Lab Report Day Twelve - Temperature Measurement System Design Lab

Temperature Measurement System Design Lab 

In this lab assignment, we need to designed a simple temperature system which outputs a DC voltage that indicates temperature. Our system will use a thermistor to indicate the temperature. We use a Wheatstone bridge circuit to convert this resistance change into a voltage change. The voltage output of the Wheatstone bridge circuit is small relative to the amount of temperature change, so we use a difference amplifier to increase the overall sensitivity of the temperature measurement system.

This is a circuit diagram for a Wheatstone bridge. It can detect small changes in temperature and amplify it in a circuit.

We want the output of the circuit to be 0 V +/- 20 mV at initial room temperature at 25 degrees and have minimum increase of 2V when it reaches 37 degrees, which is approximately body temperature.

R1 and R2 are two identical 10K ohm resistor.

R3 is where we put in series a potentiometer with a 10K resistor.

Rt in this case is where we put our thermistor. Our calculated value is 12.30 K ohm.

We build Wheatstone bridge with R1/R2 = R3/Rt

This is our Wheatstone bridge

Our data table. We measure that R1=R2=9.89k Ohm, and R3=RT= 12.30 k Ohm.

This is our circuit for our designed system.

We have R5=R6=7.9k Ohm, R7=R8=100k Ohm. And the gain is 12.5

This is our designed circuit that can amplify the output voltage.The thermistor can detect changes in temperature and as a result the voltage will drop when measuring Vout from the difference amplifier.

 

We calculate that our gain is 12.5. So the output voltage should be amplified for 12.5 times. 

This shows how the Vout changes without amplifying. When the thermistor detects a change in temperature, the output voltage starts to drop.

This shows us the original output value and the amplified output voltage. Before it reaches saturation, it has a ratio of 12.5. The saturation voltage is about 3.52V. 

Summary Data:

Vab measured at room temperature: 0 mV

Vab measured at body temperature 579 mV

Vout measured at room temperature: 0 mV

Vout measured at 37 degrees: 3.52 V (It should be bigger, but it has reached its saturation)

Summary:

Today, we learn cascaded amplifiers, which includes multiple amplifiers. We also design a temperature measurement circuit, and learn how to build a balanced Wheatstone bridge. We learn that in a Wheatstone bridge, the resistors in the circuit has to have the ratio R1/R2=R3/R4, thus the two related resistor need to be identical. Since we cannot get this, this could cause some errors in this lab.