Re: gEDA-user: basic anti-EMI design q

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Thu, Mar 23, 2006 at 09:39:31PM -0500, DJ Delorie wrote:
> 
> Ok, you all know about the furnace project.  I'm starting to think
> about the next rev of the board (cpu, ethernet, etc onboard).  Mostly,
> I want to eliminate the beige box on the floor and redesign the system
> to be a lot more EMI tolerant.
> 
> Keep in mind, this board is mounted inside a five-sided metal box,
> which is mounted on the metal air duct coming out of our furnace.
> This is the same furnace that includes a couple of induction motors,
> an electrostatic air cleaner, fire, water, refrigerant, and 18 gauge
> wires running all over the house.
> 
> Needless to say, the existing board suffers from interference.  On the
> current board, the latches occasionally change state on their own, so
> I continuously set them to limit the fault time, but I can't reset
> them often enough to keep the furnace from clicking on-off
> occasionally.
> 
> For the next board, I've planned a number of changes to protect the
> board.  The power supply is onboard (it's got to have 24vac anyway, I
> get +5 from that).  All I/O lines running around the house are
> isolated behind FETs (read and write), with LCL filters, shunt diodes,
> and small resistors.  Yes, even +5/gnd going out to the thermostats is
> LCL filtered.  There will be an opto signalling the CPU at 120Hz from
> the AC line (for master RTC and re-setting the ports).
> 
> The design has 10baseT (20MHz) and a 32MHz CPU, plus the 150KHz or so
> switching power supply.  Looks like 8/8 rules suffice, which fit the
> 0.5mm pitch nicely.  I/O lines are 1wire and 9600 baud serial.  Power
> switching is 24vac alternistors with opto isolation (no relays).
> Board is 5.5 x 3.5 inches, and the four standoffs are metal, so
> chassis ground is available (if attaching yourself to a spark
> generator is a good idea).
> 
> So, the big question is - ground/power planes.  I could probably
> squeeze the design onto two layers, but power and ground traces would
> be going all over the place.  Going to four layers gives me power and
> ground planes, with easier signal routing.  An auxiliary question is,
> should I split the planes?  I'm thinking, isolating the power/gnd
> going to the I/O drive FETS and thermo power (i.e. the stuff going
> around the house) back to a common point near the power supply, and
> maybe splitting the 10baseT analog power similarly.
> 
> Based on a goal of "minimize the effect of external EMI interference",
> what makes sense?

I am just redesigning Ronja Twister because the shielding was crappy.
It will be a board with continuous ground plane all around, with
solderable edges that will be soldered to tin plated shielding box. The
lids of the box will be soldered down all around the edge.

If thing A inside will interfere with thing B inside, I'll make a
partition inside in the same style, increase number of lids by 2 and
pass signals through the partition by a tiny opening blocked by
RLC filters.

All the system will be shielded including all cables along their lenght,
the connectors will be Canon with gold-plated pins (ensure shield
connection without transition resistance) with metal shield shell and
screwed down tight.

So the Faraday cage will be effectively without any openings.
I guess this should work for anything in any situation.

CL<