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'[EE]: 120VAC Traces'
2000\05\24@103818 by frmiller

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I have a board that I recently redesigned that contains nine
T90 relays that switch 110VAC. The board goes in an
industrial controller and switches lights and contactors
(mag starters). The relays all have flyback diodes across
the coils and snubbers across the contacts.

On the old version of the board (not my design), the 110VAC
traces were mixed on the board with signal traces. It
worked, but as you can imagine, we had problems in the
field. I tried to eliminate the problem by aligning the
relays so that the 110VAC traces were all on the lower area
of the board and all other traces were along the upper.
There is a "no trace zone" about .6" wide that runs under
the relays between the coil pins and the contact pins. This
is the dividing line between the 110VAC area and the rest of
the board.

This layout helped a lot, but I'm still getting a few spikes
back in the signal traces. It is sometimes enough to lock up
the NE5090 mux on the board, especially when the contactors
fire. I have bypass caps on the chips on the board, but I
really would like to eliminate the spikes.

The whole point of this long-winded explanation is to ask if
filling the "no trace zone" with a ground plane would do any
good. If so, should it be earth ground or circuit ground? Is
there any way to hack a current board (adding foil or
something) to simulate the ground plane that would give
meaningful test results before we redo the board? The board
is two-sided, through hole.

TIA,

Ryan

2000\05\24@130031 by Dan Michaels

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Ryan Miller wrote:
>I have a board that I recently redesigned that contains nine
>T90 relays that switch 110VAC. The board goes in an
>industrial controller and switches lights and contactors
>(mag starters). The relays all have flyback diodes across
>the coils and snubbers across the contacts.
.......

Hi Ryan, for several years I consulted with a company that built
industrial controllers that switched multiple 220 VAC inductive loads.
They sold 1000s of these units each year, and saw every manner of
failure. Many previous comments by other picisters have indicated
that 1 or 2 things might fix this sort of problem, but in my
experience, we found it required multiple lines of attack and each
was only marginally effective. A heavy inductive switching environment
is probably the very worst place into which to put a microprocessor,
and I think the best results come from an overall systems-level
approach - which of course involves 20 or 30 or 60 aspects.

That being said, let me relate some of the most effective methods
in my experience:

1. EMI and spike filters at the hiVAC power input - series EMI filter,
  MOVs, etc.
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.
3. Bypass caps, of course, at all the usual points.
4. Of course, diodes across the relay coils and snubbers across relay
  contacts.
  NOTE - some loads produce significantly worse switching transients
  than others, so the same snubber may not be universally effective.
  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.
5. On the low-level signal lines, RC filters and transorbs [transient
  voltage suppressor diodes, which are essentially low-inductance
  zeners] are effective. Note - we ended up putting transorbs on
  practically ever signal line - and regular zeners are too slow here.
6. Layout issues - physically separate hi/low V, and signal/switching
  circuitry. Separate power busses and gnd planes [if possible]. Don't
  run different lines near each other, and don't cross them.
7. Use watch dog timer in uC s.w. Use debounce routines on signal
  measurements.
8. Use digital bus crowbar ckt in cases where chip latchup is an
  especially bad problem. [Note - the watchdog will not fix latchup].

There are probably a few others which I forget.
==============

>
>The whole point of this long-winded explanation is to ask if
>filling the "no trace zone" with a ground plane would do any
>good. If so, should it be earth ground or circuit ground? Is
>there any way to hack a current board (adding foil or
>something) to simulate the ground plane that would give
>meaningful test results before we redo the board? The board
>is two-sided, through hole.
>

Regarding gnds, the digital, analog, switching, and switched
should be as physically and electrically separate as possible,
and should be connected at "one point" as close to the power
entry point of the board as possible. The more current, the larger
the trace/wire.

Regarding hacking the test bd, try using aluminum duct tape
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 can
put a gnd guard ring around each other subsystem, connected to
the gnd of that subsystem.

Hope this helps,
- Dan Michaels
Oricom Technologies
http://www.sni.net/~oricom
==========================

2000\05\24@131740 by Chris Eddy

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Ryan;

This subject came up just a few weeks ago.  The relay contacts are
generating EMI with a spark.  You need to use a snubber, just a cap, or
some other means to quench the arc on the contacts.  See the archive.

Chris Eddy

Ryan Miller wrote:

> I have a board that I recently redesigned that contains nine
> T90 relays that switch 110VAC. The board goes in an

2000\05\24@132805 by Dan Michaels

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BTW, a good book on this topic is

"Noise Reduction in Electronic Systems" by Henry Ott

2000\05\24@133156 by Spehro Pefhany

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At 06:16 AM 5/24/00 -0400, you wrote:
>Ryan;
>
>This subject came up just a few weeks ago.  The relay contacts are
>generating EMI with a spark.  You need to use a snubber, just a cap, or
>some other means to quench the arc on the contacts.  See the archive.

He said he has snubbers on there.

Part of the problem is the type of relays used. This is a very complex
subject. The closest thing to a magic bullet is to go multilayer.

Best regards,


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2000\05\24@134432 by Dan Michaels

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Always the stuff forgotten the first time [or 6]:

One way to tell if a gnd plane will be effective in a particular
situation is to simply place a grounded metal plate close to the
underside of the circuitry in question [insulated from it of
course], and note the effects. Use a short, thick gnd lead directly
to the cktry being shielded. I have used this method to determine
where gnd planes are most effectively added onto 2-sided pcbs.

- Dan Michaels

2000\05\24@180514 by frmiller

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Thanks for your reply, Dan. Comments interlaced:

> Dan Michaels wrote:
>
<snip>
>
> That being said, let me relate some of the most effective
methods
> 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
transients
>    than others, so the same snubber may not be universally
effective.
>    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
[transient
>    voltage suppressor diodes, which are essentially
low-inductance
>    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
capacitance.

> 6. Layout issues - physically separate hi/low V,
> and signal/switching circuitry. Separate power busses and
gnd
> 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
(MAX706).

> 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.

><snip>

> Regarding hacking the test bd, try using aluminum duct
tape
> 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
can
> 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.

Ryan

2000\05\24@180522 by frmiller

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Thanks Chris,

I do have snubbers but they need work. Do you have a good
rule of thumb for design? Most of the books and articles
that I have read really boil down to "keep trying different
values 'til it works".

Ryan


{Quote hidden}

2000\05\24@180525 by frmiller

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Spehro,

Could you expand on this? Is there a problem with the T90
relays?

Ryan

> Spehro Pefhany wrote:
<snip>
>
> Part of the problem is the type of relays used.

2000\05\24@204125 by Dan Michaels

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Ryan Miller wrote:
....
>> 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.
>

Actually, I was referring here to general filtering on the
power input from the AC line - which you may already have - and
which any industrial controller should have. Many commercial
versions are available, usually a C-L-L-C filter across the lines,
[C's across, L's in series in each line] with 2 C's to earth gnd,
plus MOV on downstream end, etc. Among other things, it will
help protect the controller from external noise [lightning, etc],
as  well as spikes generated by the switched loads and which are
conducted into the controller via the AC lines.
==============

{Quote hidden}

As mentioned, directly measuring the transients across the relay
contacts during on/off will help identify ineffective snubbers.
Alternatively, write some test code to exercise the relays
repetitively [at 1/sec or so] and observe which loads cause
problems. Fix those snubbers/etc. Probably use a smaller R.
================

{Quote hidden}

By "signal" lines, I really meant "all" of the low-level lines
in and out of the uC [bad terminology on my part]. It is common
for spikes to be conducted from the switched circuits directly
backwards to uC pins driving the relays. Series R's and RC's
will help here. Also, I don;t recall transorb C values, but I
believe they are small enough to not have to worry about in the
comm lines - specifically regarding the *low* V transorbs.
===============

{Quote hidden}

Here, the best defense is a good offense. First, try to kill
the spikes as dead [ie, make small] as possible. After that,
it's a systems-level, "defense" problem - ie, add preventive
measures so that spike residues have as little effect as possible.

I try to err on the conservative side, and include as many
good design features as possible, rather than just try to
get by with the minimum allowable. I think the extra cost
and effort helps in the long run. The guys I mentioned
consulting for had 10,000-20,000 units in the field and a
significant amount of "warranty" costs - about 2-5% failures
per year - that's of a lot of $$$$. By working on the
protection on the controllers over time, we were able to
greatly reduce the in-field failure rate.
=============

>> 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
>(MAX706).
>

Man, you already know/have it all.
===========

{Quote hidden}

Reset vs latchup is, I think, mainly related to the severity
of the garbage picked up. Your's sounds like it's on the lesser
side of real evil. The guard rings might have some small effect,
but will probably not produce magic. 1st attack the source of
the spikes - as above.
===============

{Quote hidden}

From my experiences with actually doing this, I think a good
patch of duct tape is about as effective as a real gnd plane of
the same area/etc. If adding a guard ring will have any significant
effect on the problem at all, I think it will be highly noticeable
with the duct tape jury rig.

Re ground used - try both ways & see what happens. Earth gnd might
be better. Also, it would probably help to keep the uC and relay
gnds separate if possible - if using an ULN2803 type driver or
BJTs/MOSFETs, you could run the gnds separately to the tie point.

best regards,
- Dan Michaels
==============

2000\05\24@232821 by Robert A. LaBudde

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<x-flowed>At 02:58 PM 5/24/00 -0700, ryan wrote:
>Thanks Chris,
>
>I do have snubbers but they need work. Do you have a good
>rule of thumb for design? Most of the books and articles
>that I have read really boil down to "keep trying different
>values 'til it works".

Typical values are 0.1 uF with series 100 ohm.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: .....ralKILLspamspam@spam@lcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

</x-flowed>

2000\05\25@103413 by Dan Michaels

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Robert wrote:
>At 02:58 PM 5/24/00 -0700, ryan wrote:
>>Thanks Chris,
>>
>>I do have snubbers but they need work. Do you have a good
>>rule of thumb for design? Most of the books and articles
>>that I have read really boil down to "keep trying different
>>values 'til it works".
>
>Typical values are 0.1 uF with series 100 ohm.
>

And you probably realize it, but there are *special*
capacitors made particularly for this type of application.
They tend to be rather bulky and take up a lot of board
space. Not any old cap will do.

2000\05\26@063333 by Tom Handley

picon face
At 11:18 AM 5/24/00 -0600, Dan Michaels wrote:
>BTW, a good book on this topic is
>
>"Noise Reduction in Electronic Systems" by Henry Ott

  Dan, I also have this book and highly recommend it. For those interested:

     Noise Reduction Techniques in Electronic Systems
     Henry W. Ott
     Wiley-Interscience
     ISBN: 0-471-65726-3

  Another good one is:

     Low-Noise Electronic Design
     C.D. Motchenbacher and F.C. Fitchen
     Wiley-Interscience
     ISBN: 0-471-61950-7

  - Tom


------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

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