by Tinkerer View Post
      The peak coil current is about 4A and the TX power consumption is about 100mA or 1.2W out of 12 battery, so the coil wire needs to be thick.

      About AWG14 or AWG12 and about 300uH, not much different from the well known ML coils.

      The main difference is that the coils are induction balanced.

      OK, I will calculate some different coil specifications and post them in the days to come, say 40cm, 30cm and 20cm.

      For a general purpose coil, I suggest about 40cm diameter.

      Smaller coils will give more sensitivity and less detecting depth.

      Tinkerer


      I was thinking about just building the TX section first just to see how it actually works. The part I'm unsure about is the bucking coil arrangement, I'm sure you've posted about this coil design before but could you please explain how to build a basic coil again, or maybe you have a picture of one that you could post ?



      Cheers
      Mick

      Comment


      • Originally posted by Tinkerer View Post
        Thanks for the feedback, Midas.

        Adding filters before the pre-amp, seems a good idea, we do really not want to amplify the noise coming form the coil.

        The input is coming from a differential RX coil. Adding a low pass filter for common mode noise and a low pass filter for differential mode noise, reduces the bandwidth to the minimum necessary.
        The minimum will depend on the intended targets. If we want to look for very small hypodermic needles, we need to keep the fo above 500kHz.
        In other words, we eliminate all the noise above the desired target response frequency, or maybe we should call it Tau, since it is a single decay curve.

        By the way, do not use these filters on a traditional PI. It will not work.

        Tinkerer
        No sorry, I'm being too vague again. I meant the high pass filters on the the gate signal lines from the PIC to the 4066. More specifically C12,R60 and C32,R61.

        Comment


        • Originally posted by mickstv View Post
          I was thinking about just building the TX section first just to see how it actually works. The part I'm unsure about is the bucking coil arrangement, I'm sure you've posted about this coil design before but could you please explain how to build a basic coil again, or maybe you have a picture of one that you could post ?



          Cheers
          Mick
          http://www.geotech1.com/pages/metdet...planar_300.pdf

          The above description by Dave Emery, is about the best I can come up with. One difference is the wire thickness, due to the high peak coil current.
          Since the TINKERERS_SB TX has a resonant component, it is very important to keep the coil and cable resistance as low as possible.

          I have posted many posts on the coil building, but they might be spread out over the whole forum. It takes some time to search for them.

          Also look for my graphite shielding posts.

          Eventually I intend to make a CD with all the pertinent information, posted over the years, but that will take some time too.

          Tinkerer

          Comment


          • Originally posted by Tinkerer View Post
            http://www.geotech1.com/pages/metdet...planar_300.pdf

            The above description by Dave Emery, is about the best I can come up with. One difference is the wire thickness, due to the high peak coil current.
            Since the TINKERERS_SB TX has a resonant component, it is very important to keep the coil and cable resistance as low as possible.

            I have posted many posts on the coil building, but they might be spread out over the whole forum. It takes some time to search for them.

            Also look for my graphite shielding posts.

            Eventually I intend to make a CD with all the pertinent information, posted over the years, but that will take some time too.

            Tinkerer

            Thanks Tinkerer. I've only ever mucked around with mono coils so this bucking coil business was new to me, but I understand now.


            Mick

            Comment


            • Originally posted by Midas View Post
              No sorry, I'm being too vague again. I meant the high pass filters on the the gate signal lines from the PIC to the 4066. More specifically C12,R60 and C32,R61.
              OK, I see now. I could not remember any high pass filters on this design.

              C32,C12, R61, R60, are level shifters.

              The PIC and TX are running on the 0V-12V battery rail to keep the switching noise isolated as much as possible.
              I also suggest to make 2 separate PCB's and a shield between to keep the EMI pf the TX & PIC out of the analog circuit.

              The analog part is running on +/-5V centered on the 6V rail.

              This makes the PIC pulse to be -6V as seen by the analog GND, so the C32 and C12 bring the pulse up and centered on the Analog GND.

              I have updated the design and added a resistor of 56k to the -5V, so that the PIC pulse triggers the CD4066 properly.

              Thanks for bringing this to my attention. I will post a revised version of the design in a few days, when all the mistakes have been discovered and corrected.

              SO, PLEASE, EVERYBODY SEARCH MISTAKES AND MAKE SUGGESTIONS, SO THAT WE CAN POST ALL THE CORRECTIONS IN ONE SINGLE POSTING.


              Tinkerer

              Comment


              • Originally posted by Tinkerer View Post
                OK, I see now. I could not remember any high pass filters on this design.

                C32,C12, R61, R60, are level shifters.

                The PIC and TX are running on the 0V-12V battery rail to keep the switching noise isolated as much as possible.
                I also suggest to make 2 separate PCB's and a shield between to keep the EMI pf the TX & PIC out of the analog circuit.

                The analog part is running on +/-5V centered on the 6V rail.

                This makes the PIC pulse to be -6V as seen by the analog GND, so the C32 and C12 bring the pulse up and centered on the Analog GND.

                I have updated the design and added a resistor of 56k to the -5V, so that the PIC pulse triggers the CD4066 properly.

                Thanks for bringing this to my attention. I will post a revised version of the design in a few days, when all the mistakes have been discovered and corrected.

                SO, PLEASE, EVERYBODY SEARCH MISTAKES AND MAKE SUGGESTIONS, SO THAT WE CAN POST ALL THE CORRECTIONS IN ONE SINGLE POSTING.


                Tinkerer
                Ok cheers, I get it now. AC coupling, followed by injection of new DC level of a touch over 2v from battery ground. Which should allow the PIC's 5v span to safely cover the worst case scenario of 7v high, 3v low required by the 4066. Without the 56k resistor it wouldn't have gone low enough. 70k might be better though, just so its nicely centered, .5v over the requirements on both ends.

                Have you given any thought to where you are going to put the PIC (ie. TX board or RX board), and how you are going to organize the power supplies if you start using its on-board ADC for audio? It would seem there would have to be some compromises.

                Midas

                Edit: No sorry all these different levels are confusing aren't they... The DC level you want to inject is 2.5v above -5v right? So I think what you want is another 100k to -5v..

                Comment


                • I probably should have mentioned that its also down to the duty cycle of the signal. I was just assuming that the signal lines spend the vast majority of their time low, which actually may not be entirely true for all sample schemes. You don't have a whole lot of margin for drifting around as you change the sample time\pulse length ratio so your probably best off putting a diode in as well.

                  Midas
                  Attached Files

                  Comment


                  • Originally posted by Midas View Post
                    I probably should have mentioned that its also down to the duty cycle of the signal. I was just assuming that the signal lines spend the vast majority of their time low, which actually may not be entirely true for all sample schemes. You don't have a whole lot of margin for drifting around as you change the sample time\pulse length ratio so your probably best off putting a diode in as well.

                    Midas
                    Thanks for the excellent suggestion. The Sample pulses are between 5 and 10us high and the rest of the time low. The diode will save power.

                    Tinkerer

                    Comment


                    • Hi Tinkerer, any chance you could post up a CRO shot of one TX pulse of the output, so that I can see what is expected to be seen. Maybe the CRO shot could be done via a second coil not in null ?

                      I roughed up a basic TX today and all I got was a over heated FET with lots of ringing. Maybe I did something wrong.




                      Cheers
                      Mick

                      Comment


                      • Originally posted by Midas View Post
                        Ok cheers, I get it now. AC coupling, followed by injection of new DC level of a touch over 2v from battery ground. Which should allow the PIC's 5v span to safely cover the worst case scenario of 7v high, 3v low required by the 4066. Without the 56k resistor it wouldn't have gone low enough. 70k might be better though, just so its nicely centered, .5v over the requirements on both ends.

                        Have you given any thought to where you are going to put the PIC (ie. TX board or RX board), and how you are going to organize the power supplies if you start using its on-board ADC for audio? It would seem there would have to be some compromises.

                        Midas

                        Edit: No sorry all these different levels are confusing aren't they... The DC level you want to inject is 2.5v above -5v right? So I think what you want is another 100k to -5v..
                        The general idea is to keep all noisy parts together and away from the analog, where the noise is really not desired.

                        On the above circuit I have decoupled the audio transistor and added the capacitors to keep the hard switching square wave noise out of the supply.
                        In fact, I see no reason why the whole audio should not be on the TX board.

                        Tinkerer

                        Comment


                        • Originally posted by mickstv View Post
                          Hi Tinkerer, any chance you could post up a CRO shot of one TX pulse of the output, so that I can see what is expected to be seen. Maybe the CRO shot could be done via a second coil not in null ?

                          I roughed up a basic TX today and all I got was a over heated FET with lots of ringing. Maybe I did something wrong.




                          Cheers
                          Mick
                          There are several mistakes that can cause ringing. If you show me a CRO shot, I might be able to tell you where the problem lies.

                          Also, did I post the Hex file for the TINKERERS_SB_NM?

                          Attached is a picture of a TX voltage pulse and a TX current pulse. This is what the wave forms should approximately look like.

                          Careful with the current measurement, not to damage the CRO. I use a 100mOhm resistor in series with the drain coil wire. A smaller resistor is even better.

                          Tinkerer

                          Now, that is funny, I can not add any attachments. I will send this off and try with a different browser.

                          Comment


                          • TX CRO pictures

                            OK, the attachments seem to work again. Must have been just a quirk of the internet.


                            Tinkerer
                            Attached Files

                            Comment


                            • Originally posted by Tinkerer View Post
                              There are several mistakes that can cause ringing. If you show me a CRO shot, I might be able to tell you where the problem lies.

                              Also, did I post the Hex file for the TINKERERS_SB_NM?

                              Attached is a picture of a TX voltage pulse and a TX current pulse. This is what the wave forms should approximately look like.

                              Careful with the current measurement, not to damage the CRO. I use a 100mOhm resistor in series with the drain coil wire. A smaller resistor is even better.

                              Tinkerer

                              Now, that is funny, I can not add any attachments. I will send this off and try with a different browser.

                              Thanks for posting the pictures.

                              No you haven't posted the HEX file. I was of the understanding that the TEM was driven with a standard TX pulse so I used one of my pic's.

                              I can't post a Picture at the moment due to a issue my end, but to try and explain it, it just looked like an undampened/ringing waveform. Like a standard PI without the damping resistor fitted. I've obviously done something wrong.


                              In the pictures what wast he TX on period ?


                              Mick

                              Comment


                              • Originally posted by mickstv View Post
                                Thanks for posting the pictures.

                                No you haven't posted the HEX file. I was of the understanding that the TEM was driven with a standard TX pulse so I used one of my pic's.

                                I can't post a Picture at the moment due to a issue my end, but to try and explain it, it just looked like an undampened/ringing waveform. Like a standard PI without the damping resistor fitted. I've obviously done something wrong.


                                In the pictures what wast he TX on period ?


                                Mick
                                The pictures are only meant to show the wave shape. For example, the current picture shows a 12A pulse, while the TINKERERS_SB uses only 4A.

                                I have a HEX file ready, I just want to test run it first before posting.

                                The Mosfet driver expects a pulse that is ON for about 97us and OFF for 7us, for a total cycle of 104us or about 10k PRR. This is when using a 300uH coil and a 4.7nF capacitor.
                                You should then get a Flyback of about 550V.

                                This is the INVERSE of a traditional PI, where the TX is ON for a short time and OFF for a long time.

                                If you use a traditional PI pulse timing, the coil will ring and the Mosfet will get hot and might self destroy.

                                Tinkerer

                                Comment

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