piclist 2000\05\24\180514a >
Thread: 120VAC Traces
face BY : frmillerEraseME@plix.com

Thanks for your reply, Dan. Comments interlaced:

> Dan Michaels wrote:
> That being said, let me relate some of the most effective
> in my experience:
> 1. EMI and spike filters at the hiVAC power input
> - series EMI filter, MOVs, etc.

Could you give examples of the EMI filters. I assume you're
talking about something that goes across the contacts of the
relay switching the AC.

> 2. EMI/spike filtering on the power supplies -
> "transient voltage suppressor" diodes [eg, Microsemi 1.5KE
> series] on the P/S input side, downstream of the AC
transformer, if there is one.

Got these.

> 3. Bypass caps, of course, at all the usual points.

Got these, too.

> 4. Of course, diodes across the relay coils and
> snubbers across relay contacts.
>    NOTE - some loads produce significantly worse switching
>    than others, so the same snubber may not be universally
>    When looking across the contacts, you will see hi-speed
spikes [bad]
>    riding on a slower wave [normal] in cases where the
snubber is not
>    effective enough. You might want to use an isolation
xfrmr to do
>    this measurement.
>    NOTE - one thing you can do is sniff around with a
scope probe,
>    ungrounded, to measure presence of EMI, and determine
what it is
>    correlated to.

Got the diodes and snubbers, though obviously my snubbers
could use a redesign.

> 5. On the low-level signal lines, RC filters and transorbs
>    voltage suppressor diodes, which are essentially
>    zeners] are effective. Note - we ended up putting
transorbs on
>    practically ever signal line - and regular zeners are
too slow here.

I use TVS's at the power line and at the +5V input to
sensitive chips. Haven't put them on signal lines, except
for RS485 out. I was concerned with the amount of

> 6. Layout issues - physically separate hi/low V,
> and signal/switching circuitry. Separate power busses and
> planes [if possible]. Don't run different lines near each
other, and don't
> cross them.

This is what I was trying to accomplish by separating the
110VAC. Some of the spikes from the mag starters were
coupled back into the low V circuit either across the gap or
through the relay, which has to physically bridge that gap.
That's why I was asking about the ground plane (guard ring)
in that gap.

> 7. Use watch dog timer in uC s.w. Use debounce routines on
signal measurements.

Watch dog on the pics (2 17C44) as well as cpu supervisors

> 8. Use digital bus crowbar ckt in cases where chip latchup
is an especially
>  bad problem. [Note - the watchdog will not fix latchup].

I really miswrote. The Mux does not lock up; it resets.
Since it drives the relays, all the relays will drop out
when a spike happens. Not cool. I was actually able to fix
the problem by putting bypass caps on the Chip Enable and
Clear lines of the mux. However, I would like to give myself
some wriggle room by eliminating as much of the coupled
noise as possible. Since I have to redo the board anyway, I
was trying to find other suggestions, like the guard ring.


> Regarding hacking the test bd, try using aluminum duct
> grounded back to the hiV side [looks like relay power in
this case].
> In essence, you are putting a guard ring around the hiV
side which
> is intended to contain the internal fields. Likewise, you
> put a gnd guard ring around each other subsystem,
connected to
> the gnd of that subsystem.

This is the main info I was looking for. Will this be as
effective for testing purposes as traces on the board? Also,
would it be better to connect this gaurd ring to earth
ground or circuit ground?

Thanks for the info.



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