Hi Paul,
These resistors are designed to survive overloads for short periods. The
only thing that's going to overload a 10M resistor is a direct lighting
strike. With that scenario, it's very difficult to predict what will
happen. For instance if the fence wire makes a sharp turn on its way to
the 10M resistor the lightning might treat it as an open anyway by
virtue of the inductance.
But with amateur radio 160/80/40m receive antennas, up to thousands of
feet of wire in a "Beverage" antenna (named after a guy named Beverage)
is typically terminated with a resistor of just a few hundred ohms
connected to ground. Using the most sturdy resistors possible is a hedge
against replacing them frequently vs occasionally, as the induced
potentials of close lightning strikes regularly drives them to high
current levels. But these are very much lower value resistors than with
your application. But for this amateurs go a step further, using carbon
composition resistors vs metal film, to avoid the tuning side effects of
the inductance of metal film parts. You don't care at all about that, so
I think the ones you found will be excellent. The very high resistance
values at the beginning of the string should make the power dissipation
irrelevant (for receive antennas we'd go with the biggest available and
that's two watts for a single part that can be put in series/parallel
combinations for higher power situations).
So a string of 10M resistors followed by the lower and lower values,
with the lowest closest to ground should be safe for everything but a
close strike. The shunt resistors used to measure current from laser
cutter power supplies use this technique, just putting the resistor
between ground and the tube's ground return path so the voltage drop
across the resistor is limited. And the datasheet of the resistor you
found specifically says it can handle a 10KV pulse, so I think you're
golden.
However, in my opinion if you want the MPU to survive out there you
should take steps to avoid possible current paths in and out of it. For
example, a long dangly piece of wire such as for an antenna would be
terrible. A long cable for the "data" would be a recipe for destruction.
Just imagine everything there is like the winding of a transformer, with
a lightning surge providing the current through the other winding.
Everything but the last resistor of the string inside a metal box tied
to a ground rod with just a little plastic window for a WIFI antenna
might withstand a close strike. With an external antenna you MUST have
an antenna with an unbalanced feed where the coax can go through a
lightning surge suppressor specifically designed to survive it. But of
course if the system being blown up doesn't cost much, you could just
rebuild it and ignore the whole subject of survival with a suppressor
(here's a page of them, but designed for very much higher transmit power
than you need:
https://www.dxengineering.com/search/part-type/coaxial-lightning-protectors
There are no doubt smaller/cheaper ones designed for the type of radio
connection you would be using.)
-Pete
On 3/21/20 11:47 AM, The MacDougals via TriEmbed wrote:
This month’s problem of the month was about an electric fence monitor.
http://everycircuit.com/circuit/5798493590650880/electric-fence-voltage-monitor
During the meeting, I was pointed to Panasonic high surge resistors at
Digikey.
After a bit of searching, this is what I think would work.
https://www.digikey.com/product-detail/en/HVR2500001005JA100/BC4429CT-ND/7351859/?itemSeq=321321346
10000V
R1 – 10Mohm
7514.9V
R2 – 10Mohm
5029.8V
R3 – 10Mohm
2544.7V
R4 – 10Mohm
59.6V
R5 – 220Kohm
4.97V ßArduino monitors here
R6 – 20Kohm
GND
Any comments about these resistors?
I was thinking of soldering them in series and then potting them with
this:
https://smile.amazon.com/Clear-Electronic-Grade-Silicone-Squeeze/dp/B0063U2RPW/ref=sr_1_19?crid=2RXV4MC2DX1Y2&dchild=1&keywords=potting%2Bcompound%2Bfor%2Belectronics&qid=1584805104&sprefix=potting%2Bcom%2Caps%2C165&sr=8-19&th=1
---> Paul
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