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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?