Download Lab6- Thermistor - Department of Applied Engineering

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 temperatures.
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 compared to a typical 100 Ω RTD 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.
Β is the material constant
T is the current temperature in oK
T0 is the room temperature (typically 25oC = 298 oK
Components Needed:
-
LM741C op-amp (3)
-
Six equal resistors (may be 10KΩ or close to this value – like 11KΩ if 10KΩs are gone) - refer to
them as R in the circuit
-
Potentiometer to be used as a variable resistor ( 1KΩ or 5KΩ) potentiometer (refer to it as RG in
the circuit)
-
Any two equal resistors (choose 4.7KΩ if possible) (refer to them as R1, R2 in the circuit)
Experiment
For all circuits, the +/- Vsupply = +/- 15Volts.
Component Values are:
R=
R1 = R2 =
?
?
A- Construct the IA circuit
With the power to the board initially turned OFF (top right corner),
a-
Careful, some op-amps in the drawing are flipped vertically.
b-
Ensure all op-amps are connected to the +/-Vsupply (but power is still turned off)
c-
Take great care in making correct connections. This will make this experiment and the next go
smoothly.
d-
SET RG to 500Ω (May used a fixed resistor if available)
e-
Again, Connect circuit very nicely. This makes it easier to troubleshoot and to understand.
f-
NO input signal for Vin yet.
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
Fig.1.
IA circuit set-up
B-
Determining the Common Mode Gain, AC
a-
Turn the power to the board ON and apply Vin to both inputs as shown
Vin
Connect to V1
Connect to V2
Vin is a sine wave at 0.2Vpk and 1KHz.
b-
USE the DMM tool in ELVIS’s instrument launcher to perform the following measurements:
Notice: The AC voltage measurements on the DMM correspond to the RMS measurements
1-
Vin(RMS) = Vin(CM)
2-
Vout(RMS) = Vo(CM)
Volts (should be about 0.1414V)
Volts (Should be VERY small, NOT zero)
If the value for Vout value is large, then here are possible errors:
c-
i-
check all op-amps output, make sure all are low. If one is near saturation, then
there is a problem with connections or the applied power supply to that opamp.
ii-
double check all connections – (this is part of troubleshooting skills)
Calculate the common mode gain, AC
AC =
𝑉𝑜(𝐶𝑀)
𝑉𝑖𝑛(𝐶𝑀)
=
(should be very small – near zero, but NOT zero)
C-
Determining the Differential Mode Gain, Ad
a-
Turn the board power off and remove the common mode input connections of step B above
b-
Apply the differential input as shown below and turn the board’s power ON.
Vin
R1
Connect to V1
4.7kΩ
Connect to V2
R2
4.7kΩ
Here, V2 sees the input signal and V1 sees half of it.
c-
Set Vin to the same value as previously. Vin = 0.2Vpk sine wave at 1KHz.
d-
Verify correct operation by monitoring Vout on the scope. Vout should be near 4.1(pk) sine
wave. If far off, then troubleshoot the circuit and fix the problem (note this part assumes using
the suggested resistor values.)
e-
If step (d) is o.k, then record your measurements using the DMM tool in ELVIS’s instrument
launcher .
f-
1-
V1(RMS) =
Volts (should be near 0.0707V) !
2-
V2(RMS) =
Volts (should be near 0.1414V) !
3-
Vout(RMS) =
Volts (should be near 2.89V) !
Calculate the differential gain, Ad
𝑉
𝐴𝑑 = 𝑉 𝑜𝑢𝑡
=
−𝑉
2
g-
(should be about 41)
1
Calculate the theoretical value of the differential gain:
2𝑅
𝐴𝑑 = 1 + 𝑅 =
𝐺
h-
Do the results in steps f and g almost agree? -------------YES, --------------NO
iIf the percentage difference is more that 10%, then the work was not successful. All previous
steps will have to be checked.
Instructor Approval: ------------------------
D-
Determining the Common Mode Rejection Ratio (CMRR)
𝐴
CMRR = 𝐴𝑑
𝑐
=
CMRRdB = 20log10(CMRR)
? (should be large , possible above 1000)
=
dB ( should be high, possible more than 60dB)
E-
Typical CMRR values of standard components
From the web, obtain and record CMRR (dB) for each of the following components:
http://www.national.com/ds/LM/LM741.pdf
http://www.analog.com/static/imported-files/data_sheets/AD620.pdf
Component
CMRR(dB) (minimum)
CMRR(dB) (typical)
LM741
AD 620 (with gain, G
=1000)
Instructor Approval of Completion: -------------------------------DO NOT disassemble the circuit, it will be used in the next experiment.
Challenges
1Draw the connection diagram of an instrumentation amplifier (refer to the AD620
datasheet- see figures 1 and 2 to help understand. The symbol is shown on top of first page.)
Also, explain, briefly, what each of the 8-pins is for?
abc2Draw the electronic symbol of an instrumentation amplifier (refer to the AD620
datasheet. The symbol is shown on top of first page.) Also, explain, briefly, what each of the 8pins is for?
Instructor approval of experiment completion: