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ECE2019 Sensors, Signals, and Systems A 2013
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
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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.
T0 = 25°C = 298 K
R0 = 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
RT  R0  
T0 

(2-1)
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gives the plot in Figure 1. The parameters for this model are the nominal resistance of
R0 = 1000 Ω at temperature T0 = 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).
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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, RT is the KTY81 temperature sensor.
R1 and R2 are fixed value resistors. R1, RT, 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 VT 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 RT 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
°C
K
0
273
100
373
P1.
RT value
(from Table 1)
820 Ω
1670 Ω
VT voltage
(design goal)
0.273 V
0.373 V
Choose R1 and R2 so the circuit of Figure 1 meets the requirements in Table 2.
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In lab you will change the temperature of the sensor by using another resistor as
a heater. As shown in Figure 3, RH is a 200Ω 1/2 W resistor that will dissipate
power provided by VSUPPLY. As power dissipation in RH 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 VSUPPLY to use when making measurements in the lab so
that the power dissipation in RH increases in approximately 100mW increments;
that is, 0.1W, 0.2W, … up to the 1/2 W limit of the resistor RH.
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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 R T 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 R T 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?
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L2.
Direct Reading Thermometer
Without changing the physical configuration of RT and RH, 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 VT in mV.
Repeat the same VSUPPLY data points as you used in part L1. Record the value of VT 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 RT. The heater RH remains connected to the power
supply; it is only there to cause the temperature change. The temperature sensor
resistance RT becomes part of the voltage divider circuit you designed.
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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.
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