Download Lab7- Thermistor

California University of Pennsylvania
Department of Applied Engineering & Technology
Electrical / Computer Engineering Technology
EET 215: Introduction to Instrumentations
Lab No.7
Temperature Measurement Using NTC Thermistor
Names:
1.
2.
3.
Signature:
Date
Instrumentation Amplifier
Objective of the Experiment
The main objective of this experiment is to understand NTC thermistors and to use them to calculate
and display temperature readings.
Learning Outcomes
Students will demonstrate:
-
The ability to use Thermistor equations
the ability to derive linear equation for a Wheatstone Bridge
interpret data and design a digital thermometer using GUI
Introduction
Thermistors (thermally sensitive resistors) electrical resistors whose resistance changes with
temperature. Thermistors are manufactured from metal oxide semiconductor material which is
encapsulated in a glass or epoxy bead.
Thermistors have a very high sensitivity, making them extremely responsive to changes in temperature.
For example, a 2252 Ω thermistor has a sensitivity of -100 Ω/°C at room temperature. Thermistors also
have a low thermal mass that results in fast response times, but are limited by a small temperature
range.
Thermistors have either a negative temperature coefficient (NTC) or a positive temperature coefficient
(PTC). The first has a resistance which decreases with increasing temperature and the latter exhibits
increased resistance with increasing temperature. Figure 1 shows a typical thermistor temperature
curve:
Figure 1: NTC Thermistor characteristics
Reference: http://zone.ni.com/devzone/cda/tut/p/id/3643#toc1
The resistance of a thermistor as a function of temperature is approximated by:
𝑅(𝑇) = 𝑅0 𝑒
1 1
𝛽( − )
𝑇 𝑇0
Where:
R(T) is the resistance at a given temperature T: Resistance as a function of temperature.
β is the material constant
T is the current temperature in oK
T0 is the room temperature (typically 25oC = 298 oK
Components Needed:
-
the IA circuit of last time
-
4 - 10KΩ resistors
-
5 KΩ resistor (or close to it)
-
9 KΩ resistor (or any value less than 10 KΩ but higher than 5 K)
-
Any value between 2KΩ and 6KΩ resistor (any value)
-
2KΩ resistor (or close to it)
-
10KΩ Thermistor
Experiment
AUse the Instrumentation amplifier circuit of previous Lab (May have to rebuild it- see circuit
below)
With the power to the board initially turned OFF,
a-
SET RG to 5KΩ. (instead of the previous value)
b-
Build the Wheatstone bridge circuit shown below. The 5V dc is from the DAQ board.
5V
To V1 input on IA
10kΩ
10kΩ
To V2 input on IA
10kΩ
10kΩ
Rx
OP-AMP1
V1
R
Vy
10kΩ
R
LM741CN
10kΩ
R
RG
50%
R 10kΩ
Vout
10kΩ
10kΩ
LM741CN
R
R
10kΩ
OP-AMP3
V2
Vx
LM741CN
OP-AMP2
Verification Step (Measure voltages using the DMM of the instrument launcher)
c-
Test to verify the circuit is still operational as follows: (Turn the power to the board ON)
1-
measure Vout of the IA. It should be near zero volts ?
2-
Change Rx in the Wheatstone bridge to 5KΩ. Measure Vout.
Vout should obey the relationship: (1+2R/RG)(V2-V1) = (1+20K/5K)(1.67-2.5) = -4.15V
Note: R = 10KΩ (used in the IA circuit)
If the two above steps agree to a great extent, then all is set to go! Otherwise, fix the problem.
B- Develop a “Linear” Relationship
The goal here is to record measurements of Vout vs. RX. This will allow us to develop a linear
relationship that will be used to design the digital voltmeter.
1-
Complete the table below:
a-
Vary Rx and record measurements of Vout vs. Rx (may use fixed resistors)
b-
Calculate Vout Expected for each case and verify the measured value.)
Example:
V1= 2.5V (since two equal resistors)
At Rx = 9KΩ, V2 = 5*9K/(9K+10K) = 2.368V
Vout=(1+20K/5K)*(V2-V1) = - 0.658V
You may use slightly different values of Rx than listed below. However, make sure to record the
values used and recalculate the expected values. The example above explains the process
Vout
Vout
(expected)
(measured)
Rx
9KΩ
-0.658V
7KΩ
-2.25
5KΩ
-4.167
2KΩ
-8.333
2- Once the table is completed successfully, we are ready to develop the “linear” relationship.
3- Use Excel to develop a linear relationship between Vout and Rx. Will let Rx be the vertical axis
(independent variable) and Vout be the horizontal axis (dependent variable). The reason for this will be
apparent in later steps when we incorporate software.
4-
Follow the steps below for guidance with EXCEL.
a- Start EXCEL and fill in your data as shown (Use your values)
b-
Plot the data as follows:
i- select the data as shown
ii-
click on INSERT on the top menu then click on Line down arrow as shown
iii-In the menu that appears, click on All Charts Types
iv-
Pick the items shown and click ok
v-
a relatively linear plot appears as shown:
viOn the “linear “ plot, place the cursor on the “line”, right click the mouse and choose
“Add Trend Line”
viiin the menu that appears, click on “Linear “ and on “Display Equation on Chart” and keep the
“Trend Line Automatic “ selection. Do not check others. See below:
viii-
Click Close and record the equation that appears: (See example figure below)
c-
The equation that relates Rx to Vout is: (Record Your Equation)
Example (This example corresponds to the data from EXCEL above )
Rx = 893.44*Vout + 9190.9 Ω
To test the equation:
At Vout = -4.167Voult,
Rx = 893.44*(-4.167)+9190.9 = 5.46KΩ (a little off the 5K expected.)
dTest your equation at a different value of Vout and see how close / far off from the
actual Rx:
At Vout =
C-
--------------?, Rx = --------------? Close or way off --------?? (should be close)
Thermistor Implementation
In this portion of the lab, the following is accomplished:
1-
Rx is replaced by the 10KΩ thermistor
2The “linear “ equation obtained by you will be used to calibrate software interface to the DAQ to
better estimate the thermistor resistance.
3using the thermistor resistance, the actual temperature is displayed. Here, some software
calibration may be necessary.
a-
Replace Rx with a 10KΩ Thermistor
bFrom the course’s page, download the LabVIEW VI called thermistor and save it to your N drive
(in a folder ). Also, download the LabVIEW DAQ interface supplement document.
cConnect Vout of the instrumentation amplifier to Analog Input 0 + (AI0+) and connect the AI0to ground.
d-
Turn the board power ON
e-
Run LabVIEW from Start>>All Programs>>National Instruments>>LabVIEW.
f-
Once the “Choose environment “ window comes up, click on Start LabVIEW (Not the robotics )
g-
in LabVIEW, click File>>Open, then navigate to the VI you downloaded. To open it.
h-
Perform all of the necessary settings as mentioned in the “LabVIEW DAQ interface
supplement” document
i-
click RUN (on the arrow as shown below)
iyour VI should run nicely. You can verify that it is working by looking at the Vout reading in
the indicator shown by the bubble above. This data should match your expected output.
j-
Click Stop (to stop running the VI)
k-
Enter your own equation as follows:
1Diagram”
on the front panel of LabVIEW, click on “Windows” as shown and select “Show Block
2Edit the equation shown to enter your numbers. Make sure the equation ends with
the semicolon ;
3-
Click on Window in LabVIEW and select “Show Front Panel”
4-
Run the VI – you should see good behavior
Instructor approval of experiment completion: