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<p>Hi Paul,<br>
</p>
<p>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. <br>
</p>
<p>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).<br>
</p>
<p>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.<br>
</p>
<p>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: <a
href="https://www.dxengineering.com/search/part-type/coaxial-lightning-protectors">https://www.dxengineering.com/search/part-type/coaxial-lightning-protectors</a>
There are no doubt smaller/cheaper ones designed for the type of
radio connection you would be using.)<br>
</p>
<p>-Pete<br>
</p>
<div class="moz-cite-prefix">On 3/21/20 11:47 AM, The MacDougals via
TriEmbed wrote:<br>
</div>
<blockquote type="cite"
cite="mid:021501d5ff97$f7ca1be0$e75e53a0$@acm.org">
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<div class="WordSection1">
<p class="MsoNormal">This month’s problem of the month was about
an electric fence monitor.<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><a
href="http://everycircuit.com/circuit/5798493590650880/electric-fence-voltage-monitor"
moz-do-not-send="true">http://everycircuit.com/circuit/5798493590650880/electric-fence-voltage-monitor</a><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">During the meeting, I was pointed to
Panasonic high surge resistors at Digikey.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">After a bit of searching, this is what I
think would work.<o:p></o:p></p>
<p class="MsoNormal"><a
href="https://www.digikey.com/product-detail/en/HVR2500001005JA100/BC4429CT-ND/7351859/?itemSeq=321321346"
moz-do-not-send="true">https://www.digikey.com/product-detail/en/HVR2500001005JA100/BC4429CT-ND/7351859/?itemSeq=321321346</a><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">10000V<o:p></o:p></p>
<p class="MsoNormal" style="text-indent:.5in"><span
style="color:black">R1 – 10Mohm <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">7514.9V<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">
R2 – 10Mohm<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">5029.8V<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">
R3 – 10Mohm<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">2544.7V<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">
R4 – 10Mohm<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">59.6V<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">
R5 – 220Kohm<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">4.97V </span><span
style="font-family:Wingdings;color:black">ß</span><span
style="color:black"> Arduino monitors here<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">
R6 – 20Kohm<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">GND<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black">Any comments
about these resistors?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black">I was thinking of
soldering them in series and then potting them with this:<o:p></o:p></span></p>
<p class="MsoNormal"><a
href="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"
moz-do-not-send="true">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</a><o:p></o:p></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black">---> Paul<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<br>
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