Temperature Measurement System Design Lab

Temperature Measurement System Design Lab

Overview: In this lab we designed a circuit whose output voltage provided a temperature measurement. Using a thermistor integrated into a Wheatstone bridge, we sent a varying voltage (based on varying resistance AKA temperature) signal to a difference amplifier to output an overall high-sensitivity Vout signal.

Design:
We wanted the circuit to have a 0 V +/- 20 mV signal at room temperature (~25 degrees Centigrade) and then increase by 2V over the temperature range of 25-37 C (approximately body temperature).

^^^^ The Wheatsone bridge that we designed ^^^^

R1 in this case is where we placed our thermistor.

Rnom is where we placed a  potentiometer achieve a desired resistance that matched the thermistor's room temperature resistance.

R2 and R3 were identical 8.2 K ohm resistors. 

We balanced the Wheatstone bridge with the following relationship: R2/R1 = R3/Rnom

We then hooked up our Va and Vb from the Wheatstone bridge to a difference amplifier, designed as follows: 

^^^^ Va was the left voltage input, and Vb was the right voltage input ^^^^

Utilizing the relationship R1/R2 = R3/R4, the equation for the output voltage becomes:

Vout = (R2/R1)(Vb-Va)

With that in mind, setting R2 and R4 to 39K ohms and R1 and R3 to 1K ohms, we get an output voltage range equal to approximately 3.6V. This is desirable, because it makes the circuit more temperature sensitive. With the design and calculations finalized, we were ready to construct the circuit.

Construction and Execution:

Thermistor resistance measurements:

25 Centigrade: 11.23K ohms

37 Centigrade: 9.98K ohms

Measurements of resistors used in Wheatstone bridge:

R2: 8.22K ohms

R3: 8.21K ohms

Rnom of potentiometer: 11.23K ohms

Measurements of resistors used in Difference amplifier:

R2: 49.1K ohms

R4: 49.0K ohms

R1: 1.003K ohms

R3: .995K ohms

^^^^ (LEFT) construction of the Wheatstone bridge (RIGHT) prelab calculations and design ^^^^

 ^^^^ Views of the completed circuit, as viewed from above and the side ^^^^

Results:
Vab measured at room temperature: 2 mV
Vab measured at body temperature 96 mV
Vout measured at room temperature: 78 mV
Vout measured at ~30 degrees (guesstimated, as it was about halfway through the resistance range): 1.9 V
Vout measured at 37 degrees: 3.45 V

Analysis: The system overall was a success. The only issue was that the difference amplifier didn't seem to amplify quite as much as it was supposed to. I believe this is because of slight imbalances within the resistors of the Wheatstone bridge and the difference amplifier system. The difference amplifier did however amplify the signal to a magnitude close to what it was supposed to amplify to (+/- 3.5%).  Overall, this lab illustrates that Wheatstone bridges can be used to effectively relay minor changes in resistance, and then difference amplifiers can greatly magnify that difference. The lab was a success.