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'Industrial Electronics Problem [OT]'
1999\05\10@115723 by Kelly Schauf

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face
To the forum:

The problem I have does not involve a PIC; however, there appears to be
an abundance of knowledge in this forum regarding electronics based on
some of the messages I've read so far.

Is there anyone out there in the forum that has experience with Design of
Experiments (Taguchi-type) in addition to their electronics
troubleshooting skills.  I have been seeing repetitive problems with a
MM54C14J Hex Inverter (an old ceramic-style part) where the Vcc is
getting creamed.

Because the explanation of the failure will be a bit too long to
adequately put into a message, the best place to get further details of
the failure type is to go to
http://web.gmtcom.com/~k3jsch/sixsigma/access.htm

The problem occurs regardless of the following:

1) Whether it has been in service for only a few hours or at least 8000
hours.
2) Whether it is operating at a temperature of 55 degrees C or slightly
past 100 degrees C

3) Whether it is operating in a 60Hz domestic unit or a 50Hz overseas unit.

4) date code of IC

Any suggestions, etc., are welcome.  What I am trying to do is set up an
experiment at our production facility to inject noise, etc. into the
design as it is running.  Factors I am looking at are :

1) Levels of noise voltage to inject into the cable between the
microprocessor unit that this IC resides on and the gate firing board of
the SCR bridge that is directly mounted over the 3-phase, 6SCR bridge
that ultimately runs either the left track or right track motor.

2) Brand name of chip.

3) Whether or not modification to the shielded cable and/or its placement
in the circuit would make a difference.


Regards,

Kelly Schauf
spam_OUTk3jschTakeThisOuTspammail.gmtcom.com

1999\05\10@140319 by Mark Willis

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Not that I'm any great expert on RF, but this looks familiar.  One of
those "This one bites everyone" things that is good for "Breadth of
experience" for newbie & old-timer alike.

 It's probably not a ground loop, I'd guess you've looked at that!

 Here's MY best guess;  Is the capacitance of that shielded cable,
what's killing the 54C14J's?  That's in common between all the failing
parts.  And I know I've almost been BIT by this one before, driving
MOSFET's gate capacitances would have killed my 4000 series CMOS
circuits dead if I hadn't thought it through in advance.  (Current
limiting resistance...  Could be FET oscillation, I know I'm not an
expert on that just now <G>

 My "quick dirty" solution:  Split the 1k resistor at the output of
that shielded cable, into 2 resistors - one, say 100R, between the
54C14J and the cable, to limit 54C14J max output current, and the other,
say 900R at the input to the FET.  (You want to prevent both FET
oscillation and drawing too much current out of that FET.)  There's
probably a better optimized resistor value splitting to be done there,
there are some past posts in the PICList archives that covered this
IIRC.  (http://www.iversoft.com/piclist/ - I'd go look there.)  I don't
usually memorize solutions, just "Gotchas", and look up the solutions
when needed.  And check datasheets <G>

 Mark

Kelly Schauf wrote:
{Quote hidden}

1999\05\10@161227 by Dave VanHorn

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Here's one from the fringe:

Cables are triboelectric. Some more than others.
With the wrong cable, it's possible to kill a chip, by smacking the cable
hard.
They make special coaxes for mic cables, so they won't be bothered by noise
as the cable is drug along the floor.

1999\05\10@203638 by Russell McMahon

picon face
Kelly,


I couldn't guarantee that I know what is happening here but I suspect
that I could STOP it happening for very little effort, BIMBW *

i    Doesn't seem likely but - High dv/dt on FET switching can
reflect into gate. Oscillation can occur on gate under "correct"
conditions. The voltage here would need to be high enough to drive IC
into latch up via 1K drive resistor.

ii    I don't know how long the cable is but significant energy *may*
be able to be stored in the cable. What is it's capacitance. You are
driving it directly (NO resistor from gate output. Is this within
spec? (ie - depending on connection method there may effectively be
capacitance to ground bypassing the FET gate resistor).

iii    Regardless of mechanism, the *cause* would *seem* to be energy
from the cable getting into the gate's output.
Faced with this problem I would.

a)    Strongly consider placing the 1K series resistor at the gate
output rather than at the FET (isolates gate from direct capacitance)
or split the resistor to eg 470r at each end of cable. At present the
IC output is directly exposed to anything that *may* appear on the
cable.

b) Place Schottky catch diodes to ground and supply at the IC output.

With 1K resistor and say 30ma capability (a very small Schottky
(BAT85 etc?)) you can withstand 30 volt continuous above or below
power supply rails without sinking ANY current in the ICs substrate
diodes.




As long as the problem is not from somewhere else (eg spikes via the
Vcc rail) these steps would be very very (very ...) likely to prevent
damage, regardless of the cause.

With the amount of energy you have in the general vicinity I would
want to be extremely certain that my power supply was not the source
of problems BUT I would pick the above as most probable.

regards


               Russell McMahon


*  BIMBW = But, I May Be Wrong :-)


PS - if I had to "test" this situation rather than fixing it I would
consider applying, to the IC output, capacitors of the value
equivalent to that of the cable, charged to increasingly greater
voltage until I replicated the fault (or couldn't).




From: Kelly Schauf <k3jschspamKILLspamMAIL.GMTCOM.COM>


>To the forum:
>
>The problem I have does not involve a PIC; however, there appears to
be
>an abundance of knowledge in this forum regarding electronics based
on
>some of the messages I've read so far.
>
>Is there anyone out there in the forum that has experience with
Design of
>Experiments (Taguchi-type) in addition to their electronics
>troubleshooting skills.  I have been seeing repetitive problems with
a
>MM54C14J Hex Inverter (an old ceramic-style part) where the Vcc is
>getting creamed.
>
>Because the explanation of the failure will be a bit too long to
>adequately put into a message, the best place to get further details
of
>the failure type is to go to
>http://web.gmtcom.com/~k3jsch/sixsigma/access.htm
>
>The problem occurs regardless of the following:
>
>1) Whether it has been in service for only a few hours or at least
8000
>hours.
>2) Whether it is operating at a temperature of 55 degrees C or
slightly
>past 100 degrees C
>
>3) Whether it is operating in a 60Hz domestic unit or a 50Hz
overseas unit.
>
>4) date code of IC
>
>Any suggestions, etc., are welcome.  What I am trying to do is set
up an
>experiment at our production facility to inject noise, etc. into the
>design as it is running.  Factors I am looking at are :
>
>1) Levels of noise voltage to inject into the cable between the
>microprocessor unit that this IC resides on and the gate firing
board of
>the SCR bridge that is directly mounted over the 3-phase, 6SCR
bridge
>that ultimately runs either the left track or right track motor.
>
>2) Brand name of chip.
>
>3) Whether or not modification to the shielded cable and/or its
placement
>in the circuit would make a difference.
>
>
>Regards,
>
>Kelly Schauf
>.....k3jschKILLspamspam.....mail.gmtcom.com
>

1999\05\10@214333 by Mark Willis

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(Hmmm, Russell, your post just got here.  Time lag?)

 One minor suggestion/question for the list, would it be better to put
the Zener or catch Schottky's on the output pin of the 54C14J, or at the
outside (past the output resistor), at the cable connection?  That's
where I'd be tempted to put them, then any HV noise put into the the
cable is solved right there - you cannot get up *to* even 30 volts on
the other end of that 54C14J output resistor.

 Also, Kelly, one question:  Your web page doesn't give relative
quantities made of the various units, some sort of "The failure rate IS
(or IS NOT) fairly uniform across models made" information might be good
to provide people in some cases, you could provide that without
disclosing sales data directly (in case that's sensitive information.)
In this case, I suspect it's that cable/54C14J connection.

 (I learned something good that I keep losing, from Dave VanHorn,
reminder of Triboelectricity.  Good to be reminded of that.  I need to
go write a note to optoisolate one thing <G>)

 Mark

Russell McMahon wrote:
{Quote hidden}

1999\05\10@214545 by Richard Prosser

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Hope I'm not repeating what someone else has already suggested but-

How well bonded are the earths between the control board with the failing IC
and the FET driving the pulse transformer. Any earth potential rise at the
FET end could cause the output of the IC to exceed its max (or min) limits
causing latchup and failure. If the cable is longish or there is any sort of
inductance or signal coupling in the earth loop then this could create a
damaging situation - particularly with the power levels and risetimes likely
to be occuring in the environment.

Note that the earth return inductance will not need to be very high to
generate substantial voltage from the high di/dt values you're likely to
find in this area. (V=L * di/dt) If L=1uH I=500A in 1uS then V=500V & more
significantly perhaps E=1/2 * L * I^2 = 125mJ = more than enough to do
damage!

My reccommendation would be to move the driver IC down to the same PCB as
the FET and drive it from a differential signal, rather than the single
ended one shown. Standard RS485 driver / receiver ICs might be an option.

The differential receiver would need to have a good common mode performance
but this is not too hard to arrange with even just a "long tail pair" of
transistors and a constant current source.

Alternatively, isolate the FET driver from the logic using an opto isolator
and keep the earths and supplies separate.

The other replies with respect to bypass diodes and series resistors should
also help but I'd look carefully at the return path impedance and coupling.

Richard
(rprosserspamspam_OUTswichtec.co.nz)


From: Kelly Schauf <@spam@k3jschKILLspamspamMAIL.GMTCOM.COM>


>To the forum:
>
>The problem I have does not involve a PIC; however, there appears to
be
>an abundance of knowledge in this forum regarding electronics based
on
>some of the messages I've read so far.
>
>Is there anyone out there in the forum that has experience with
Design of
>Experiments (Taguchi-type) in addition to their electronics
>troubleshooting skills.  I have been seeing repetitive problems with
a
>MM54C14J Hex Inverter (an old ceramic-style part) where the Vcc is
>getting creamed.
>
>Because the explanation of the failure will be a bit too long to
>adequately put into a message, the best place to get further details
of
>the failure type is to go to
>http://web.gmtcom.com/~k3jsch/sixsigma/access.htm
>
>The problem occurs regardless of the following:
>
>1) Whether it has been in service for only a few hours or at least
8000
>hours.
>2) Whether it is operating at a temperature of 55 degrees C or
slightly
>past 100 degrees C
>
>3) Whether it is operating in a 60Hz domestic unit or a 50Hz
overseas unit.
>
>4) date code of IC
>
>Any suggestions, etc., are welcome.  What I am trying to do is set
up an
>experiment at our production facility to inject noise, etc. into the
>design as it is running.  Factors I am looking at are :
>
>1) Levels of noise voltage to inject into the cable between the
>microprocessor unit that this IC resides on and the gate firing
board of
>the SCR bridge that is directly mounted over the 3-phase, 6SCR
bridge
>that ultimately runs either the left track or right track motor.
>
>2) Brand name of chip.
>
>3) Whether or not modification to the shielded cable and/or its
placement
>in the circuit would make a difference.
>
>
>Regards,
>
>Kelly Schauf
>KILLspamk3jschKILLspamspammail.gmtcom.com
>

1999\05\11@072526 by Russell McMahon

picon face
>  One minor suggestion/question for the list, would it be better to
put
>the Zener or catch Schottky's on the output pin of the 54C14J, or at
the
>outside (past the output resistor), at the cable connection?  That's
>where I'd be tempted to put them, then any HV noise put into the the
>cable is solved right there - you cannot get up *to* even 30 volts
on
>the other end of that 54C14J output resistor.


With the diodes at the IC output AND a resistor on the PCB before the
cable the diode sinks current when the "spike" is dissipated across
the resistor. The resistor takes the brunt of any voltage spike.
Diodes should be Schottky to keep drop below 1 silicon junction drop
and also are "fast". Diode power is low as it is 1 Schottky junction
drop x the available fault current. If a zener is placed outside an
on board resistor it has to dissipate all the energy itself. Gruntier
diode needed and runs the risk of being driven quite high if spike is
fast.

>  Also, Kelly, one question:  Your web page doesn't give relative
>quantities made of the various units, some sort of "The failure rate
IS
>(or IS NOT) fairly uniform across models made" information might be
good
>to provide people in some cases, you could provide that without
>disclosing sales data directly (in case that's sensitive
information.)
>In this case, I suspect it's that cable/54C14J connection.


He implied that there were probably about 1000 all up - he has not
been given the exact figure I think.




regards

           Russell McMahon

1999\05\11@132920 by Mark Willis

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Russell McMahon wrote:
{Quote hidden}

 Sure, good explanation; I'd thought that even with a FAST dv/dt the
zener/Transzorb on the cable end would catch the spike sooner than if
the zener was at the gate end, due to higher voltage across it
(Transzorb is what I'd use - I think of them as the "Same device",
transzorb's a "Rough hewn" Zener from a black box level, really.  And
sure takes higher wattage!)  With my headache yesterday, I wasn't
getting how the Zener can be driven higher with a fast spike on the
outside of the resistor, finally with less headache today it dawns on
me:  Duh!:  That gate's an ACTIVE output, Mark.  Not tri-stated, which I
do a lot.  Trying to drive that active output 'past the stops', should
do a GOOD job of pulse width stretching, your "the resistor takes the
brunt" finally clued me in! <G>

 For production stuff, the gate end is the way to go - I do custom
few-at-a-time stuff, some of my stuff HAS to have huge "herking"
protection to prevent any failures if I can;  I'd guess Schottky's on
the gate end with a Transzorb ("cheap P6KE6.8", 600W, 88 cents or so,
beats a service call any day!) on probably EACH end of the cable, and
perhaps a Zener at the FET's gate as a backup?  I'd rather have some
cheap redundancy, than drive 300 miles to replace a $1 part, or trust
"Joe Ten-Thumbs" there who's more likely to use Acid core solder than
not, to solder in replacement parts, and definitely more likely to melt
the parts soldered in & still end up with cold solder joints, than not.
Motivates me to put everything in connectorized modules, it's hard to
plug the Red DB-25M into a Green DB-09F socket!  He'll probably use a
hammer <G>

 ("Joe" does what he does really well, it's nice and technical and
incomprehensible to me - he just doesn't "Grok" electronics at all;  I
wouldn't ask my Doctor to solder for me, he doesn't ask me to diagnose
medical problems for him, same with "Joe" here, the world's a safer
place if he doesn't solder...)

 Different markets, different techniques!  Consumer appliances are
different than industrial embedded stuff <G>

> >  Also, Kelly, one question:  Your web page doesn't give relative
> >quantities made of the various units, some sort of "The failure rate
> <snipped>
> He implied that there were probably about 1000 all up - he has not
> been given the exact figure I think.
>
> regards
>
>             Russell McMahon

 Missed that number - Thanks, Russell!  Still would be good to know (in
other circumstances) if failure rate was "uniformish" or not, a minor
design change is the solution here, though, though the more I think
about it the more uncomfortable I am with my ASSUMPTION that ground
loops weren't the culprit;  I'd be tempted (in addition to definitely a
series resistor at that gate!) to consider going Optoisolated at the end
of the driven cable, and have the optoisolator drive that FET, just
because any ground loop that changes the Voltage at the ground end of
the FET could change whether the FET's driven hard or not, potentially.
I'm probably being paranoid, though;  I definitely don't think that's
the problem we're against here <G>

 Mark

1999\05\13@084303 by paulb
flavicon
face
Mark Willis wrote:

> Motivates me to put everything in connectorized modules, it's hard to
> plug the Red DB-25M into a Green DB-09F socket!

 I still can't figure out what these "DB-09F" and similar fittings are
that people say they use!

 DB-25s I understand, and DE-9s and DE-15s and similar, but DB-09 must
be an odd species indeed.  Does this refer to a partially-populated
shell with insertable crimp pins?  Do you blank out the unused
positions?  You can certainly produce "coded" connectors, but they're
not particularly goon-proof!

> I wouldn't ask my Doctor to solder for me, he doesn't ask me to
> diagnose medical problems for him,

 Well, not being *your* doctor, I suppose I couldn't object to that but
otherwise I probably would! ;-)

> same with "Joe" here, the world's a safer place if he doesn't
> solder...
>   Different markets, different techniques!  Consumer appliances are
> different than industrial embedded stuff <G>

 I don't follow.  You reference to "acid-core" solder denotes a
plumber.  I should certainly not expect a *plumber* to be working on
appliances (except I have seen one or two replace a heater element).
An appliance serviceman or electrician however - have you *really* seen
one using plumber's solder?  That *is* more than a bit slanderous!
--
 Cheers,
       Paul B.

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