Randy

11-24-2006, 12:37 PM

Trial and error have gotten me where I need to be but I'd like to know how to figure this with an equation. I am using nine LM34 temperature sensors with the ground terminal bias to 0.33 volts. This lets me measure negative temperatures ( to -33 degrees ) without negative values. The outputs go to an eleven channelˇTLC1543 A/D converterˇand then to a BS2pe where I subtract 33 for the corect temperature. The -ref is to ground and the +ref to 2.55 volts for scaleing. If you look at the attached drawing you will see that I am using a voltage divider consisting of two resistors (R1 and R2)ˇand a pot that is drawn as R3 and R4, all in series. The 2.55 volts at the wiper, or node between R3 and R4,ˇis applied to the +ref pin and the current draw is too low for my meter to measure. The 0.33 volts at the R1 / R2 node is used for the biased ground to all nine LM34ˇ sensors. Combined they draw 0.567 ma. It seems to me that using the voltage and current I could figure the effective resistance of the nine sensors and using that number I should be able to prove all of the other values with ohm's law. Either I am screwing something up with the combination of series and parallell resistances or I am wrong about being able to deduce a resistance value for the sensors. I have seperated the dividers using the pot between ground and five volts and adjusting for 2.55 volts and using just R1 and R2 between ground and five volts but I cannot get to the 0.33 volts except by trial and error. How do you figure the values?

In the drawing I give the measured values of the pot from the wiper to the corresponding pin. It is a 10k pot that measures 10.5k and yes I know the numbers don't quite add up.

Randy

In the drawing I give the measured values of the pot from the wiper to the corresponding pin. It is a 10k pot that measures 10.5k and yes I know the numbers don't quite add up.

Randy