At 07:23 PM 2/15/02 +0000, you wrote:

>However, if it gets really cold the resistance will go >200 and the voltage
>goes >2.5v. The resistance could easily go much higher and the PIC ADC will
>see more than Vref+0.3v (from the 16F877 datasheet)....

What happens under these conditions? The A/D converter "should" read 0x3FF if
Vin >= Vref. What does it actually do?  Are there "side affects" on other
channels or some such thing? Anyone know? I don't see anything in the midrange
reference manual, but the 0.3V > Vref thing IS on the data sheet.

>So I drew up this circuit. Theoretically if the middle of the second pot
>divider is 2.5v, any voltage on the first greater than this will cause the
>diode to conduct... But where will the "volts" go???
>My gut instinct on this circuit is that it will become an oscillator very
>quickly... Any ideas??

No, but the diode won't limit it to 0.3V either. The current will go into
the Vref divider so Vin will be > Vref by no more than about 0.6 or 0.7V.

>How else does anyone limit the voltage... Low value zener diode? I know
>recently these have been discussed and no-one was impressed.....

Zeners are not very good- you can make an almost perfect op-amp clamp.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu