Day 12

Temperature Measurement Design System

For this lab we needed to design a simple temperature measurement system. The system will output a DC voltage that will indicate the temperature. Here we are introduced to a Wheatstone Bridge circuit that allows us to take a change in resistance and we find the Vab across this bridge. At the end we use a difference Amplifier which takes tiny changes in voltages and amplifies them so we can have a more accurate reading.

Pre Lab: Balancing the Wheatstone Bridge

Below is a Wheatstone Bridge circuit diagram which illustrates all of the major components. This bridge is designed at finding a change in resistance, the resistance is then translated across the Vab bridge. Resistances R2 and R3 will be equivalent and on the right side we have a Potentiometer and our thermistor at room temperature. In order to balance the Wheatstone Bridge we will adjust the Potentiometer until Vab reads 0 (across nodes a and b). We do this adjustment by using a multimeter.

Usually 10k potentiometers won't yield a high enough resistance to be useful in real world applications. So to calibrate the circuit we just add another resistor in series with the potentiometer until we reach the desired range we need to work in. The below Wheatstone Bridge was able to be balanced to 0.02V.

Difference Amplifier

Now we have a voltage provided by the Wheatstone Bridge. However, this voltage change is very small and won't be much use to use. So we need to run the voltage difference through a Difference Amplifier which is one of the four different setups we learned with op amps. Difference Amplifiers are used in many instrumental tools where small measurements need to be quantified. Cascading the Wheatstone Bridge with the Difference Amplifier will allow us to see the change in resistance to the thermistor due to temperature.

By having all the resistors in the Difference Amplifier being the same (10k in this case) we simplify the equation to allow the resistors to cancel themselves out. This allows us to have a very simple equation of V(out) = Vb - Va. No heavy math required and the instrument is very simple to use. Pack it up and ship it out.