On 09.10.21 02:31, Gene Heskett wrote: > On Friday 08 October 2021 21:15:12 John Dammeyer wrote: > > > I disagree. Grounding won't fix what is inherently a bad design. > > Might make it appear less often if that indeed is the reason but it's > > still a bad design. > > > HAving taken a third look at that link, I have to agree. That family of > chips have a proclivity to set off a substrate scr, which can muck up > the logic or even crowbar the power supply when an input pin is pulled > above the supply line, a possibility given that the encoder is running > on 12 volts and the 74hc4050 is on 5 volts. Bad dog, no biscuit.
Yup, been bitten by that one too. Back in '78 I took over a supposedly complete digital clock design for supply to Ford. But they reported that "Your clock is dead the second time we turn it on." A drive to their proving grounds, where the prototype XD Falcons were being put through their paces, and some probing with a storage scope, showed a -1 kV transient on the accessories line on ignition switch-off, due to a couple of relay coils. That led to: Q: "What's wrong with your flyback diodes?" A: "What are flyback diodes?" We drove back to Melbourne from out near Geelong, my boss and his went home, and I sat in the design lab with the instruction "Fix it, but it's not allowed to cost anything. The product price is fixed by contract." It took a couple of hours, but the data book's logic family characteristics section revealed that if I limited the current of the input overvoltage to a few mA, then the parasitic SCR wouldn't latch. The 220 kOhm resistor I added on the CMOS input did cost a cent at the time, but they allowed me that. My recollection is that we just sent the new prototypes, and waited for confirmation that our clocks were now automotive compatible. What boggles my mind is that on that day, the scope trace was recorded with a polaroid camera bolted over the high persistence CRT screen. Now we don't have CRTs or film cameras, EVs drive themselves, and fossil fuelled cars are set to not be mass manufactured by the end of the decade at the latest - due to having been more expensive to own for more than half the decade. (Well, from now, if you can get delivery of an Xpeng 5 or BYD Dolphin or ...) OK, it doesn't help when VW takes 30 hrs to build a car, while Tesla does it in 10, for a projected fully-on-line output of 10,000 cars per week from the Grünheide factory alone. VW is already planning 30,000 layoffs in order to avoid bankruptcy as they hand over European #1 carmaker title to Tesla, and try to avoid doing a Kodak. Just wait till the Texas factory arcs up. Erik _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
