More Op-Amp Circuits; Temperature Sensing

Transcription

1 ECE 2A Lab #5 Lab 5 More OpAmp Circuits; Temperature Sensing Overview In this lab we will continue our exploration of opamps 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 opamp current source ; the other will use a diode pair and differential amplifier configuration. You will also learn how to operate the oscilloscope in an XY mode, which is useful for characterizing the characteristics of many electronic devices and circuits. Table of Contents Background Information 2 Thermistors 2 Diodes as Temperature Sensors 2 References 3 Prelab Preparation 4 Before Coming to the Lab 4 Required Equipment 4 Parts List 4 InLab Procedure OpAmp CurrentSource; Thermistors 4 First Step: DC Biasing 4 A Simple Thermistor Circuit 5 Testing with a Resistive Heater 6 Oscilloscope XY Mode Differential Measurements; Diode Sensors 7 1 Bob York

2 2 More OpAmp Circuits; Temperature Sensing Background Information Thermistors All metallic conductors have a temperaturedependent 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 R 0 is the resistance at the reference temperature T 0, most commonly taken as 25ºC or T K. 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, 3 copper has TCR /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 are available in two types: positive temperature coefficient Figure 51 Resistance vs. temperature for an NTC thermistor with R0 50k and 4400 K (PTC) and negative temperature coefficient (NTC). A simple model for the temperaturedependence of the thermistor resistance for NTC devices is given by / T / T 0 0 R( T) R e e (5.2) where R 0 is again the resistance at the reference temperature T 0. In this lab we will use an NTCtype of thermistor. The datasheet provides the appropriate values for R 0 and. Figure 51 illustrates the resistancetemperature dependence predicted by (5.2) for a device similar to the one we will use in our lab. Diodes as Temperature Sensors Figure 52 illustrates the currentvoltage 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 qv / nkt I I0 e (5.3) where q C, k 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 3 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 ( V V ) So the difference in voltage between two diodes should scale linearly with temperature. The key assumptions in (5.4) are that the diodes have the identical saturation currents and ideality factors, and that they both at exactly the same temperature. The only good way to satisfy these conditions is to use two diodes that are fabricated sidebyside on the same chip, using identical materials and fabrication processes. The device we will use in the lab integrates two identical devices in this fashion. We will discuss diodes in greater depth in ECE 2B. For now, all you I nkt I (5.4) I q I enkt V1 V2 ln have to do is accept that the IV 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 temperaturedependent 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 D. Sheingold, Transducer Interfacing Handbook, Analog Devices, 1981 Current I, ma K 300K Diode Voltage V, Volts 280K Figure 52 Temperaturedependent IV curves for a representative silicon pn junction.

4 4 More OpAmp Circuits; Temperature Sensing Prelab 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 Description 1 MSD6100G Dual switching diode, TO92 common cathode 1 NTC Thermistor 50K 4.7/C 1 20 Ohm 5W, Cement Resistor 1 Large Binder Clips; Clip size 2 ; Cap. 11/ Ohm 1/4 Watt resistor kohm 1/4 Watt resistor 1 12 kohm 1/4 Watt resistor kohm 1/4 Watt resistor InLab Procedure 5.1 OpAmp CurrentSource; Thermistors First Step: DC Biasing The lab uses opamps again, so remember our advice: always V s 0.1μF make setting up the bias circuit your first step. This time we 7 will use ±12V bias: 2 Insert the opamp in your breadboard and add the wires and bypass capacitors as shown in Figure 53. To avoid 3 6 problems later you may want to attach a small label to 4 the breadboard to indicate which rails correspond to V s, Vs, and ground. V s 0.1μF Next, set the powersupply to produce ±12V and attach the supply and COM connections to the terminals on Figure bias circuit. your breadboard. The use jumper wires to power the rails as shown. Remember, the powersupply COM terminal will be our circuit ground reference. Once you have your bias connections, monitor the output current on the powersupply to be sure that there aren t any inadvertent shortcircuits. 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 OpAmp CurrentSource; Thermistors 5 A Simple Thermistor Circuit As described in the background section the thermistor resistance varies with temperature in a predictable way. Our goal is to design a 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 constant current, and adjust the magnitude of this current so that the voltage varies in a prescribed manner over temperature. Figure 54 illustrates one way of doing this. The current I is set by resistor R 1 and the voltage Vs, which is most conveniently taken as the negative supply voltage, 12V. In this case the output voltage at each temperature will be R ( ) out ( ) 12 T T V T (5.5) R1 The circuit resembles an inverting amplifier, but when used in this fashion it is essentially a voltagecontrolled current source, since the current through the load (the thermistor) is a constant, independent of the value of R T, 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 R 0 at this temperature; you can look this up in the data sheet or just measure it manually using your ohmmeter: Find or measure the roomtemperature resistance of your thermistor, and with the powersupply disconnected or turned off, construct the circuit shown in Figure 54. Use the bench decade box for R 1. Using (5.5), calculate the approximate value of R 1 needed to make Vout 1V at room temperature, and set your decade box to this value. Turn on power and finetune the value of R 1 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 51 or the figure in the datasheet to compute the temperature, or alternatively compute it from (5.2) which can be inverted to give T (5.6) RT ln R0 T0 A value for should be given in the data sheet. V s R 1 I Thermistor R T LM741 V out Figure 54 Thermistor driven by a constantcurrent as set by R 1.

6 6 More OpAmp 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 nickelchrome 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) V s R 1 20Ω/5W R T LM741 V 1 Thermistor and power resistor in thermal contact V out Figure 55 Using a power resistor as a heat source for temperature sensing. Construct the circuit shown in Figure 55, 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 V 1. Use jumper wires as needed for the electrical connections, but take care not to allow the thermistor leads to electrically touch the powerresistor 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 V 1 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 V out. 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 3minute mark. Again use the charts or (5.6) to estimate the temperature of the power resistor. Turn V 1 back to 0V and allow the power resistor to cool. Monitor V out to see when it has returned to 25C ( V out =1.00V). Fanning or blowing on the device may speed this up. Oscilloscope XY Mode So far in ECE 2 we have used the oscilloscope in a timebase or YT 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 XY mode, and is useful for studying the currentvoltage (IV) characteristics of devices, as well as phase relationships of Bob York

7 Differential Measurements; Diode Sensors 7 sinusoidal signals. We will use this mode to display the currentvoltage characteristic of the thermistor. The desired input signal is a triangle wave at 1 khz, with a 10V amplitude (peaktopeak 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. Power resistor Disconnect R 1 from the 12V supply (heat source) V 1 20Ω/5W and reconnect it to the output of the Thermistor function generator, as indicated in and power R Figure 56. Monitor V in using 1 resistor in R T thermal channel 2 of the scope, and monitor the opamp output V out with channel 1. It should be obvious by now that Ch1 will represent the voltage across the thermistor. Remember that this is an inverting amplifier configuration, so the relationship between thermistor current and input voltage is I Vin / R1. To obtain a noninverted 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 IV characteristic with slope equal to (1/R). What resistance do you calculate from your display for the thermistor? Turn the voltage V 1 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 V 1 back to zero when you are finished. 5.2 Differential Measurements; Diode Sensors V in 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 opamp differential amplifier LM741 contact V out Figure 56 Same circuit as in Figure 55, but with a function generator as the stimulus for current in R 1.

8 8 More OpAmp Circuits; Temperature Sensing circuit. The diode pair device chosen for this lab is the MSD6100 shown in Figure 57. The two devices are fabricated on the same chip and hence are very well matched in terms of their IV characteristics. Leaving the ±12V power supply connections to the op amp intact, and verifying that V 1 is still set to 0, modify the circuit to that of Figure 58. 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. Turn on power and make sure V 1 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 Differential Amplifier Figure 57 MSD6100 integrated diode pair in a TO92 package. 1.2kΩ V 1 12kΩ 100 kω 100 kω 100 kω V out LM741 20Ω 5W 100 kω MSD6100 thermally coupled to power resistor Figure 58 Temperature sensor using diodepair and differential amplifier. Now measure V out with the DMM. The opamp 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 opamp circuit is not perfect, however, owing to the tolerances in the resistor values and the internal offset voltage of the opamp. Now turn up V 1 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 Congratulations! You have now completed Lab 5 As noted in the previous lab: keep all your leftover electrical components! Bob York