ECE 2A Lab #5
Lab 5
More Op-Amp Circuits;
Temperature Sensing
Overview
In this lab we will continue our exploration of op-amps but this time in the context of a
specific application: temperature sensing. One temperature sensor will use a device called a
thermistor with a simple op-amp “current source”; the other will use a diode pair and
differential amplifier configuration. You will also learn how to operate the oscilloscope in an
X-Y mode, which is useful for characterizing the characteristics of many electronic devices
and circuits.
Table of Contents
Background Information
Thermistors
Diodes as Temperature Sensors
References
Pre-lab Preparation
Before Coming to the Lab
Required Equipment
Parts List
In-Lab Procedure
5.1
Op-Amp Current-Source; Thermistors
First Step: DC Biasing
A Simple Thermistor Circuit
Testing with a Resistive Heater
Oscilloscope X-Y Mode
5.2
Differential Measurements; Diode Sensors
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© Bob York
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More Op-Amp Circuits; Temperature Sensing
Background Information
Thermistors
All metallic conductors have a temperature-dependent resistance. This is not always
desirable but can be exploited for temperature sensing applications. Over a small range of
temperature the resistance of most conductors follows an approximately linear relationship
R (T )  R0 1  TCR (T  T0 ) 
(5.1)
where R0 is the resistance at the reference temperature T0 , most commonly taken as 25ºC or
T0  298.15K . The parameter TCR quantifies the fractional change in resistance per degree
of temperature change, and is called the “temperature coefficient of resistance”, or TCR. For
ordinary metallic conductors the temperature coefficient is always positive. For example,
copper has TCR  4.04  103 /K . Platinum also has a similar TCR but is a better choice for
simple resistance temperature detectors (RTDs), especially at high temperatures (e.g. above
600ºC) because it is a refractory
metal that does not oxidize
easily and its resistance follows
(5.1) quite well over a wide
range of temperature.
Thermistors have a much
greater sensitivity to temperature
than platinum RTDs, but the
dependence
is
strongly
nonlinear and limited to a
smaller operating temperature
range, typically from -20ºC to
+120ºC. Thermistors are often
made
from
a
ceramic
semiconducting material, and Figure 5-1 – Resistance vs. temperature for an NTC thermistor
are available in two types: with R0  50 k and   4400 K
positive temperature coefficient
(PTC) and negative temperature coefficient (NTC). A simple model for the temperaturedependence of the thermistor resistance for NTC devices is given by
R (T )  R0 e   /T0 e  /T
(5.2)
where R0 is again the resistance at the reference temperature T0 . In this lab we will use an
NTC-type of thermistor. The datasheet provides the appropriate values for R0 and  .
Figure 5-1 illustrates the resistance-temperature dependence predicted by (5.2) for a device
similar to the one we will use in our lab.
Diodes as Temperature Sensors
Figure 5-2 illustrates the current-voltage relationship for a simple silicon diode. This is also
similar to the devices we used in earlier labs. In forward conduction the diodes are well
modeled by
I  I 0 eqV / nkT
19
23
(5.3)
where q  1.6  10 C , k  1.38  10 J/K , I 0 is the saturation current, and n is a
dimensionless “ideality” factor. Usually measurements are required to determine the
© Bob York
3
Background Information
saturation current and ideality factor. For two identical devices at the same temperature but
biased at different current levels, the ratio of the currents is given by
q
(V1 V2 )
I1
 e nkT

I2
V1  V2 
nkT I1
ln
q
I2
(5.4)
Current I, mA
So the difference in voltage between
30
two diodes should scale linearly with
temperature. The key assumptions in
320K
25
300K
(5.4) are that the diodes have the
20
280K
identical saturation currents and
ideality factors, and that they both at
15
exactly the same temperature. The
10
only good way to satisfy these
conditions is to use two diodes that are
5
fabricated side-by-side on the same
0
chip, using identical materials and
0.40
0.50
0.60
0.70
0.80
0.90
fabrication processes. The device we
Diode
Voltage
V,
Volts
will use in the lab integrates two
identical devices in this fashion.
Figure 5-2 – Temperature-dependent I-V curves for a
We will discuss diodes in greater representative silicon p-n junction.
depth in ECE 2B. For now, all you
have to do is accept that the I-V curves are modeled by an exponential relationship like (5.3).
The important takeaway here is that the diode characteristics are temperature dependent, and
anytime that happens we might try to exploit that device as a temperature sensor. In more
advanced coursework like ECE 137 you will learn how circuits can be designed to deal with
temperature-dependent effects and continue to function properly over a wide range of
temperatures. One particular circuit, commonly called a “bandgap voltage reference”, is a
clever example of how circuits can be designed to cancel out temperature effects completely.
Stable voltage references are important in many applications such as data acquisition and test
instruments.
References
http://en.wikipedia.org/wiki/Thermistor
D. Sheingold, Transducer Interfacing Handbook, Analog Devices, 1981
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More Op-Amp Circuits; Temperature Sensing
Pre-lab Preparation
Before Coming to the Lab
□
Read through the details of the lab experiment to familiarize yourself with the
components and testing sequence.
□
One person from each lab group should obtain a parts kit from the ECE Shop.
Required Equipment
■ Provided in lab: Bench power supply, Function Generator, Oscilloscope, Decade Box
■ Student equipment: Solderless breadboard, and jumper wire kit
Parts List
Qty
1
1
1
1
1
1
1
4
Description
MSD6100G Dual switching diode, TO-92 common cathode
NTC Thermistor - 50K Ohms @25C, -4.7/C
20 Ohm 5W, Cement Resistor
Large Binder Clips; Clip size 2”; Cap. 1-1/16”
100-Ohm 1/4 Watt resistor
1.2 k-Ohm 1/4 Watt resistor
12 k-Ohm 1/4 Watt resistor
100 k-Ohm 1/4 Watt resistor
In-Lab Procedure
5.1 Op-Amp Current-Source; Thermistors
First Step: DC Biasing
The lab uses op-amps again, so remember our advice: always
make setting up the bias circuit your first step. This time we
will use ±12V bias:
□
□
Insert the op-amp in your breadboard and add the wires
and bypass capacitors as shown in Figure 5-3. To avoid
problems later you may want to attach a small label to
the breadboard to indicate which rails correspond to
Vs , Vs , and ground.
0.1μF
+Vs
2
3
7
+
6
4
-Vs
0.1μF
Next, set the power-supply to produce ±12V and attach Figure 5-3 – 741 bias circuit.
the supply and COM connections to the terminals on
your breadboard. The use jumper wires to power the rails as shown. Remember, the
power-supply COM terminal will be our circuit “ground” reference. Once you have your
bias connections, monitor the output current on the power-supply to be sure that there
aren’t any inadvertent short-circuits. You may wish to use a DMM to probe the IC pins
directly to insure that pin 7 is at +12V and pin 4 is at -12V.
© Bob York
5
Op-Amp Current-Source; Thermistors
A Simple Thermistor Circuit
As described in the background section the
Thermistor
thermistor resistance varies with temperature in
a predictable way. Our goal is to design a
R1
I
RT
circuit that uses the thermistor to generate a
voltage proportional to temperature. One way
to do this is to drive the thermistor with a known
-Vs
Vout
constant current, and adjust the magnitude of
+
this current so that the voltage varies in a
LM741
prescribed manner over temperature. Figure 5-4
illustrates one way of doing this. The current I
is set by resistor R1 and the voltage Vs , which Figure 5-4 – Thermistor driven by a constantcurrent as set by R1 .
is most conveniently taken as the negative
supply voltage, -12V. In this case the output voltage at each temperature will be
R (T )
Vout (T )  12 T
(5.5)
R1
The circuit resembles an inverting amplifier, but when used in this fashion it is essentially a
voltage-controlled current source, since the current through the “load” (the thermistor) is a
constant, independent of the value of RT , and set by the input voltage Vs .
The next step is to choose the range of output voltage. In a real application the output
voltage of the sensor is often fed into a digital panel meter or other data acquisition system, in
which case there might be some constraints on the allowable voltages. But in this lab we can
choose the voltage range to be anything we like. We will be content to simply monitor the
voltage with our DMM and perform the mathematical conversion between voltage and
temperature by hand.
Let’s design the circuit to produce Vout  1V when the thermistor is at room temperature
( T  25 C ). In most cases the thermistors are specified to have a nominal resistance R0 at
this temperature; you can look this up in the data sheet or just measure it manually using your
ohmmeter:
□
Find or measure the room-temperature resistance of your thermistor, and with the powersupply disconnected or turned off, construct the circuit shown in Figure 5-4. Use the
bench decade box for R1 .
□
Using (5.5), calculate the approximate value of R1 needed to make Vout  1V at room
temperature, and set your decade box to this value.
□
Turn on power and fine-tune the value of R1 to make Vout  1V .
□
Now pinch the thermistor between your thumb and finger and wait until the output
voltage reaches a steady value. Record this number. From (5.5) you can estimate the
thermistor resistance and hence its temperature. You can use a plot like that of Figure
5-1 or the figure in the datasheet to compute the temperature, or alternatively compute it
from (5.2) which can be inverted to give
T

R
ln  T
 R0
A value for  should be given in the data sheet.
 

 T0
(5.6)
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More Op-Amp Circuits; Temperature Sensing
Testing with a Resistive Heater
Next we will create a very simple heat source using a power resistor. Such resistors are
designed to dissipate more power than the conventional ¼ Watt resistors that we normally
use, but as with all resistors that power is dissipated as heat. Many commercial heaters
operate on similar principles: water heaters, space heaters for small rooms, toaster ovens, etc.
all use resistive filaments (usually a nickel-chrome alloy) as the heat source. So the idea here
is to use the power resistor to create a programmable heat source, and put the thermistor in
contact with it so that we can sense how hot the resistor becomes. The power resistor that we
have chosen has a large rectangular shape which gives us a nice, flat, “hot” surface:
Power resistor
(heat source)
20Ω/5W
R1
-Vs
RT
+
+V1
Thermistor
and power
resistor in
thermal
contact
Vout
LM741
Figure 5-5 – Using a power resistor as a heat source for temperature sensing.
□
Construct the circuit shown in Figure 5-5, with the power resistor and thermistor clamped
together using the large binding clip that is included in your kit. Use the remaining
independently adjustable output on your power supply for V1 . Use jumper wires as
needed for the electrical connections, but take care not to allow the thermistor leads to
electrically touch the power-resistor leads. Those components should be thermally
connected, but not electrically connected.
□
The 20Ω power resistor is rated for maximum power dissipation of 5W. What value of
supply voltage will result in exactly 5W dissipation? Turn up supply V1 to the voltage
you just calculated and monitor the ammeter on the power supply to verify that the
current drawn is approximately 0.5A.
□
Allow about 3 minutes for the temperature to stabilize, and then record the output voltage
Vout . Since the heat flow from the power resistor to its surroundings is not managed well
its temperature may not be completely stable; if so, just note the fact in your lab notebook
and record the average value at the 3-minute mark. Again use the charts or (5.6) to
estimate the temperature of the power resistor.
□
Turn V1 back to 0V and allow the power resistor to cool. Monitor Vout to see when it has
returned to 25C ( Vout =1.00V). Fanning or blowing on the device may speed this up.
Oscilloscope X-Y Mode
So far in ECE 2 we have used the oscilloscope in a time-base or “Y-T” mode for displaying
the input signals as functions of time. But most oscilloscopes can also be configured to
display one signal as a function of another. This is called the X-Y mode, and is useful for
studying the current-voltage (I-V) characteristics of devices, as well as phase relationships of
© Bob York
7
Differential Measurements; Diode Sensors
sinusoidal signals. We will use this mode to display the current-voltage characteristic of the
thermistor.
□
The desired input signal is a triangle wave at 1 kHz, with a 10V amplitude (peak-to-peak
voltage of 20V). To verify and/or change the scope's display mode, press the
"DISPLAY" button, then look at the "Format" field. The two possibilities are YT (time
base), or XY. For now we want YT.
□
+V1
Disconnect R1 from the -12V supply
(heat source)
20Ω/5W
and reconnect it to the output of the
Thermistor
and power
function generator, as indicated in
R1
resistor in
Figure 5-6.
Monitor Vin using
RT
thermal
channel 2 of the scope, and monitor
contact
the op-amp output Vout with channel Vin
1. It should be obvious by now that
Vout
+
Ch1 will represent the voltage across
the thermistor. Remember that this
LM741
is
an
inverting
amplifier
configuration, so the relationship Figure 5-6 – Same circuit as in Figure 5-5, but with a
between
thermistor current and function generator as the stimulus for current in R1 .
input voltage is I  Vin / R1 . To
obtain a non-inverted measurement of the thermistor current we cannot simply reverse
the black and red clip leads from channel 2 of the scope, since the black lead is connected
to ground inside the scope (remember Lab #3!). We can, however, invert the measured
signal using the scope's invert function. On the Ch2 menu screen, press the Invert button
to obtain “Invert On”. The two traces, Ch1 and Ch2, should now be in phase.
□
As a convenience in reading the thermistor current, change the decade resistor to 100kΩ.
This will make the Channel 2 voltage exactly 100,000 times the thermistor current.
□
Verify that both traces are clean triangle waves with reasonable amplitudes. Adjust the
two traces on the scope so that they are both centered vertically, i.e., their zero levels are
both exactly on the middle horizontal gridline. Now change to XY mode, using the
DISPLAY menu. In this mode you can still adjust the Volts/Div sensitivity of each
channel. You can also adjust the offsets, but be careful if you do, since that moves the
(0,0) position away from the center of the display. Channel 1 is the horizontal variable
(X), and Channel 2 is the vertical variable (Y).
□
Sketch the trace from the scope in your notebook. For the vertical scale, indicate both the
actual volts/div for channel 2, and the corresponding current in the thermistor (V/100k)
□
A resistor has a linear I-V characteristic with slope equal to (1/R). What resistance do
you calculate from your display for the thermistor?
□
Turn the voltage V1 back up to 10V and watch the slope of the XY trace change as the
power resistor heats up. What resistance value do you calculate after the temperature has
settled? Turn V1 back to zero when you are finished.
Power resistor
5.2 Differential Measurements; Diode Sensors
The second method for measuring temperature will take advantage of the inherent
temperature variation of the forward voltage of a silicon diode, as described in the
background section. This will illustrate the use of a single op-amp differential amplifier
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More Op-Amp Circuits; Temperature Sensing
circuit. The diode pair device chosen for this lab is the MSD6100 shown in Figure 5-7. The
two devices are fabricated on the same chip and hence are very well matched in terms of their
I-V characteristics.
□
□
Leaving the ±12V power supply connections to the
op amp intact, and verifying that V1 is still set to 0,
modify the circuit to that of Figure 5-8. Couple the
MSD6100 to the power resistor as shown, similar to
the method we used with the thermistor. Notice that
the two resistors connecting the diodes to the +12V
supply are different by a factor of 10. This will make
the currents in the two diodes different by about the
same factor. Measure the two resistors using your
ohmmeter prior to inserting them in the circuit.
Figure 5-7 – MSD6100 integrated
diode pair in a TO-92 package.
Turn on power and make sure V1 is set to 0V. Record the voltage drops across the diode
biasing resistors (the 1.2k and 12k resistors). From this information and the actual
measured resistances, determine the current flowing in each diode.
+12V
1.2kΩ
Differential Amplifier
12kΩ
100 kΩ
+V1
100 kΩ
100 kΩ
+
Vout
LM741
20Ω
5W
100 kΩ
MSD6100 thermally
coupled to power resistor
Figure 5-8 – Temperature sensor using diode-pair and differential amplifier.
□
Now measure Vout with the DMM. The op-amp circuit is a simple differential amplifier
with a nominal differential gain of one, so the output voltage should be approximately the
same as the difference in the voltage drops across the two diodes. You should use your
DMM to confirm this by recording the diode voltage drops directly. The op-amp circuit
is not perfect, however, owing to the tolerances in the resistor values and the internal
offset voltage of the op-amp.
□
Now turn up V1 to 10V. Again allow 3 minutes for the temperature to settle. What is
Vo? Using the information in the background section to calculate the temperature of the
diodes and power resistor. For the MSD6100, the information in the datasheet is
consistent with an ideality factor of   1.75 .
Congratulations!
You have now completed Lab 5
As noted in the previous lab: keep all your leftover electrical components!
© Bob York