Dom Bragge,

I would like to add a brief caution regarding Bob Holme's response to the
1500 volt, 12-mil spacing question.

Bob stated:

"Air breakdown should not be a problem. The Paschen curve says that the maximum
spark gap for 1500 volts is about 8.5 mils.  If you assure a 12 mil space air 
gap breakdown will not be an issue."

The Paschen curve data assumes spherical electrodes suspended in space.  
This is not the case with typical etched traces on circuit boards.  As a result,
significant field enhancement will occur at the square edges of the traces.  My 
guess is you can't even hold off 1500 volts in air at the 12-mil gap without 
some sort of dielectric overcoat (soldermask or conformal coating).

I agree with Bob that the surface flashover is the biggest problem whether 
enhanced by dust and moisture or not (this can only be solved by completely 
encapsulating the traces in a good dielectric medium).  1500 volts over a 
gap of 12-mils is equivalent to 125 Kv/mil.  Almost any (pinhole-free)
dielectric material can withstand this voltage gradient.  We have solved these
kinds of design challenges in our high-voltage systems on our ion-beam 
accelerators by submerging the entire circuit in a box filled with Freon-TF 
(not always a practical solution but effective in eliminating even the dust 
problem).

Gary P.
---
                                  Gary D. Peterson
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