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Departement Elektriese en Elektroniese Ingenieurswese
Elektrotegnologie 144
Practical 2 - Temperature sensors and operational amplfiiers
The purpose of the practical is:
1. Get to know power sources
2. get to know temperature sensors
3. use an operational amplifier (see notes on operational amplifiers on WebCT)
What do you need? Temperature sensor LM335, resistors (3 X 10 kΩ, 33 kΩ en 1 X 330
kΩ) and terminal board.
1.
Temperature sensor
The LM355 is a temperature sensor which produces an output proportional to temperature.
The connection is shown in figure 1. The unused connection can be used to calibrate the
sensor, but we will not use it in this practical.
Figuur 1. Temperature sensor
A calibrated sensor wil produce an output voltage of 0 V at 0 K (-272 °C). For every 1 °C
increase in temperature the voltage will increase with 10 mV. At 20 °C the output voltage
will be 2.92 V. Since we will not calibrate the sensor, it is possible that the output voltage
will not be exactly correct. However, one should still observe the 10mV/°C increase with
temperature.
a) Build the circuit of figure 1. Make sure that the connections are correct before applying
power to the circuit.
b) Measure the output voltage Vt with the multimeter and calculate the temperature.
c) Heat up or cool the sensor and measure the voltage, observing how the output changes.
Keep this circuit on the breadbord, you will use it again.
2
2. The inverting amplifier
The voltage change of 10 mV/°C is quite small and we would typically prefer a more
sensitive measurement. To do this we first need to amplify the output voltage. The easiest
way to do this is to use an operational amplifier. We will use the simplest configuration, the
inverting amplifier.
Power sources:
The supply voltage required is +15 V and -15 V. Figure 2(a) demonstrates how the connection
is made. The 0 V (ground) is the reference voltage and all measurements are made relative to
this.
+15 V
(rooi draad)
0 V (grond)
(swart draad)
-15 V
(rooi of ander kleur draad)
Figuur 2. Connection for ±15 V supply voltage
The operational amplifier:
Figure 3 shows the pin connections of the op-amp. The
hald circle on the edge is a reference to indicate the position
of pin 1. Pins 1 and 5 are not used.
Figuur 3
3
The amplifier circuit in figure 4 has a gain (amplification) of
A = Vuit/Vin = - R2/R1.
Our output voltage of the temperature sensor is about 3 V. If the amplification is 4 times, the
output voltage will be 12 V. If the amplification is too high (i.e. 6) the output voltage will be
too high and the amplifier will saturate.
a) Connect the power supply wires as indicated in figure 2 and make sure you measure +15
and -15 V and that you know which is which!
b) Build the circuit in figure 4.
+15V
R2= 33 k
10 k
R1= 10 k
+15V
2
-
7
6
+
Temperatuur
sensor
+
Vin
3
4
-15V
-
+
Vuit
-
Figuur 4. Omkeerder versterker
c) Measure the voltage at pins 4 and 7 to make sure the connection is correct.
d) Calculate the gain of the amplifier, using the formula for gain given above.
e) Measure the input and output voltages and determine the gain A= -Vuit/Vin How does this
compare with the answer in d) ?
f) Heat or cool the sensor and observe how the output voltage changes.
4
3. The voltage summer
There is a problem with the previous circuit: it saturates if we want to amplify the input too
much. One solution is to subtract a constant value from the input voltage and then amplify it.
This is referred to as an offset voltage.
Figure 5 shows a summer amplifier. The two voltages at A and B are summed and then
amplified so that
Vuit = - R2/R1(V1 + V2)
+15V
R2= 330 k
(LW: Ander waarde as in 2!)
10 k
V
A
R1= 10 k
+15V
2
-
7
6
Temperatuur
sensor
+15V
+
R1= 10 k
10 k
3
+
4
Vuit
-15V
V
B
-
-15V
Figuur 5. Sommeerder
-15V
+15V
-15V
If the voltage at VA = 2.95 V, we can add -2.95 V to VB to obtain 0 V. If VA now increases
with
10 mV, it is small enough to amplify it considerably before saturation. The ouput
voltage Vuit will increase with 10 * R2/R1 mV/°C .
The variable resistor in figure 5 is used to obtain the offset voltage at VB. This can be adjusted
between -15 V en +15 V.
a) Build the summer circuit of figure 5.
b) Set the voltage at VB so that it is the negative of VA. If the summer works, the output
voltage should be 0 V. (It is very difficult to adjust the output voltage precisely, but try
to make it 0 V)
c) Calculate the gain of the amplifier.
d) Measure VA and Vuit and calculate the gain (A = Vuit/Vin) . Compare with c).
e) Heat or cool the sensor and observe how Vuit changes.
f)
How does the measurements compare to those of the circuit in figure 4?