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1264Re: [Raspberry_Pi_4-Ham_RADIO] WsprryPi for WSPR15

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  • Jim Thisdale
    Apr 9, 2013
      Couple thoughts:

      1 - most wall wart power supplies will NOT provide a GROUND, they can't if they are the simple 2-pin
      ac plug type since they only use hot and neutral. Even if you found a 3-pin one, most likely the DC
      output will be isolated from the AC connector.

      2 - the ethernet connection will not normally supply a ground to the pi either.

      3 - If you want the Pi grounded you are probably best off to supply a good ground to the pi separately.

      4 - If your using the GPIO pins as OUTPUTS some isolation/buffer chips/transistors are highly
      recommended, especially to drive an antenna.

      5 - If using the GPIO pins as inputs, buffering/isolating is almost mandatory, especially since they
      are 3.3v max.

      6 - The micro-usb power connector is the only fused power input. Power applied anywhere else needs
      to be fused externally from the board. (option - modify the board)

      7 - good news, Pi's are in-expensive... Not like frying a $800+ computer.

      -Jim- N1JMM, KB1YPL_B
      Visit my Pi running ircddb, dvrptr, Apache & many other things:

      On 4/7/13 8:02 PM, Robert Nickels wrote:
      > Sad news to report ;-( After two days of amazing spots on WRPR it
      > seems my Pi has died while in Wsprry Pi service - although I have no
      > idea whether it was KIA or just coincidence.
      > I'd switched the antenna to a vertical last night and started the beacon
      > around 0445 UT in order to evaluate it's performance against the beam
      > I'd been using on 20 meters. This morning I was surprised to only see
      > no spots after 12:22 UT, and upon investigation I discovered that the
      > beacon had failed and the Pi would not boot up. Upon further
      > investigation using a bench power supply, I observed excessive current
      > consumption (1.1 A from the 5 V supply) and the output of the 3.3 volt
      > regulator is low, and drops even lower the longer power is applied - to
      > less than 2 volts. One time I was able to observe a partial boot
      > sequence before everything dropped out. The 3.3 volt regulator package
      > is hot to the touch after such attempts and the 3.3 V line measures < 2
      > ohms to ground. Fried Pi!
      > My theory is that some part of the SOC has been damaged, and while the
      > CPU part is still intact (and tries to run), the damaged part is
      > drawing so much current that the regulator fails to achieve the 3.3 VDC
      > needed (or drops below that level very quickly). The polyfuse is
      > fine, and was not in the circuit at the time as I was feeding power to
      > the Pi via the GPIO connector based on a suggestion I found on one of
      > the R-Pi sites to eliminate having to use the microUSB cable. I
      > wasn't aware until I looked at the schematic today that this method
      > bypasses the fuse, although I'm not sure how that would have made a
      > difference in this case.
      > For Wsprry use, the low pass filter is capacitively coupled to GPIO pin
      > 7 (R-Pi) which is connected directly to the GPIO4 (Broadcom) pin on the
      > BCM2835. Hindsight being 20/20, I now wonder about the wisdom of
      > coupling a outside antenna to a GPIO pin that goes directly to the CPU
      > chip without any kind of isolation. Transient voltages such as
      > lightning or static buildup as well as RF from other transmitters
      > operating into nearby antennas are a concern. And, a ground loop is
      > created between the shielded side of the antenna coax and an other
      > device connected to the Pi that wind up grounded via the AC powerline
      > neutral or grounding conductors. My theory is that the GPIO was
      > damaged by one of these means, or in some other way that I haven't
      > thought of yet.
      > I think that one stage of isolation using a simple BJT amplifier would
      > provide cheap insurance, as would galvanic isolation between the antenna
      > and the Pi. I plan to try both when the replacement Pi arrives.
      > Meanwhile, I thought others could benefit from this experience.
      > Any thoughts or theories on this would be appreciated as well,
      > especially by Guido and others who have been using Wsprry Pi longer than
      > I have.
      > 73, Bob W9RAN
      > ------------------------------------
      > Yahoo! Groups Links
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