Re: [Raspberry_Pi_4-Ham_RADIO] Pi protection
- 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
- On 08/04/2013 19:00, Robert Nickels wrote:
> I've isolated the cause of my Pi failure I think, and to no greatInteresting.
> surprise "the enemy is me".
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
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
The project does need some careful investigation and testing as to the
best way of buffering and earthing.
73 Eddie G3ZJO