On 4/28/2019 9:38 AM, Its Me wrote:
On Sunday, April 28, 2019 at 1:19:56 AM UTC-4, 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.

The perfect isolator for those common mode spikes is a transformer. It doesn't have any reference unless you give it one. You can certainly use an op amp to terminate and receive that twisted pair signal (and we did at times), but a large enough spike will take it out. There's no way to completely protect it. Once you start jumping air gaps inside the IC, or exceeding the reverse voltage spec on junctions in there, things start blowing up.



The vacuum chamber systems I used to design and build often had multiple
banks of quartz lamps inside used to heat optics to be coated to a
temperature that permitted ideal conditions for the evaporated materials
to condense. It was also to add energy to drive off any remaining
water vapor molecules that might still be on the optics. The lamps
ran on up to 240 vac, controlled by a proportional SCR controller.
Under vacuum conditions, remaining gas molecules on the optics or
fixturing often caused arcs to occur, very similar in nature to
mini-lightening strikes. Those arcs (to ground) would wipe out either
the fast acting (and expensive) SCR fuse or sometimes the SCR itself.

To minimize this we used a large, 1:1 (ratio) isolation transformer
that removed the steel chamber as the primary ground potential. It
solved most of the nuisance but expensive fuse blowing and provided some
extra protection for the SCR itself. Wasn't 100 percent effective
but reduced the blowing of the fuse to maybe once or twice a year.



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