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 <k3jsch@MAIL.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
>k3jsch@mail.gmtcom.com
>