ECE2019 Sensors, Circuits, and Systems A2015. Lab #2: Temperature Sensing

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1 ECE2019 Sensors, Circuits, and Systems A2015 Lab #2: Temperature Sensing Introduction This lab investigates the use of a resistor as a temperature sensor. Using the temperature-sensitive resistance as part of a suitably chosen voltage divider circuit allows design of a circuit that gives an output voltage proportional to absolute temperature with a scale factor of 1 mv/k. Materials you will need from TA (to be returned at end of lab period): KTY81 series resistive temperature sensor 200Ω 1/2W resistor 1

2 Introduction This lab uses the KTY81 series resistive temperature sensor. The full data sheet is available on the course website; the table below shows resistance vs. temperature over a range of -55 C to +150 C. Table 1. KTY81 sensor resistance vs. temperature. T 0 = 25 C = 298 K R 0 = 1000 Ω As described in class, the temperature dependence of carrier mobility is what underlies the temperature dependence of resistance for this sensor. Plotting the data from Table 1 and fitting the data to a mobility-based model T n R T = R 0 (2-1) T 0 2

3 gives the plot in Figure 1. The parameters for this model are the nominal resistance of R 0 = 1000 Ω at temperature T 0 = 25 C = 298 K, and the exponent of n=2.314 that gives a best fit to the data over this temperature range. Figure 1. Data and model for KTY81 temperature sensor behavior. So, to design with this sensor, we could use the Table 1 data for temperatures in the table; if we needed to estimate resistance for a temperature not given in the table, we could either interpolate or use Equation (2-1). 3

4 Prelab All of your prelab work is to be done in your laboratory notebook and will be checked by a lab TA prior to the start of lab. Design: Direct Reading Thermometer For the circuit shown in Figure 2 below, R T is the KTY81 temperature sensor. R1 and R2 are fixed value resistors. R1, R T, and R2 are connected in a voltage divider configuration with a 5V source, which will be provided from your lab power supply. The goal of your design is to choose R1 and R2 so that the output voltage V T in millivolts provides the temperature in degrees Kelvin. Figure 2. Thermometer circuit using KTY81 sensor. Since there are two degrees of freedom in the design the values of R1 and R2 one way to proceed is to choose two temperatures at which the V T voltage is correct for the R T resistance value at those temperatures. If we choose 0 C and 100 C as our design points, then we have the following constraints: Table 2: Design requirements for direct reading thermometer. Temperature R T value V T voltage C K (from Table 1) (design goal) Ω V Ω V P1. Choose R1 and R2 so the circuit of Figure 1 meets the requirements in Table 2. 4

5 In lab you will change the temperature of the sensor by using another resistor as a heater. As shown in Figure 3, R H is a 200Ω 1/2 W resistor that will dissipate power provided by V SUPPLY. As power dissipation in R H increases, it will heat up, increasing its temperature and also the temperature of R T since the resistors are thermally coupled. Figure 3. Resistive heater / temperature sensor configuration. P2. Determine values of V SUPPLY to use when making measurements in the lab so that the power dissipation in R H increases in approximately 100mW increments; that is, 0.1W, 0.2W, up to the 1/2 W limit of the resistor R H. 5

6 Lab L1. Temperature vs. Power Dissipation To construct the heater/sensor configuration of Figure 3, get a 200Ω 1/2W resistor and a KTY81 sensor from the lab TA and connect them in your breadboard as shown in Figure 4. The paper clip presses the two resistors together to improve thermal coupling make sure it s insulated so you don t short out either resistor! Top View Figure 4. Heater / Sensor Perspective View RT TO POWER SUPPLY TO DMM RH Using the test values from prelab part P2, measure the RT values as a function of VSUPPLY. Be sure to measure the value with VSUPPLY=0, so you have a room temperature baseline. Be patient the thermal time constant of the sensor is 20 seconds, so you may need to wait a minute or so at each point to be sure that the DMM reading of RT resistance is stable. At each point, calculate the power dissipation in RH and the temperature corresponding to the measured value of RT. Plot the temperature as a function of power dissipation what is the relationship? Does this make sense? 6

7 L2. Direct Reading Thermometer Without changing the physical configuration of R T and R H, modify your breadboard circuit to implement your design for the direct reading thermometer of prelab part P1. Set up the DMM so you are now measuring the voltage V T in mv. Repeat the same V SUPPLY data points as you used in part L1. Record the value of V T in mv at each point. How well do the mv readings of temperature correspond to the temperatures you recorded in part L1? If there is a disagreement, think of as many reasons as you can that might explain the discrepancy. If you have time, modify your circuit to improve the agreement, and repeat your measurements to verify the improved performance. CAUTION! Be sure not to confuse the roles of the heater resistor R H and the temperature sensor resistance R T. The heater R H remains connected to the power supply; it is only there to cause the temperature change. The temperature sensor resistance R T becomes part of the voltage divider circuit you designed. 7

8 Lab Writeup W1. Temperature vs. Power Dissipation Present your measured results from part L1. Comment on any interesting aspects of the measured data. W2. Direct Reading Thermometer Present your design calculations from part P1, as well as your measured results from part L2. Comment on any interesting aspects of the measured data, especially any discrepancies you may have observed as well as any improvements you may have made. 8