(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:
>
> 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>
> <snipped>