Which brings up my other question from this discussion, what are the
failure mechanisms that would cause the Optocoupler to hold of 2 Kv
for a minute, but fail after an unspecified number of hours?
Is this dirt accumulation caused by the fields?, is it breakdown of
the plastics? is it too many electrons sitting for too long that they
finally make a run for it? Is it arcing
The answer is, of course, Fowler-Nordheim tunnelling and Poole-Frenkel
hopping, as anyone nows, errrrr....
The position I have now, which appears to be workable, is to use the
acnv2601 which will stand of Viorm of over 2.2kV. To do a straight
highspeed, bidirectional interface I can use 8 (!!!) for i2c, 4 for SPI
but 2 connecting using UARTs should work and only need 2 optos. Ok, the
device I want to talk to is still i2c but I can reform the i2c
conversation using a 8pin microcontroller.
One technology I was pointed at in a PM was using pulse transformers. I
read one manufacture's datasheet that said the high voltage impulse
tests should not be made repeatedly as the plastics degrade during the
test. I would guess plastic -> carbon -> conductor -> arcing -> fried
low voltage electronics. If the presence of the EHT causes gradually
degradation of the plastic in the same way plastics degrade in heat or
UV or other radiation then it is bad news at the end of the day.
Another guess is that one factor is the separation distance between the
light source and sensor. It's why 1/2 metre of fibre optic works so
well when you want to stand off 40kV. The acnv2601 datasheets makes a
lot out of the 13mm separation distance between the led and
phototransistor. Most other optos (eg 6n137) the distances are much
smaller. And in those big EHT switching opto diodes, very much bigger.
The problem with a lot of manufacture's datasheets is they tell you what
impulse voltage their devices will standoff but not what they will stand
off continuously. If you are connection two low voltage systems
together (low being, say, < 500V) then the question of wear probably
doesn't arise, but you get your protection from high voltage spikes.
The answer to your question is, of course, Fowler-Nordheim tunnelling
and Poole-Frenkel hopping, as anyone on the Clapham Omnibus knows,
errrrr....
Not a physicist or a chemist! I'm just glad I have a part I can use.
Grahame
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