> Thoughts on this little guy?
>
> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3DBZX84C=
5V1TS-FDICT-ND
>
> 6 pin TSSOP, 3 independent 5.1V Zeners.

Pricing of zeners seems a bit odd. I recently got through-hole 15V
1W zeners from Mouser and paid NZ$0.067 each. After a very
quick search I can't see 5V1 through-hole from Mouser or Digikey
less than $0.12. There are some SMT that are easy to work with,
eg the MiniMELF SOD-80 package (like an unleaded 1N4148)
at 6.7c, 1-9. The part you quote is NZ 18c/diode. As we've found
out before though, US pricing can be wildly different from what I
see on pages presented for NZ customers

I'd have thought 5V1 would be a much bigger seller than 15V.
Perhaps that's why the price is higher (cf 16F84)

TSSOP is fiddly to work with but I'd use them if needs be, they do
the job just as well. You'd need a resistor with each too, plus the
Schottky and perhaps also a small ceramic cap, a few 10s of nF

A 1k - 2k series resistor should do. This will allow enough clamping
current through. You can go higher in a digital circuit (depending on
the '0' > '1' transition threshold) but I wouldn't for analogue

For a typical 400mW 5V1 zener

12V in, 100k, Vout 3.8V. 12V in, 1k, Vout 5.1V
5V in, 100k, Vout 3.2V. 5V in, 1k, Vout 4.4V

You can see that passive protection will involve some losses with
a 5V signal, but you can scale the ADC result to compensate. I
haven't tried but a 5V6 + 2k may be closer to 5V, and be under
the pin Vmax for a 5V part (if you run the PIC at > 5.3V so that
Vpin < Vdd + 0.3V) if 12V is applied and clamped

You should be OK at the sort of signal levels you're using, with
the considerations of price, space, ease of assembly and aesthetics
(if that's important)

Joe

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.