In this lab we are going to design a temperature measurement system that could make a voltage difference greater than 2V when temperature changes from 25 to 37.
we are going to use OPAMP to amplifier the output voltage and use a Wheatstone bridge circuit to calibrate the measurement
Practice in class.
lab circuit
Resistor value in this lab.
For Wheatstone Bridge, we make R1 = R2 = 10K, We know at room temperature, Rn=11.8K, so we calculated that we need a 4.7K resistor to balance the circuit. Actually, we use a 10 K potentiometers to balance the circuit.
Below is the measured values for true resistance of those resitors that been used in the experiment. and the measured output range when heating the thermistor to 37 degree. output range is 0 - 0.276V.
The opamp circuit.
Below is the resistors value we choose for this circuit, the ratio between R2 and R1 is the multiplaction of the circuit which could amplify the signal 10 times.
after done the experiment, the actual output range is 0 to 1.8V the multiplication is around 9 times.
the real circuit could been seen in the videos.
Summary:
Today, we learn cascaded amplifiers and how to build a balanced Wheatstone bridge. There are some very important applications of opamp, while we figure out that the opapm we uses is not ideal, so the feedback is very strong,we initially use the around 10K resistor for R2 and R4 and the feedback is too strong that the amplifier not functioning well. Then we switch to high resistance 1M and 100K, the circuit works much better. All in all we kind successfully finished this experiment with the output range is slightly smaller than 2V. if we could use larger resistors for opamp circuit we could get even better results.
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