Russell McMahon wrote: > > > 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. 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! 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 ("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 > > Also, Kelly, one question: Your web page doesn't give relative > >quantities made of the various units, some sort of "The failure rate > > 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 Mark