9-Apr-2015: Temperature Measurement System Design

PURPOSE

The purpose of this experiment was to design a circuit that would result in an output voltage of at least 2 V.

PROCEDURES

Figure 1

To begin, we measured the resistance values of the different circuit elements of the Wheatstone bridge (Figure 1). For example, the resistance of the thermistor was 12.1 kΩ at room temperature and decreased to 10.8 kΩ at ~37°C. 

Figure 2

Figure 3

Then, we set up the circuit as shown in Figure 2. We applied 5 V (Vs) to the circuit and adjusted the resistance of the potentiometer until the output of the circuit was 0 V i.e. balanced the bridge (Figure 3). We also measured the output voltage when the thermistor was warmed up to approximately 37°C, which was -244 mV. 

Figure 4

Next, we designed a difference amplifier as displayed in Figure 4. The design had a theoretical gain of the 15.29, which we believed was sufficient enough to meet the design requirements (Vout =         |(Gain)(Vab)| = |(15.29)(-0.244)| = |-3.73 V| > 2V).

Figure 5

Afterwards, we set up the circuit as shown in Figure 5 and applied two voltages, Va and Vb. We kept Va constant at 300 mV and varied Vb from 100 mV to 500 mV in 50 mV increments. We then plotted the resulting output voltage (Vout) with respect to the potential difference between the two voltage sources (Vab). This graph is illustrated below in Figure 6.

Figure 6: Vout vs Vab

The resulting graph shows that the gain of the circuit is -15.34. If we ignore the negative sign, which was most likely a result of incorrect wiring, we can see that this value was very close to the theoretical value of 15.29. In fact, the percent difference between the values was only 0.327 percent. Therefore, we concluded that our circuit was functioning properly.

Figure 7

Finally, we combined the two circuits together to end up with the circuit in Figure 7. We applied 5 V to the circuit and measured an output voltage of 0 V at room temperature (~25°C). Then, we warmed the thermistor up with our hands and observed an output voltage of -3.56 V. According to the theoretical gain of the difference amplifier, the output voltage was supposed to be -3.73 V. Therefore, the percent difference between the theoretical and experimental values was 4.558 percent. Since the percent difference was less than 5 percent, we concluded that our design was a success.