03/03/2016

03/03/2016: Temperature Measurement System

I. Introduction:

For today lab, we were going to desing a circuit whose output voltage provides a temperature measurement. A thermistor- a device with resistance changes with temperature- is used to sense the temperature. There were three things about this lab we had to pay attention to:

-5V input voltage to the system

-output voltage varies by a minimum of 0.5 V over a temperature range of 25 to 37 degree celcius

-output voltage must increase a temperature increases

here we had 8 bit machine, therefore, there were a total of 256 values. the voltage resolution would be 5/255 =0.01996V. Since delta V=0.5, we would have 25 counts

II. Calculation:

Here we were back to the voltage divider. We know that Vout/5= 5/(R_th+R). And we were given delta Vout as 0.5 Solve the quadratic equation, we came up with two values for R. We picked the lower resistance to be our choice of resistance

III. Setting up the circuit

The circuit was easy to set up since everything was in circuit to each other.

We picked the blue resistor to be our resistor (there was some random resistor, we had to double check to make sure we got the right one. We let the circuit sit in the room temperature (25) and read the measurement, which was about 1.51V. Then we firmly pressed the thermistor, the voltage increased to 2.04V, we found the experimental delta V to be 0.53V, which was bout 6% percet error

V. Post Lab Question:
Using the thermistor provided in this lab, we had to design a temperature measurement system which meets the following requirements:
1. The output must increase as the temperature increases
2. The output sensitivity of the device must be at least 0.1V/degree Cel

Vmax here is Vout. V max must be 44.4V or V=0.563, which is impossible to implement given all the lab device

IV. Node Voltage Method:
Applied the voltage method to annalyze the circuit

Here we chose the ground to be the lower node. Applied node voltage to essential nodes, we were able to find the V1, V2 respectively

VI. Summary:

Today we spent most of our time on temperature measurement system lab. The percent error was 6 percent, which was acceptable. We also learned the new techique to annalyze and solve for circuit element using node method. Node method is utilized to take away one variable in the system of equations, which makes it easier to solve for.