Re: gEDA-user: OFF: triac

[DJ Delorie <dj@xxxxxxxxxxx>]

> I'm not sure what the polarized things at the top are - but you
> could consider bringing then closer to the centre of the board,

The top strip is the power supply.  R-L: Fuse, bridge rectifier, bulk
caps, isolated switching regulator.  Putting it along the top keeps it
out of the way.  There are probably some minor components I haven't
pictured.

The power/ground planes are fed from the upper left corner, split into
three wedges - one for MCU digital, one for ethernet analog, and one
for the I/O blocks.  At least, that's my thinking so far.  It keeps
transients from the wiring away from the logic.  I hope.

> perhaps also consider 5.08 pitch pcb sockets and matching line plugs,

I've looked at them, but furnace controllers are almost always
screw-blocks off stripped 18ga solid wires.  The wires already exist,
I have to accomodate them.  The Weidmuller blocks at least are spring
loaded and top-access only (the hole for the wire is on the top, just
push it in to engage; the button to release is also on the top).

The wires don't change much, and I don't have a problem with the TB's
I currently have, except for how much space they take up.

> If you get hit by lightning lots of ESD protection won't help much.

The board is mounted in a metal box on the side of my furnace.  The
furnace includes a couple of motors and relays, and an electrostatic
air filter.  It's not lightning I'm worried about, it's the zaps from
the filter and stray pickups from the long wires to the thermostats.

> ESD protection does help against a nearby strike.  Make sure you
> provide an easier path to ground - which has to be complemented by a
> good high-current ground path that is _not_ back through your PC.

I'm contemplating grounding it through the furnace itself, since it's
a big chunk of metal anyway.  I have the 120VAC ground available to me
too, with some extra wiring, and some plumbing.  I worry that I'll
pick up more junk from that though.  Hmmm... the LCL protectors I'm
looking at have an independent ground; I could tie those and the spike
zeners to the chassis ground directly, and the logic ground through
its own ESD circuit.

But the general idea was to have both power and ground isolated at all
points, keeping it symmetrical, with only weak ties to earth ground
just to keep the potential low.  My main goal is to keep noise away
from the MCU, since the current board suffers from that.

The new board won't need a PC - eventually.  Hence the ethernet port :-)

At first I'll use the serial port to tie it into the existing PC to
test it out and get it working.  I have windows of opportunity in the
spring and fall to work on these types of things.

> make heatsink and don't touch labels both easier :)

The alternistors are way overrated for their load; heatsinks aren't
required.  Plus, adjacent ones are rarely on at the same time.  In
operation, the board is inside a metal box anyway.  I don't think
"don't touch" is needed, it should be safe to touch at any time.