An extremely good alternative to zener diodes is a clamp regulator such as a TL431(which I tend to recommend to all and sundry). The lower voltage TLV431 has some advantages. This has a much sharper "knee" than a zener and can be obtained in 0.5%, 1% & 2% accuracy versions. "Programming" requires 2 resistors. If you are committed to an existing PCB then a small add in PCB could be made to insert where the zener no longer is. My constant refrain is that any overvoltage risks unexpected PIC operation - but I'll admit that a genuine 5.2V and maybe 5.3V on a genuine 5V supply is statistically not too too likely to cause problems. A TL431 or TLV431 would allow you to clamp very close to 5V. 680r alone probably meets absolute max ratings (see relevant data sheet) BUT certainly violates "normal operating conditions". You can be utterly certain that some PICs will perform bizarrely if you inject 10 mA (or even 1 mA ) via protection diodes. As long as data sheet guarantees non destruction and as long as your system does not mind processor going insance when Vin > ~=3D 5V then no zener at all is an OK choice. A small Shottky diode from Vin to Vcc =3D 5V clamps wellish but Shottky leakage may be annoying if you have very high temperatures. The Schottky drop also notionally exceed sthe limits that I preach about but often does OK in practice. Very good is a small Schottky from the middle of a split Rin to Vcc so you clamp to about 5.3V or less with a largish resistor between V and pin. Also violates spec but statistically better. Russell On 6 September 2011 21:07, Matt Rhys-Roberts wrote: > > I'm reviewing an ugly ADC input circuit which so far has worked very > adequately. An op amp's output signal (nom. 0-5v, but could rise to 12v > in fault condition) is clamped for safety by a 680R/5V1 zener diode > combination. So the voltage into the PIC ADC pin is externally limited > to 5v1 and the current from the opamp is externally limited to 10mA if > the op amp ever hits 12v. > > Possibly due to some 5v1 zener diodes going obsolete and being replaced > by a close alternative, I find that some PIC ADC inputs are being > dragged down by the presence of the zener diodes, as the incoming > voltage approaches 5v. This seems to suggest that the current batch of > diodes are turning on earlier than those previously specified. > > Removing the zeners seems to make the problem go away. I'm then left > with the worry that merely limiting the over-voltage ADC input current > to 10mA may be risky, even though the ADC inputs are very unlikely to > rise above 5v during the service life of the equipment. > > Questions: > a) is just a current limiting resistor of 680R (for 10mA) sufficient due > diligence, for protecting the PIC ADC input clamping diodes if the > source voltage rises to 12v? > b) is relying on limiting the internal protection diode current, asking > for SCR latchup? > > Ponderings appreciated, > > Matt. > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .