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gEDA-user: earth ground & EMI
Ok, I'm working on the new furnace board (yes, finally!). The board
has four mounting holes, which go to four hard points in the box. The
points and box are metal, attached to the metal shell of the furnace
(recall that the furnace has motors, relays, and an electrostatic
filter in it). So, I have four "earth ground" points available, one
in each corner.
Two of the corners are near the 24vac circuits, I don't think they
need to deal with them at all. One of those two is near the
fuse/rectifier of the power supply, though.
Another corner is near the output of the power supply, both the
isolated switcher and the two LDOs.
The fourth corner is near the I/O block for the cabling that runs to
the thermostats. Each line has an LCL ("T") filter that can block 12+
MHz. The capacitor in those filters needs to be connected
to... something. Normally, I'd just ground them, but I'm going to put
filters on the power and ground terminals too (unless that's stupid).
Where do I connect those caps? I'm thinking earth ground, but I don't
want to pull in *more* EMI that way. The only other choice is to use
gnd for the power filter, and power for the gnd filter, but even then
- connect it before or after the other line's filter?
PCB GND ----OOO----*----OOO---- GND cable to thermo
|
-----
-----
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?
Another question is if it would be beneficial to have a copper trace
that goes around the edge of the board on all layers to block EMI
coming in from the edges. I've seen papers suggesting a grounded
"fence" to keep EMI from *escaping* from the edges (mostly of power
planes), but nothing about keeping it out. Does anyone know if such a
fence would be advantageous in my case? If so, should it be earth
ground, or pcb ground?
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