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Re: [Raspberry_Pi_4-Ham_RADIO] Pi protection

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  • Robert Nickels
    ... FET or BJT should work I think John. I ve isolated the cause of my Pi failure I think, and to no great surprise the enemy is me . I enjoy checking into
    Message 1 of 6 , Apr 8, 2013
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      On 4/8/2013 11:36 AM, John wrote:
      >
      > Given W9RAN's experience with an overcooked pi, we could all benefit
      > from a discussion on protective connection to the device. I am
      > thinking in terms of a fet (2n7000?) buffer between the pi and the
      > outside world.
      >
      FET or BJT should work I think John.

      I've isolated the cause of my Pi failure I think, and to no great
      surprise "the enemy is me". I enjoy checking into the DX-60 Net on AM
      mode Sunday mornings, and curiously, the last WSPR spot was logged just
      about the time I did so. I got to wondering about how much RF the
      vertical antenna connected to the Pi could pick up from other
      transmitters/antennas in use here, and so I did a little experiment last
      night. With 100 watts into the dipole maybe 50 ft away - I measured 90
      volts peak-to-peak across the coax leading to the vertical that was
      attached to the Pi at the time! This voltage would have appeared at
      the output side of my low-pass filter which consists of inductors in
      series and capacitors to ground (which would have attenuated it
      somewhat) - and then to GPIO4 pin via a 0.01uf capacitor, which has an
      impedance of 4 ohms at the frequency in use. So, it seems likely that
      sufficient RF was present to damage the transistors on that I/O pin
      which are now causing excessive current consumption.

      Yeah, I should have known better, but made the mistake of thinking of
      the Pi like it was an actual transmitter instead of what it is. It's
      neat to prove the point but I'll never remember to disconnect or never
      transmit with an antenna connected to the Pi, so some kind of galvanic
      isolation and buffering is in the future. It would be a good way to
      include control over the output power as well as proper harmonic filtering.

      73, Bob W9RAN
    • John Ferrell
      I am pretty good at remembering to disconnect antennas. It is not unusual (even in a modest electrical storm) for the coax cables laying on the floor to arc
      Message 2 of 6 , Apr 8, 2013
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        I am pretty good at remembering to disconnect antennas. It is not unusual (even in a modest electrical storm) for the coax cables laying on the floor to arc across. Wind will generate a noisy arc as well.

        On 4/8/2013 2:00 PM, Robert Nickels wrote:
         

        On 4/8/2013 11:36 AM, John wrote:
        >
        > Given W9RAN's experience with an overcooked pi, we could all benefit
        > from a discussion on protective connection to the device. I am
        > thinking in terms of a fet (2n7000?) buffer between the pi and the
        > outside world.
        >
        FET or BJT should work I think John.

        I've isolated the cause of my Pi failure I think, and to no great
        surprise "the enemy is me". I enjoy checking into the DX-60 Net on AM
        mode Sunday mornings, and curiously, the last WSPR spot was logged just
        about the time I did so. I got to wondering about how much RF the
        vertical antenna connected to the Pi could pick up from other
        transmitters/antennas in use here, and so I did a little experiment last
        night. With 100 watts into the dipole maybe 50 ft away - I measured 90
        volts peak-to-peak across the coax leading to the vertical that was
        attached to the Pi at the time! This voltage would have appeared at
        the output side of my low-pass filter which consists of inductors in
        series and capacitors to ground (which would have attenuated it
        somewhat) - and then to GPIO4 pin via a 0.01uf capacitor, which has an
        impedance of 4 ohms at the frequency in use. So, it seems likely that
        sufficient RF was present to damage the transistors on that I/O pin
        which are now causing excessive current consumption.

        Yeah, I should have known better, but made the mistake of thinking of
        the Pi like it was an actual transmitter instead of what it is. It's
        neat to prove the point but I'll never remember to disconnect or never
        transmit with an antenna connected to the Pi, so some kind of galvanic
        isolation and buffering is in the future. It would be a good way to
        include control over the output power as well as proper harmonic filtering.

        73, Bob W9RAN


        -- 
        John Ferrell W8CCW
        That which can be destroyed by the truth should be.
        P.C. HODGELL
          
      • psk31
        ... Interesting. During all my tests with WsprryPi I disconnected all other TX s, even my 100mW MEPT on 28MHz. My last tests were on 474KHz driving a 1Watt PA
        Message 3 of 6 , Apr 9, 2013
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          On 08/04/2013 19:00, Robert Nickels wrote:
          > I've isolated the cause of my Pi failure I think, and to no great
          > surprise "the enemy is me".
          Interesting.

          During all my tests with WsprryPi I disconnected all other TX's, even my
          100mW MEPT on 28MHz. My last tests were on 474KHz driving a 1Watt PA via
          a FET Driver. I had second thoughts about putting my 10Watt 474KHz PA
          on, as I well know the RF field produced at these frequencies using Hi Q
          antennas.

          I was also paranoid about the earthing arrangements, my shack is
          elevated and isolated from mains Earth (which isn't Earth at all, PME,
          yuck), so the shack Earth lead is long and can produce some tingles
          between it and antennas.

          The Pi is not designed for radio use of course, powered by the mini USB
          port that will be the earth point. When using LPF's on GPIO4 I used a
          nearby earth pin as the return, I am not sure about the wisdom of that
          and where the track goes, in RF design it would be direct to an earth
          plane.

          The project does need some careful investigation and testing as to the
          best way of buffering and earthing.

          73 Eddie G3ZJO
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