On 4/28/2019 6:18 AM, John H. wrote:
On Sun, 28 Apr 2019 01:19:50 -0400, wrote:

On Sat, 27 Apr 2019 21:43:45 -0700 (PDT), Its Me
wrote:

On Sunday, April 28, 2019 at 12:31:07 AM UTC-4, Bill wrote:


Why were the MOV to ground, instead of across the lines? Or a bigger value
Varistor to ground?

Because of the twisted pair, the danger isn't usually voltage spike across the pair, but rather the spike potential from the pair to ground. That's what we were trying to protect from. And what protection components on 66 punch blocks from back in the day did as well.

As far as the value, it's a bit of a tightrope. Too low of a value, and it's always firing and causing issues like we experienced. Too big of a value, and you may as well not have any protection on there at all. Even a transformer doesn't protect you, as it has an arc-over value. We thought the 180v parts would be OK, but we didn't realize that the lines would be as dirty as they were.


===

I'm wondering if you couldn't use an isolated op amp with differential
inputs to extract the signal. It would have to be totally isolated
from any ground reference to protect from the high voltage common mode
spikes. It would need a floating power supply of course, with the
output through an opto isolator or some such.

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I'm wondering if we could talk about artillery and tanks for a while, with maybe a bit of C-4
discussion thrown in! :)



Heh. I learned a long time ago that anyone who claims to understand the
physics of lightning and can reliably predict it's effects should be
taken with a grain of salt. :-)