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  • Coil Damping question

    I am building a surf pi and I am having some issues with it so I tried a couple different coils. I put a damping pot in place of the resistor to adjust the damping to the different coils.Watching the preamp output I adjusted for no ringing but just no more. value was 650 ohms,I noticed that when I move a coin target near the coil a peak rises in the curve of the preamp output instead of the normal whole curve moving away from the TX pulse.I suppose I am wrong in thinking that you need just enough resistance to stop ringing.I thought lower value damping resistors hurt sensitivity.What is the method for optimizing the damping resistor? Coil used is 450uH 24awg and 2ohms or so 11.5" round

  • #2
    What MOSFET do you use? In case you have somewhere some old one try and see what happens. Coss as the consequence of low Rdson of the nowadays MOSFETs might be a culprit.

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    • #3
      Originally posted by Davor View Post
      What MOSFET do you use? In case you have somewhere some old one try and see what happens. Coss as the consequence of low Rdson of the nowadays MOSFETs might be a culprit.
      It is the one supplied in the kit irf9640 I think,It has alot of Coss over 300pF. I tried another with less Coss but it was only 100v so it clipped the flyback to much I think.I have not found any good high volt rated pnp mosfets yet. I want to investigate changing the pnp to an npn so I can use igbt,I have some of those on order 600v and 21pF output capacitance. I am going to work on it some more today.

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      • #4
        I am not sure how the voltage rating of the mosfet really affects these pi machines I am building,I worked on the barracuda today and I used the irf9520 100v device and I have better results than ever,I damped the same 440uh coil on the barra with 1.5kohms resistance (dont know how that happened yet)and the mosfet just gets warm.If I put a sink on it I dont think it will be a problem.It does take the preamp 20us to come out of saturation though so with this setup I can only sample at 25us.

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        • #5
          Originally posted by Brian Deese View Post
          I am not sure how the voltage rating of the mosfet really affects these pi machines I am building,I worked on the barracuda today and I used the irf9520 100v device and I have better results than ever,I damped the same 440uh coil on the barra with 1.5kohms resistance (dont know how that happened yet)and the mosfet just gets warm.If I put a sink on it I dont think it will be a problem.It does take the preamp 20us to come out of saturation though so with this setup I can only sample at 25us.
          Resistance of coil for your barra should be 1.5Ω not 1.5KΩ, try it with inductance from 250uH to 450uH, and try to use lower dumping resistance you will get even better results.
          Try to set up sample abouth 10uS it will be much more sensitiwe to small gold.
          When you set up your dumping resistance with pot, connect is series resistance abouth 100-200Ω to prewent shortcut.

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          • #6
            Originally posted by detecto View Post
            Resistance of coil for your barra should be 1.5Ω not 1.5KΩ, try it with inductance from 250uH to 450uH, and try to use lower dumping resistance you will get even better results.
            Try to set up sample abouth 10uS it will be much more sensitiwe to small gold.
            When you set up your dumping resistance with pot, connect is series resistance abouth 100-200Ω to prewent shortcut.
            You misunderstand me,Let me clarify. The coil is 440uh,24awg wire,1.6ohms resistance,with no faraday sheild.I do have test pot with series resistance.The damping resistor is 1.5k which is higher than I expected,but there is no ringing on the preamp output.Looks exactly the same from 550ohm-1500ohms,after 1500 it starts to ring.

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