Bob Ammerman wrote: > Roman, > Would you find a schottky diode wired with its anode to the I/O pin and its > cathode to the Vdd rail an acceptable alternative to a 5.1V Zener? No.:o) > If not, why not? I knew you'd ask. Schottky diodes are generally a low voltage part. I do not rely on them in spike circumstances. They are useful in switching and for some high A low V rect apps. I also prefer to reference all voltages to gnd and pass spike energy to gnd, pref keeping it as far from the semi's Vdd as possible. It wasn't just a clamp diode argument to start with, more like connecting 170vdc and mains spikes to the clamp diode. > If so, what's wrong with putting that diode inside the chip. > That is, of course, what mChip has done. Of course. The important difference is the feeding of high voltages directly to the little silicon chip. I understand the clamp diode mechanism but unlike an external clamp diode that may feed it to the filter cap (for example), you are actually connecting a high voltage potential to the chip itself. Then relying on microscopic etchings in that PIC wafer (the diodes) to save the chip from the excessive volts. I don't like it! > Re: this input protection thread, I want you to know that what I really > looking for is the underlying 'why' and 'how', rather than just a 'never'. That's the problem with practical experience, you always see the WHAT happened but usually left guessing the WHY. :o) > Many designs are highly cost-limited, and adding a zener could make a big > difference. As I am sure you know, any sort of life-cycle cost analysis > requires understanding the relative costs of prevention and cure, and the > latter depends on failure rate. With a low-enough failure rate almost any > prevention expenditure is a waste of money. Fair enough I am in a different boat to most designers. I get paid to build things that last. Just means I have a different focus, not that my way is better. The thing that gets me is the ISOLATION issue. In TV's I get to work with 240vac, 5v, 12v, 26v, 120v, 180v, 320v, 550v, 4.5kv, 24kv in every TV set. Most TV's have ALL those voltages in them somewhere, and connected in many ways. I see the things that fail, and the ways the clever designers try to protect from these failures with their very tight budgets. Maybe voltage specs mean more to me than many people who usually work on battery/low powered equipment??:o) A simple voltage divider to the PIC pin goes a long way towards keeping the potential pin voltage within the 5v spec. You only need >2.0v to guarantee high threshold so bias it about 3.0v with a two resistor divider. I would also stick a small (ceramic?) cap across the bottom resistor, cheap and reasonably small. Doesn't offer the level of protection a zener does but is better than nothing. Save the clamp diodes for ESD protection and infrequent fault currents they were meant for. A series resistor itself doesn't offer protection from voltage. Especially high voltage spikes which are VERY ac in terms of performance. Connecting a high voltage through a resistor into a chip is still exceeding it's max voltage spec. 0v-5v. That's what my PICs get. Sorry for the ranting! Here's my soapbox: ;o) -Roman -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads