Day Four - Temperature Measurement System 3/9/2017

Hello Guys, in day four of our ENGR 44 class, we first had our experiment, and we talked about nodal analysis.

For this lab, we are asked to solve a design problem which electrical engineer will always face in their career. The problem is to find a value for R so that Vout increases by a minimum of 0.5V over a temperature range of 25 degree Celsius to 37 degree Celsius.

PreLab:
Using Voltage Divider to get the R in different situations.

I

In our case, we want 0.5 volt voltage increase on our output from 25 to 37. 

First we calculate the resistance of the thermistor at room temperature, we had 11k ohms.

Second we calculate the resistance of the thermistor at human temperature, we had 7k ohms.

Now we compare these two values using Voltage Divider to get the output difference of 0.5v.

From calculator, we had two resistance value (17633 and 4367 ohms); however, we chose the average of these two resistance (10k ohms) because it is the most accurate value from the bell curve.

From Measurement:

For our thermistor, in room temperature, the thermistor has 9.9k ohms; furthermore, in human temperature, the thermistor has 5.9k ohms. For our 10k ohms resistor, we measured that it has 9.8k ohms.

The output voltage at the high temperature condition is 2.46v and at the low temperature condition is 3.09v, so the voltage difference is 0.65v which passes the requirement for the design. 

Using the equation from the prelab, the calculated value of using 9.9k/5.9k ohms thermistor and 9.8k ohms resistor in circuit has a 0.633 output voltage.

Percentage difference is very low in this lab, it is around 2.65%.

Based on the calculation and implementation, we think that our design has met the design requirement, which is a minimum of 0.5V increase of 25 to 37.

If we need a more sensitive device that has at least 0.1V/degree Celsius, we are not able to design it with the current thermistor because it only has maximum 0.64 voltage increase.

Concern:

When we are using our finger tip to increase the voltage, our human temperature might not be as high as 37 degree Celsius; therefore, the output voltage can be much larger. Similarly, when we are waiting for the thermistor to cool down, the actual temperature on the thermistor might not be 25 degree Celsius. Also, the thermal conduction might take time for heat to transfer from our finger tip to the thermistor.

After the lab, we learned about nodal analysis, which I hate.

We are using the voltage difference between a voltage node and a non reference node to calculate the current flowing on that element.

Then using KCL to derive multiple equations from the circuit, and calculate the voltage.

Summary:

We learned about nodal analysis to deal with circuit that is problematic to solve using KCL and KVL.

Also, we learned to create our own temperature sensor device. From this device, we conclude that we are not always able to meet the design requirement. In reality, we have to know the reason of it and we must learn to explain to the customer when these situations are encountered.