Day 5: Temperature Measurement System

Lab 5: Temperature Measurement System

Purpose/Prelab:

This lab was our first design centered challenge in which we had to develop a circuit that increase V(out) with a variable resistance. More specifically we had a set of criteria to meet:

• +5 voltage input to the system
• Output voltage varies by a minimum of 0.5V over a temperature range of 25 - 37 degrees celsius
• Output voltage must increase as temperature increases

In order to achieve these specifications a circuit element called a Thermistor was used rather than a standard potentiometer.

A thermistor is basically a resistor whose resistance value changes with temperature changes within it! The specific model we used was a negative one that decreased the resistance value the higher the temperature got.

Preliminary schematic of circuit

Using a handy chart supplied to us regarding the thermistor, we were able to find the predicable resistances associated with that temperature. 

Chart of decreasing resistance with increased temperature

The only thing needed after that was to find a fixed value of R to use aside the variable R(Thermistor)

After some voltage divider algebra, we found a value we were satisfied with!

We had calculated that the value of the fixed resistor was to be between 4.67k - 17.63k Ohms. Since that would not be an easy part to find in the lab, we were directed to a more realistic value of 10k ohms.

Procedure:

We first measured our thermistor to get some realistic R values from room temp to human touch temp (37 degrees Celsius)

Real value of R(Thermistor) at room temp (10.95K Ohms)

Physically touching unit to get applicable resistance (6.40K Ohms)

We then measured the fixed Resistor value (expected 10K) and put together the circuit for implementation.

Measuring R of fixed resistor (9.95K Ohms)

Prototype circuit built

Observing the V(out) values from the circuit in the footage, we can see that the resistance from the thermistor decreases the longer it is touched.

From this we obtain the values:

V(25 Degrees Celsius) = 2.41 V

V(37 Degrees Celsius) = 2.93 V

This confirms that we met the 2nd requirement for the change in voltage to be at least 0.5 V between our target temperatures.

Calculating a percentage error:

[(Experimental - Theoretical)/Theoretical]*100%

Where the experimental value is the actual change in voltage in the circuit.

Experimental = 2.93V - 2.41V = 0.52V

[(0.52V - 0.5V)/0.5]*100% = 4.00%

Lecture:

Today, we learned about Nodal analysis and it's benefits/process.

Using this we found the voltages contained at specific nodes in the circuit.

Further, if we analyze another styled circuit we can see that nodes can be combined to make nodal analysis easier.