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Dear all,

I wonder if anyone has any ideas on how this circuit will act?
I'm sure it won't do what I want to do but I'm curious as to what anyone
thinks it will do!!

I've using a 2.5v reference, using an op-amp voltage doubler to bump up the
current output and double the voltage.. I feed this through a potential
divider with the bottom resistor being of variable resistance (actually a
sensor). The sensor is usually between 0 and 200 ohms so I've used a 200 ohm
resistor for the top resistor..... This gives between 0 and 2.5v from the
middle of the pot divider....
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)....
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??
How else does anyone limit the voltage... Low value zener diode? I know
recently these have been discussed and no-one was impressed.....

Thanks for any input,

Ben

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