Well the best place to start is specifying exactly what "customer error" you expect to encounter. Connecting to 5v, or just touched, or what? Where does this ESD come from? I do recommend the spec sheets above measurement, because of the tolerance issues. Danny Forrest W Christian wrote: > I finally found a datasheet with curves.... > http://www.onsemi.com/pub_link/Collateral/1N5333B-D.PDF > > I see what you mean... The zeners at the low end of the scale are not > very regulated....(Fig 7). The 3.3V varies widely... and it looks like > the curves are opposite of what I would have liked to see. > > Hmm... well, that probably isn't going to work. I'll likely have to > figure out another way to protect these inputs from customer error. > Will probably try a few on the protoboard and verify that I'm reading > the curves correctly. > > Sean Breheny wrote: > >> Hi Forrest, >> >> Beware that the "knee" of such low voltage zeners is often very soft. >> In other words, a 3.3V zener may start to break down at 2.5V and not >> really conduct well until 4.5V. >> >> Sean >> >> >> On Sun, Oct 26, 2008 at 8:59 PM, Forrest W Christian wrote: >> >> >>> I'm using a Zener diode as an input protection diode for a 16f886 >>> running at 3.3V. Currently I have the circuit designed with a 3.0V >>> zener (1n4619) between the input pin and ground (reverse biased of >>> course). At 3.3V, Vih (min) on the '886 looks like either 1.9V or >>> 2.64V depending on whether I am talking a TTL or Schmitt trigger input, >>> so I'm expecting a 3.0V zener to be perfect for this purpose - high >>> enough so that the voltage is over Vih, but low enough that the input >>> will never exceed Vcc. >>> >>> That said, a lot of the sample circuits I've seen seem to prefer a 3.3V >>> zener for this purpose. Other than not thinking through the design >>> completely (and the intuitive idea that we want to clamp at 3.3V, so we >>> should use a 3.3V zener), is there a reason why the examples show a 3.3V >>> zener and not something lower, like the 3.0V zener I currently have >>> specified? >>> >>> For those of you who will want more information such as what the input >>> circuit looks like.... This is for a switch-closure input. From VCC >>> to the PIC Input: I have a 1n5819 Schottky diode connected through a 10K >>> pullup resistor to the external switch input. From the external switch >>> input, it goes through another 10K pullup resisitor to the PIC input and >>> the cathode of the diode.. Of course, the anode of the diode is >>> connected to Vss, (as is the other "side" of the switch input). The >>> theory being that a voltage accidentally injected at the input (in >>> relation to the ground), will not be able to raise the VCC rail due to >>> the 1n5819, and the 10K current limiting resistor will limit current >>> through the zener to ground. >>> >>> So, am I correct in thinking that the 3.0V zener is the correct diode >>> for the job, or is the 3.3V more appropriate? (It's too bad that 3.15 >>> or 3.2V isn't an option). >>> >>> -forrest >>> >>> >>> -- >>> http://www.piclist.com PIC/SX FAQ & list archive >>> View/change your membership options at >>> http://mailman.mit.edu/mailman/listinfo/piclist >>> >>> >>> > > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist