Converting 0 - 80 V to 0 - 3.3 V range
Well, if you want to convert an exact ratio of whatever/80 to its equivalent voltage /3.3v (for e.g. 40v to 1.65V) then you find the ratio of resistors needed and implement it, with the output being in the middle of the two.

Your ratio is 3.3V max to 80V max - so it is 3.3/80 for your Vout to Vin. Assuming your top resistor is r1 and bottom is r2, for 80V across R1 plus R2, we want 3.3v dissipated across R2. Thus, 3.3(R1+R2) = 80R2. (1+R1/R2) = 80/3.3. R1/R2 = ([80/3.3] - 1) 

Use this ratio to find suitable resistors. The resistor value magnitude depends on the current you want flowing into the microcontroller also - a Kilo-ohm range will result in milli-amps - it may suffice depending on the microcontroller specification.





This solution is a fixed voltage divider, and is an answer on how you would equally divide voltage. This does NOT cover how to regulate an input to 3.3v if that's what you might have been looking for, but based on your question I believe you are looking for the voltage divider method.